File: bashref.html

package info (click to toggle)
bash 5.3-1
links: PTS
area: main
in suites: forky, sid
size: 43,860 kB
sloc: ansic: 134,738; sh: 8,866; yacc: 5,966; makefile: 4,697; perl: 4,105; asm: 48; awk: 23; sed: 16
file content (17852 lines) | stat: -rw-r--r-- 1,040,870 bytes
<!DOCTYPE html>
<html>
<!-- Created by GNU Texinfo 7.2, https://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<!-- This text is a brief description of the features that are present in
the Bash shell (version 5.3, 18 May 2025).

This is Edition 5.3, last updated 18 May 2025,
of The GNU Bash Reference Manual,
for Bash, Version 5.3.

Copyright © 1988-2025 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled
"GNU Free Documentation License". -->
<title>Bash Reference Manual</title>

<meta name="description" content="Bash Reference Manual">
<meta name="keywords" content="Bash Reference Manual">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta name="viewport" content="width=device-width,initial-scale=1">

<link href="#Top" rel="start" title="Top">
<link href="#Indexes" rel="index" title="Indexes">
<link href="#SEC_Contents" rel="contents" title="Table of Contents">
<link href="dir.html#Top" rel="up" title="(dir)">
<link href="#Introduction" rel="next" title="Introduction">
<link href="dir.html#Top" rel="prev" title="(dir)">
<style type="text/css">
<!--
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
a.summary-letter-printindex {text-decoration: none}
div.center {text-align:center}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
kbd.kbd {font-style: oblique}
kbd.key {font-style: normal}
pre.display-preformatted {font-family: inherit}
span.r {font-family: initial; font-weight: normal; font-style: normal}
span:hover a.copiable-link {visibility: visible}
td.printindex-index-entry {vertical-align: top}
td.printindex-index-section {vertical-align: top; padding-left: 1em}
th.entries-header-printindex {text-align:left}
th.sections-header-printindex {text-align:left; padding-left: 1em}
ul.mark-bullet {list-style-type: disc}
ul.toc-numbered-mark {list-style: none}
-->
</style>


</head>

<body lang="en">











<div class="top-level-extent" id="Top">
<div class="nav-panel">
<p>
Next: <a href="#Introduction" accesskey="n" rel="next">Introduction</a>, Previous: <a href="dir.html#Top" accesskey="p" rel="prev">(dir)</a>, Up: <a href="dir.html#Top" accesskey="u" rel="up">(dir)</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h1 class="top" id="Bash-Features-1"><span>Bash Features<a class="copiable-link" href="#Bash-Features-1"> &para;</a></span></h1>

<p>This text is a brief description of the features that are present in
the Bash shell (version 5.3, 18 May 2025).
The Bash home page is <a class="url" href="http://www.gnu.org/software/bash/">http://www.gnu.org/software/bash/</a>.
</p>
<p>This is Edition 5.3, last updated 18 May 2025,
of <cite class="cite">The GNU Bash Reference Manual</cite>,
for <code class="code">Bash</code>, Version 5.3.
</p>
<p>Bash contains features that appear in other popular shells, and some
features that only appear in Bash.
Some of the shells that Bash has
borrowed concepts from are the Bourne Shell (<samp class="file">sh</samp>), the Korn Shell
(<samp class="file">ksh</samp>), and the C-shell (<samp class="file">csh</samp> and its successor,
<samp class="file">tcsh</samp>).
The following menu breaks the features up into
categories, noting which features were inspired by other shells and
which are specific to Bash.
</p>
<p>This manual is meant as a brief introduction to features found in Bash.
The Bash manual page should be used as the definitive
reference on shell behavior.
</p>

<div class="region-contents" id="SEC_Contents">
<h2 class="contents-heading">Table of Contents</h2>

<div class="contents">

<ul class="toc-numbered-mark">
  <li><a id="toc-Introduction" href="#Introduction">1 Introduction</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-What-is-Bash_003f" href="#What-is-Bash_003f">1.1 What is Bash?</a></li>
    <li><a id="toc-What-is-a-shell_003f" href="#What-is-a-shell_003f">1.2 What is a shell?</a></li>
  </ul></li>
  <li><a id="toc-Definitions" href="#Definitions">2 Definitions</a></li>
  <li><a id="toc-Basic-Shell-Features" href="#Basic-Shell-Features">3 Basic Shell Features</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Shell-Syntax" href="#Shell-Syntax">3.1 Shell Syntax</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Shell-Operation" href="#Shell-Operation">3.1.1 Shell Operation</a></li>
      <li><a id="toc-Quoting" href="#Quoting">3.1.2 Quoting</a>
      <ul class="toc-numbered-mark">
        <li><a id="toc-Escape-Character" href="#Escape-Character">3.1.2.1 Escape Character</a></li>
        <li><a id="toc-Single-Quotes" href="#Single-Quotes">3.1.2.2 Single Quotes</a></li>
        <li><a id="toc-Double-Quotes" href="#Double-Quotes">3.1.2.3 Double Quotes</a></li>
        <li><a id="toc-ANSI_002dC-Quoting" href="#ANSI_002dC-Quoting">3.1.2.4 ANSI-C Quoting</a></li>
        <li><a id="toc-Locale_002dSpecific-Translation" href="#Locale-Translation">3.1.2.5 Locale-Specific Translation</a></li>
      </ul></li>
      <li><a id="toc-Comments" href="#Comments">3.1.3 Comments</a></li>
    </ul></li>
    <li><a id="toc-Shell-Commands" href="#Shell-Commands">3.2 Shell Commands</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Reserved-Words" href="#Reserved-Words">3.2.1 Reserved Words</a></li>
      <li><a id="toc-Simple-Commands" href="#Simple-Commands">3.2.2 Simple Commands</a></li>
      <li><a id="toc-Pipelines" href="#Pipelines">3.2.3 Pipelines</a></li>
      <li><a id="toc-Lists-of-Commands" href="#Lists">3.2.4 Lists of Commands</a></li>
      <li><a id="toc-Compound-Commands" href="#Compound-Commands">3.2.5 Compound Commands</a>
      <ul class="toc-numbered-mark">
        <li><a id="toc-Looping-Constructs" href="#Looping-Constructs">3.2.5.1 Looping Constructs</a></li>
        <li><a id="toc-Conditional-Constructs" href="#Conditional-Constructs">3.2.5.2 Conditional Constructs</a></li>
        <li><a id="toc-Grouping-Commands" href="#Command-Grouping">3.2.5.3 Grouping Commands</a></li>
      </ul></li>
      <li><a id="toc-Coprocesses" href="#Coprocesses">3.2.6 Coprocesses</a></li>
      <li><a id="toc-GNU-Parallel" href="#GNU-Parallel">3.2.7 GNU Parallel</a></li>
    </ul></li>
    <li><a id="toc-Shell-Functions" href="#Shell-Functions">3.3 Shell Functions</a></li>
    <li><a id="toc-Shell-Parameters" href="#Shell-Parameters">3.4 Shell Parameters</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Positional-Parameters" href="#Positional-Parameters">3.4.1 Positional Parameters</a></li>
      <li><a id="toc-Special-Parameters" href="#Special-Parameters">3.4.2 Special Parameters</a></li>
    </ul></li>
    <li><a id="toc-Shell-Expansions" href="#Shell-Expansions">3.5 Shell Expansions</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Brace-Expansion" href="#Brace-Expansion">3.5.1 Brace Expansion</a></li>
      <li><a id="toc-Tilde-Expansion" href="#Tilde-Expansion">3.5.2 Tilde Expansion</a></li>
      <li><a id="toc-Shell-Parameter-Expansion" href="#Shell-Parameter-Expansion">3.5.3 Shell Parameter Expansion</a></li>
      <li><a id="toc-Command-Substitution" href="#Command-Substitution">3.5.4 Command Substitution</a></li>
      <li><a id="toc-Arithmetic-Expansion" href="#Arithmetic-Expansion">3.5.5 Arithmetic Expansion</a></li>
      <li><a id="toc-Process-Substitution" href="#Process-Substitution">3.5.6 Process Substitution</a></li>
      <li><a id="toc-Word-Splitting" href="#Word-Splitting">3.5.7 Word Splitting</a></li>
      <li><a id="toc-Filename-Expansion" href="#Filename-Expansion">3.5.8 Filename Expansion</a>
      <ul class="toc-numbered-mark">
        <li><a id="toc-Pattern-Matching" href="#Pattern-Matching">3.5.8.1 Pattern Matching</a></li>
      </ul></li>
      <li><a id="toc-Quote-Removal" href="#Quote-Removal">3.5.9 Quote Removal</a></li>
    </ul></li>
    <li><a id="toc-Redirections" href="#Redirections">3.6 Redirections</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Redirecting-Input" href="#Redirecting-Input">3.6.1 Redirecting Input</a></li>
      <li><a id="toc-Redirecting-Output" href="#Redirecting-Output">3.6.2 Redirecting Output</a></li>
      <li><a id="toc-Appending-Redirected-Output" href="#Appending-Redirected-Output">3.6.3 Appending Redirected Output</a></li>
      <li><a id="toc-Redirecting-Standard-Output-and-Standard-Error" href="#Redirecting-Standard-Output-and-Standard-Error">3.6.4 Redirecting Standard Output and Standard Error</a></li>
      <li><a id="toc-Appending-Standard-Output-and-Standard-Error" href="#Appending-Standard-Output-and-Standard-Error">3.6.5 Appending Standard Output and Standard Error</a></li>
      <li><a id="toc-Here-Documents" href="#Here-Documents">3.6.6 Here Documents</a></li>
      <li><a id="toc-Here-Strings" href="#Here-Strings">3.6.7 Here Strings</a></li>
      <li><a id="toc-Duplicating-File-Descriptors" href="#Duplicating-File-Descriptors">3.6.8 Duplicating File Descriptors</a></li>
      <li><a id="toc-Moving-File-Descriptors" href="#Moving-File-Descriptors">3.6.9 Moving File Descriptors</a></li>
      <li><a id="toc-Opening-File-Descriptors-for-Reading-and-Writing" href="#Opening-File-Descriptors-for-Reading-and-Writing">3.6.10 Opening File Descriptors for Reading and Writing</a></li>
    </ul></li>
    <li><a id="toc-Executing-Commands" href="#Executing-Commands">3.7 Executing Commands</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Simple-Command-Expansion" href="#Simple-Command-Expansion">3.7.1 Simple Command Expansion</a></li>
      <li><a id="toc-Command-Search-and-Execution" href="#Command-Search-and-Execution">3.7.2 Command Search and Execution</a></li>
      <li><a id="toc-Command-Execution-Environment" href="#Command-Execution-Environment">3.7.3 Command Execution Environment</a></li>
      <li><a id="toc-Environment" href="#Environment">3.7.4 Environment</a></li>
      <li><a id="toc-Exit-Status" href="#Exit-Status">3.7.5 Exit Status</a></li>
      <li><a id="toc-Signals" href="#Signals">3.7.6 Signals</a></li>
    </ul></li>
    <li><a id="toc-Shell-Scripts" href="#Shell-Scripts">3.8 Shell Scripts</a></li>
  </ul></li>
  <li><a id="toc-Shell-Builtin-Commands" href="#Shell-Builtin-Commands">4 Shell Builtin Commands</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Bourne-Shell-Builtins" href="#Bourne-Shell-Builtins">4.1 Bourne Shell Builtins</a></li>
    <li><a id="toc-Bash-Builtin-Commands" href="#Bash-Builtins">4.2 Bash Builtin Commands</a></li>
    <li><a id="toc-Modifying-Shell-Behavior" href="#Modifying-Shell-Behavior">4.3 Modifying Shell Behavior</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-The-Set-Builtin" href="#The-Set-Builtin">4.3.1 The Set Builtin</a></li>
      <li><a id="toc-The-Shopt-Builtin" href="#The-Shopt-Builtin">4.3.2 The Shopt Builtin</a></li>
    </ul></li>
    <li><a id="toc-Special-Builtins" href="#Special-Builtins">4.4 Special Builtins</a></li>
  </ul></li>
  <li><a id="toc-Shell-Variables" href="#Shell-Variables">5 Shell Variables</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Bourne-Shell-Variables" href="#Bourne-Shell-Variables">5.1 Bourne Shell Variables</a></li>
    <li><a id="toc-Bash-Variables" href="#Bash-Variables">5.2 Bash Variables</a></li>
  </ul></li>
  <li><a id="toc-Bash-Features-1" href="#Bash-Features">6 Bash Features</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Invoking-Bash" href="#Invoking-Bash">6.1 Invoking Bash</a></li>
    <li><a id="toc-Bash-Startup-Files" href="#Bash-Startup-Files">6.2 Bash Startup Files</a></li>
    <li><a id="toc-Interactive-Shells" href="#Interactive-Shells">6.3 Interactive Shells</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-What-is-an-Interactive-Shell_003f" href="#What-is-an-Interactive-Shell_003f">6.3.1 What is an Interactive Shell?</a></li>
      <li><a id="toc-Is-this-Shell-Interactive_003f" href="#Is-this-Shell-Interactive_003f">6.3.2 Is this Shell Interactive?</a></li>
      <li><a id="toc-Interactive-Shell-Behavior" href="#Interactive-Shell-Behavior">6.3.3 Interactive Shell Behavior</a></li>
    </ul></li>
    <li><a id="toc-Bash-Conditional-Expressions" href="#Bash-Conditional-Expressions">6.4 Bash Conditional Expressions</a></li>
    <li><a id="toc-Shell-Arithmetic" href="#Shell-Arithmetic">6.5 Shell Arithmetic</a></li>
    <li><a id="toc-Aliases" href="#Aliases">6.6 Aliases</a></li>
    <li><a id="toc-Arrays" href="#Arrays">6.7 Arrays</a></li>
    <li><a id="toc-The-Directory-Stack" href="#The-Directory-Stack">6.8 The Directory Stack</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Directory-Stack-Builtins" href="#Directory-Stack-Builtins">6.8.1 Directory Stack Builtins</a></li>
    </ul></li>
    <li><a id="toc-Controlling-the-Prompt" href="#Controlling-the-Prompt">6.9 Controlling the Prompt</a></li>
    <li><a id="toc-The-Restricted-Shell" href="#The-Restricted-Shell">6.10 The Restricted Shell</a></li>
    <li><a id="toc-Bash-and-POSIX" href="#Bash-POSIX-Mode">6.11 Bash and POSIX</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-What-is-POSIX_003f" href="#What-is-POSIX_003f">6.11.1 What is POSIX?</a></li>
      <li><a id="toc-Bash-POSIX-Mode" href="#Bash-POSIX-Mode-1">6.11.2 Bash POSIX Mode</a></li>
    </ul></li>
    <li><a id="toc-Shell-Compatibility-Mode" href="#Shell-Compatibility-Mode">6.12 Shell Compatibility Mode</a></li>
  </ul></li>
  <li><a id="toc-Job-Control" href="#Job-Control">7 Job Control</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Job-Control-Basics" href="#Job-Control-Basics">7.1 Job Control Basics</a></li>
    <li><a id="toc-Job-Control-Builtins" href="#Job-Control-Builtins">7.2 Job Control Builtins</a></li>
    <li><a id="toc-Job-Control-Variables" href="#Job-Control-Variables">7.3 Job Control Variables</a></li>
  </ul></li>
  <li><a id="toc-Command-Line-Editing" href="#Command-Line-Editing">8 Command Line Editing</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Introduction-to-Line-Editing" href="#Introduction-and-Notation">8.1 Introduction to Line Editing</a></li>
    <li><a id="toc-Readline-Interaction" href="#Readline-Interaction">8.2 Readline Interaction</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Readline-Bare-Essentials" href="#Readline-Bare-Essentials">8.2.1 Readline Bare Essentials</a></li>
      <li><a id="toc-Readline-Movement-Commands" href="#Readline-Movement-Commands">8.2.2 Readline Movement Commands</a></li>
      <li><a id="toc-Readline-Killing-Commands" href="#Readline-Killing-Commands">8.2.3 Readline Killing Commands</a></li>
      <li><a id="toc-Readline-Arguments" href="#Readline-Arguments">8.2.4 Readline Arguments</a></li>
      <li><a id="toc-Searching-for-Commands-in-the-History" href="#Searching">8.2.5 Searching for Commands in the History</a></li>
    </ul></li>
    <li><a id="toc-Readline-Init-File" href="#Readline-Init-File">8.3 Readline Init File</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Readline-Init-File-Syntax" href="#Readline-Init-File-Syntax">8.3.1 Readline Init File Syntax</a></li>
      <li><a id="toc-Conditional-Init-Constructs" href="#Conditional-Init-Constructs">8.3.2 Conditional Init Constructs</a></li>
      <li><a id="toc-Sample-Init-File" href="#Sample-Init-File">8.3.3 Sample Init File</a></li>
    </ul></li>
    <li><a id="toc-Bindable-Readline-Commands" href="#Bindable-Readline-Commands">8.4 Bindable Readline Commands</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Commands-For-Moving" href="#Commands-For-Moving">8.4.1 Commands For Moving</a></li>
      <li><a id="toc-Commands-For-Manipulating-The-History" href="#Commands-For-History">8.4.2 Commands For Manipulating The History</a></li>
      <li><a id="toc-Commands-For-Changing-Text" href="#Commands-For-Text">8.4.3 Commands For Changing Text</a></li>
      <li><a id="toc-Killing-And-Yanking" href="#Commands-For-Killing">8.4.4 Killing And Yanking</a></li>
      <li><a id="toc-Specifying-Numeric-Arguments" href="#Numeric-Arguments">8.4.5 Specifying Numeric Arguments</a></li>
      <li><a id="toc-Letting-Readline-Type-For-You" href="#Commands-For-Completion">8.4.6 Letting Readline Type For You</a></li>
      <li><a id="toc-Keyboard-Macros" href="#Keyboard-Macros">8.4.7 Keyboard Macros</a></li>
      <li><a id="toc-Some-Miscellaneous-Commands" href="#Miscellaneous-Commands">8.4.8 Some Miscellaneous Commands</a></li>
    </ul></li>
    <li><a id="toc-Readline-vi-Mode" href="#Readline-vi-Mode">8.5 Readline vi Mode</a></li>
    <li><a id="toc-Programmable-Completion" href="#Programmable-Completion">8.6 Programmable Completion</a></li>
    <li><a id="toc-Programmable-Completion-Builtins" href="#Programmable-Completion-Builtins">8.7 Programmable Completion Builtins</a></li>
    <li><a id="toc-A-Programmable-Completion-Example" href="#A-Programmable-Completion-Example">8.8 A Programmable Completion Example</a></li>
  </ul></li>
  <li><a id="toc-Using-History-Interactively" href="#Using-History-Interactively">9 Using History Interactively</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Bash-History-Facilities" href="#Bash-History-Facilities">9.1 Bash History Facilities</a></li>
    <li><a id="toc-Bash-History-Builtins" href="#Bash-History-Builtins">9.2 Bash History Builtins</a></li>
    <li><a id="toc-History-Expansion" href="#History-Interaction">9.3 History Expansion</a>
    <ul class="toc-numbered-mark">
      <li><a id="toc-Event-Designators" href="#Event-Designators">9.3.1 Event Designators</a></li>
      <li><a id="toc-Word-Designators" href="#Word-Designators">9.3.2 Word Designators</a></li>
      <li><a id="toc-Modifiers" href="#Modifiers">9.3.3 Modifiers</a></li>
    </ul></li>
  </ul></li>
  <li><a id="toc-Installing-Bash" href="#Installing-Bash">10 Installing Bash</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Basic-Installation" href="#Basic-Installation">10.1 Basic Installation</a></li>
    <li><a id="toc-Compilers-and-Options" href="#Compilers-and-Options">10.2 Compilers and Options</a></li>
    <li><a id="toc-Compiling-For-Multiple-Architectures" href="#Compiling-For-Multiple-Architectures">10.3 Compiling For Multiple Architectures</a></li>
    <li><a id="toc-Installation-Names" href="#Installation-Names">10.4 Installation Names</a></li>
    <li><a id="toc-Specifying-the-System-Type" href="#Specifying-the-System-Type">10.5 Specifying the System Type</a></li>
    <li><a id="toc-Sharing-Defaults" href="#Sharing-Defaults">10.6 Sharing Defaults</a></li>
    <li><a id="toc-Operation-Controls" href="#Operation-Controls">10.7 Operation Controls</a></li>
    <li><a id="toc-Optional-Features" href="#Optional-Features">10.8 Optional Features</a></li>
  </ul></li>
  <li><a id="toc-Reporting-Bugs" href="#Reporting-Bugs">Appendix A Reporting Bugs</a></li>
  <li><a id="toc-Major-Differences-From-The-Bourne-Shell" href="#Major-Differences-From-The-Bourne-Shell">Appendix B Major Differences From The Bourne Shell</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Implementation-Differences-From-The-SVR4_002e2-Shell" href="#Implementation-Differences-From-The-SVR4_002e2-Shell">B.1 Implementation Differences From The SVR4.2 Shell</a></li>
  </ul></li>
  <li><a id="toc-GNU-Free-Documentation-License" href="#GNU-Free-Documentation-License">Appendix C GNU Free Documentation License</a></li>
  <li><a id="toc-Indexes" href="#Indexes">Appendix D Indexes</a>
  <ul class="toc-numbered-mark">
    <li><a id="toc-Index-of-Shell-Builtin-Commands" href="#Builtin-Index" rel="index">D.1 Index of Shell Builtin Commands</a></li>
    <li><a id="toc-Index-of-Shell-Reserved-Words" href="#Reserved-Word-Index" rel="index">D.2 Index of Shell Reserved Words</a></li>
    <li><a id="toc-Parameter-and-Variable-Index" href="#Variable-Index" rel="index">D.3 Parameter and Variable Index</a></li>
    <li><a id="toc-Function-Index" href="#Function-Index" rel="index">D.4 Function Index</a></li>
    <li><a id="toc-Concept-Index" href="#Concept-Index" rel="index">D.5 Concept Index</a></li>
  </ul></li>
</ul>
</div>
</div>
<hr>
<div class="chapter-level-extent" id="Introduction">
<div class="nav-panel">
<p>
Next: <a href="#Definitions" accesskey="n" rel="next">Definitions</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Introduction-1"><span>1 Introduction<a class="copiable-link" href="#Introduction-1"> &para;</a></span></h2>

<ul class="mini-toc">
<li><a href="#What-is-Bash_003f" accesskey="1">What is Bash?</a></li>
<li><a href="#What-is-a-shell_003f" accesskey="2">What is a shell?</a></li>
</ul>
<hr>
<div class="section-level-extent" id="What-is-Bash_003f">
<div class="nav-panel">
<p>
Next: <a href="#What-is-a-shell_003f" accesskey="n" rel="next">What is a shell?</a>, Up: <a href="#Introduction" accesskey="u" rel="up">Introduction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="What-is-Bash_003f-1"><span>1.1 What is Bash?<a class="copiable-link" href="#What-is-Bash_003f-1"> &para;</a></span></h3>

<p>Bash is the shell, or command language interpreter,
for the <small class="sc">GNU</small> operating system.
The name is an acronym for the &lsquo;<samp class="samp">Bourne-Again SHell</samp>&rsquo;,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell <code class="code">sh</code>, 
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
</p>
<p>Bash is largely compatible with <code class="code">sh</code> and incorporates useful
features from the Korn shell <code class="code">ksh</code> and the C shell <code class="code">csh</code>.
It is intended to be a conformant implementation of the <small class="sc">IEEE</small>
<small class="sc">POSIX</small> Shell and Tools portion of the <small class="sc">IEEE</small> <small class="sc">POSIX</small>
specification (<small class="sc">IEEE</small> Standard 1003.1).
It offers functional improvements over <code class="code">sh</code> for both interactive and
programming use.
</p>
<p>While the <small class="sc">GNU</small> operating system provides other shells, including
a version of <code class="code">csh</code>, Bash is the default shell.
Like other <small class="sc">GNU</small> software, Bash is quite portable.  It currently runs
on nearly every version of Unix and a few other operating systems &minus;
independently-supported ports exist for Windows and other platforms.
</p>
<hr>
</div>
<div class="section-level-extent" id="What-is-a-shell_003f">
<div class="nav-panel">
<p>
Previous: <a href="#What-is-Bash_003f" accesskey="p" rel="prev">What is Bash?</a>, Up: <a href="#Introduction" accesskey="u" rel="up">Introduction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="What-is-a-shell_003f-1"><span>1.2 What is a shell?<a class="copiable-link" href="#What-is-a-shell_003f-1"> &para;</a></span></h3>

<p>At its base, a shell is simply a macro processor that executes commands.
The term macro processor means functionality where text
and symbols are expanded to create larger expressions.
</p>
<p>A Unix shell is both a command interpreter and a programming language.
As a command interpreter, the shell provides the user
interface to the rich set of <small class="sc">GNU</small> utilities.
The programming language features allow these utilities to be combined.
Users can create files containing commands, and these become
commands themselves. 
These new commands have the same status as
system commands in directories such as <samp class="file">/bin</samp>, allowing users
or groups to establish custom environments to automate their common
tasks.
</p>
<p>Shells may be used interactively or non-interactively.  In
interactive mode, they accept input typed from the keyboard.
When executing non-interactively, shells execute commands read
from a file or a string.
</p>
<p>A shell allows execution of <small class="sc">GNU</small> commands, both synchronously and
asynchronously.
The shell waits for synchronous commands to complete before accepting
more input; asynchronous commands continue to execute in parallel
with the shell while it reads and executes additional commands.
The <em class="dfn">redirection</em> constructs permit
fine-grained control of the input and output of those commands.
Moreover, the shell allows control over the contents of commands&rsquo;
environments.
</p>
<p>Shells also provide a small set of built-in
commands (<em class="dfn">builtins</em>) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, <code class="code">cd</code>, <code class="code">break</code>, <code class="code">continue</code>, and
<code class="code">exec</code> cannot be implemented outside of the shell because
they directly manipulate the shell itself.
The <code class="code">history</code>, <code class="code">getopts</code>, <code class="code">kill</code>, or <code class="code">pwd</code>
builtins, among others, could be implemented in separate utilities,
but they are more convenient to use as builtin commands.
All of the shell builtins are described in subsequent sections.
</p>
<p>While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming languages. 
Like any high-level language, the shell provides variables,
flow control constructs, quoting, and functions. 
</p>
<p>Shells offer features geared specifically for
interactive use rather than to augment the programming language. 
These interactive features include job control, command line
editing, command history and aliases.
This manual describes how Bash provides all of these features.
</p>
<hr>
</div>
</div>
<div class="chapter-level-extent" id="Definitions">
<div class="nav-panel">
<p>
Next: <a href="#Basic-Shell-Features" accesskey="n" rel="next">Basic Shell Features</a>, Previous: <a href="#Introduction" accesskey="p" rel="prev">Introduction</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Definitions-1"><span>2 Definitions<a class="copiable-link" href="#Definitions-1"> &para;</a></span></h2>
<p>These definitions are used throughout the remainder of this manual.
</p>
<dl class="table">
<dt><a id="index-POSIX"></a><span><code class="code">POSIX</code><a class="copiable-link" href="#index-POSIX"> &para;</a></span></dt>
<dd><p>A family of open system standards based on Unix.  Bash
is primarily concerned with the Shell and Utilities portion of the
<small class="sc">POSIX</small> 1003.1 standard. 
</p>
</dd>
<dt><code class="code">blank</code></dt>
<dd><p>A space or tab character.
</p>
</dd>
<dt><code class="code">whitespace</code></dt>
<dd><p>A character belonging to the <code class="code">space</code> character class in the
current locale, or for which <code class="code">isspace()</code> returns true.
</p>
</dd>
<dt><a id="index-builtin-1"></a><span><code class="code">builtin</code><a class="copiable-link" href="#index-builtin-1"> &para;</a></span></dt>
<dd><p>A command that is implemented internally by the shell itself, rather
than by an executable program somewhere in the file system.
</p>
</dd>
<dt><a id="index-control-operator"></a><span><code class="code">control operator</code><a class="copiable-link" href="#index-control-operator"> &para;</a></span></dt>
<dd><p>A <code class="code">token</code> that performs a control function.
It is a <code class="code">newline</code> or one of the following:
&lsquo;<samp class="samp">||</samp>&rsquo;, &lsquo;<samp class="samp">&amp;&amp;</samp>&rsquo;, &lsquo;<samp class="samp">&amp;</samp>&rsquo;, &lsquo;<samp class="samp">;</samp>&rsquo;, &lsquo;<samp class="samp">;;</samp>&rsquo;, &lsquo;<samp class="samp">;&amp;</samp>&rsquo;, &lsquo;<samp class="samp">;;&amp;</samp>&rsquo;,
&lsquo;<samp class="samp">|</samp>&rsquo;, &lsquo;<samp class="samp">|&amp;</samp>&rsquo;, &lsquo;<samp class="samp">(</samp>&rsquo;, or &lsquo;<samp class="samp">)</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-exit-status"></a><span><code class="code">exit status</code><a class="copiable-link" href="#index-exit-status"> &para;</a></span></dt>
<dd><p>The value returned by a command to its caller.
The value is restricted to eight bits, so the maximum value is 255.
</p>
</dd>
<dt><a id="index-field"></a><span><code class="code">field</code><a class="copiable-link" href="#index-field"> &para;</a></span></dt>
<dd><p>A unit of text that is the result of one of the shell expansions.
After expansion, when executing a command, the resulting fields are
used as the command name and arguments.
</p>
</dd>
<dt><a id="index-filename"></a><span><code class="code">filename</code><a class="copiable-link" href="#index-filename"> &para;</a></span></dt>
<dd><p>A string of characters used to identify a file.
</p>
</dd>
<dt><a id="index-job"></a><span><code class="code">job</code><a class="copiable-link" href="#index-job"> &para;</a></span></dt>
<dd><p>A set of processes comprising a pipeline, and any processes descended
from it, that are all in the same process group.
</p>
</dd>
<dt><a id="index-job-control"></a><span><code class="code">job control</code><a class="copiable-link" href="#index-job-control"> &para;</a></span></dt>
<dd><p>A mechanism by which users can selectively stop (suspend) and restart
(resume) execution of processes.
</p>
</dd>
<dt><a id="index-metacharacter"></a><span><code class="code">metacharacter</code><a class="copiable-link" href="#index-metacharacter"> &para;</a></span></dt>
<dd><p>A character that, when unquoted, separates words.
A metacharacter is a <code class="code">space</code>, <code class="code">tab</code>, <code class="code">newline</code>,
or one of the following characters:
&lsquo;<samp class="samp">|</samp>&rsquo;, &lsquo;<samp class="samp">&amp;</samp>&rsquo;, &lsquo;<samp class="samp">;</samp>&rsquo;, &lsquo;<samp class="samp">(</samp>&rsquo;, &lsquo;<samp class="samp">)</samp>&rsquo;, &lsquo;<samp class="samp">&lt;</samp>&rsquo;, or
&lsquo;<samp class="samp">&gt;</samp>&rsquo;.
</p>
</dd>
<dt><a class="index-entry-id" id="index-identifier"></a>
<a id="index-name"></a><span><code class="code">name</code><a class="copiable-link" href="#index-name"> &para;</a></span></dt>
<dd><p>A <code class="code">word</code> consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore.
<code class="code">Name</code>s are used as shell variable and function names.
Also referred to as an <code class="code">identifier</code>.
</p>
</dd>
<dt><a id="index-operator_002c-shell"></a><span><code class="code">operator</code><a class="copiable-link" href="#index-operator_002c-shell"> &para;</a></span></dt>
<dd><p>A <code class="code">control operator</code> or a <code class="code">redirection operator</code>.
See <a class="xref" href="#Redirections">Redirections</a>, for a list of redirection operators.
Operators contain at least one unquoted <code class="code">metacharacter</code>.
</p>
</dd>
<dt><a id="index-process-group"></a><span><code class="code">process group</code><a class="copiable-link" href="#index-process-group"> &para;</a></span></dt>
<dd><p>A collection of related processes each having the same process
group <small class="sc">ID</small>.
</p>
</dd>
<dt><a id="index-process-group-ID"></a><span><code class="code">process group ID</code><a class="copiable-link" href="#index-process-group-ID"> &para;</a></span></dt>
<dd><p>A unique identifier that represents a <code class="code">process group</code>
during its lifetime.
</p>
</dd>
<dt><a id="index-reserved-word"></a><span><code class="code">reserved word</code><a class="copiable-link" href="#index-reserved-word"> &para;</a></span></dt>
<dd><p>A <code class="code">word</code> that has a special meaning to the shell.
Most reserved words introduce shell flow control constructs, such as
<code class="code">for</code> and <code class="code">while</code>.
</p>
</dd>
<dt><a id="index-return-status"></a><span><code class="code">return status</code><a class="copiable-link" href="#index-return-status"> &para;</a></span></dt>
<dd><p>A synonym for <code class="code">exit status</code>.
</p>
</dd>
<dt><a id="index-signal"></a><span><code class="code">signal</code><a class="copiable-link" href="#index-signal"> &para;</a></span></dt>
<dd><p>A mechanism by which a process may be notified by the kernel
of an event occurring in the system.
</p>
</dd>
<dt><a id="index-special-builtin"></a><span><code class="code">special builtin</code><a class="copiable-link" href="#index-special-builtin"> &para;</a></span></dt>
<dd><p>A shell builtin command that has been classified as special by the
<small class="sc">POSIX</small> standard.
</p>
</dd>
<dt><a id="index-token"></a><span><code class="code">token</code><a class="copiable-link" href="#index-token"> &para;</a></span></dt>
<dd><p>A sequence of characters considered a single unit by the shell.
It is either a <code class="code">word</code> or an <code class="code">operator</code>.
</p>
</dd>
<dt><a id="index-word"></a><span><code class="code">word</code><a class="copiable-link" href="#index-word"> &para;</a></span></dt>
<dd><p>A sequence of characters treated as a unit by the shell.
Words may not include unquoted <code class="code">metacharacters</code>.
</p></dd>
</dl>

<hr>
</div>
<div class="chapter-level-extent" id="Basic-Shell-Features">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Builtin-Commands" accesskey="n" rel="next">Shell Builtin Commands</a>, Previous: <a href="#Definitions" accesskey="p" rel="prev">Definitions</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Basic-Shell-Features-1"><span>3 Basic Shell Features<a class="copiable-link" href="#Basic-Shell-Features-1"> &para;</a></span></h2>
<a class="index-entry-id" id="index-Bourne-shell"></a>

<p>Bash is an acronym for &lsquo;<samp class="samp">Bourne-Again SHell</samp>&rsquo;.
The Bourne shell is
the traditional Unix shell originally written by Stephen Bourne.
All of the Bourne shell builtin commands are available in Bash, and
the rules for evaluation and quoting are taken from the <small class="sc">POSIX</small>
specification for the &lsquo;standard&rsquo; Unix shell.
</p>
<p>This chapter briefly summarizes the shell&rsquo;s &lsquo;building blocks&rsquo;:
commands, control structures, shell functions, shell <i class="i">parameters</i>,
shell expansions,
<i class="i">redirections</i>, which are a way to direct input and output from
and to named files, and how the shell executes commands.
</p>

<ul class="mini-toc">
<li><a href="#Shell-Syntax" accesskey="1">Shell Syntax</a></li>
<li><a href="#Shell-Commands" accesskey="2">Shell Commands</a></li>
<li><a href="#Shell-Functions" accesskey="3">Shell Functions</a></li>
<li><a href="#Shell-Parameters" accesskey="4">Shell Parameters</a></li>
<li><a href="#Shell-Expansions" accesskey="5">Shell Expansions</a></li>
<li><a href="#Redirections" accesskey="6">Redirections</a></li>
<li><a href="#Executing-Commands" accesskey="7">Executing Commands</a></li>
<li><a href="#Shell-Scripts" accesskey="8">Shell Scripts</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Shell-Syntax">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Commands" accesskey="n" rel="next">Shell Commands</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Syntax-1"><span>3.1 Shell Syntax<a class="copiable-link" href="#Shell-Syntax-1"> &para;</a></span></h3>

<p>When the shell reads input, it proceeds through a
sequence of operations.  If the input indicates the beginning of a
comment, the shell ignores the comment symbol (&lsquo;<samp class="samp">#</samp>&rsquo;), and the rest
of that line.
</p>                                
<p>Otherwise, roughly speaking,  the shell reads its input and
divides the input into words and operators, employing the quoting rules
to select which meanings to assign various words and characters.
</p>
<p>The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others, redirects input and output as needed, executes the specified
command, waits for the command&rsquo;s exit status, and makes that exit status
available for further inspection or processing.
</p>
<ul class="mini-toc">
<li><a href="#Shell-Operation" accesskey="1">Shell Operation</a></li>
<li><a href="#Quoting" accesskey="2">Quoting</a></li>
<li><a href="#Comments" accesskey="3">Comments</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Shell-Operation">
<div class="nav-panel">
<p>
Next: <a href="#Quoting" accesskey="n" rel="next">Quoting</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Shell-Operation-1"><span>3.1.1 Shell Operation<a class="copiable-link" href="#Shell-Operation-1"> &para;</a></span></h4>

<p>The following is a brief description of the shell&rsquo;s operation when it
reads and executes a command.
Basically, the shell does the following:
</p>
<ol class="enumerate">
<li> Reads its input from a file (see <a class="pxref" href="#Shell-Scripts">Shell Scripts</a>), from a string
supplied as an argument to the <samp class="option">-c</samp> invocation option
(see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>), or from the user&rsquo;s terminal.

</li><li> Breaks the input into words and operators, obeying the quoting rules
described in <a class="ref" href="#Quoting">Quoting</a>.
These tokens are separated by <code class="code">metacharacters</code>.
This step performs alias expansion (see <a class="pxref" href="#Aliases">Aliases</a>).

</li><li> Parses the tokens into simple and compound commands
(see <a class="pxref" href="#Shell-Commands">Shell Commands</a>).

</li><li> Performs the various shell expansions (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>), breaking
the expanded tokens into lists of filenames (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>)
and commands and arguments.

</li><li> Performs any necessary redirections (see <a class="pxref" href="#Redirections">Redirections</a>) and removes
the redirection operators and their operands from the argument list.

</li><li> Executes the command (see <a class="pxref" href="#Executing-Commands">Executing Commands</a>).

</li><li> Optionally waits for the command to complete and collects its exit
status (see <a class="pxref" href="#Exit-Status">Exit Status</a>).

</li></ol>

<hr>
</div>
<div class="subsection-level-extent" id="Quoting">
<div class="nav-panel">
<p>
Next: <a href="#Comments" accesskey="n" rel="next">Comments</a>, Previous: <a href="#Shell-Operation" accesskey="p" rel="prev">Shell Operation</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Quoting-1"><span>3.1.2 Quoting<a class="copiable-link" href="#Quoting-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-quoting"></a>

<p>Quoting is used to remove the special meaning of certain
characters or words to the shell.
Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
</p>
<p>Each of the shell metacharacters (see <a class="pxref" href="#Definitions">Definitions</a>)
has special meaning to the shell and must be quoted if it is to
represent itself.
</p>
<p>When the command history expansion facilities are being used
(see <a class="pxref" href="#History-Interaction">History Expansion</a>), the
<em class="dfn">history expansion</em> character, usually &lsquo;<samp class="samp">!</samp>&rsquo;, must be quoted
to prevent history expansion.
See <a class="xref" href="#Bash-History-Facilities">Bash History Facilities</a>, for more details concerning history
expansion.
</p>
<p>There are four quoting mechanisms: the
<em class="dfn">escape character</em>, single quotes, double quotes,
and dollar-single quotes.
</p>
<ul class="mini-toc">
<li><a href="#Escape-Character" accesskey="1">Escape Character</a></li>
<li><a href="#Single-Quotes" accesskey="2">Single Quotes</a></li>
<li><a href="#Double-Quotes" accesskey="3">Double Quotes</a></li>
<li><a href="#ANSI_002dC-Quoting" accesskey="4">ANSI-C Quoting</a></li>
<li><a href="#Locale-Translation" accesskey="5">Locale-Specific Translation</a></li>
</ul>
<hr>
<div class="subsubsection-level-extent" id="Escape-Character">
<div class="nav-panel">
<p>
Next: <a href="#Single-Quotes" accesskey="n" rel="next">Single Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Escape-Character-1"><span>3.1.2.1 Escape Character<a class="copiable-link" href="#Escape-Character-1"> &para;</a></span></h4>
<p>A non-quoted backslash &lsquo;<samp class="samp">\</samp>&rsquo; is the Bash escape character.
It preserves the literal value of the next character that follows,
removing any special meaning it has,
with the exception of <code class="code">newline</code>.
If a <code class="code">\newline</code> pair appears, and the backslash itself is not quoted,
the <code class="code">\newline</code> is treated as a line continuation (that is, it is
removed from the input stream and effectively ignored).
</p>
<hr>
</div>
<div class="subsubsection-level-extent" id="Single-Quotes">
<div class="nav-panel">
<p>
Next: <a href="#Double-Quotes" accesskey="n" rel="next">Double Quotes</a>, Previous: <a href="#Escape-Character" accesskey="p" rel="prev">Escape Character</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Single-Quotes-1"><span>3.1.2.2 Single Quotes<a class="copiable-link" href="#Single-Quotes-1"> &para;</a></span></h4>

<p>Enclosing characters in single quotes (&lsquo;<samp class="samp">'</samp>&rsquo;) preserves the literal value
of each character within the quotes.
A single quote may not occur
between single quotes, even when preceded by a backslash.
</p>
<hr>
</div>
<div class="subsubsection-level-extent" id="Double-Quotes">
<div class="nav-panel">
<p>
Next: <a href="#ANSI_002dC-Quoting" accesskey="n" rel="next">ANSI-C Quoting</a>, Previous: <a href="#Single-Quotes" accesskey="p" rel="prev">Single Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Double-Quotes-1"><span>3.1.2.3 Double Quotes<a class="copiable-link" href="#Double-Quotes-1"> &para;</a></span></h4>

<p>Enclosing characters in double quotes (&lsquo;<samp class="samp">&quot;</samp>&rsquo;) preserves the literal value
of all characters within the quotes, with the exception of
&lsquo;<samp class="samp">$</samp>&rsquo;, &lsquo;<samp class="samp">`</samp>&rsquo;, &lsquo;<samp class="samp">\</samp>&rsquo;,
and, when history expansion is enabled, &lsquo;<samp class="samp">!</samp>&rsquo;.
When the shell is in
<small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>),
the &lsquo;<samp class="samp">!</samp>&rsquo; has no special meaning
within double quotes, even when history expansion is enabled.
The characters &lsquo;<samp class="samp">$</samp>&rsquo; and &lsquo;<samp class="samp">`</samp>&rsquo;
retain their special meaning within double quotes (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>).
The backslash retains its special meaning only when followed by one of
the following characters:
&lsquo;<samp class="samp">$</samp>&rsquo;, &lsquo;<samp class="samp">`</samp>&rsquo;, &lsquo;<samp class="samp">&quot;</samp>&rsquo;, &lsquo;<samp class="samp">\</samp>&rsquo;, or <code class="code">newline</code>.
Within double quotes, backslashes that are followed by one of these
characters are removed.
Backslashes preceding characters without a
special meaning are left unmodified.
</p>
<p>A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an
&lsquo;<samp class="samp">!</samp>&rsquo;
appearing in double quotes is escaped using a backslash.
The backslash preceding the &lsquo;<samp class="samp">!</samp>&rsquo; is not removed.
</p>
<p>The special parameters &lsquo;<samp class="samp">*</samp>&rsquo; and &lsquo;<samp class="samp">@</samp>&rsquo; have special meaning
when in double quotes (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).
</p>
<hr>
</div>
<div class="subsubsection-level-extent" id="ANSI_002dC-Quoting">
<div class="nav-panel">
<p>
Next: <a href="#Locale-Translation" accesskey="n" rel="next">Locale-Specific Translation</a>, Previous: <a href="#Double-Quotes" accesskey="p" rel="prev">Double Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="ANSI_002dC-Quoting-1"><span>3.1.2.4 ANSI-C Quoting<a class="copiable-link" href="#ANSI_002dC-Quoting-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-quoting_002c-ANSI"></a>
<a class="index-entry-id" id="index-dollar_002dsingle-quote-quoting"></a>

<p>Character sequences of the form <code class="code">$'<var class="var">string</var>'</code> are treated as
a special kind of single quotes.
The sequence expands to <var class="var">string</var>, with backslash-escaped characters
in <var class="var">string</var> replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
</p>
<dl class="table">
<dt><code class="code">\a</code></dt>
<dd><p>alert (bell)
</p></dd>
<dt><code class="code">\b</code></dt>
<dd><p>backspace
</p></dd>
<dt><code class="code">\e</code></dt>
<dt><code class="code">\E</code></dt>
<dd><p>An escape character (not in ANSI C).
</p></dd>
<dt><code class="code">\f</code></dt>
<dd><p>form feed
</p></dd>
<dt><code class="code">\n</code></dt>
<dd><p>newline
</p></dd>
<dt><code class="code">\r</code></dt>
<dd><p>carriage return
</p></dd>
<dt><code class="code">\t</code></dt>
<dd><p>horizontal tab
</p></dd>
<dt><code class="code">\v</code></dt>
<dd><p>vertical tab
</p></dd>
<dt><code class="code">\\</code></dt>
<dd><p>backslash
</p></dd>
<dt><code class="code">\'</code></dt>
<dd><p>single quote
</p></dd>
<dt><code class="code">\&quot;</code></dt>
<dd><p>double quote
</p></dd>
<dt><code class="code">\?</code></dt>
<dd><p>question mark
</p></dd>
<dt><code class="code">\<var class="var">nnn</var></code></dt>
<dd><p>The eight-bit character whose value is the octal value <var class="var">nnn</var>
(one to three octal digits).
</p></dd>
<dt><code class="code">\x<var class="var">HH</var></code></dt>
<dd><p>The eight-bit character whose value is the hexadecimal value <var class="var">HH</var>
(one or two hex digits).
</p></dd>
<dt><code class="code">\u<var class="var">HHHH</var></code></dt>
<dd><p>The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var class="var">HHHH</var> (one to four hex digits).
</p></dd>
<dt><code class="code">\U<var class="var">HHHHHHHH</var></code></dt>
<dd><p>The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var class="var">HHHHHHHH</var> (one to eight hex digits).
</p></dd>
<dt><code class="code">\c<var class="var">x</var></code></dt>
<dd><p>A control-<var class="var">x</var> character.
</p></dd>
</dl>

<p>The expanded result is single-quoted, as if the dollar sign had not
been present.
</p>
<hr>
</div>
<div class="subsubsection-level-extent" id="Locale-Translation">
<div class="nav-panel">
<p>
Previous: <a href="#ANSI_002dC-Quoting" accesskey="p" rel="prev">ANSI-C Quoting</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Locale_002dSpecific-Translation"><span>3.1.2.5 Locale-Specific Translation<a class="copiable-link" href="#Locale_002dSpecific-Translation"> &para;</a></span></h4>
<a class="index-entry-id" id="index-localization"></a>
<a class="index-entry-id" id="index-internationalization"></a>
<a class="index-entry-id" id="index-native-languages"></a>
<a class="index-entry-id" id="index-translation_002c-native-languages"></a>

<p>Prefixing a double-quoted string with a dollar sign (&lsquo;<samp class="samp">$</samp>&rsquo;), such
as <code class="verb">$&quot;hello,&nbsp;world&quot;</code>,
causes the string to be translated according to the current locale.
The <code class="code">gettext</code> infrastructure performs the lookup and 
translation, using the <code class="code">LC_MESSAGES</code>, <code class="code">TEXTDOMAINDIR</code>,
and <code class="code">TEXTDOMAIN</code> shell variables, as explained below.
See the gettext documentation for additional details not covered here.
If the current locale is <code class="code">C</code> or <code class="code">POSIX</code>,
if there are no translations available,
or if the string is not translated, the dollar sign is ignored,
and the string is treated as double-quoted as described above.
Since this is a form of double quoting, the string remains double-quoted
by default, whether or not it is translated and replaced.
If the <code class="code">noexpand_translation</code> option is enabled
using the <code class="code">shopt</code> builtin (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
translated strings are single-quoted instead of double-quoted.
</p>
<p>The rest of this section is a brief overview of how you use gettext to
create translations for strings in a shell script named <var class="var">scriptname</var>.
There are more details in the gettext documentation.
</p>
<hr>
<a class="node-id" id="Creating-Internationalized-Scripts"></a><div class="nav-panel">
<p>
[<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="node"><span>Creating Internationalized Scripts<a class="copiable-link" href="#Creating-Internationalized-Scripts"> &para;</a></span></h4>
<a class="index-entry-id" id="index-internationalized-scripts"></a>
<a class="index-entry-id" id="index-string-translations"></a>
<p>Once you&rsquo;ve marked the strings in your script
that you want to translate using $&quot;&hellip;&quot;,
you create a gettext &quot;template&quot; file using the command
</p>
<div class="example">
<pre class="example-preformatted">bash --dump-po-strings <var class="var">scriptname</var> &gt; <var class="var">domain</var>.pot
</pre></div>

<p>The <var class="var">domain</var> is your <em class="dfn">message domain</em>.
It&rsquo;s just an arbitrary string that&rsquo;s used to identify the files gettext
needs, like a package or script name.
It needs to be unique among all
the message domains on systems where you install the translations, so
gettext knows which translations correspond to your script.
You&rsquo;ll use the template file to create translations for each target language.
The template file conventionally has the suffix &lsquo;<samp class="samp">.pot</samp>&rsquo;.
</p>
<p>You copy this template file to a separate file for each target language
you want to support (called &quot;PO&quot; files, which use the suffix &lsquo;<samp class="samp">.po</samp>&rsquo;).
PO files use various naming conventions, but
when you are working to translate a template file into a particular
language, you first copy the template file to a file whose name is the
language you want to target, with the &lsquo;<samp class="samp">.po</samp>&rsquo; suffix.
For instance, the Spanish translations of your strings would be
in a file named &lsquo;<samp class="samp">es.po</samp>&rsquo;, and to get started using a message
domain named &quot;example,&quot; you would run
</p>
<div class="example">
<pre class="example-preformatted">cp example.pot es.po
</pre></div>

<p>Ultimately, PO files are often named <var class="var">domain</var>.po and installed in
directories that contain multiple translation files for a particular language.
</p>
<p>Whichever naming convention you choose, you will need to translate the
strings in the PO files into the appropriate languages.
This has to be done manually.
</p>
<p>When you have the translations and PO files complete, you&rsquo;ll use the
gettext tools to produce what are called &quot;MO&quot; files, which are compiled
versions of the PO files the gettext tools use to look up translations
efficiently.
MO files are also called &quot;message catalog&quot; files.
You use the <code class="command">msgfmt</code> program to do this.
For instance, if you had a file with Spanish translations, you could run
</p>
<div class="example">
<pre class="example-preformatted">msgfmt -o es.mo es.po
</pre></div>

<p>to produce the corresponding MO file.
</p>
<p>Once you have the MO files, you decide where to install them and use the
<code class="code">TEXTDOMAINDIR</code> shell variable to tell the gettext tools where they are.
Make sure to use the same message domain to name the MO files 
as you did for the PO files when you install them.
</p>
<a class="index-entry-id" id="index-LANG"></a>
<a class="index-entry-id" id="index-LC_005fMESSAGES"></a>
<a class="index-entry-id" id="index-TEXTDOMAIN"></a>
<a class="index-entry-id" id="index-TEXTDOMAINDIR"></a>
<p>Your users will use the <code class="env">LANG</code> or <code class="env">LC_MESSAGES</code> shell variables to
select the desired language.
</p>
<p>You set the <code class="env">TEXTDOMAIN</code> variable to the script&rsquo;s message domain.
As above, you use the message domain to name your translation files.
</p>
<p>You, or possibly your users, set the <code class="env">TEXTDOMAINDIR</code> variable to the
name of a directory where the message catalog files are stored.
If you install the message files into the system&rsquo;s standard message catalog
directory, you don&rsquo;t need to worry about this variable.
</p>
<p>The directory where the message catalog files are stored varies between
systems.
Some use the message catalog selected by the <code class="env">LC_MESSAGES</code>
shell variable.
Others create the name of the message catalog from the value of the
<code class="env">TEXTDOMAIN</code> shell variable, possibly adding the &lsquo;<samp class="samp">.mo</samp>&rsquo; suffix.
If you use the <code class="env">TEXTDOMAIN</code> variable, you may need to set the
<code class="env">TEXTDOMAINDIR</code> variable to the location of the message catalog files,
as above.
It&rsquo;s common to use both variables in this fashion:
<code class="env">$TEXTDOMAINDIR</code>/<code class="env">$LC_MESSAGES</code>/LC_MESSAGES/<code class="env">$TEXTDOMAIN</code>.mo.
</p>
<p>If you used that last convention, and you wanted to store the message
catalog files with Spanish (es) and Esperanto (eo) translations into a
local directory you use for custom translation files, you could run
</p>
<div class="example">
<pre class="example-preformatted">TEXTDOMAIN=example
TEXTDOMAINDIR=/usr/local/share/locale

cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo
</pre></div>

<p>When all of this is done, and the message catalog files containing the
compiled translations are installed in the correct location,
your users will be able to see translated strings
in any of the supported languages by setting the <code class="env">LANG</code> or
<code class="env">LC_MESSAGES</code> environment variables before running your script.
</p>
<hr>
</div>
</div>
<div class="subsection-level-extent" id="Comments">
<div class="nav-panel">
<p>
Previous: <a href="#Quoting" accesskey="p" rel="prev">Quoting</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Comments-1"><span>3.1.3 Comments<a class="copiable-link" href="#Comments-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-comments_002c-shell"></a>

<p>In a non-interactive shell, or an interactive shell in which the
<code class="code">interactive_comments</code> option to the <code class="code">shopt</code>
builtin is enabled (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
a word beginning with &lsquo;<samp class="samp">#</samp>&rsquo; introduces a comment.
A word begins at the beginning of a line, after unquoted whitespace, or
after an operator.
The comment causes that word and all remaining characters on that line to
be ignored.
An interactive shell without the <code class="code">interactive_comments</code>
option enabled does not allow comments.
The <code class="code">interactive_comments</code>
option is enabled by default in interactive shells.
See <a class="xref" href="#Interactive-Shells">Interactive Shells</a>, for a description of what makes
a shell interactive.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Shell-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Functions" accesskey="n" rel="next">Shell Functions</a>, Previous: <a href="#Shell-Syntax" accesskey="p" rel="prev">Shell Syntax</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Commands-1"><span>3.2 Shell Commands<a class="copiable-link" href="#Shell-Commands-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-commands_002c-shell"></a>

<p>A simple shell command such as <code class="code">echo a b c</code> consists of the command
itself followed by arguments, separated by spaces.
</p>
<p>More complex shell commands are composed of simple commands arranged together
in a variety of ways: in a pipeline in which the output of one command
becomes the input of a second, in a loop or conditional construct, or in
some other grouping.
</p>

<ul class="mini-toc">
<li><a href="#Reserved-Words" accesskey="1">Reserved Words</a></li>
<li><a href="#Simple-Commands" accesskey="2">Simple Commands</a></li>
<li><a href="#Pipelines" accesskey="3">Pipelines</a></li>
<li><a href="#Lists" accesskey="4">Lists of Commands</a></li>
<li><a href="#Compound-Commands" accesskey="5">Compound Commands</a></li>
<li><a href="#Coprocesses" accesskey="6">Coprocesses</a></li>
<li><a href="#GNU-Parallel" accesskey="7">GNU Parallel</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Reserved-Words">
<div class="nav-panel">
<p>
Next: <a href="#Simple-Commands" accesskey="n" rel="next">Simple Commands</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Reserved-Words-1"><span>3.2.1 Reserved Words<a class="copiable-link" href="#Reserved-Words-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-reserved-words"></a>

<p>Reserved words are words that have special meaning to the shell.
They are used to begin and end the shell&rsquo;s compound commands.
</p>
<p>The following words are recognized as reserved when unquoted and 
the first word of a command (see below for exceptions):
</p>
<table class="multitable">
<tbody><tr><td width="10%"><code class="code">if</code></td><td width="10%"><code class="code">then</code></td><td width="10%"><code class="code">elif</code></td><td width="10%"><code class="code">else</code></td><td width="12%"><code class="code">fi</code></td><td width="10%"><code class="code">time</code></td></tr>
<tr><td width="10%"><code class="code">for</code></td><td width="10%"><code class="code">in</code></td><td width="10%"><code class="code">until</code></td><td width="10%"><code class="code">while</code></td><td width="12%"><code class="code">do</code></td><td width="10%"><code class="code">done</code></td></tr>
<tr><td width="10%"><code class="code">case</code></td><td width="10%"><code class="code">esac</code></td><td width="10%"><code class="code">coproc</code></td><td width="10%"><code class="code">select</code></td><td width="12%"><code class="code">function</code></td></tr>
<tr><td width="10%"><code class="code">{</code></td><td width="10%"><code class="code">}</code></td><td width="10%"><code class="code">[[</code></td><td width="10%"><code class="code">]]</code></td><td width="12%"><code class="code">!</code></td></tr>
</tbody>
</table>

<p><code class="code">in</code> is recognized as a reserved word if it is the third word of a
<code class="code">case</code> or <code class="code">select</code> command.
<code class="code">in</code> and <code class="code">do</code> are recognized as reserved
words if they are the third word in a <code class="code">for</code> command.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Simple-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Pipelines" accesskey="n" rel="next">Pipelines</a>, Previous: <a href="#Reserved-Words" accesskey="p" rel="prev">Reserved Words</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Simple-Commands-1"><span>3.2.2 Simple Commands<a class="copiable-link" href="#Simple-Commands-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-simple"></a>

<p>A simple command is the kind of command that&rsquo;s executed most often.
It&rsquo;s just a sequence of words separated by <code class="code">blank</code>s, terminated
by one of the shell&rsquo;s control operators (see <a class="pxref" href="#Definitions">Definitions</a>).
The first word generally specifies a command to be executed, with the
rest of the words being that command&rsquo;s arguments.
</p>
<p>The return status (see <a class="pxref" href="#Exit-Status">Exit Status</a>) of a simple command is
its exit status as provided
by the <small class="sc">POSIX</small> 1003.1 <code class="code">waitpid</code> function, or 128+<var class="var">n</var> if
the command was terminated by signal <var class="var">n</var>.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Pipelines">
<div class="nav-panel">
<p>
Next: <a href="#Lists" accesskey="n" rel="next">Lists of Commands</a>, Previous: <a href="#Simple-Commands" accesskey="p" rel="prev">Simple Commands</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Pipelines-1"><span>3.2.3 Pipelines<a class="copiable-link" href="#Pipelines-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-pipeline"></a>
<a class="index-entry-id" id="index-commands_002c-pipelines"></a>

<p>A <code class="code">pipeline</code> is a sequence of one or more commands separated by
one of the control operators &lsquo;<samp class="samp">|</samp>&rsquo; or &lsquo;<samp class="samp">|&amp;</samp>&rsquo;.
</p>
<a class="index-entry-id" id="index-time"></a>
<a class="index-entry-id" id="index-_0021"></a>
<a class="index-entry-id" id="index-command-timing"></a>
<p>The format for a pipeline is
</p><div class="example">
<pre class="example-preformatted">[time [-p]] [!] <var class="var">command1</var> [ | or |&amp; <var class="var">command2</var> ] ...
</pre></div>

<p>The output of each command in the pipeline is connected via a pipe
to the input of the next command.
That is, each command reads the previous command&rsquo;s output.
This connection is performed before any redirections specified by
<var class="var">command1</var>.
</p>
<p>If &lsquo;<samp class="samp">|&amp;</samp>&rsquo; is the pipeline operator,
<var class="var">command1</var>&rsquo;s standard error, in addition to
its standard output, is connected to
<var class="var">command2</var>&rsquo;s standard input through the pipe;
it is shorthand for <code class="code">2&gt;&amp;1 |</code>.
This implicit redirection of the standard error to the standard output is
performed after any redirections specified by <var class="var">command1</var>,
consistent with that shorthand.
</p>
<p>If the reserved word <code class="code">time</code> precedes the pipeline,
Bash prints timing statistics for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command&rsquo;s execution.
The <samp class="option">-p</samp> option changes the output format to that specified
by <small class="sc">POSIX</small>.
When the shell is in <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>),
it does not recognize <code class="code">time</code> as a reserved word if the next
token begins with a &lsquo;<samp class="samp">-</samp>&rsquo;.
The value of the <code class="env">TIMEFORMAT</code> variable is a format string that
specifies how the timing information should be displayed.
See <a class="xref" href="#Bash-Variables">Bash Variables</a>, for a description of the available formats.
Providing <code class="code">time</code> as a reserved word permits the timing of
shell builtins, shell functions, and pipelines.
An external <code class="code">time</code> command cannot time these easily.
</p>
<p>When the shell is in <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>),
you can use <code class="code">time</code> by itself as a simple command.
In this case, the shell displays the
total user and system time consumed by the shell and its children.
The <code class="env">TIMEFORMAT</code> variable specifies the format of the time information.
</p>
<p>If a pipeline is not executed asynchronously (see <a class="pxref" href="#Lists">Lists of Commands</a>), the
shell waits for all commands in the pipeline to complete.
</p>
<p>Each command in a multi-command pipeline,
where pipes are created,
is executed in its own <em class="dfn">subshell</em>, which is a
separate process (see <a class="pxref" href="#Command-Execution-Environment">Command Execution Environment</a>).
If the <code class="code">lastpipe</code> option is enabled using the <code class="code">shopt</code> builtin
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
and  job control is not active,
the last element of a pipeline may be run by the shell process.
</p>
<p>The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the <code class="code">pipefail</code> option is enabled
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
If <code class="code">pipefail</code> is enabled, the pipeline&rsquo;s return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word &lsquo;<samp class="samp">!</samp>&rsquo; precedes the pipeline, the
exit status is the logical negation of the exit status as described
above.
If a pipeline is not executed asynchronously (see <a class="pxref" href="#Lists">Lists of Commands</a>), the
shell waits for all commands in the pipeline to terminate before
returning a value.
The return status of an asynchronous pipeline is 0.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Lists">
<div class="nav-panel">
<p>
Next: <a href="#Compound-Commands" accesskey="n" rel="next">Compound Commands</a>, Previous: <a href="#Pipelines" accesskey="p" rel="prev">Pipelines</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Lists-of-Commands"><span>3.2.4 Lists of Commands<a class="copiable-link" href="#Lists-of-Commands"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-lists"></a>

<p>A <code class="code">list</code> is a sequence of one or more pipelines separated by one
of the operators &lsquo;<samp class="samp">;</samp>&rsquo;, &lsquo;<samp class="samp">&amp;</samp>&rsquo;, &lsquo;<samp class="samp">&amp;&amp;</samp>&rsquo;, or &lsquo;<samp class="samp">||</samp>&rsquo;,
and optionally terminated by one of &lsquo;<samp class="samp">;</samp>&rsquo;, &lsquo;<samp class="samp">&amp;</samp>&rsquo;, or a
<code class="code">newline</code>.
</p>
<p>Of these list operators, &lsquo;<samp class="samp">&amp;&amp;</samp>&rsquo; and &lsquo;<samp class="samp">||</samp>&rsquo;
have equal precedence, followed by &lsquo;<samp class="samp">;</samp>&rsquo; and &lsquo;<samp class="samp">&amp;</samp>&rsquo;,
which have equal precedence.
</p>
<p>A sequence of one or more newlines may appear in a <code class="code">list</code>
to delimit commands, equivalent to a semicolon.
</p>
<p>If a command is terminated by the control operator &lsquo;<samp class="samp">&amp;</samp>&rsquo;,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the <em class="dfn">background</em>,
and these are referred to as <em class="dfn">asynchronous</em> commands.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see <a class="pxref" href="#Job-Control">Job Control</a>),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from <code class="code">/dev/null</code>.
</p>
<p>Commands separated by a &lsquo;<samp class="samp">;</samp>&rsquo; are executed sequentially; the shell
waits for each command to terminate in turn.
The return status is the exit status of the last command executed.
</p>
<p><small class="sc">AND</small> and <small class="sc">OR</small> lists are sequences of one or more pipelines
separated by the control operators &lsquo;<samp class="samp">&amp;&amp;</samp>&rsquo; and &lsquo;<samp class="samp">||</samp>&rsquo;,
respectively.
<small class="sc">AND</small> and <small class="sc">OR</small> lists are executed with left associativity.
</p>
<p>An <small class="sc">AND</small> list has the form
</p><div class="example">
<pre class="example-preformatted"><var class="var">command1</var> &amp;&amp; <var class="var">command2</var>
</pre></div>

<p><var class="var">command2</var> is executed if, and only if, <var class="var">command1</var>
returns an exit status of zero (success).
</p>
<p>An <small class="sc">OR</small> list has the form
</p><div class="example">
<pre class="example-preformatted"><var class="var">command1</var> || <var class="var">command2</var>
</pre></div>

<p><var class="var">command2</var> is executed if, and only if, <var class="var">command1</var>
returns a non-zero exit status.
</p>
<p>The return status of
<small class="sc">AND</small> and <small class="sc">OR</small> lists is the exit status of the last command
executed in the list.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Compound-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Coprocesses" accesskey="n" rel="next">Coprocesses</a>, Previous: <a href="#Lists" accesskey="p" rel="prev">Lists of Commands</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Compound-Commands-1"><span>3.2.5 Compound Commands<a class="copiable-link" href="#Compound-Commands-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-compound"></a>


<p>Compound commands are the shell programming language constructs.
Each construct begins with a reserved word or control operator and is
terminated by a corresponding reserved word or operator.
Any redirections (see <a class="pxref" href="#Redirections">Redirections</a>) associated with a compound command
apply to all commands within that compound command unless explicitly overridden.
</p>
<p>In most cases a list of commands in a compound command&rsquo;s description may be
separated from the rest of the command by one or more newlines, and may be
followed by a newline in place of a semicolon.
</p>
<p>Bash provides looping constructs, conditional commands, and mechanisms
to group commands and execute them as a unit.
</p>
<ul class="mini-toc">
<li><a href="#Looping-Constructs" accesskey="1">Looping Constructs</a></li>
<li><a href="#Conditional-Constructs" accesskey="2">Conditional Constructs</a></li>
<li><a href="#Command-Grouping" accesskey="3">Grouping Commands</a></li>
</ul>
<hr>
<div class="subsubsection-level-extent" id="Looping-Constructs">
<div class="nav-panel">
<p>
Next: <a href="#Conditional-Constructs" accesskey="n" rel="next">Conditional Constructs</a>, Up: <a href="#Compound-Commands" accesskey="u" rel="up">Compound Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Looping-Constructs-1"><span>3.2.5.1 Looping Constructs<a class="copiable-link" href="#Looping-Constructs-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-looping"></a>

<p>Bash supports the following looping constructs.
</p>
<p>Note that wherever a &lsquo;<samp class="samp">;</samp>&rsquo; appears in the description of a
command&rsquo;s syntax, it may be replaced with one or more newlines.
</p>
<dl class="table">
<dt><a class="index-entry-id" id="index-do"></a>
<a class="index-entry-id" id="index-done"></a>
<a id="index-until"></a><span><code class="code">until</code><a class="copiable-link" href="#index-until"> &para;</a></span></dt>
<dd><p>The syntax of the <code class="code">until</code> command is:
</p>
<div class="example">
<pre class="example-preformatted">until <var class="var">test-commands</var>; do <var class="var">consequent-commands</var>; done
</pre></div>

<p>Execute <var class="var">consequent-commands</var> as long as
<var class="var">test-commands</var> has an exit status which is not zero.
The return status is the exit status of the last command executed
in <var class="var">consequent-commands</var>, or zero if none was executed.
</p>
</dd>
<dt><a id="index-while"></a><span><code class="code">while</code><a class="copiable-link" href="#index-while"> &para;</a></span></dt>
<dd><p>The syntax of the <code class="code">while</code> command is:
</p>
<div class="example">
<pre class="example-preformatted">while <var class="var">test-commands</var>; do <var class="var">consequent-commands</var>; done
</pre></div>

<p>Execute <var class="var">consequent-commands</var> as long as
<var class="var">test-commands</var> has an exit status of zero.
The return status is the exit status of the last command executed
in <var class="var">consequent-commands</var>, or zero if none was executed.
</p>
</dd>
<dt><a id="index-for"></a><span><code class="code">for</code><a class="copiable-link" href="#index-for"> &para;</a></span></dt>
<dd><p>The syntax of the <code class="code">for</code> command is:
</p>
<div class="example">
<pre class="example-preformatted">for <var class="var">name</var> [ [in <var class="var">words</var> ...] ; ] do <var class="var">commands</var>; done
</pre></div>

<p>Expand <var class="var">words</var> (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>), and then
execute <var class="var">commands</var> once for each word
in the resultant list, with <var class="var">name</var> bound to the current word.
If &lsquo;<samp class="samp">in <var class="var">words</var></samp>&rsquo; is not present, the <code class="code">for</code> command
executes the <var class="var">commands</var> once for each positional parameter that is
set, as if &lsquo;<samp class="samp">in &quot;$@&quot;</samp>&rsquo; had been specified
(see <a class="pxref" href="#Special-Parameters">Special Parameters</a>).
</p>
<p>The return status is the exit status of the last command that executes.
If there are no items in the expansion of <var class="var">words</var>, no commands are
executed, and the return status is zero.
</p>
<p>There is an alternate form of the <code class="code">for</code> command which is similar to the
C language:
</p>
<div class="example">
<pre class="example-preformatted">for (( <var class="var">expr1</var> ; <var class="var">expr2</var> ; <var class="var">expr3</var> )) [;] do <var class="var">commands</var> ; done
</pre></div>

<p>First, evaluate the arithmetic expression <var class="var">expr1</var> according
to the rules described below (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).
Then, repeatedly evaluate the arithmetic expression <var class="var">expr2</var>
until it evaluates to zero.   
Each time <var class="var">expr2</var> evaluates to a non-zero value, execute <var class="var">commands</var>
and evaluate the arithmetic expression <var class="var">expr3</var>.
If any expression is omitted, it behaves as if it evaluates to 1.
The return value is the exit status of the last command in <var class="var">commands</var>
that is executed, or non-zero if any of the expressions is invalid.
</p></dd>
</dl>

<p>Use the <code class="code">break</code> and <code class="code">continue</code> builtins
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
to control loop execution.
</p>
<hr>
</div>
<div class="subsubsection-level-extent" id="Conditional-Constructs">
<div class="nav-panel">
<p>
Next: <a href="#Command-Grouping" accesskey="n" rel="next">Grouping Commands</a>, Previous: <a href="#Looping-Constructs" accesskey="p" rel="prev">Looping Constructs</a>, Up: <a href="#Compound-Commands" accesskey="u" rel="up">Compound Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Conditional-Constructs-1"><span>3.2.5.2 Conditional Constructs<a class="copiable-link" href="#Conditional-Constructs-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-conditional"></a>

<dl class="table">
<dt><a class="index-entry-id" id="index-then"></a>
<a class="index-entry-id" id="index-else"></a>
<a class="index-entry-id" id="index-elif"></a>
<a class="index-entry-id" id="index-fi"></a>
<a id="index-if"></a><span><code class="code">if</code><a class="copiable-link" href="#index-if"> &para;</a></span></dt>
<dd><p>The syntax of the <code class="code">if</code> command is:
</p>
<div class="example">
<pre class="example-preformatted">if <var class="var">test-commands</var>; then
  <var class="var">consequent-commands</var>;
[elif <var class="var">more-test-commands</var>; then
  <var class="var">more-consequents</var>;]
[else <var class="var">alternate-consequents</var>;]
fi
</pre></div>

<p>The <var class="var">test-commands</var> list is executed, and if its return status is zero,
the <var class="var">consequent-commands</var> list is executed.
If <var class="var">test-commands</var> returns a non-zero status, each <code class="code">elif</code> list
is executed in turn, and if its exit status is zero,
the corresponding <var class="var">more-consequents</var> is executed and the   
command completes.
If &lsquo;<samp class="samp">else <var class="var">alternate-consequents</var></samp>&rsquo; is present, and
the final command in the final <code class="code">if</code> or <code class="code">elif</code> clause
has a non-zero exit status, then <var class="var">alternate-consequents</var> is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.
</p>
</dd>
<dt><a class="index-entry-id" id="index-in"></a>
<a class="index-entry-id" id="index-esac"></a>
<a id="index-case"></a><span><code class="code">case</code><a class="copiable-link" href="#index-case"> &para;</a></span></dt>
<dd><p>The syntax of the <code class="code">case</code> command is:
</p>
<div class="example">
<pre class="example-preformatted">case <var class="var">word</var> in
    [ [(] <var class="var">pattern</var> [| <var class="var">pattern</var>]...) <var class="var">command-list</var> ;;]...
esac
</pre></div>

<p><code class="code">case</code> will selectively execute the <var class="var">command-list</var> corresponding to
the first <var class="var">pattern</var> that matches <var class="var">word</var>,
proceeding from the first pattern to the last.
The match is performed according
to the rules described below in <a class="ref" href="#Pattern-Matching">Pattern Matching</a>.
If the <code class="code">nocasematch</code> shell option
(see the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The &lsquo;<samp class="samp">|</samp>&rsquo; is used to separate multiple patterns in a pattern list,
and the &lsquo;<samp class="samp">)</samp>&rsquo; operator terminates the pattern list.
A pattern list and an associated <var class="var">command-list</var> is known
as a <var class="var">clause</var>.
</p>
<p>Each clause must be terminated with &lsquo;<samp class="samp">;;</samp>&rsquo;, &lsquo;<samp class="samp">;&amp;</samp>&rsquo;, or &lsquo;<samp class="samp">;;&amp;</samp>&rsquo;.
The <var class="var">word</var> undergoes tilde expansion, parameter expansion, command
substitution, process substitution, arithmetic expansion, and quote removal
(see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>)
before the shell attempts to match the pattern.
Each <var class="var">pattern</var> undergoes tilde expansion, parameter expansion,
command substitution, arithmetic expansion, process substitution, and
quote removal.
</p>
<p>There may be an arbitrary number of <code class="code">case</code> clauses, each terminated
by a &lsquo;<samp class="samp">;;</samp>&rsquo;, &lsquo;<samp class="samp">;&amp;</samp>&rsquo;, or &lsquo;<samp class="samp">;;&amp;</samp>&rsquo;.
The first pattern that matches determines the
command-list that is executed.
It&rsquo;s a common idiom to use &lsquo;<samp class="samp">*</samp>&rsquo; as the final pattern to define the
default case, since that pattern will always match.
</p>
<p>Here is an example using <code class="code">case</code> in a script that could be used to
describe one interesting feature of an animal:
</p>
<div class="example">
<pre class="example-preformatted">echo -n &quot;Enter the name of an animal: &quot;
read ANIMAL
echo -n &quot;The $ANIMAL has &quot;
case $ANIMAL in
  horse | dog | cat) echo -n &quot;four&quot;;;
  man | kangaroo ) echo -n &quot;two&quot;;;
  *) echo -n &quot;an unknown number of&quot;;;
esac
echo &quot; legs.&quot;
</pre></div>


<p>If the &lsquo;<samp class="samp">;;</samp>&rsquo; operator is used, the <code class="code">case</code> command completes
after the first pattern match.
Using &lsquo;<samp class="samp">;&amp;</samp>&rsquo;  in place of &lsquo;<samp class="samp">;;</samp>&rsquo; causes execution to continue with
the <var class="var">command-list</var> associated with the next clause, if any.
Using &lsquo;<samp class="samp">;;&amp;</samp>&rsquo; in place of &lsquo;<samp class="samp">;;</samp>&rsquo; causes the shell to test the patterns
in the next clause, if any, and execute any associated <var class="var">command-list</var>
if the match succeeds,
continuing the case statement execution as if the pattern list had not matched.
</p>
<p>The return status is zero if no <var class="var">pattern</var> matches. 
Otherwise, the return status is the exit status of the
last <var class="var">command-list</var> executed.
</p>
</dd>
<dt><a id="index-select"></a><span><code class="code">select</code><a class="copiable-link" href="#index-select"> &para;</a></span></dt>
<dd>
<p>The <code class="code">select</code> construct allows the easy generation of menus.
It has almost the same syntax as the <code class="code">for</code> command:
</p>
<div class="example">
<pre class="example-preformatted">select <var class="var">name</var> [in <var class="var">words</var> ...]; do <var class="var">commands</var>; done
</pre></div>

<p>First, expand the list of words following <code class="code">in</code>, generating a list
of items, and print the set of expanded words on the standard
error stream, each preceded by a number.
If the &lsquo;<samp class="samp">in <var class="var">words</var></samp>&rsquo; is omitted, print the positional parameters,
as if &lsquo;<samp class="samp">in &quot;$@&quot;</samp>&rsquo; had been specified.
<code class="code">select</code> then displays the <code class="env">PS3</code>
prompt and reads a line from the standard input.
If the line consists of a number corresponding to one of the displayed
words, then <code class="code">select</code> sets the value of <var class="var">name</var> to that word.
If the line is empty, <code class="code">select</code> displays the words and prompt again.
If <code class="code">EOF</code> is read, <code class="code">select</code> completes and returns 1.
Any other value read causes <var class="var">name</var> to be set to null.
The line read is saved in the variable <code class="env">REPLY</code>.
</p>
<p>The <var class="var">commands</var> are executed after each selection until a
<code class="code">break</code> command is executed, at which
point the <code class="code">select</code> command completes.
</p>
<p>Here is an example that allows the user to pick a filename from the
current directory, and displays the name and index of the file
selected.
</p>
<div class="example">
<pre class="example-preformatted">select fname in *;
do
	echo you picked $fname \($REPLY\)
	break;
done
</pre></div>

</dd>
<dt><code class="code">((&hellip;))</code></dt>
<dd><div class="example">
<pre class="example-preformatted">(( <var class="var">expression</var> ))
</pre></div>

<p>The arithmetic <var class="var">expression</var> is evaluated according to the rules
described below (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).
The <var class="var">expression</var> undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in <var class="var">expression</var> are not treated
specially and are removed.
Since this can potentially result in empty strings,
this command treats those as expressions that evaluate to 0.
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1. 
</p>
</dd>
<dt><a class="index-entry-id" id="index-_005d_005d"></a>
<a id="index-_005b_005b"></a><span><code class="code">[[&hellip;]]</code><a class="copiable-link" href="#index-_005b_005b"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">[[ <var class="var">expression</var> ]]
</pre></div>

<p>Evaluate the conditional expression <var class="var">expression</var> and
return a status of zero (true) or non-zero (false).
Expressions are composed of the primaries described below in
<a class="ref" href="#Bash-Conditional-Expressions">Bash Conditional Expressions</a>.
The words between the <code class="code">[[</code> and <code class="code">]]</code> do not undergo word splitting
and filename expansion.
The shell performs tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal on those words.
Conditional operators such as &lsquo;<samp class="samp">-f</samp>&rsquo; must be unquoted to be recognized
as primaries.
</p>
<p>When used with <code class="code">[[</code>, the &lsquo;<samp class="samp">&lt;</samp>&rsquo; and &lsquo;<samp class="samp">&gt;</samp>&rsquo; operators sort
lexicographically using the current locale.
</p>
<p>When the &lsquo;<samp class="samp">==</samp>&rsquo; and &lsquo;<samp class="samp">!=</samp>&rsquo; operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below in <a class="ref" href="#Pattern-Matching">Pattern Matching</a>,
as if the <code class="code">extglob</code> shell option were enabled.
The &lsquo;<samp class="samp">=</samp>&rsquo; operator is identical to &lsquo;<samp class="samp">==</samp>&rsquo;.
If the <code class="code">nocasematch</code> shell option
(see the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches (&lsquo;<samp class="samp">==</samp>&rsquo;) or does not
match (&lsquo;<samp class="samp">!=</samp>&rsquo;) the pattern, and 1 otherwise.
</p>
<p>If you quote any part of the pattern,
using any of the shell&rsquo;s quoting mechanisms,
the quoted portion is matched literally.
This means every character in the quoted portion matches itself,
instead of having any special pattern matching meaning.
</p>
<p>An additional binary operator, &lsquo;<samp class="samp">=~</samp>&rsquo;, is available, with the same
precedence as &lsquo;<samp class="samp">==</samp>&rsquo; and &lsquo;<samp class="samp">!=</samp>&rsquo;.
When you use &lsquo;<samp class="samp">=~</samp>&rsquo;, the string to the right of the operator is considered
a <small class="sc">POSIX</small> extended regular expression pattern and matched accordingly
(using the <small class="sc">POSIX</small> <code class="code">regcomp</code> and <code class="code">regexec</code> interfaces
usually described in <i class="i">regex</i>(3)).
The return value is 0 if the string matches the pattern, and 1 if it does not.
If the regular expression is syntactically incorrect, the conditional
expression returns 2.
If the <code class="code">nocasematch</code> shell option
(see the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
is enabled, the match is performed without regard to the case
of alphabetic characters.
</p>
<p>You can quote any part of the pattern
to force the quoted portion to be matched literally
instead of as a regular expression (see above).
If the pattern is stored in a shell variable, quoting the variable
expansion forces the entire pattern to be matched literally.
</p>
<p>The match succeeds if the pattern matches any part of the string.
If you want to force the pattern to match the entire string,
anchor the pattern using the &lsquo;<samp class="samp">^</samp>&rsquo; and &lsquo;<samp class="samp">$</samp>&rsquo; regular expression
operators.
</p>
<p>For example, the following will match a line
(stored in the shell variable <code class="code">line</code>)
if there is a sequence of characters anywhere in the value consisting of
any number, including zero, of 
characters in the <code class="code">space</code> character class,
immediately followed by zero or one instances of &lsquo;<samp class="samp">a</samp>&rsquo;,
then a &lsquo;<samp class="samp">b</samp>&rsquo;:
</p>
<div class="example">
<pre class="example-preformatted">[[ $line =~ [[:space:]]*(a)?b ]]
</pre></div>

<p>That means values for <code class="code">line</code> like
&lsquo;<samp class="samp">aab</samp>&rsquo;, &lsquo;<samp class="samp">  aaaaaab</samp>&rsquo;, &lsquo;<samp class="samp">xaby</samp>&rsquo;, and &lsquo;<samp class="samp"> ab</samp>&rsquo;
will all match,
as will a line containing a &lsquo;<samp class="samp">b</samp>&rsquo; anywhere in its value.
</p>
<p>If you want to match a character that&rsquo;s special to the regular expression
grammar (&lsquo;<samp class="samp">^$|[]()\.*+?</samp>&rsquo;), it has to be quoted to remove its special
meaning.
This means that in the pattern &lsquo;<samp class="samp">xxx.txt</samp>&rsquo;, the &lsquo;<samp class="samp">.</samp>&rsquo; matches any
character in the string (its usual regular expression meaning), but in the
pattern &lsquo;<samp class="samp">&quot;xxx.txt&quot;</samp>&rsquo;, it can only match a literal &lsquo;<samp class="samp">.</samp>&rsquo;.
</p>
<p>Likewise, if you want to include a character in your pattern that has a
special meaning to the regular expression grammar, you must make sure it&rsquo;s
not quoted.
If you want to anchor a pattern at the beginning or end of the string,
for instance, you cannot quote the &lsquo;<samp class="samp">^</samp>&rsquo; or &lsquo;<samp class="samp">$</samp>&rsquo;
characters using any form of shell quoting.
</p>
<p>If you want to match &lsquo;<samp class="samp">initial string</samp>&rsquo; at the start of a line,
the following will work:
</p><div class="example">
<pre class="example-preformatted">[[ $line =~ ^&quot;initial string&quot; ]]
</pre></div>
<p>but this will not:
</p><div class="example">
<pre class="example-preformatted">[[ $line =~ &quot;^initial string&quot; ]]
</pre></div>
<p>because in the second example the &lsquo;<samp class="samp">^</samp>&rsquo; is quoted and doesn&rsquo;t have its
usual special meaning.
</p>
<p>It is sometimes difficult to specify a regular expression properly
without using quotes, or to keep track of the quoting used by regular
expressions while paying attention to
shell quoting and the shell&rsquo;s quote removal.
Storing the regular expression in a shell variable is often a useful
way to avoid problems with quoting characters that are special to the
shell.
For example, the following is equivalent to the pattern used above:
</p>
<div class="example">
<pre class="example-preformatted">pattern='[[:space:]]*(a)?b'
[[ $line =~ $pattern ]]
</pre></div>

<p>Shell programmers should take special care with backslashes, since
backslashes are used by both the shell and regular expressions to remove
the special meaning from the following character.
This means that after the shell&rsquo;s word expansions complete
(see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>),
any backslashes remaining in parts of the pattern
that were originally not quoted can remove the
special meaning of pattern characters.
If any part of the pattern is quoted, the shell does its best to ensure that
the regular expression treats those remaining backslashes as literal,
if they appeared in a quoted portion.
</p>
<p>The following two sets of commands are <em class="emph">not</em> equivalent:
</p>
<div class="example">
<pre class="example-preformatted">pattern='\.'

[[ . =~ $pattern ]]
[[ . =~ \. ]]

[[ . =~ &quot;$pattern&quot; ]]
[[ . =~ '\.' ]]
</pre></div>

<p>The first two matches will succeed, but the second two will not, because
in the second two the backslash will be part of the pattern to be matched.
In the first two examples, the pattern passed to the regular expression
parser is &lsquo;<samp class="samp">\.</samp>&rsquo;.
The backslash removes the special meaning from
&lsquo;<samp class="samp">.</samp>&rsquo;, so the literal &lsquo;<samp class="samp">.</samp>&rsquo; matches.
In the second two examples, the pattern passed to the regular expression
parser has the backslash quoted (e.g., &lsquo;<samp class="samp">\\\.</samp>&rsquo;), which will not match
the string, since it does not contain a backslash.
If the string in the first examples were anything other than &lsquo;<samp class="samp">.</samp>&rsquo;, say
&lsquo;<samp class="samp">a</samp>&rsquo;, the pattern would not match, because the quoted &lsquo;<samp class="samp">.</samp>&rsquo; in the
pattern loses its special meaning of matching any single character.
</p>
<p>Bracket expressions in regular expressions can be sources of errors as well,
since characters that are normally special in regular expressions
lose their special meanings between brackets.
However, you can use bracket expressions to match special pattern characters
without quoting them, so they are sometimes useful for this purpose.
</p>
<p>Though it might seem like a strange way to write it, the following pattern
will match a &lsquo;<samp class="samp">.</samp>&rsquo; in the string:
</p>
<div class="example">
<pre class="example-preformatted">[[ . =~ [.] ]]
</pre></div>

<p>The shell performs any word expansions before passing the pattern
to the regular expression functions,
so you can assume that the shell&rsquo;s quoting takes precedence.
As noted above, the regular expression parser will interpret any
unquoted backslashes remaining in the pattern after shell expansion
according to its own rules.
The intention is to avoid making shell programmers quote things twice
as much as possible, so shell quoting should be sufficient to quote
special pattern characters where that&rsquo;s necessary.
</p>
<p>The array variable <code class="code">BASH_REMATCH</code> records which parts of the string
matched the pattern.
The element of <code class="code">BASH_REMATCH</code> with index 0 contains the portion of
the string matching the entire regular expression.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the remaining <code class="code">BASH_REMATCH</code> indices.
The element of <code class="code">BASH_REMATCH</code> with index <var class="var">n</var> is the portion of the
string matching the <var class="var">n</var>th parenthesized subexpression.
</p>
<p>Bash sets
<code class="code">BASH_REMATCH</code>
in the global scope; declaring it as a local variable will lead to
unexpected results.
</p>
<p>Expressions may be combined using the following operators, listed
in decreasing order of precedence:
</p>
<dl class="table">
<dt><code class="code">( <var class="var">expression</var> )</code></dt>
<dd><p>Returns the value of <var class="var">expression</var>.
This may be used to override the normal precedence of operators.
</p>
</dd>
<dt><code class="code">! <var class="var">expression</var></code></dt>
<dd><p>True if <var class="var">expression</var> is false.
</p>
</dd>
<dt><code class="code"><var class="var">expression1</var> &amp;&amp; <var class="var">expression2</var></code></dt>
<dd><p>True if both <var class="var">expression1</var> and <var class="var">expression2</var> are true.
</p>
</dd>
<dt><code class="code"><var class="var">expression1</var> || <var class="var">expression2</var></code></dt>
<dd><p>True if either <var class="var">expression1</var> or <var class="var">expression2</var> is true.
</p></dd>
</dl>

<p>The <code class="code">&amp;&amp;</code> and <code class="code">||</code> operators do not evaluate <var class="var">expression2</var> if the
value of <var class="var">expression1</var> is sufficient to determine the return
value of the entire conditional expression.
</p></dd>
</dl>

<hr>
</div>
<div class="subsubsection-level-extent" id="Command-Grouping">
<div class="nav-panel">
<p>
Previous: <a href="#Conditional-Constructs" accesskey="p" rel="prev">Conditional Constructs</a>, Up: <a href="#Compound-Commands" accesskey="u" rel="up">Compound Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Grouping-Commands"><span>3.2.5.3 Grouping Commands<a class="copiable-link" href="#Grouping-Commands"> &para;</a></span></h4>
<a class="index-entry-id" id="index-commands_002c-grouping"></a>

<p>Bash provides two ways to group a list of commands to be executed
as a unit.  When commands are grouped, redirections may be applied
to the entire command list.  For example, the output of all the
commands in the list may be redirected to a single stream.
</p>
<dl class="table">
<dt><code class="code">()</code></dt>
<dd><div class="example">
<pre class="example-preformatted">( <var class="var">list</var> )
</pre></div>

<p>Placing a list of commands between parentheses forces the shell to create
a subshell (see <a class="pxref" href="#Command-Execution-Environment">Command Execution Environment</a>), and each
of the commands in <var class="var">list</var> is executed in that subshell environment.
Since the <var class="var">list</var> is executed in a subshell, variable assignments do not
remain in effect after the subshell completes. 
</p>
</dd>
<dt><a class="index-entry-id" id="index-_007d"></a>
<a id="index-_007b"></a><span><code class="code">{}</code><a class="copiable-link" href="#index-_007b"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">{ <var class="var">list</var>; }
</pre></div>

<p>Placing a list of commands between curly braces causes the list to
be executed in the current shell environment.
No subshell is created.
The semicolon (or newline) following <var class="var">list</var> is required.
</p></dd>
</dl>

<p>In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons.  The braces
are reserved words, so they must be separated from the <var class="var">list</var>
by <code class="code">blank</code>s or other shell metacharacters.
The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from the <var class="var">list</var> by whitespace.
</p>
<p>The exit status of both of these constructs is the exit status of
<var class="var">list</var>.
</p>
<hr>
</div>
</div>
<div class="subsection-level-extent" id="Coprocesses">
<div class="nav-panel">
<p>
Next: <a href="#GNU-Parallel" accesskey="n" rel="next">GNU Parallel</a>, Previous: <a href="#Compound-Commands" accesskey="p" rel="prev">Compound Commands</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Coprocesses-1"><span>3.2.6 Coprocesses<a class="copiable-link" href="#Coprocesses-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-coprocess"></a>

<p>A <code class="code">coprocess</code> is a shell command preceded by the <code class="code">coproc</code>
reserved word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the &lsquo;<samp class="samp">&amp;</samp>&rsquo; control operator, with a two-way pipe
established between the executing shell and the coprocess.
</p>
<p>The syntax for a coprocess is:
</p>
<div class="example">
<pre class="example-preformatted">coproc [<var class="var">NAME</var>] <var class="var">command</var> [<var class="var">redirections</var>]
</pre></div>

<p>This creates a coprocess named <var class="var">NAME</var>.
<var class="var">command</var> may be either a simple command (see <a class="pxref" href="#Simple-Commands">Simple Commands</a>)
or a compound command (see <a class="pxref" href="#Compound-Commands">Compound Commands</a>).
<var class="var">NAME</var> is a shell variable name.
If <var class="var">NAME</var> is not supplied, the default name is <code class="code">COPROC</code>.
</p>
<p>The recommended form to use for a coprocess is
</p>
<div class="example">
<pre class="example-preformatted">coproc <var class="var">NAME</var> { <var class="var">command</var>; }
</pre></div>

<p>This form is preferred because simple commands result in the coprocess
always being named <code class="code">COPROC</code>, and it is simpler to use and more complete
than the other compound commands.
</p>
<p>There are other forms of coprocesses:
</p>
<div class="example">
<pre class="example-preformatted">coproc <var class="var">NAME</var> <var class="var">compound-command</var>
coproc <var class="var">compound-command</var>
coproc <var class="var">simple-command</var>
</pre></div>

<p>If <var class="var">command</var> is a compound command, <var class="var">NAME</var> is optional. The
word following <code class="code">coproc</code> determines whether that word is interpreted
as a variable name: it is interpreted as <var class="var">NAME</var> if it is not a
reserved word that introduces a compound command.
If <var class="var">command</var> is a simple command, <var class="var">NAME</var> is not allowed; this
is to avoid confusion between <var class="var">NAME</var> and the first word of the simple
command.
</p>
<p>When the coprocess is executed, the shell creates an array variable
(see <a class="pxref" href="#Arrays">Arrays</a>)
named <var class="var">NAME</var> in the context of the executing shell.
The standard output of <var class="var">command</var>
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to <var class="var">NAME</var>[0].
The standard input of <var class="var">command</var>
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to <var class="var">NAME</var>[1].
This pipe is established before any redirections specified by the
command (see <a class="pxref" href="#Redirections">Redirections</a>).
The file descriptors can be utilized as arguments to shell commands
and redirections using standard word expansions.
Other than those created to execute command and process substitutions,
the file descriptors are not available in subshells.
</p>
<p>The process ID of the shell spawned to execute the coprocess is
available as the value of the variable <code class="env"><var class="var">NAME</var>_PID</code>.
The <code class="code">wait</code>
builtin may be used to wait for the coprocess to terminate.
</p>
<p>Since the coprocess is created as an asynchronous command,
the <code class="code">coproc</code> command always returns success.
The return status of a coprocess is the exit status of <var class="var">command</var>.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="GNU-Parallel">
<div class="nav-panel">
<p>
Previous: <a href="#Coprocesses" accesskey="p" rel="prev">Coprocesses</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="GNU-Parallel-1"><span>3.2.7 GNU Parallel<a class="copiable-link" href="#GNU-Parallel-1"> &para;</a></span></h4>

<p>There are ways to run commands in parallel that are not built into Bash.
GNU Parallel is a tool to do just that.
</p>
<p>GNU Parallel, as its name suggests, can be used to build and run commands
in parallel.  You may run the same command with different arguments, whether
they are filenames, usernames, hostnames, or lines read from files.  GNU
Parallel provides shorthand references to many of the most common operations
(input lines, various portions of the input line, different ways to specify
the input source, and so on).  Parallel can replace <code class="code">xargs</code> or feed
commands from its input sources to several different instances of Bash.
</p>
<p>For a complete description, refer to the GNU Parallel documentation, which
is available at
<a class="url" href="https://www.gnu.org/software/parallel/parallel_tutorial.html">https://www.gnu.org/software/parallel/parallel_tutorial.html</a>.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Shell-Functions">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Parameters" accesskey="n" rel="next">Shell Parameters</a>, Previous: <a href="#Shell-Commands" accesskey="p" rel="prev">Shell Commands</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Functions-1"><span>3.3 Shell Functions<a class="copiable-link" href="#Shell-Functions-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-shell-function"></a>
<a class="index-entry-id" id="index-functions_002c-shell"></a>

<p>Shell functions are a way to group commands for later execution
using a single name for the group.
They are executed just like a &quot;regular&quot; simple command.
When the name of a shell function is used as a simple command name,
the shell executes
the list of commands associated with that function name.
Shell functions are executed in the current
shell context; there is no new process created to interpret them.
</p>
<p>Functions are declared using this syntax:
<a class="index-entry-id" id="index-function"></a>
</p><div class="example">
<pre class="example-preformatted"><var class="var">fname</var> () <var class="var">compound-command</var> [ <var class="var">redirections</var> ]
</pre></div>

<p>or
</p>
<div class="example">
<pre class="example-preformatted">function <var class="var">fname</var> [()] <var class="var">compound-command</var> [ <var class="var">redirections</var> ]
</pre></div>

<p>This defines a shell function named <var class="var">fname</var>.
The reserved word <code class="code">function</code> is optional.
If the <code class="code">function</code> reserved word is supplied, the parentheses are optional.
The <em class="dfn">body</em> of the function is the compound command
<var class="var">compound-command</var> (see <a class="pxref" href="#Compound-Commands">Compound Commands</a>).
That command is usually a <var class="var">list</var> enclosed between { and }, but
may be any compound command listed above.
If the <code class="code">function</code> reserved word is used, but the
parentheses are not supplied, the braces are recommended.
When the shell is in <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>),
<var class="var">fname</var> must be a valid shell name and
may not be the same as one of the special builtins
(see <a class="pxref" href="#Special-Builtins">Special Builtins</a>).
When not in <small class="sc">POSIX</small>  mode,
a function name can be any unquoted shell word that does
not contain &lsquo;<samp class="samp">$</samp>&rsquo;.
</p>
<p>Any redirections (see <a class="pxref" href="#Redirections">Redirections</a>) associated with the shell function
are performed when the function is executed.
Function definitions are deleted using the <samp class="option">-f</samp> option to the
<code class="code">unset</code> builtin (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
</p>
<p>The exit status of a function definition is zero unless a syntax error
occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.
</p>
<p>Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
<code class="code">blank</code>s or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated from the command list
by whitespace or another shell metacharacter.
When using the braces, the <var class="var">list</var> must be terminated by a semicolon,
a &lsquo;<samp class="samp">&amp;</samp>&rsquo;, or a newline.
</p>
<p><var class="var">compound-command</var> is executed whenever <var class="var">fname</var> is specified
as the name of a simple command.
Functions are executed in the context of the
calling shell; there is no new process created to interpret
them (contrast this with the execution of a shell script).
</p>
<p>When a function is executed, the arguments to the
function become the positional parameters
during its execution (see <a class="pxref" href="#Positional-Parameters">Positional Parameters</a>).
The special parameter &lsquo;<samp class="samp">#</samp>&rsquo; that expands to the number of
positional parameters
is updated to reflect the new set of positional parameters.
Special parameter <code class="code">0</code> is unchanged.
The first element of the <code class="env">FUNCNAME</code> variable is set to the
name of the function while the function is executing.
</p>
<p>All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions:
the <code class="env">DEBUG</code> and <code class="env">RETURN</code> traps
are not inherited unless the function has been given the
<code class="code">trace</code> attribute using the <code class="code">declare</code> builtin or
the <code class="code">-o functrace</code> option has been enabled with
the <code class="code">set</code> builtin,
(in which case all functions inherit the <code class="env">DEBUG</code> and <code class="env">RETURN</code> traps),
and the <code class="env">ERR</code> trap is not inherited unless the <code class="code">-o errtrace</code>
shell option has been enabled.
See <a class="xref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>, for the description of the
<code class="code">trap</code> builtin.
</p>
<p>The <code class="env">FUNCNEST</code> variable, if set to a numeric value greater
than 0, defines a maximum function nesting level.  Function
invocations that exceed the limit cause the entire command to
abort.
</p>
<p>If the builtin command <code class="code">return</code>
is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the <code class="code">RETURN</code> trap is executed
before execution resumes.
When a function completes, the values of the
positional parameters and the special parameter &lsquo;<samp class="samp">#</samp>&rsquo;
are restored to the values they had prior to the function&rsquo;s
execution.
If <code class="code">return</code> is supplied a numeric argument,
that is the function&rsquo;s return status; otherwise the function&rsquo;s
return status is the exit status of the last command executed
before the <code class="code">return</code>.
</p>
<p>Variables local to the function are declared with the
<code class="code">local</code> builtin (<em class="dfn">local variables</em>).
Ordinarily, variables and their values
are shared between a function and its caller.
These variables are visible only to
the function and the commands it invokes.
This is particularly
important when a shell function calls other functions.
</p>
<p>In the following description, the <em class="dfn">current scope</em> is a currently-
executing function.
Previous scopes consist of that function&rsquo;s caller and so on,
back to the &quot;global&quot; scope, where the shell is not executing
any shell function.
A local variable at the current local scope is a variable
declared using the <code class="code">local</code> or <code class="code">declare</code> builtins in the
function that is currently executing.
</p>
<p>Local variables &quot;shadow&quot;
variables with the same name declared at previous scopes.
For instance, a local variable declared in a function
hides variables with the same name declared at previous scopes,
including global variables: references and assignments
refer to the local variable, leaving the variables
at previous scopes  unmodified.
When the function returns, the global variable is once again visible.
</p>
<p>The shell uses <em class="dfn">dynamic scoping</em> to control a variable&rsquo;s visibility
within functions.
With dynamic scoping, visible variables and their values
are a result of the sequence of function calls that caused execution
to reach the current function.
The value of a variable that a function sees depends
on its value within its caller, if any, whether that caller is
the global scope or another shell function.
This is also the value that a local variable
declaration shadows, and the value that is restored when the function
returns.
</p>
<p>For example, if a variable <code class="env">var</code> is declared as local in function
<code class="code">func1</code>, and <code class="code">func1</code> calls another function <code class="code">func2</code>,
references to <code class="env">var</code> made from within <code class="code">func2</code> resolve to the
local variable <code class="env">var</code> from <code class="code">func1</code>, shadowing any global variable
named <code class="env">var</code>.
</p>
<p>The following script demonstrates this behavior.
When executed, the script displays
</p>
<div class="example">
<pre class="example-preformatted">In func2, var = func1 local
</pre></div>

<div class="example">
<pre class="example-preformatted">func1()
{
    local var='func1 local'
    func2
}

func2()
{
    echo &quot;In func2, var = $var&quot;
}

var=global
func1
</pre></div>

<p>The <code class="code">unset</code> builtin also acts using the same dynamic scope: if a
variable is local to the current scope, <code class="code">unset</code> unsets it;       
otherwise the unset will refer to the variable found in any calling scope
as described above.
If a variable at the current local scope is unset, it remains so
(appearing as unset)
until it is reset in that scope or until the function returns.
Once the function returns, any instance of the variable at a previous
scope becomes visible.
If the unset acts on a variable at a previous scope, any instance of a
variable with that name that had been shadowed becomes visible
(see below how the <code class="code">localvar_unset</code> shell option changes this behavior).
</p>
<p>The <samp class="option">-f</samp> option to the <code class="code">declare</code> (<code class="code">typeset</code>)
builtin command (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>)
lists function names and definitions.
The <samp class="option">-F</samp> option to <code class="code">declare</code> or <code class="code">typeset</code>
lists the function names only
(and optionally the source file and line number, if the <code class="code">extdebug</code>
shell option is enabled).
Functions may be exported so that child shell processes
(those created when executing a separate shell invocation)
automatically have them defined with the
<samp class="option">-f</samp> option to the <code class="code">export</code> builtin
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
The <samp class="option">-f</samp> option to
the <code class="code">unset</code> builtin
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
deletes a function definition.
</p>
<p>Functions may be recursive.
The <code class="code">FUNCNEST</code> variable may be used to limit the depth of the
function call stack and restrict the number of function invocations.
By default, Bash places no limit on the number of recursive  calls.
</p>
<hr>
</div>
<div class="section-level-extent" id="Shell-Parameters">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Expansions" accesskey="n" rel="next">Shell Expansions</a>, Previous: <a href="#Shell-Functions" accesskey="p" rel="prev">Shell Functions</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Parameters-1"><span>3.4 Shell Parameters<a class="copiable-link" href="#Shell-Parameters-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-parameters"></a>
<a class="index-entry-id" id="index-variable_002c-shell"></a>
<a class="index-entry-id" id="index-shell-variable"></a>


<p>A <em class="dfn">parameter</em> is an entity that stores values.
It can be a <code class="code">name</code>, a number, or one of the special characters
listed below.
A <em class="dfn">variable</em> is a parameter denoted by a <code class="code">name</code>.
A variable has a <code class="code">value</code> and zero or more <code class="code">attributes</code>.
Attributes are assigned using the <code class="code">declare</code> builtin command
(see the description of the <code class="code">declare</code> builtin in <a class="ref" href="#Bash-Builtins">Bash Builtin Commands</a>).
The <code class="code">export</code> and <code class="code">readonly</code> builtins assign specific attributes.
</p>
<p>A parameter is set if it has been assigned a value.
The null string is a valid value.
Once a variable is set, it may be unset only by using
the <code class="code">unset</code> builtin command.
</p>
<p>A variable is assigned to using a statement of the form
</p><div class="example">
<pre class="example-preformatted"><var class="var">name</var>=[<var class="var">value</var>]
</pre></div>
<p>If <var class="var">value</var> is not given, the variable is assigned the null string.
All <var class="var">value</var>s undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).
If the variable has its <code class="code">integer</code>
attribute set, then <var class="var">value</var> 
is evaluated as an arithmetic expression even if the <code class="code">$((&hellip;))</code>
expansion is not used (see <a class="pxref" href="#Arithmetic-Expansion">Arithmetic Expansion</a>).
Word splitting and filename expansion are not performed.
Assignment statements may also appear as arguments to the
<code class="code">alias</code>, 
<code class="code">declare</code>, <code class="code">typeset</code>, <code class="code">export</code>, <code class="code">readonly</code>,
and <code class="code">local</code> builtin commands (<em class="dfn">declaration commands</em>).
When in <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>), these builtins may appear
in a command after one or more instances of the <code class="code">command</code> builtin
and retain these assignment statement properties.
For example,
</p><div class="example">
<pre class="example-preformatted">command export var=value
</pre></div>

<p>In the context where an assignment statement is assigning a value  
to a shell variable or array index (see <a class="pxref" href="#Arrays">Arrays</a>), the
&lsquo;<samp class="samp">+=</samp>&rsquo; operator appends to or adds to
the variable&rsquo;s previous value.
This includes arguments to declaration commands such as
<code class="code">declare</code>
that accept assignment statements.
When &lsquo;<samp class="samp">+=</samp>&rsquo;
is applied to a variable for which the
<code class="code">integer</code> attribute has been set,
the variable&rsquo;s current value and <var class="var">value</var> are each evaluated as
arithmetic expressions,
and the sum of the results is assigned as the variable&rsquo;s value.
The current value is usually an integer constant, but may be an expression.
When &lsquo;<samp class="samp">+=</samp>&rsquo;
is applied to an array variable using compound assignment (see <a class="pxref" href="#Arrays">Arrays</a>),
the variable&rsquo;s value is not unset
(as it is when using &lsquo;<samp class="samp">=</samp>&rsquo;),
and new values are appended to the array
beginning at one greater than the array&rsquo;s maximum index (for indexed arrays),
or added as additional key-value pairs in an associative array.
When applied to a string-valued variable, <var class="var">value</var> is expanded and
appended to the variable&rsquo;s value.
</p>
<p>A variable can be assigned the <code class="code">nameref</code> attribute using the
<samp class="option">-n</samp> option to the <code class="code">declare</code> or <code class="code">local</code> builtin commands
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>)
to create a <em class="dfn">nameref</em>, or a reference to another variable.
This allows variables to be manipulated indirectly.
Whenever the nameref variable is referenced, assigned to, unset, or has
its attributes modified (other than using or changing the nameref
attribute itself), the
operation is actually performed on the variable specified by the nameref
variable&rsquo;s value.
A nameref is commonly used within shell functions to refer to a variable
whose name is passed as an argument to the function.
For instance, if a variable name is passed to a shell function as its first
argument, running
</p><div class="example">
<pre class="example-preformatted">declare -n ref=$1
</pre></div>
<p>inside the function creates a local nameref variable <code class="env">ref</code> whose value
is the variable name passed as the first argument.
References and assignments to <code class="env">ref</code>, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as <code class="code">$1</code>.
</p>
<p>If the control variable in a <code class="code">for</code> loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
is established for each word in the list, in turn, when the loop is
executed.
Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
array variables.
Namerefs can be unset using the <samp class="option">-n</samp> option to the <code class="code">unset</code> builtin
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
Otherwise, if <code class="code">unset</code> is executed with the name of a nameref variable
as an argument, the variable referenced by the nameref variable is unset.
</p>
<p>When the shell starts, it reads its environment and creates a shell
variable from each environment variable that has a valid name,
as described below (see <a class="pxref" href="#Environment">Environment</a>).
</p>
<ul class="mini-toc">
<li><a href="#Positional-Parameters" accesskey="1">Positional Parameters</a></li>
<li><a href="#Special-Parameters" accesskey="2">Special Parameters</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Positional-Parameters">
<div class="nav-panel">
<p>
Next: <a href="#Special-Parameters" accesskey="n" rel="next">Special Parameters</a>, Up: <a href="#Shell-Parameters" accesskey="u" rel="up">Shell Parameters</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Positional-Parameters-1"><span>3.4.1 Positional Parameters<a class="copiable-link" href="#Positional-Parameters-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-parameters_002c-positional"></a>

<p>A <em class="dfn">positional parameter</em> is a parameter denoted by one or more
digits, other than the single digit <code class="code">0</code>.
Positional parameters are
assigned from the shell&rsquo;s arguments when it is invoked,
and may be reassigned using the <code class="code">set</code> builtin command.
Positional parameter <code class="code">N</code> may be referenced as <code class="code">${N}</code>, or
as <code class="code">$N</code> when <code class="code">N</code> consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The <code class="code">set</code> and <code class="code">shift</code> builtins are used to set and
unset them (see <a class="pxref" href="#Shell-Builtin-Commands">Shell Builtin Commands</a>).
The positional parameters are
temporarily replaced when a shell function is executed
(see <a class="pxref" href="#Shell-Functions">Shell Functions</a>).
</p>
<p>When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces.
Without braces, a digit following &lsquo;<samp class="samp">$</samp>&rsquo; can only refer to
one of the first nine positional parameters ($1\-$9) or the
special parameter $0 (see below).
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Special-Parameters">
<div class="nav-panel">
<p>
Previous: <a href="#Positional-Parameters" accesskey="p" rel="prev">Positional Parameters</a>, Up: <a href="#Shell-Parameters" accesskey="u" rel="up">Shell Parameters</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Special-Parameters-1"><span>3.4.2 Special Parameters<a class="copiable-link" href="#Special-Parameters-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-parameters_002c-special"></a>

<p>The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.
Special parameters are denoted by one of the following characters.
</p>
<dl class="vtable">
<dt><a id="index-_002a"></a><span><code class="code">*</code><a class="copiable-link" href="#index-_002a"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_002a"></a>
<p>($*) Expands to the positional parameters, starting from one.
When the expansion is not within double quotes, each positional parameter
expands to a separate word.
In contexts where word expansions are performed, those words
are subject to further word splitting and filename expansion.
When the expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character of the
<code class="env">IFS</code> variable.
That is, <code class="code">&quot;$*&quot;</code> is equivalent
to <code class="code">&quot;$1<var class="var">c</var>$2<var class="var">c</var>&hellip;&quot;</code>, where <var class="var">c</var>
is the first character of the value of the <code class="code">IFS</code>
variable.
If <code class="env">IFS</code> is unset, the parameters are separated by spaces.
If <code class="env">IFS</code> is null, the parameters are joined without intervening
separators.
</p>
</dd>
<dt><a id="index-_0040"></a><span><code class="code">@</code><a class="copiable-link" href="#index-_0040"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_0040"></a>
<p>($@) Expands to the positional parameters, starting from one.
In contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting.
In contexts where word splitting is not performed,
such as the value portion of an assignment statement,
this expands to a single word
with each positional parameter separated by a space.
When the expansion occurs within double quotes,
and word splitting is performed,
each parameter expands to a separate word.
That is, <code class="code">&quot;$@&quot;</code> is equivalent to
<code class="code">&quot;$1&quot; &quot;$2&quot; &hellip;</code>.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the expansion of the
beginning part of the original
word, and the expansion of the last parameter is joined with the
expansion of the last part of the original word.
When there are no positional parameters, <code class="code">&quot;$@&quot;</code> and
<code class="code">$@</code>
expand to nothing (i.e., they are removed).
</p>
</dd>
<dt><a id="index-_0023"></a><span><code class="code">#</code><a class="copiable-link" href="#index-_0023"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_0023"></a>
<p>($#) Expands to the number of positional parameters in decimal.
</p>
</dd>
<dt><a id="index-_003f"></a><span><code class="code">?</code><a class="copiable-link" href="#index-_003f"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_003f"></a>
<p>($?) Expands to the exit status of the most recently executed command.
</p>
</dd>
<dt><a id="index-_002d"></a><span><code class="code">-</code><a class="copiable-link" href="#index-_002d"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_002d"></a>
<p>($-, a hyphen.)  Expands to the current option flags as specified upon
invocation, by the <code class="code">set</code>
builtin command, or those set by the shell itself
(such as the <samp class="option">-i</samp> option).
</p>
</dd>
<dt><a id="index-_0024"></a><span><code class="code">$</code><a class="copiable-link" href="#index-_0024"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_0024"></a>
<p>($$) Expands to the process <small class="sc">ID</small> of the shell.
In a subshell, it expands to the process <small class="sc">ID</small> of the invoking shell,
not the subshell.
</p>
</dd>
<dt><a id="index-_0021-1"></a><span><code class="code">!</code><a class="copiable-link" href="#index-_0021-1"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_0021"></a>
<p>($!) Expands to the process <small class="sc">ID</small> of the job most recently placed into the
background, whether executed as an asynchronous command or using
the <code class="code">bg</code> builtin (see <a class="pxref" href="#Job-Control-Builtins">Job Control Builtins</a>).
</p>
</dd>
<dt><a id="index-0"></a><span><code class="code">0</code><a class="copiable-link" href="#index-0"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_00240"></a>
<p>($0) Expands to the name of the shell or shell script.
This is set at shell initialization.
If Bash is invoked with a file of commands (see <a class="pxref" href="#Shell-Scripts">Shell Scripts</a>),
<code class="code">$0</code> is set to the name of that file.
If Bash is started with the <samp class="option">-c</samp> option (see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>),
then <code class="code">$0</code> is set to the first argument after the string to be
executed, if one is present.
Otherwise, it is set to the filename used to invoke Bash, as given by
argument zero.
</p></dd>
</dl>

<hr>
</div>
</div>
<div class="section-level-extent" id="Shell-Expansions">
<div class="nav-panel">
<p>
Next: <a href="#Redirections" accesskey="n" rel="next">Redirections</a>, Previous: <a href="#Shell-Parameters" accesskey="p" rel="prev">Shell Parameters</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Expansions-1"><span>3.5 Shell Expansions<a class="copiable-link" href="#Shell-Expansions-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-expansion"></a>

<p>Expansion is performed on the command line after it has been split into
<code class="code">token</code>s.
Bash performs these expansions:
</p>
<ul class="itemize mark-bullet">
<li>brace expansion
</li><li>tilde expansion
</li><li>parameter and variable expansion
</li><li>command substitution
</li><li>arithmetic expansion
</li><li>word splitting
</li><li>filename expansion
</li><li>quote removal
</li></ul>


<p>The order of expansions is:
brace expansion;
tilde expansion, parameter and variable expansion, arithmetic expansion,
and command substitution (done in a left-to-right fashion);
word splitting;
filename expansion;
and quote removal.
</p>
<p>On systems that can support it, there is an additional expansion
available: <em class="dfn">process substitution</em>.
This is performed at the
same time as tilde, parameter, variable, and arithmetic expansion and
command substitution.
</p>
<p><em class="dfn">Quote removal</em> is always performed last.
It removes quote characters present in the original word,
not ones resulting from one of the other expansions,
unless they have been quoted themselves.
See <a class="xref" href="#Quote-Removal">Quote Removal</a> for more details.
</p>
<p>Only brace expansion, word splitting, and filename expansion
can increase the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
<code class="code">&quot;$@&quot;</code> and <code class="code">$*</code> (see <a class="pxref" href="#Special-Parameters">Special Parameters</a>), and
<code class="code">&quot;${<var class="var">name</var>[@]}&quot;</code> and <code class="code">${<var class="var">name</var>[*]}</code>
(see <a class="pxref" href="#Arrays">Arrays</a>).
</p>
<ul class="mini-toc">
<li><a href="#Brace-Expansion" accesskey="1">Brace Expansion</a></li>
<li><a href="#Tilde-Expansion" accesskey="2">Tilde Expansion</a></li>
<li><a href="#Shell-Parameter-Expansion" accesskey="3">Shell Parameter Expansion</a></li>
<li><a href="#Command-Substitution" accesskey="4">Command Substitution</a></li>
<li><a href="#Arithmetic-Expansion" accesskey="5">Arithmetic Expansion</a></li>
<li><a href="#Process-Substitution" accesskey="6">Process Substitution</a></li>
<li><a href="#Word-Splitting" accesskey="7">Word Splitting</a></li>
<li><a href="#Filename-Expansion" accesskey="8">Filename Expansion</a></li>
<li><a href="#Quote-Removal" accesskey="9">Quote Removal</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Brace-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Tilde-Expansion" accesskey="n" rel="next">Tilde Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Brace-Expansion-1"><span>3.5.1 Brace Expansion<a class="copiable-link" href="#Brace-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-brace-expansion"></a>
<a class="index-entry-id" id="index-expansion_002c-brace"></a>

<p>Brace expansion is a mechanism to generate arbitrary strings
sharing a common prefix and suffix, either of which can be empty.
This mechanism is similar to
<em class="dfn">filename expansion</em> (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>),
but the filenames generated need not exist.
Patterns to be brace expanded are formed from an optional <var class="var">preamble</var>,
followed by either a series of comma-separated strings or
a sequence expression between a pair of braces,
followed by an optional
<var class="var">postscript</var>.
The preamble is prefixed to each string contained within the braces,
and the postscript is then appended to each resulting string,
expanding left to right.
</p>
<p>Brace expansions may be nested.
The results of each expanded string are not sorted;
brace expansion preserves left to right order.
For example,
</p><div class="example">
<pre class="example-preformatted">bash$ echo a{d,c,b}e
ade ace abe
</pre></div>

<p>A sequence expression takes the form
<code class="code"><var class="var">x</var>..<var class="var">y</var>[..<var class="var">incr</var>]</code>,
where <var class="var">x</var> and <var class="var">y</var> are either integers or letters,
and <var class="var">incr</var>, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
<var class="var">x</var> and <var class="var">y</var>, inclusive.
If either <var class="var">x</var> or <var class="var">y</var> begins with a zero,
each generated term will contain the same number of digits,
zero-padding where necessary.
When letters are supplied, the expression expands to each character
lexicographically between <var class="var">x</var> and <var class="var">y</var>, inclusive,
using the C locale.
Note that both <var class="var">x</var> and <var class="var">y</var> must be of the same type
(integer or letter).
When the increment is supplied, it is used as the difference between
each term.
The default increment is 1 or -1 as appropriate.
</p>
<p>Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result.
It is strictly textual.
Bash does not apply any syntactic interpretation to the context
of the expansion or the text between the braces.
</p>
<p>A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma or a valid
sequence expression.
Any incorrectly formed brace expansion is left unchanged.
</p>
<p>A &lsquo;<samp class="samp">{</samp>&rsquo; or &lsquo;<samp class="samp">,</samp>&rsquo;
may be quoted with a backslash to prevent its
being considered part of a brace expression.
To avoid conflicts with parameter expansion, the string &lsquo;<samp class="samp">${</samp>&rsquo;
is not considered eligible for brace expansion,
and inhibits brace expansion until the closing
&lsquo;<samp class="samp">}</samp>&rsquo;.
</p>
<p>This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the
above example:
</p><div class="example">
<pre class="example-preformatted">mkdir /usr/local/src/bash/{old,new,dist,bugs}
</pre></div>
<p>or
</p><div class="example">
<pre class="example-preformatted">chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
</pre></div>

<p>Brace expansion introduces a slight incompatibility with
historical versions of
<code class="command">sh</code>.
<code class="command">sh</code>
does not treat opening or closing braces specially when they
appear as part of a word, and preserves them in the output.
Bash
removes braces from words as a consequence of brace
expansion.
For example, a word entered to
<code class="command">sh</code>
as
&lsquo;<samp class="samp">file{1,2}</samp>&rsquo;
appears identically in the output.
Bash
outputs that word as
&lsquo;<samp class="samp">file1 file2</samp>&rsquo;
after brace expansion.
Start
Bash
with the
<samp class="option">+B</samp>
option or disable brace expansion with the
<samp class="option">+B</samp>
option to the
<code class="code">set</code>
command
(see <a class="pxref" href="#Shell-Builtin-Commands">Shell Builtin Commands</a>)
for strict
<code class="command">sh</code>
compatibility.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Tilde-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Parameter-Expansion" accesskey="n" rel="next">Shell Parameter Expansion</a>, Previous: <a href="#Brace-Expansion" accesskey="p" rel="prev">Brace Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Tilde-Expansion-1"><span>3.5.2 Tilde Expansion<a class="copiable-link" href="#Tilde-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-tilde-expansion"></a>
<a class="index-entry-id" id="index-expansion_002c-tilde"></a>

<p>If a word begins with an unquoted tilde character (&lsquo;<samp class="samp">~</samp>&rsquo;), all of the
characters up to the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a <em class="dfn">tilde-prefix</em>.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible <em class="dfn">login name</em>.
If this login name is the null string, the tilde is replaced with the
value of the <code class="env">HOME</code> shell variable.
If <code class="env">HOME</code> is unset, the tilde expands to
the home directory of the user executing the shell instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
</p>
<p>If the tilde-prefix is &lsquo;<samp class="samp">~+</samp>&rsquo;, the value of
the shell variable <code class="env">PWD</code> replaces the tilde-prefix.
If the tilde-prefix is &lsquo;<samp class="samp">~-</samp>&rsquo;, the shell substitutes
the value of the shell variable
<code class="env">OLDPWD</code>, if it is set.
</p>
<p>If the characters following the tilde in the tilde-prefix consist of a
number <var class="var">N</var>, optionally prefixed by a &lsquo;<samp class="samp">+</samp>&rsquo; or a &lsquo;<samp class="samp">-</samp>&rsquo;,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the <code class="code">dirs</code> builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see <a class="pxref" href="#The-Directory-Stack">The Directory Stack</a>).
If the tilde-prefix, sans the tilde, consists of a number without a
leading &lsquo;<samp class="samp">+</samp>&rsquo; or &lsquo;<samp class="samp">-</samp>&rsquo;, tilde expansion assumes &lsquo;<samp class="samp">+</samp>&rsquo;.
</p>
<p>The results of tilde expansion are treated as if they were quoted, so
the replacement is not subject to word splitting and filename expansion.
</p>
<p>If the login name is invalid, or the tilde expansion fails, the
tilde-prefix is left unchanged.
</p>
<p>Bash checks each variable assignment
for unquoted tilde-prefixes immediately
following a &lsquo;<samp class="samp">:</samp>&rsquo; or the first &lsquo;<samp class="samp">=</samp>&rsquo;,
and performs tilde expansion in these cases.
Consequently, one may use filenames with tildes in assignments to
<code class="env">PATH</code>, <code class="env">MAILPATH</code>, and <code class="env">CDPATH</code>,
and the shell assigns the expanded value.
</p>
<p>The following table shows how Bash treats unquoted tilde-prefixes:
</p>
<dl class="table">
<dt><code class="code">~</code></dt>
<dd><p>The value of <code class="code">$HOME</code>.
</p></dd>
<dt><code class="code">~/foo</code></dt>
<dd><p><samp class="file">$HOME/foo</samp>
</p>
</dd>
<dt><code class="code">~fred/foo</code></dt>
<dd><p>The directory or file <code class="code">foo</code> in the home directory of the user
<code class="code">fred</code>.
</p>
</dd>
<dt><code class="code">~+/foo</code></dt>
<dd><p><samp class="file">$PWD/foo</samp>
</p>
</dd>
<dt><code class="code">~-/foo</code></dt>
<dd><p><samp class="file">${OLDPWD-'~-'}/foo</samp>
</p>
</dd>
<dt><code class="code">~<var class="var">N</var></code></dt>
<dd><p>The string that would be displayed by &lsquo;<samp class="samp">dirs +<var class="var">N</var></samp>&rsquo;.
</p>
</dd>
<dt><code class="code">~+<var class="var">N</var></code></dt>
<dd><p>The string that would be displayed by &lsquo;<samp class="samp">dirs +<var class="var">N</var></samp>&rsquo;.
</p>
</dd>
<dt><code class="code">~-<var class="var">N</var></code></dt>
<dd><p>The string that would be displayed by &lsquo;<samp class="samp">dirs -<var class="var">N</var></samp>&rsquo;.
</p></dd>
</dl>

<p>Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (see <a class="pxref" href="#Shell-Parameters">Shell Parameters</a>)
when they appear as arguments to simple commands.
Bash does not do this, except for the declaration commands listed
above, when in <small class="sc">POSIX</small> mode.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Shell-Parameter-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Command-Substitution" accesskey="n" rel="next">Command Substitution</a>, Previous: <a href="#Tilde-Expansion" accesskey="p" rel="prev">Tilde Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Shell-Parameter-Expansion-1"><span>3.5.3 Shell Parameter Expansion<a class="copiable-link" href="#Shell-Parameter-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-parameter-expansion"></a>
<a class="index-entry-id" id="index-expansion_002c-parameter"></a>

<p>The &lsquo;<samp class="samp">$</samp>&rsquo; character introduces parameter expansion,
command substitution, or arithmetic expansion.
The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be expanded from
characters immediately following it which could be
interpreted as part of the name.
For example, if the first positional parameter has the value &lsquo;<samp class="samp">a</samp>&rsquo;,
then <code class="code">${11}</code> expands to the value of the eleventh positional
parameter, while <code class="code">$11</code> expands to &lsquo;<samp class="samp">a1</samp>&rsquo;.
</p>
<p>When braces are used, the matching ending brace is the first &lsquo;<samp class="samp">}</samp>&rsquo;
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
</p>
<p>The basic form of parameter expansion is ${<var class="var">parameter</var>},
which substitutes the value of <var class="var">parameter</var>.
The <var class="var">parameter</var> is a shell parameter as described above
(see <a class="pxref" href="#Shell-Parameters">Shell Parameters</a>) or an array reference (see <a class="pxref" href="#Arrays">Arrays</a>).
The braces are required when <var class="var">parameter</var>
is a positional parameter with more than one digit,
or when <var class="var">parameter</var> is followed by a character that is not to be
interpreted as part of its name.
</p>
<p>If the first character of <var class="var">parameter</var> is an exclamation point (!),
and <var class="var">parameter</var> is not a nameref,
it introduces a level of indirection.
Bash uses the value formed by expanding the rest of
<var class="var">parameter</var> as the new <var class="var">parameter</var>;
this new parameter is then
expanded and that value is used
in the rest of the expansion, rather
than the expansion of the original <var class="var">parameter</var>.
This is known as <code class="code">indirect expansion</code>.
The value is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
If <var class="var">parameter</var> is a nameref, this expands to the name of the
variable referenced by <var class="var">parameter</var> instead of performing the
complete indirect expansion, for compatibility.
The exceptions to this are the expansions of ${!<var class="var">prefix</var>*}
and ${!<var class="var">name</var>[@]}
described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.
</p>
<p>In each of the cases below, <var class="var">word</var> is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
</p>
<p>When not performing substring expansion, using the forms described
below (e.g., &lsquo;<samp class="samp">:-</samp>&rsquo;), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is unset.
Put another way, if the colon is included,
the operator tests for both <var class="var">parameter</var>&rsquo;s existence and that its value
is not null; if the colon is omitted, the operator tests only for existence.
</p>
<dl class="table">
<dt><code class="code">${<var class="var">parameter</var>:&minus;<var class="var">word</var>}</code></dt>
<dd><p>If <var class="var">parameter</var> is unset or null, the expansion of
<var class="var">word</var> is substituted.
Otherwise, the value of <var class="var">parameter</var> is substituted.
</p>
<div class="example">
<pre class="example-preformatted">$ v=123
$ echo ${v-unset}
123
$ echo ${v:-unset-or-null}
123
$ unset v
$ echo ${v-unset}
unset
$ v=
$ echo ${v-unset}

$ echo ${v:-unset-or-null}
unset-or-null
</pre></div>

</dd>
<dt><code class="code">${<var class="var">parameter</var>:=<var class="var">word</var>}</code></dt>
<dd><p>If <var class="var">parameter</var> is unset or null, the expansion of <var class="var">word</var>
is assigned to <var class="var">parameter</var>,
and the result of the expansion
is the final value of <var class="var">parameter</var>.
Positional parameters and special parameters may not be assigned
in this way.
</p>
<div class="example">
<pre class="example-preformatted">$ unset var
$ : ${var=DEFAULT}
$ echo $var
DEFAULT
$ var=
$ : ${var=DEFAULT}
$ echo $var

$ var=
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
$ unset var
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
</pre></div>

</dd>
<dt><code class="code">${<var class="var">parameter</var>:?<var class="var">word</var>}</code></dt>
<dd><p>If <var class="var">parameter</var>
is null or unset, the shell writes
the expansion of <var class="var">word</var> (or a message
to that effect if <var class="var">word</var>
is not present) to the standard error and, if it
is not interactive, exits with a non-zero status.
An interactive shell does not exit, but does not execute the command
associated with the expansion.
Otherwise, the value of <var class="var">parameter</var> is substituted.
</p>
<div class="example">
<pre class="example-preformatted">$ var=
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ echo ${var?var is unset}

$ unset var
$ : ${var?var is unset}
bash: var: var is unset
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ var=123
$ echo ${var:?var is unset or null}
123
</pre></div>

</dd>
<dt><code class="code">${<var class="var">parameter</var>:+<var class="var">word</var>}</code></dt>
<dd><p>If <var class="var">parameter</var>
is null or unset, nothing is substituted, otherwise the expansion of
<var class="var">word</var> is substituted.
The value of <var class="var">parameter</var> is not used.
</p>
<div class="example">
<pre class="example-preformatted">$ var=123
$ echo ${var:+var is set and not null}
var is set and not null
$ echo ${var+var is set}
var is set
$ var=
$ echo ${var:+var is set and not null}

$ echo ${var+var is set}
var is set
$ unset var
$ echo ${var+var is set}

$ echo ${var:+var is set and not null}

$ 
</pre></div>

</dd>
<dt><code class="code">${<var class="var">parameter</var>:<var class="var">offset</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>:<var class="var">offset</var>:<var class="var">length</var>}</code></dt>
<dd><p>This is referred to as Substring Expansion.
It expands to up to <var class="var">length</var> characters of the value of <var class="var">parameter</var>
starting at the character specified by <var class="var">offset</var>.
If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;, an indexed array subscripted by
&lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;, or an associative array name, the results differ as
described below.
If :<var class="var">length</var> is omitted (the first form above), this
expands to the substring of the value of
<var class="var">parameter</var> starting at the character specified by <var class="var">offset</var>
and extending to the end of the value.
If <var class="var">offset</var> is omitted,
it is treated as 0.
If <var class="var">length</var> is omitted,
but the colon after <var class="var">offset</var> is present,
it is treated as 0.
<var class="var">length</var> and <var class="var">offset</var> are arithmetic expressions
(see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).
</p>
<p>If <var class="var">offset</var> evaluates to a number less than zero, the value
is used as an offset in characters
from the end of the value of <var class="var">parameter</var>.
If <var class="var">length</var> evaluates to a number less than zero,
it is interpreted as an offset in characters
from the end of the value of <var class="var">parameter</var> rather than
a number of characters, and the expansion is the characters between
<var class="var">offset</var> and that result.
</p>
<p>Note that a negative offset must be separated from the colon by at least
one space to avoid being confused with the &lsquo;<samp class="samp">:-</samp>&rsquo; expansion.
</p>
<p>Here are some examples illustrating substring expansion on parameters and
subscripted arrays:
</p>
<pre class="verbatim">$ string=01234567890abcdefgh
$ echo ${string:7}
7890abcdefgh
$ echo ${string:7:0}

$ echo ${string:7:2}
78
$ echo ${string:7:-2}
7890abcdef
$ echo ${string: -7}
bcdefgh
$ echo ${string: -7:0}

$ echo ${string: -7:2}
bc
$ echo ${string: -7:-2}
bcdef
$ set -- 01234567890abcdefgh
$ echo ${1:7}
7890abcdefgh
$ echo ${1:7:0}

$ echo ${1:7:2}
78
$ echo ${1:7:-2}
7890abcdef
$ echo ${1: -7}
bcdefgh
$ echo ${1: -7:0}

$ echo ${1: -7:2}
bc
$ echo ${1: -7:-2}
bcdef
$ array[0]=01234567890abcdefgh
$ echo ${array[0]:7}
7890abcdefgh
$ echo ${array[0]:7:0}

$ echo ${array[0]:7:2}
78
$ echo ${array[0]:7:-2}
7890abcdef
$ echo ${array[0]: -7}
bcdefgh
$ echo ${array[0]: -7:0}

$ echo ${array[0]: -7:2}
bc
$ echo ${array[0]: -7:-2}
bcdef
</pre>
<p>If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;, the result is <var class="var">length</var>
positional parameters beginning at <var class="var">offset</var>.
A negative <var class="var">offset</var> is taken relative to one greater than the greatest
positional parameter, so an offset of -1 evaluates to the last positional
parameter (or 0 if there are no positional parameters).
It is an expansion error if <var class="var">length</var> evaluates to a number less than zero.
</p>
<p>The following examples illustrate substring expansion using positional
parameters:
</p>
<pre class="verbatim">$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:7}
7 8 9 0 a b c d e f g h
$ echo ${@:7:0}

$ echo ${@:7:2}
7 8
$ echo ${@:7:-2}
bash: -2: substring expression &lt; 0
$ echo ${@: -7:2}
b c
$ echo ${@:0}
./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:0:2}
./bash 1
$ echo ${@: -7:0}

</pre>
<p>If <var class="var">parameter</var> is an indexed array name subscripted
by &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;, the result is the <var class="var">length</var>
members of the array beginning with <code class="code">${<var class="var">parameter</var>[<var class="var">offset</var>]}</code>.
A negative <var class="var">offset</var> is taken relative to one greater than the maximum
index of the specified array.
It is an expansion error if <var class="var">length</var> evaluates to a number less than zero.
</p>
<p>These examples show how you can use substring expansion with indexed
arrays:
</p>
<pre class="verbatim">$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
$ echo ${array[@]:7}
7 8 9 0 a b c d e f g h
$ echo ${array[@]:7:2}
7 8
$ echo ${array[@]: -7:2}
b c
$ echo ${array[@]: -7:-2}
bash: -2: substring expression &lt; 0
$ echo ${array[@]:0}
0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${array[@]:0:2}
0 1
$ echo ${array[@]: -7:0}

</pre>
<p>Substring expansion applied to an associative array produces undefined
results.
</p>
<p>Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default.
If <var class="var">offset</var> is 0, and the positional parameters are used, <code class="code">$0</code> is
prefixed to the list.
</p>
</dd>
<dt><code class="code">${!<var class="var">prefix</var>*}</code></dt>
<dt><code class="code">${!<var class="var">prefix</var>@}</code></dt>
<dd><p>Expands to the names of variables whose names begin with <var class="var">prefix</var>,
separated by the first character of the <code class="env">IFS</code> special variable.
When &lsquo;<samp class="samp">@</samp>&rsquo; is used and the expansion appears within double quotes, each
variable name expands to a separate word.
</p>
</dd>
<dt><code class="code">${!<var class="var">name</var>[@]}</code></dt>
<dt><code class="code">${!<var class="var">name</var>[*]}</code></dt>
<dd><p>If <var class="var">name</var> is an array variable, expands to the list of array indices
(keys) assigned in <var class="var">name</var>.
If <var class="var">name</var> is not an array, expands to 0 if <var class="var">name</var> is set and null
otherwise.
When &lsquo;<samp class="samp">@</samp>&rsquo; is used and the expansion appears within double quotes, each
key expands to a separate word.
</p>
</dd>
<dt><code class="code">${#<var class="var">parameter</var>}</code></dt>
<dd><p>Substitutes the length in characters of the value of <var class="var">parameter</var>.
If <var class="var">parameter</var> is &lsquo;<samp class="samp">*</samp>&rsquo; or &lsquo;<samp class="samp">@</samp>&rsquo;, the value substituted
is the number of positional parameters.
If <var class="var">parameter</var> is an array name subscripted by &lsquo;<samp class="samp">*</samp>&rsquo; or &lsquo;<samp class="samp">@</samp>&rsquo;, 
the value substituted is the number of elements in the array.
If <var class="var">parameter</var>
is an indexed array name subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
<var class="var">parameter</var>, so negative indices count back from the end of the
array, and an index of -1 references the last element.
</p>
</dd>
<dt><code class="code">${<var class="var">parameter</var>#<var class="var">word</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>##<var class="var">word</var>}</code></dt>
<dd><p>The <var class="var">word</var>
is expanded to produce a pattern and matched
against the expanded value of <var class="var">parameter</var>
according to the rules
described below (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).
If the pattern matches
the beginning of the expanded value of <var class="var">parameter</var>,
then the result of the expansion is the expanded value of <var class="var">parameter</var>
with the shortest matching pattern (the &lsquo;<samp class="samp">#</samp>&rsquo; case) or the
longest matching pattern (the &lsquo;<samp class="samp">##</samp>&rsquo; case) deleted.
If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If <var class="var">parameter</var> is an array variable subscripted with
&lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
</p>
</dd>
<dt><code class="code">${<var class="var">parameter</var>%<var class="var">word</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>%%<var class="var">word</var>}</code></dt>
<dd><p>The <var class="var">word</var>
is expanded to produce a pattern and matched
against the expanded value of <var class="var">parameter</var>
according to the rules
described below (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).
If the pattern matches a trailing portion of the expanded value of
<var class="var">parameter</var>, then the result of the expansion is the value of
<var class="var">parameter</var> with the shortest matching pattern (the &lsquo;<samp class="samp">%</samp>&rsquo; case)
or the longest matching pattern (the &lsquo;<samp class="samp">%%</samp>&rsquo; case) deleted.
If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If <var class="var">parameter</var>
is an array variable subscripted with &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.
</p>
</dd>
<dt><code class="code">${<var class="var">parameter</var>/<var class="var">pattern</var>/<var class="var">string</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>//<var class="var">pattern</var>/<var class="var">string</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>/#<var class="var">pattern</var>/<var class="var">string</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>/%<var class="var">pattern</var>/<var class="var">string</var>}</code></dt>
<dd><p>The <var class="var">pattern</var> is expanded to produce a pattern
and matched against the expanded value of <var class="var">parameter</var>
as described below (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).
The longest match of <var class="var">pattern</var>
in the expanded value is replaced with <var class="var">string</var>.
<var class="var">string</var> undergoes tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, and quote removal.
</p>
<p>In the first form above, only the first match is replaced.
If there are two slashes separating <var class="var">parameter</var> and <var class="var">pattern</var>
(the second form above), all matches of <var class="var">pattern</var> are
replaced with <var class="var">string</var>.
If <var class="var">pattern</var> is preceded by &lsquo;<samp class="samp">#</samp>&rsquo; (the third form above),
it must match at the beginning of the expanded value of <var class="var">parameter</var>.
If <var class="var">pattern</var> is preceded by &lsquo;<samp class="samp">%</samp>&rsquo; (the fourth form above),
it must match at the end of the expanded value of <var class="var">parameter</var>.
</p>
<p>If the expansion of <var class="var">string</var> is null,
matches of <var class="var">pattern</var> are deleted
and the &lsquo;<samp class="samp">/</samp>&rsquo; following <var class="var">pattern</var> may be omitted.
</p>
<p>If the <code class="code">patsub_replacement</code> shell option is enabled using <code class="code">shopt</code>
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
any unquoted instances of &lsquo;<samp class="samp">&amp;</samp>&rsquo; in <var class="var">string</var> are replaced with the
matching portion of <var class="var">pattern</var>.
This is intended to duplicate a common <code class="code">sed</code> idiom.
</p>
<p>Quoting any part of <var class="var">string</var> inhibits replacement in the
expansion of the quoted portion, including replacement strings stored
in shell variables.
Backslash escapes &lsquo;<samp class="samp">&amp;</samp>&rsquo; in <var class="var">string</var>; the backslash is removed
in order to permit a literal &lsquo;<samp class="samp">&amp;</samp>&rsquo; in the replacement string.
Users should take care if <var class="var">string</var> is double-quoted to avoid
unwanted interactions between the backslash and double-quoting, since
backslash has special meaning within double quotes.
Pattern substitution performs the check for unquoted &lsquo;<samp class="samp">&amp;</samp>&rsquo; after
expanding <var class="var">string</var>,
so users should ensure to properly quote any occurrences of &lsquo;<samp class="samp">&amp;</samp>&rsquo;
they want to be taken literally in the replacement
and ensure any instances of &lsquo;<samp class="samp">&amp;</samp>&rsquo; they want to be replaced are unquoted.
</p>
<p>For instance,
</p>
<div class="example">
<pre class="example-preformatted">var=abcdef
rep='&amp; '
echo ${var/abc/&amp; }
echo &quot;${var/abc/&amp; }&quot;
echo ${var/abc/$rep}
echo &quot;${var/abc/$rep}&quot;
</pre></div>

<p>will display four lines of &quot;abc def&quot;, while
</p>
<div class="example">
<pre class="example-preformatted">var=abcdef
rep='&amp; '
echo ${var/abc/\&amp; }
echo &quot;${var/abc/\&amp; }&quot;
echo ${var/abc/&quot;&amp; &quot;}
echo ${var/abc/&quot;$rep&quot;}
</pre></div>

<p>will display four lines of &quot;&amp; def&quot;.
Like the pattern removal operators, double quotes surrounding the
replacement string quote the expanded characters, while double quotes
enclosing the entire parameter substitution do not, since
the expansion is performed in a
context that doesn&rsquo;t take any enclosing double quotes into account.
</p>
<p>Since backslash can escape &lsquo;<samp class="samp">&amp;</samp>&rsquo;, it can also escape a backslash in
the replacement string.
This means that &lsquo;<samp class="samp">\\</samp>&rsquo; will insert a literal
backslash into the replacement, so these two <code class="code">echo</code> commands
</p>
<div class="example">
<pre class="example-preformatted">var=abcdef
rep='\\&amp;xyz'
echo ${var/abc/\\&amp;xyz}
echo ${var/abc/$rep}
</pre></div>

<p>will both output &lsquo;<samp class="samp">\abcxyzdef</samp>&rsquo;.
</p>
<p>It should rarely be necessary to enclose only <var class="var">string</var> in double
quotes.
</p>
<p>If the <code class="code">nocasematch</code> shell option   
(see the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
is enabled, the match is performed without regard to the case   
of alphabetic characters.
</p>
<p>If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If <var class="var">parameter</var>
is an array variable subscripted with &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
</p>
</dd>
<dt><code class="code">${<var class="var">parameter</var>^<var class="var">pattern</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>^^<var class="var">pattern</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>,<var class="var">pattern</var>}</code></dt>
<dt><code class="code">${<var class="var">parameter</var>,,<var class="var">pattern</var>}</code></dt>
<dd><p>This expansion modifies the case of alphabetic characters in <var class="var">parameter</var>.
First, the <var class="var">pattern</var> is expanded to produce a pattern
as described below in <a class="ref" href="#Pattern-Matching">Pattern Matching</a>.
</p>
<p><code class="code">Bash</code>
then examines characters in the expanded value of <var class="var">parameter</var>
against <var class="var">pattern</var> as described below.
If a character matches the pattern, its case is converted.
The pattern should not attempt to match more than one character.
</p>
<p>Using
&lsquo;<samp class="samp">^</samp>&rsquo;
converts lowercase letters matching <var class="var">pattern</var> to uppercase;
&lsquo;<samp class="samp">,</samp>&rsquo;
converts matching uppercase letters to lowercase.
The
&lsquo;<samp class="samp">^</samp>&rsquo; and &lsquo;<samp class="samp">,</samp>&rsquo; variants
examine the first character in the expanded value
and convert its case if it matches <var class="var">pattern</var>;
the
&lsquo;<samp class="samp">^^</samp>&rsquo; and &lsquo;<samp class="samp">,,</samp>&rsquo; variants
examine all characters in the expanded value
and convert each one that matches <var class="var">pattern</var>.
If <var class="var">pattern</var> is omitted, it is treated like a &lsquo;<samp class="samp">?</samp>&rsquo;, which matches
every character.
</p>
<p>If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the case modification operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If <var class="var">parameter</var>
is an array variable subscripted with &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the case modification operation is applied to each member of the
array in turn, and the expansion is the resultant list.
</p>
</dd>
<dt><code class="code">${<var class="var">parameter</var>@<var class="var">operator</var>}</code></dt>
<dd><p>The expansion is either a transformation of the value of <var class="var">parameter</var>
or information about <var class="var">parameter</var> itself, depending on the value of
<var class="var">operator</var>.  Each <var class="var">operator</var> is a single letter:
</p>
<dl class="table">
<dt><code class="code">U</code></dt>
<dd><p>The expansion is a string that is the value of <var class="var">parameter</var> with lowercase
alphabetic characters converted to uppercase.
</p></dd>
<dt><code class="code">u</code></dt>
<dd><p>The expansion is a string that is the value of <var class="var">parameter</var> with the first
character converted to uppercase, if it is alphabetic.
</p></dd>
<dt><code class="code">L</code></dt>
<dd><p>The expansion is a string that is the value of <var class="var">parameter</var> with uppercase
alphabetic characters converted to lowercase.
</p></dd>
<dt><code class="code">Q</code></dt>
<dd><p>The expansion is a string that is the value of <var class="var">parameter</var> quoted in a
format that can be reused as input.
</p></dd>
<dt><code class="code">E</code></dt>
<dd><p>The expansion is a string that is the value of <var class="var">parameter</var> with backslash
escape sequences expanded as with the <code class="code">$'&hellip;'</code> quoting mechanism.
</p></dd>
<dt><code class="code">P</code></dt>
<dd><p>The expansion is a string that is the result of expanding the value of
<var class="var">parameter</var> as if it were a prompt string (see <a class="pxref" href="#Controlling-the-Prompt">Controlling the Prompt</a>).
</p></dd>
<dt><code class="code">A</code></dt>
<dd><p>The expansion is a string in the form of
an assignment statement or <code class="code">declare</code> command that, if
evaluated, recreates <var class="var">parameter</var> with its attributes and value.
</p></dd>
<dt><code class="code">K</code></dt>
<dd><p>Produces a possibly-quoted version of the value of <var class="var">parameter</var>,
except that it prints the values of
indexed and associative arrays as a sequence of quoted key-value pairs
(see <a class="pxref" href="#Arrays">Arrays</a>).
The keys and values are quoted in a format that can be reused as input.
</p></dd>
<dt><code class="code">a</code></dt>
<dd><p>The expansion is a string consisting of flag values representing
<var class="var">parameter</var>&rsquo;s attributes.
</p></dd>
<dt><code class="code">k</code></dt>
<dd><p>Like the &lsquo;<samp class="samp">K</samp>&rsquo; transformation, but expands the keys and values of
indexed and associative arrays to separate words after word splitting.
</p></dd>
</dl>

<p>If <var class="var">parameter</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If <var class="var">parameter</var>
is an array variable subscripted with &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the operation is applied to each member of the
array in turn, and the expansion is the resultant list.
</p>
<p>The result of the expansion is subject to word splitting and filename
expansion as described below.
</p></dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Command-Substitution">
<div class="nav-panel">
<p>
Next: <a href="#Arithmetic-Expansion" accesskey="n" rel="next">Arithmetic Expansion</a>, Previous: <a href="#Shell-Parameter-Expansion" accesskey="p" rel="prev">Shell Parameter Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Command-Substitution-1"><span>3.5.4 Command Substitution<a class="copiable-link" href="#Command-Substitution-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-command-substitution"></a>

<p>Command substitution allows the output of a command to replace
the command itself.
The standard form of command substitution occurs when a command is
enclosed as follows:
</p><div class="example">
<pre class="example-preformatted">$(<var class="var">command</var>)
</pre></div>
<p>or (deprecated)
</p><div class="example">
<pre class="example-preformatted">`<var class="var">command</var>`.
</pre></div>

<p>Bash performs command substitution by executing <var class="var">command</var> in a subshell
environment and replacing the command substitution with the standard
output of the command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution <code class="code">$(cat <var class="var">file</var>)</code> can be
replaced by the equivalent but faster <code class="code">$(&lt; <var class="var">file</var>)</code>.
</p>
<p>With the old-style backquote form of substitution,
backslash retains its literal meaning except when followed by
&lsquo;<samp class="samp">$</samp>&rsquo;, &lsquo;<samp class="samp">`</samp>&rsquo;, or &lsquo;<samp class="samp">\</samp>&rsquo;. 
The first backquote not preceded by a backslash terminates the
command substitution.
When using the <code class="code">$(<var class="var">command</var>)</code> form, all characters between
the parentheses make up the command; none are treated specially.
</p>
<p>There is an alternate form of command substitution:
</p>
<div class="example">
<pre class="example-preformatted">${<var class="var">c</var> <var class="var">command</var>; }
</pre></div>

<p>which executes <var class="var">command</var> in the current execution environment
and captures its output, again with trailing newlines removed.
</p>
<p>The character <var class="var">c</var> following the open brace must be a space, tab,
newline, or &lsquo;<samp class="samp">|</samp>&rsquo;, and the close brace must be in a position
where a reserved word may appear (i.e., preceded by a command terminator
such as semicolon).
Bash allows the close brace to be joined to the remaining characters in
the word without being followed by a shell metacharacter as a reserved
word would usually require.
</p>
<p>Any side effects of <var class="var">command</var> take effect immediately
in the current execution environment and persist in the current 
environment after the command completes (e.g., the <code class="code">exit</code> builtin
exits the shell).
</p>
<p>This type of command substitution superficially resembles executing an
unnamed shell function: local variables are created as when a shell
function is executing, and the <code class="code">return</code> builtin forces
<var class="var">command</var> to complete;
however, the rest of the execution environment,
including the positional parameters, is shared with the caller.
</p>
<p>If the first character following the open brace
is a &lsquo;<samp class="samp">|</samp>&rsquo;, the construct expands to the
value of the <code class="code">REPLY</code> shell variable after <var class="var">command</var> executes,
without removing any trailing newlines,
and the standard output of <var class="var">command</var> remains the same as in the
calling shell.
Bash creates <code class="code">REPLY</code> as an initially-unset local variable when
<var class="var">command</var> executes, and restores <code class="code">REPLY</code> to the value it had
before the command substitution after <var class="var">command</var> completes,
as with any local variable.
</p>
<p>For example, this construct expands to &lsquo;<samp class="samp">12345</samp>&rsquo;, and leaves the
shell variable <code class="code">X</code> unchanged in the current execution environment:
</p>
<div class="example">
<pre class="example-preformatted">

${ local X=12345 ; echo $X; }
</pre></div>

<p>(not declaring <code class="code">X</code> as local would modify its value in the current
environment, as with normal shell function execution),
while this construct does not require any output to expand to
&lsquo;<samp class="samp">12345</samp>&rsquo;:
</p>
<div class="example">
<pre class="example-preformatted">${| REPLY=12345; }
</pre></div>

<p>and restores <code class="code">REPLY</code> to the value it had before the command substitution.
</p>
<p>Command substitutions may be nested.  To nest when using the backquoted
form, escape the inner backquotes with backslashes.
</p>
<p>If the substitution appears within double quotes, Bash does not perform
word splitting and filename expansion on the results.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Arithmetic-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Process-Substitution" accesskey="n" rel="next">Process Substitution</a>, Previous: <a href="#Command-Substitution" accesskey="p" rel="prev">Command Substitution</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Arithmetic-Expansion-1"><span>3.5.5 Arithmetic Expansion<a class="copiable-link" href="#Arithmetic-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-expansion_002c-arithmetic"></a>
<a class="index-entry-id" id="index-arithmetic-expansion"></a>

<p>Arithmetic expansion evaluates an arithmetic expression
and substitutes the result.
The format for arithmetic expansion is:
</p>
<div class="example">
<pre class="example-preformatted">$(( <var class="var">expression</var> ))
</pre></div>

<p>The <var class="var">expression</var> undergoes the same expansions
as if it were within double quotes,
but unescaped double quote characters
in <var class="var">expression</var> are not treated
specially and are removed.
All tokens in the expression undergo parameter and variable expansion,
command substitution, and quote removal.
The result is treated as the arithmetic expression to be evaluated.
Since the way Bash handles double quotes
can potentially result in empty strings,
arithmetic expansion treats
those as expressions that evaluate to 0.
Arithmetic expansions may be nested. 
</p>
<p>The evaluation is performed according to the rules listed below
(see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).
If the expression is invalid, Bash prints a message indicating
failure to the standard error,
does not perform the substitution,
and does not execute the command associated with the expansion.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Process-Substitution">
<div class="nav-panel">
<p>
Next: <a href="#Word-Splitting" accesskey="n" rel="next">Word Splitting</a>, Previous: <a href="#Arithmetic-Expansion" accesskey="p" rel="prev">Arithmetic Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Process-Substitution-1"><span>3.5.6 Process Substitution<a class="copiable-link" href="#Process-Substitution-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-process-substitution"></a>

<p>Process substitution allows a process&rsquo;s input or output to be
referred to using a filename.
It takes the form of 
</p><div class="example">
<pre class="example-preformatted">&lt;(<var class="var">list</var>)
</pre></div>
<p>or
</p><div class="example">
<pre class="example-preformatted">&gt;(<var class="var">list</var>)
</pre></div>
<p>The process <var class="var">list</var> is run asynchronously, and its input or output 
appears as a filename.
This filename is
passed as an argument to the current command as the result of the
expansion.
</p>
<p>If the
<code class="code">&gt;(<var class="var">list</var>)</code> form is used, writing to the file
provides input for <var class="var">list</var>.
If the
<code class="code">&lt;(<var class="var">list</var>)</code> form is used, reading the file 
obtains the output of <var class="var">list</var>.
Note that no space may appear between the <code class="code">&lt;</code> or <code class="code">&gt;</code>
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
</p>
<p>Process substitution is supported on systems that support named
pipes (<small class="sc">FIFO</small>s) or the <samp class="file">/dev/fd</samp> method of naming open files.
</p>
<p>When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Word-Splitting">
<div class="nav-panel">
<p>
Next: <a href="#Filename-Expansion" accesskey="n" rel="next">Filename Expansion</a>, Previous: <a href="#Process-Substitution" accesskey="p" rel="prev">Process Substitution</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Word-Splitting-1"><span>3.5.7 Word Splitting<a class="copiable-link" href="#Word-Splitting-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-word-splitting"></a>

<p>The shell scans the results of parameter expansion, command substitution,
and arithmetic expansion that did not occur within double quotes for
word splitting.
Words that were not expanded are not split.
</p>
<p>The shell treats each character of <code class="env">$IFS</code> as a delimiter,
and splits the results of the other expansions into fields
using these characters as field terminators.
</p>
<p>An <em class="dfn">IFS whitespace</em> character is whitespace as defined above
(see <a class="pxref" href="#Definitions">Definitions</a>) that appears in the value of <code class="env">IFS</code>.
Space, tab, and newline are always considered IFS whitespace, even
if they don&rsquo;t appear in the locale&rsquo;s <code class="code">space</code> category.
</p>
<p>If <code class="env">IFS</code> is unset, word splitting behaves as if its value were
<code class="code">&lt;space&gt;&lt;tab&gt;&lt;newline&gt;</code>,
and treats these characters as IFS whitespace.
If the value of <code class="env">IFS</code> is null, no word splitting occurs,
but implicit null arguments (see below) are still removed.
</p>
<p>Word splitting begins by removing sequences of IFS whitespace characters
from the beginning and end of the results of the previous expansions,
then splits the remaining words.
</p>
<p>If the value of <code class="env">IFS</code> consists solely of IFS whitespace,
any sequence of IFS whitespace characters delimits a field,
so a field consists of characters that are not unquoted IFS
whitespace, and null fields result only from quoting.
</p>
<p>If <code class="env">IFS</code> contains a non-whitespace character, then any
character in the value of <code class="env">IFS</code> that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delimits a field.
This means that adjacent non-IFS-whitespace delimiters produce a
null field.
A sequence of IFS whitespace characters also delimits a field.
</p>
<p>Explicit null arguments (<code class="code">&quot;&quot;</code> or <code class="code">''</code>) are retained
and passed to commands as empty strings.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
Expanding a parameter with no value within double quotes
produces a null field,
which is retained and passed to a command as an empty string.
</p>
<p>When a quoted null argument appears as part of a word whose expansion
is non-null, word splitting removes the null argument portion,
leaving the non-null expansion.
That is, the word
<code class="code">-d''</code> becomes <code class="code">-d</code> after word splitting and
null argument removal.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Filename-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Quote-Removal" accesskey="n" rel="next">Quote Removal</a>, Previous: <a href="#Word-Splitting" accesskey="p" rel="prev">Word Splitting</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Filename-Expansion-1"><span>3.5.8 Filename Expansion<a class="copiable-link" href="#Filename-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-expansion_002c-filename"></a>
<a class="index-entry-id" id="index-expansion_002c-pathname"></a>
<a class="index-entry-id" id="index-filename-expansion"></a>
<a class="index-entry-id" id="index-pathname-expansion"></a>

<p>After word splitting, unless the <samp class="option">-f</samp> option has been set
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>), Bash scans each word for the characters
&lsquo;<samp class="samp">*</samp>&rsquo;, &lsquo;<samp class="samp">?</samp>&rsquo;, and &lsquo;<samp class="samp">[</samp>&rsquo;.
If one of these characters appears, and is not quoted, then the word is
regarded as a <var class="var">pattern</var>,
and replaced with a sorted list of filenames matching the pattern
(see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>),
subject to the value of the <code class="code">GLOBSORT</code> shell variable
(see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
</p>
<p>If no matching filenames are found,
and the shell option <code class="code">nullglob</code> is disabled, the word is left
unchanged.
If the <code class="code">nullglob</code> option is set, and no matches are found, the word
is removed.
If the <code class="code">failglob</code> shell option is set, and no matches are found,
Bash prints an error message and does not execute the command.
If the shell option <code class="code">nocaseglob</code> is enabled, the match is performed
without regard to the case of alphabetic characters.
</p>
<p>When a pattern is used for filename expansion, the character &lsquo;<samp class="samp">.</samp>&rsquo;
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option <code class="code">dotglob</code> is set.
In order to match the filenames <samp class="file">.</samp> and <samp class="file">..</samp>,
the pattern must begin with &lsquo;<samp class="samp">.</samp>&rsquo; (for example, &lsquo;<samp class="samp">.?</samp>&rsquo;),
even if <code class="code">dotglob</code> is set.
If the <code class="code">globskipdots</code> shell option is enabled, the filenames
<samp class="file">.</samp> and <samp class="file">..</samp> never match, even if the pattern begins
with a &lsquo;<samp class="samp">.</samp>&rsquo;.
When not matching filenames, the &lsquo;<samp class="samp">.</samp>&rsquo; character is not treated specially.
</p>
<p>When matching a filename, the slash character must always be
matched explicitly by a slash in the pattern, but in other matching
contexts it can be matched by a special pattern character as described
below (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).
</p>
<p>See the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>,
for a description of the <code class="code">nocaseglob</code>, <code class="code">nullglob</code>,
<code class="code">globskipdots</code>,
<code class="code">failglob</code>, and <code class="code">dotglob</code> options.
</p>
<p>The <code class="env">GLOBIGNORE</code>
shell variable may be used to restrict the set of file names matching a
pattern.
If <code class="env">GLOBIGNORE</code>
is set, each matching file name that also matches one of the patterns in
<code class="env">GLOBIGNORE</code> is removed from the list of matches.
If the <code class="code">nocaseglob</code> option is set, the matching against the patterns in
<code class="env">GLOBIGNORE</code> is performed without regard to case.
The filenames
<samp class="file">.</samp> and <samp class="file">..</samp>
are always ignored when <code class="env">GLOBIGNORE</code>
is set and not null.
However, setting <code class="env">GLOBIGNORE</code>
to a non-null value has the effect of enabling the
<code class="code">dotglob</code>
shell option, so all other filenames beginning with a
&lsquo;<samp class="samp">.</samp>&rsquo;
match.
To get the old behavior of ignoring filenames beginning with a
&lsquo;<samp class="samp">.</samp>&rsquo;,
make &lsquo;<samp class="samp">.*</samp>&rsquo; one of the patterns in <code class="env">GLOBIGNORE</code>.
The <code class="code">dotglob</code> option is disabled when <code class="env">GLOBIGNORE</code>
is unset.
The <code class="code">GLOBIGNORE</code>
pattern matching honors the setting of the <code class="code">extglob</code> shell
option.
</p>
<p>The value of the
<code class="env">GLOBSORT</code>
shell variable controls how the results of pathname expansion are sorted,
as described below (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
</p>
<ul class="mini-toc">
<li><a href="#Pattern-Matching" accesskey="1">Pattern Matching</a></li>
</ul>
<hr>
<div class="subsubsection-level-extent" id="Pattern-Matching">
<div class="nav-panel">
<p>
Up: <a href="#Filename-Expansion" accesskey="u" rel="up">Filename Expansion</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsubsection" id="Pattern-Matching-1"><span>3.5.8.1 Pattern Matching<a class="copiable-link" href="#Pattern-Matching-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-pattern-matching"></a>
<a class="index-entry-id" id="index-matching_002c-pattern"></a>

<p>Any character that appears in a pattern, other than the special pattern
characters described below, matches itself.
The <small class="sc">NUL</small> character may not occur in a pattern.
A backslash escapes the following character; the
escaping backslash is discarded when matching.
The special pattern characters must be quoted if they are to be matched
literally.
</p>
<p>The special pattern characters have the following meanings:
</p><dl class="table">
<dt><code class="code">*</code></dt>
<dd><p>Matches any string, including the null string.
When the <code class="code">globstar</code> shell option is enabled, and &lsquo;<samp class="samp">*</samp>&rsquo; is used in
a filename expansion context, two adjacent &lsquo;<samp class="samp">*</samp>&rsquo;s used as a single
pattern match all files and zero or more directories and
subdirectories.
If followed by a &lsquo;<samp class="samp">/</samp>&rsquo;, two adjacent &lsquo;<samp class="samp">*</samp>&rsquo;s match only
directories and subdirectories.
</p></dd>
<dt><code class="code">?</code></dt>
<dd><p>Matches any single character.
</p></dd>
<dt><code class="code">[&hellip;]</code></dt>
<dd><p>Matches any one of the characters enclosed between the brackets.
This is known as a <em class="dfn">bracket expression</em>
and matches a single character.
A pair of characters separated by a hyphen denotes a <em class="dfn">range expression</em>;
any character that falls between those two characters, inclusive,
using the current locale&rsquo;s collating sequence and character set, matches.
If the first character following the
&lsquo;<samp class="samp">[</samp>&rsquo; is a &lsquo;<samp class="samp">!</samp>&rsquo; or a &lsquo;<samp class="samp">^</samp>&rsquo;
then any character not within the range matches.
To match a &lsquo;<samp class="samp">&minus;</samp>&rsquo;, include it as the first
or last character in the set.
To match a &lsquo;<samp class="samp">]</samp>&rsquo;, include it as the first
character in the set.
</p>
<p>The sorting order of characters in range expressions,
and the characters included in the range,
are determined by the current locale and the values of the
<code class="env">LC_COLLATE</code> and <code class="env">LC_ALL</code> shell variables, if set.
</p>
<p>For example, in the default C locale, &lsquo;<samp class="samp">[a-dx-z]</samp>&rsquo; is equivalent to
&lsquo;<samp class="samp">[abcdxyz]</samp>&rsquo;.
Many locales sort characters in dictionary order, and in these locales
&lsquo;<samp class="samp">[a-dx-z]</samp>&rsquo; is typically not equivalent to &lsquo;<samp class="samp">[abcdxyz]</samp>&rsquo;;
it might be equivalent to &lsquo;<samp class="samp">[aBbCcDdxYyZz]</samp>&rsquo;, for example.
To obtain
the traditional interpretation of ranges in bracket expressions, you can
force the use of the C locale by setting the <code class="env">LC_COLLATE</code> or
<code class="env">LC_ALL</code> environment variable to the value &lsquo;<samp class="samp">C</samp>&rsquo;, or enable the
<code class="code">globasciiranges</code> shell option.
</p>
<p>Within a bracket expression, <em class="dfn">character classes</em> can be specified
using the syntax
<code class="code">[:</code><var class="var">class</var><code class="code">:]</code>, where <var class="var">class</var> is one of the
following classes defined in the
<small class="sc">POSIX</small>
standard:
</p><div class="example">
<pre class="example-preformatted">alnum   alpha   ascii   blank   cntrl   digit   graph   lower
print   punct   space   upper   word    xdigit
</pre></div>
<p>A character class matches any character belonging to that class.
The <code class="code">word</code> character class matches letters, digits, and the character
&lsquo;<samp class="samp">_</samp>&rsquo;.
</p>
<p>For instance, the following pattern will match any character belonging
to the <code class="code">space</code> character class in the current locale, then any
upper case letter or &lsquo;<samp class="samp">!</samp>&rsquo;, a dot, and finally any lower case letter
or a hyphen.
</p>
<div class="example">
<pre class="example-preformatted">[[:space:]][[:upper:]!].[-[:lower:]]
</pre></div>

<p>Within a bracket expression, an <em class="dfn">equivalence class</em> can be
specified using the syntax <code class="code">[=</code><var class="var">c</var><code class="code">=]</code>, which
matches all characters with the same collation weight (as defined
by the current locale) as the character <var class="var">c</var>.
</p>
<p>Within a bracket expression, the syntax <code class="code">[.</code><var class="var">symbol</var><code class="code">.]</code>
matches the collating symbol <var class="var">symbol</var>.
</p></dd>
</dl>

<p>If the <code class="code">extglob</code> shell option is enabled using the <code class="code">shopt</code>
builtin, the shell recognizes several extended pattern matching operators.
In the following description, a <var class="var">pattern-list</var> is a list of one
or more patterns separated by a &lsquo;<samp class="samp">|</samp>&rsquo;.
When matching filenames, the <code class="code">dotglob</code> shell option determines
the set of filenames that are tested, as described above.
Composite patterns may be formed using one or more of the following
sub-patterns:
</p>
<dl class="table">
<dt><code class="code">?(<var class="var">pattern-list</var>)</code></dt>
<dd><p>Matches zero or one occurrence of the given patterns.
</p>
</dd>
<dt><code class="code">*(<var class="var">pattern-list</var>)</code></dt>
<dd><p>Matches zero or more occurrences of the given patterns.
</p>
</dd>
<dt><code class="code">+(<var class="var">pattern-list</var>)</code></dt>
<dd><p>Matches one or more occurrences of the given patterns.
</p>
</dd>
<dt><code class="code">@(<var class="var">pattern-list</var>)</code></dt>
<dd><p>Matches one of the given patterns.
</p>
</dd>
<dt><code class="code">!(<var class="var">pattern-list</var>)</code></dt>
<dd><p>Matches anything except one of the given patterns.
</p></dd>
</dl>

<p>The <code class="code">extglob</code> option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic meaning.
To ensure that extended matching patterns are parsed correctly, make sure
that <code class="code">extglob</code> is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.
</p>
<p>When matching filenames, the <code class="code">dotglob</code> shell option determines
the set of filenames that are tested:
when <code class="code">dotglob</code> is enabled, the set of filenames includes all files
beginning with &lsquo;<samp class="samp">.</samp>&rsquo;, but the filenames
<samp class="file">.</samp> and <samp class="file">..</samp> must be matched by a
pattern or sub-pattern that begins with a dot;
when it is disabled, the set does not
include any filenames beginning with &lsquo;<samp class="samp">.</samp>&rsquo; unless the pattern
or sub-pattern begins with a &lsquo;<samp class="samp">.</samp>&rsquo;.
If the <code class="code">globskipdots</code>
shell option is enabled, the filenames
<samp class="file">.</samp> and <samp class="file">..</samp>
never appear in the set.
As above, &lsquo;<samp class="samp">.</samp>&rsquo; only has a special meaning when matching filenames. 
</p>
<p>Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches.
Using separate matches against shorter strings, or using arrays of
strings instead of a single long string, may be faster.
</p>
<hr>
</div>
</div>
<div class="subsection-level-extent" id="Quote-Removal">
<div class="nav-panel">
<p>
Previous: <a href="#Filename-Expansion" accesskey="p" rel="prev">Filename Expansion</a>, Up: <a href="#Shell-Expansions" accesskey="u" rel="up">Shell Expansions</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Quote-Removal-1"><span>3.5.9 Quote Removal<a class="copiable-link" href="#Quote-Removal-1"> &para;</a></span></h4>

<p>After the preceding expansions, all unquoted occurrences of the
characters &lsquo;<samp class="samp">\</samp>&rsquo;, &lsquo;<samp class="samp">'</samp>&rsquo;, and &lsquo;<samp class="samp">&quot;</samp>&rsquo; that did not
result from one of the above expansions are removed.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Redirections">
<div class="nav-panel">
<p>
Next: <a href="#Executing-Commands" accesskey="n" rel="next">Executing Commands</a>, Previous: <a href="#Shell-Expansions" accesskey="p" rel="prev">Shell Expansions</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Redirections-1"><span>3.6 Redirections<a class="copiable-link" href="#Redirections-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-redirection"></a>

<p>Before a command is executed, its input and output may be
<em class="dfn">redirected</em>
using a special notation interpreted by the shell.
<em class="dfn">Redirection</em> allows commands&rsquo; file handles to be
duplicated, opened, closed, made to refer to different files,
and can change the files the command reads from and writes to.
When used with the <code class="code">exec</code> builtin,
redirections modify file handles in the current shell execution environment.
The following redirection
operators may precede or appear anywhere within a
simple command or may follow a command.
Redirections are processed in the order they appear, from
left to right.
</p>
<p>Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {<var class="var">varname</var>}.
In this case, for each redirection operator except
<code class="code">&gt;&amp;-</code>
and
<code class="code">&lt;&amp;-</code>,
the shell allocates a file descriptor greater
than or equal to 10 and assigns it to {<var class="var">varname</var>}.
If {<var class="var">varname</var>} precedes
<code class="code">&gt;&amp;-</code>
or
<code class="code">&lt;&amp;-</code>,
the value of <var class="var">varname</var> defines the file descriptor to close.
If {<var class="var">varname</var>} is supplied, the redirection persists beyond
the scope of the command, which allows the shell programmer to
manage the file descriptor&rsquo;s lifetime manually without using
the <code class="code">exec</code> builtin.
The <code class="code">varredir_close</code> shell option manages this behavior
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
</p>
<p>In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
&lsquo;<samp class="samp">&lt;</samp>&rsquo;,
the redirection refers to the standard input (file descriptor 0).
If the first character of the redirection operator is
&lsquo;<samp class="samp">&gt;</samp>&rsquo;,
the redirection refers to the standard output (file descriptor 1).
</p>
<p>The <var class="var">word</var> following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to
brace expansion,
tilde expansion,
parameter and variable expansion,
command substitution,
arithmetic expansion,
quote removal,
filename expansion,
and word splitting.
If it expands to more than one word, Bash reports an error.
</p>
<p>The order of redirections is significant.
For example,
the command
</p><div class="example">
<pre class="example-preformatted">ls &gt; <var class="var">dirlist</var> 2&gt;&amp;1
</pre></div>
<p>directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file <var class="var">dirlist</var>, while the command
</p><div class="example">
<pre class="example-preformatted">ls 2&gt;&amp;1 &gt; <var class="var">dirlist</var>
</pre></div>
<p>directs only the standard output to file <var class="var">dirlist</var>,
because the standard error was made a copy of the standard output
before the standard output was redirected to <var class="var">dirlist</var>.
</p>
<p>Bash handles several filenames specially when they are used in
redirections, as described in the following table.
If the operating system on which Bash is running provides these
special files, Bash uses them; otherwise it emulates them
internally with the behavior described below.
</p>
<dl class="table">
<dt><code class="code">/dev/fd/<var class="var">fd</var></code></dt>
<dd><p>If <var class="var">fd</var> is a valid integer, duplicate file descriptor <var class="var">fd</var>.
</p>
</dd>
<dt><code class="code">/dev/stdin</code></dt>
<dd><p>File descriptor 0 is duplicated.
</p>
</dd>
<dt><code class="code">/dev/stdout</code></dt>
<dd><p>File descriptor 1 is duplicated.
</p>
</dd>
<dt><code class="code">/dev/stderr</code></dt>
<dd><p>File descriptor 2 is duplicated.
</p>
</dd>
<dt><code class="code">/dev/tcp/<var class="var">host</var>/<var class="var">port</var></code></dt>
<dd><p>If <var class="var">host</var> is a valid hostname or Internet address, and <var class="var">port</var>
is an integer port number or service name, Bash attempts to open
the corresponding TCP socket.
</p>
</dd>
<dt><code class="code">/dev/udp/<var class="var">host</var>/<var class="var">port</var></code></dt>
<dd><p>If <var class="var">host</var> is a valid hostname or Internet address, and <var class="var">port</var>
is an integer port number or service name, Bash attempts to open 
the corresponding UDP socket.
</p></dd>
</dl>

<p>A failure to open or create a file causes the redirection to fail.
</p>
<p>Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
internally.
</p>
<ul class="mini-toc">
<li><a href="#Redirecting-Input" accesskey="1">Redirecting Input</a></li>
<li><a href="#Redirecting-Output" accesskey="2">Redirecting Output</a></li>
<li><a href="#Appending-Redirected-Output" accesskey="3">Appending Redirected Output</a></li>
<li><a href="#Redirecting-Standard-Output-and-Standard-Error" accesskey="4">Redirecting Standard Output and Standard Error</a></li>
<li><a href="#Appending-Standard-Output-and-Standard-Error" accesskey="5">Appending Standard Output and Standard Error</a></li>
<li><a href="#Here-Documents" accesskey="6">Here Documents</a></li>
<li><a href="#Here-Strings" accesskey="7">Here Strings</a></li>
<li><a href="#Duplicating-File-Descriptors" accesskey="8">Duplicating File Descriptors</a></li>
<li><a href="#Moving-File-Descriptors" accesskey="9">Moving File Descriptors</a></li>
<li><a href="#Opening-File-Descriptors-for-Reading-and-Writing">Opening File Descriptors for Reading and Writing</a></li>
</ul>
<div class="subsection-level-extent" id="Redirecting-Input">
<h4 class="subsection"><span>3.6.1 Redirecting Input<a class="copiable-link" href="#Redirecting-Input"> &para;</a></span></h4>
<p>Redirecting input opens the file whose name results from
the expansion of <var class="var">word</var> for reading on file descriptor <code class="code">n</code>,
or the standard input (file descriptor 0) if <code class="code">n</code>
is not specified.
</p>
<p>The general format for redirecting input is:
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;<var class="var">word</var>
</pre></div>

</div>
<div class="subsection-level-extent" id="Redirecting-Output">
<h4 class="subsection"><span>3.6.2 Redirecting Output<a class="copiable-link" href="#Redirecting-Output"> &para;</a></span></h4>
<p>Redirecting output opens the file whose name results from
the expansion of <var class="var">word</var>
for writing on file descriptor <var class="var">n</var>,
or the standard output (file descriptor 1) if <var class="var">n</var>
is not specified.
If the file does not exist it is created;
if it does exist it is truncated to zero size.
</p>
<p>The general format for redirecting output is:
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&gt;[|]<var class="var">word</var>
</pre></div>

<p>If the redirection operator is
&lsquo;<samp class="samp">&gt;</samp>&rsquo;,
and the
<code class="code">noclobber</code>
option to the
<code class="code">set</code>
builtin command has been enabled, the redirection fails if the file
whose name results from the expansion of <var class="var">word</var> exists and is
a regular file.
If the redirection operator is &lsquo;<samp class="samp">&gt;|</samp>&rsquo;,
or the redirection operator is &lsquo;<samp class="samp">&gt;</samp>&rsquo; and
the <code class="code">noclobber</code> option to the <code class="code">set</code>
builtin is not enabled,
Bash attempts the redirection
even if the file named by <var class="var">word</var> exists.
</p>
</div>
<div class="subsection-level-extent" id="Appending-Redirected-Output">
<h4 class="subsection"><span>3.6.3 Appending Redirected Output<a class="copiable-link" href="#Appending-Redirected-Output"> &para;</a></span></h4>
<p>Redirecting output in this fashion opens
the file whose name results from the expansion of
<var class="var">word</var>
for appending on file descriptor <var class="var">n</var>,
or the standard output (file descriptor 1) if <var class="var">n</var>
is not specified.
If the file does not exist it is created.
</p>
<p>The general format for appending output is:
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&gt;&gt;<var class="var">word</var>
</pre></div>

</div>
<div class="subsection-level-extent" id="Redirecting-Standard-Output-and-Standard-Error">
<h4 class="subsection"><span>3.6.4 Redirecting Standard Output and Standard Error<a class="copiable-link" href="#Redirecting-Standard-Output-and-Standard-Error"> &para;</a></span></h4>
<p>This construct redirects both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to the file whose name is the expansion of <var class="var">word</var>.
</p>
<p>There are two formats for redirecting standard output and
standard error:
</p><div class="example">
<pre class="example-preformatted">&amp;&gt;<var class="var">word</var>
</pre></div>
<p>and
</p><div class="example">
<pre class="example-preformatted">&gt;&amp;<var class="var">word</var>
</pre></div>
<p>Of the two forms, the first is preferred.
This is semantically equivalent to
</p><div class="example">
<pre class="example-preformatted">&gt;<var class="var">word</var> 2&gt;&amp;1
</pre></div>
<p>When using the second form, <var class="var">word</var> may not expand to a number or
&lsquo;<samp class="samp">-</samp>&rsquo;.
If it does, other redirection operators apply
(see Duplicating File Descriptors below) for compatibility reasons.
</p>
</div>
<div class="subsection-level-extent" id="Appending-Standard-Output-and-Standard-Error">
<h4 class="subsection"><span>3.6.5 Appending Standard Output and Standard Error<a class="copiable-link" href="#Appending-Standard-Output-and-Standard-Error"> &para;</a></span></h4>
<p>This construct appends both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to the file whose name is the expansion of <var class="var">word</var>.
</p>
<p>The format for appending standard output and standard error is:
</p><div class="example">
<pre class="example-preformatted">&amp;&gt;&gt;<var class="var">word</var>
</pre></div>
<p>This is semantically equivalent to
</p><div class="example">
<pre class="example-preformatted">&gt;&gt;<var class="var">word</var> 2&gt;&amp;1
</pre></div>
<p>(see Duplicating File Descriptors below).
</p>
</div>
<div class="subsection-level-extent" id="Here-Documents">
<h4 class="subsection"><span>3.6.6 Here Documents<a class="copiable-link" href="#Here-Documents"> &para;</a></span></h4>
<p>This type of redirection instructs the shell to read input from the
current source until it reads a line containing only <var class="var">delimiter</var>
(with no trailing blanks).
All of the lines read up to that point then become the standard
input (or file descriptor <var class="var">n</var> if <var class="var">n</var> is specified) for a command.
</p>
<p>The format of here-documents is:
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;&lt;[&minus;]<var class="var">word</var>
        <var class="var">here-document</var>
<var class="var">delimiter</var>
</pre></div>

<p>The shell does not perform
parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion on
<var class="var">word</var>.
</p>
<p>If any part of <var class="var">word</var> is quoted, the
<var class="var">delimiter</var> is the result of quote removal on <var class="var">word</var>,
and the lines in the here-document are not expanded.
If <var class="var">word</var> is unquoted,
<var class="var">delimiter</var> is <var class="var">word</var> itself,
and the here-document text is treated similarly to a double-quoted string:
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence <code class="code">\newline</code> is treated literally,
and &lsquo;<samp class="samp">\</samp>&rsquo; must be used to quote the characters
&lsquo;<samp class="samp">\</samp>&rsquo;, &lsquo;<samp class="samp">$</samp>&rsquo;, and &lsquo;<samp class="samp">`</samp>&rsquo;;
however, double quote characters have no special meaning.
</p>
<p>If the redirection operator is &lsquo;<samp class="samp">&lt;&lt;-</samp>&rsquo;,
the shell strips leading tab characters are stripped from input lines
and the line containing <var class="var">delimiter</var>.
This allows here-documents within shell scripts to be indented in a
natural fashion.
</p>
<p>If the delimiter is not quoted, the
<code class="code">\&lt;newline&gt;</code>
sequence is treated as a line continuation: the two lines are joined
and the backslash-newline is removed.
This happens while reading the here-document, before the check for
the ending delimiter, so joined lines can form the end delimiter.
</p>
</div>
<div class="subsection-level-extent" id="Here-Strings">
<h4 class="subsection"><span>3.6.7 Here Strings<a class="copiable-link" href="#Here-Strings"> &para;</a></span></h4>
<p>A variant of here documents, the format is:
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;&lt;&lt; <var class="var">word</var>
</pre></div>

<p>The <var class="var">word</var> undergoes
tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.
Filename expansion and word splitting are not performed.
The result is supplied as a single string, with a newline appended,
to the command on its
standard input (or file descriptor <var class="var">n</var> if <var class="var">n</var> is specified).
</p>
</div>
<div class="subsection-level-extent" id="Duplicating-File-Descriptors">
<h4 class="subsection"><span>3.6.8 Duplicating File Descriptors<a class="copiable-link" href="#Duplicating-File-Descriptors"> &para;</a></span></h4>
<p>The redirection operator
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;&amp;<var class="var">word</var>
</pre></div>
<p>is used to duplicate input file descriptors.
If <var class="var">word</var>
expands to one or more digits, file descriptor <var class="var">n</var>
is made to be a copy of that file descriptor.
It is a redirection error if the digits in
<var class="var">word</var> do not specify a file descriptor open for input.
If <var class="var">word</var>
evaluates to &lsquo;<samp class="samp">-</samp>&rsquo;, file descriptor <var class="var">n</var> is closed.
If <var class="var">n</var> is not specified, this uses the standard input (file descriptor 0).
</p>
<p>The operator
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&gt;&amp;<var class="var">word</var>
</pre></div>
<p>is used similarly to duplicate output file descriptors.
If <var class="var">n</var> is not specified, this uses the standard output (file descriptor 1).
It is a redirection error if the digits in
<var class="var">word</var> do not specify a file descriptor open for output.
If <var class="var">word</var>
evaluates to &lsquo;<samp class="samp">-</samp>&rsquo;, file descriptor <var class="var">n</var> is closed.
As a special case, if <var class="var">n</var> is omitted, and
<var class="var">word</var> does not expand to one or more digits or &lsquo;<samp class="samp">-</samp>&rsquo;,
this redirects the standard output and standard error as described
previously.
</p>
</div>
<div class="subsection-level-extent" id="Moving-File-Descriptors">
<h4 class="subsection"><span>3.6.9 Moving File Descriptors<a class="copiable-link" href="#Moving-File-Descriptors"> &para;</a></span></h4>
<p>The redirection operator
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;&amp;<var class="var">digit</var>-
</pre></div>
<p>moves the file descriptor <var class="var">digit</var> to file descriptor <var class="var">n</var>,
or the standard input (file descriptor 0) if <var class="var">n</var> is not specified.
<var class="var">digit</var> is closed after being duplicated to <var class="var">n</var>.
</p>
<p>Similarly, the redirection operator
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&gt;&amp;<var class="var">digit</var>-
</pre></div>
<p>moves the file descriptor <var class="var">digit</var> to file descriptor <var class="var">n</var>,
or the standard output (file descriptor 1) if <var class="var">n</var> is not specified.
</p>
</div>
<div class="subsection-level-extent" id="Opening-File-Descriptors-for-Reading-and-Writing">
<h4 class="subsection"><span>3.6.10 Opening File Descriptors for Reading and Writing<a class="copiable-link" href="#Opening-File-Descriptors-for-Reading-and-Writing"> &para;</a></span></h4>
<p>The redirection operator
</p><div class="example">
<pre class="example-preformatted">[<var class="var">n</var>]&lt;&gt;<var class="var">word</var>
</pre></div>
<p>opens the file whose name is the expansion of <var class="var">word</var>
for both reading and writing on file descriptor
<var class="var">n</var>, or on file descriptor 0 if <var class="var">n</var>
is not specified.
If the file does not exist, it is created.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Executing-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Scripts" accesskey="n" rel="next">Shell Scripts</a>, Previous: <a href="#Redirections" accesskey="p" rel="prev">Redirections</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Executing-Commands-1"><span>3.7 Executing Commands<a class="copiable-link" href="#Executing-Commands-1"> &para;</a></span></h3>


<ul class="mini-toc">
<li><a href="#Simple-Command-Expansion" accesskey="1">Simple Command Expansion</a></li>
<li><a href="#Command-Search-and-Execution" accesskey="2">Command Search and Execution</a></li>
<li><a href="#Command-Execution-Environment" accesskey="3">Command Execution Environment</a></li>
<li><a href="#Environment" accesskey="4">Environment</a></li>
<li><a href="#Exit-Status" accesskey="5">Exit Status</a></li>
<li><a href="#Signals" accesskey="6">Signals</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Simple-Command-Expansion">
<div class="nav-panel">
<p>
Next: <a href="#Command-Search-and-Execution" accesskey="n" rel="next">Command Search and Execution</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Simple-Command-Expansion-1"><span>3.7.1 Simple Command Expansion<a class="copiable-link" href="#Simple-Command-Expansion-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-command-expansion"></a>

<p>When the shell executes a simple command, it performs the following
expansions, assignments, and redirections, from left to right, in
the following order.
</p>
<ol class="enumerate">
<li> The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.

</li><li> The words that are not variable assignments or redirections are
expanded (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>).
If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
the arguments.

</li><li> Redirections are performed as described above (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li> The text after the &lsquo;<samp class="samp">=</samp>&rsquo; in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.
</li></ol>

<p>If no command name results, the variable assignments affect the current
shell environment.
In the case of such a command (one that consists only of assignment
statements and redirections), assignment statements are performed before
redirections.
Otherwise, the variables are added to the environment
of the executed command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a readonly variable,
an error occurs, and the command exits with a non-zero status.
</p>
<p>If no command name results, redirections are performed, but do not
affect the current shell environment.
A redirection error causes the command to exit with a non-zero status.
</p>
<p>If there is a command name left after expansion, execution proceeds as
described below.
Otherwise, the command exits.
If one of the expansions contained a command substitution,
the exit status of the command is the exit status of
the last command substitution performed.
If there were no command substitutions,
the command exits with a zero status.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Command-Search-and-Execution">
<div class="nav-panel">
<p>
Next: <a href="#Command-Execution-Environment" accesskey="n" rel="next">Command Execution Environment</a>, Previous: <a href="#Simple-Command-Expansion" accesskey="p" rel="prev">Simple Command Expansion</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Command-Search-and-Execution-1"><span>3.7.2 Command Search and Execution<a class="copiable-link" href="#Command-Search-and-Execution-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-command-execution"></a>
<a class="index-entry-id" id="index-command-search"></a>

<p>After a command has been split into words, if it results in a
simple command and an optional list of arguments, the shell performs
the following actions.
</p>
<ol class="enumerate">
<li> If the command name contains no slashes, the shell attempts to
locate it.
If there exists a shell function by that name, that
function is invoked as described in <a class="ref" href="#Shell-Functions">Shell Functions</a>.

</li><li> If the name does not match a function, the shell searches for
it in the list of shell builtins.
If a match is found, that builtin is invoked.

</li><li> If the name is neither a shell function nor a builtin,
and contains no slashes, Bash searches each element of
<code class="env">$PATH</code> for a directory containing an executable file
by that name.
Bash uses a hash table to remember the full
pathnames of executable files to avoid multiple <code class="env">PATH</code> searches
(see the description of <code class="code">hash</code> in <a class="ref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
Bash performs a full search of the directories in <code class="env">$PATH</code>
only if the command is not found in the hash table.
If the search is unsuccessful, the shell searches for a defined shell
function named <code class="code">command_not_found_handle</code>.
If that function exists, it is invoked in a separate execution environment
with the original command and
the original command&rsquo;s arguments as its arguments, and the function&rsquo;s
exit status becomes the exit status of that subshell.
If that function is not defined, the shell prints an error
message and returns an exit status of 127.

</li><li> If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in
a separate execution environment.
Argument 0 is set to the name given, and the remaining arguments
to the command are set to the arguments supplied, if any.

</li><li> If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
<em class="dfn">shell script</em>,
a file containing shell commands,
and the shell executes it as described in <a class="ref" href="#Shell-Scripts">Shell Scripts</a>.

</li><li> If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.

</li></ol>

<hr>
</div>
<div class="subsection-level-extent" id="Command-Execution-Environment">
<div class="nav-panel">
<p>
Next: <a href="#Environment" accesskey="n" rel="next">Environment</a>, Previous: <a href="#Command-Search-and-Execution" accesskey="p" rel="prev">Command Search and Execution</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Command-Execution-Environment-1"><span>3.7.3 Command Execution Environment<a class="copiable-link" href="#Command-Execution-Environment-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-execution-environment"></a>

<p>The shell has an <em class="dfn">execution environment</em>, which consists of the
following:
</p>
<ul class="itemize mark-bullet">
<li>Open files inherited by the shell at invocation, as modified by
redirections supplied to the <code class="code">exec</code> builtin.

</li><li>The current working directory as set by <code class="code">cd</code>, <code class="code">pushd</code>, or
<code class="code">popd</code>, or inherited by the shell at invocation.

</li><li>The file creation mode mask as set by <code class="code">umask</code> or inherited from
the shell&rsquo;s parent.

</li><li>Current traps set by <code class="code">trap</code>.

</li><li>Shell parameters that are set by variable assignment or with <code class="code">set</code>
or inherited from the shell&rsquo;s parent in the environment.

</li><li>Shell functions defined during execution or inherited from the shell&rsquo;s
parent in the environment.

</li><li>Options enabled at invocation (either by default or with command-line
arguments) or by <code class="code">set</code>.

</li><li>Options enabled by <code class="code">shopt</code> (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).

</li><li>Shell aliases defined with <code class="code">alias</code> (see <a class="pxref" href="#Aliases">Aliases</a>).

</li><li>Various process <small class="sc">ID</small>s, including those of background jobs
(see <a class="pxref" href="#Lists">Lists of Commands</a>), the value of <code class="code">$$</code>, and the value of <code class="env">$PPID</code>.

</li></ul>

<p>When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following.
Unless otherwise noted, the values are inherited from the shell.
</p>
<ul class="itemize mark-bullet">
<li>The shell&rsquo;s open files, plus any modifications and additions specified
by redirections to the command.

</li><li>The current working directory.

</li><li>The file creation mode mask.

</li><li>Shell variables and functions marked for export, along with variables
exported for the command, passed in the environment (see <a class="pxref" href="#Environment">Environment</a>).

</li><li>Traps caught by the shell are reset to the values inherited from the
shell&rsquo;s parent, and traps ignored by the shell are ignored.

</li></ul>

<p>A command invoked in this separate environment cannot affect the
shell&rsquo;s execution environment.
</p>
<p>A <em class="dfn">subshell</em> is a copy of the shell process.
</p>
<p>Command substitution, commands grouped with parentheses,
and asynchronous commands are invoked in a
subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values
that the shell inherited from its parent at invocation.
Builtin commands that are invoked as part of a pipeline,
except possibly in the last element depending on the value of the
<code class="code">lastpipe</code> shell option (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
are also executed in a subshell environment.
Changes made to the subshell environment
cannot affect the shell&rsquo;s execution environment.
</p>
<p>When the shell is in <small class="sc">POSIX</small> mode,
subshells spawned to execute command substitutions inherit the value of
the <samp class="option">-e</samp> option from the parent shell.
When not in <small class="sc">POSIX</small> mode,
Bash clears the <samp class="option">-e</samp> option in such subshells
See the description of the <code class="code">inherit_errexit</code> shell option
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>) for how to control this behavior when not
in <small class="sc">POSIX</small> mode.
</p>
<p>If a command is followed by a &lsquo;<samp class="samp">&amp;</samp>&rsquo; and job control is not active, the
default standard input for the command is the empty file <samp class="file">/dev/null</samp>.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Environment">
<div class="nav-panel">
<p>
Next: <a href="#Exit-Status" accesskey="n" rel="next">Exit Status</a>, Previous: <a href="#Command-Execution-Environment" accesskey="p" rel="prev">Command Execution Environment</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Environment-1"><span>3.7.4 Environment<a class="copiable-link" href="#Environment-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-environment"></a>

<p>When a program is invoked it is given an array of strings
called the <em class="dfn">environment</em>.
This is a list of name-value pairs, of the form <code class="code">name=value</code>.
</p>
<p>Bash provides several ways to manipulate the environment.
On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for <code class="code">export</code> to child processes.
Executed commands inherit the environment.
The <code class="code">export</code>, &lsquo;<samp class="samp">declare -x</samp>&rsquo;, and <code class="code">unset</code>
commands modify the environment by
adding and deleting parameters and functions.
If the value of a parameter in the environment is modified,
the new value automatically becomes part
of the environment, replacing the old.
The environment
inherited by any executed command consists of the shell&rsquo;s
initial environment, whose values may be modified in the shell,
less any pairs removed by the <code class="code">unset</code> and &lsquo;<samp class="samp">export -n</samp>&rsquo;
commands, plus any additions via the <code class="code">export</code> and
&lsquo;<samp class="samp">declare -x</samp>&rsquo; commands.
</p>
<p>If any parameter assignment statements,
as described in <a class="ref" href="#Shell-Parameters">Shell Parameters</a>,
appear before a simple command,
the variable assignments are part of that command&rsquo;s environment
for as long as it executes.
These assignment statements affect only the environment seen
by that command.
If these assignments precede a call to a shell function, the variables
are local to the function and exported to that function&rsquo;s children.
</p>
<p>If the <samp class="option">-k</samp> option is set (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>), then all
parameter assignments are placed in the environment for a command,
not just those that precede the command name.
</p>
<p>When Bash invokes an external command, the variable &lsquo;<samp class="samp">$_</samp>&rsquo;
is set to the full pathname of the command and passed to that
command in its environment.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Exit-Status">
<div class="nav-panel">
<p>
Next: <a href="#Signals" accesskey="n" rel="next">Signals</a>, Previous: <a href="#Environment" accesskey="p" rel="prev">Environment</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Exit-Status-1"><span>3.7.5 Exit Status<a class="copiable-link" href="#Exit-Status-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-exit-status-1"></a>

<p>The exit status of an executed command is the value returned by the
<code class="code">waitpid</code> system call or equivalent function.
Exit statuses fall between 0 and 255, though, as explained below,
the shell may use values above 125 specially.
Exit statuses from shell builtins and compound commands are also limited
to this range.
Under certain circumstances, the shell will use special values to
indicate specific failure modes.
</p>
<p>For the shell&rsquo;s purposes, a command which exits with a
zero exit status has succeeded.
So while an exit status of zero indicates success, a non-zero
exit status indicates failure.
This seemingly counter-intuitive scheme is used so there
is one well-defined way to indicate success and a variety of
ways to indicate various failure modes.
</p>
<p>When a command terminates on a fatal signal whose number is <var class="var">N</var>,
Bash uses the value 128+<var class="var">N</var> as the exit status.
</p>
<p>If a command is not found, the child process created to
execute it returns a status of 127.
If a command is found but is not executable, the return status is 126.
</p>
<p>If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
</p>
<p>The exit status is used by the Bash conditional commands
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>) and some of the list
constructs (see <a class="pxref" href="#Lists">Lists of Commands</a>).
</p>
<p>All of the Bash builtins return an exit status of zero if they succeed
and a non-zero status on failure, so they may be used by the
conditional and list constructs.
All builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.
</p>
<p>The exit status of the last command is available in the special
parameter $? (see <a class="pxref" href="#Special-Parameters">Special Parameters</a>).
</p>
<p>Bash itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it exits
with a non-zero value.
See also the <code class="code">exit</code> builtin command (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Signals">
<div class="nav-panel">
<p>
Previous: <a href="#Exit-Status" accesskey="p" rel="prev">Exit Status</a>, Up: <a href="#Executing-Commands" accesskey="u" rel="up">Executing Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Signals-1"><span>3.7.6 Signals<a class="copiable-link" href="#Signals-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-signal-handling"></a>

<p>When Bash is interactive, in the absence of any traps,
it ignores <code class="code">SIGTERM</code>
(so that &lsquo;<samp class="samp">kill 0</samp>&rsquo; does not kill an interactive shell),
and catches and handles <code class="code">SIGINT</code>
(so that the <code class="code">wait</code> builtin is interruptible).
When Bash receives a <code class="code">SIGINT</code>, it breaks out of any executing loops.
In all cases, Bash ignores <code class="code">SIGQUIT</code>.
If job control is in effect (see <a class="pxref" href="#Job-Control">Job Control</a>), Bash
ignores <code class="code">SIGTTIN</code>, <code class="code">SIGTTOU</code>, and <code class="code">SIGTSTP</code>.
</p>
<p>The <code class="code">trap</code> builtin modifies the shell&rsquo;s signal handling, as
described below (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>.
</p>
<p>Non-builtin commands Bash executes have signal handlers set to the
values inherited by the shell from its parent,
unless <code class="code">trap</code> sets them to be ignored, in which case the child
process will ignore them as well.
When job control is not in effect, asynchronous commands
ignore <code class="code">SIGINT</code> and <code class="code">SIGQUIT</code> in addition to these inherited
handlers.
Commands run as a result of
command substitution ignore the keyboard-generated job control signals
<code class="code">SIGTTIN</code>, <code class="code">SIGTTOU</code>, and <code class="code">SIGTSTP</code>.
</p>
<p>The shell exits by default upon receipt of a <code class="code">SIGHUP</code>.
Before exiting, an interactive shell resends the <code class="code">SIGHUP</code> to
all jobs, running or stopped.
The shell sends <code class="code">SIGCONT</code> to stopped jobs to ensure that they
receive the <code class="code">SIGHUP</code>
(See <a class="xref" href="#Job-Control">Job Control</a>, for more information about running and stopped jobs).
To prevent the shell from sending the <code class="code">SIGHUP</code> signal to a
particular job, remove it from the jobs table with the <code class="code">disown</code>
builtin (see <a class="pxref" href="#Job-Control-Builtins">Job Control Builtins</a>) or mark it
not to receive <code class="code">SIGHUP</code> using <code class="code">disown -h</code>.
</p>
<p>If the  <code class="code">huponexit</code> shell option has been set using <code class="code">shopt</code>
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), Bash sends a <code class="code">SIGHUP</code> to all jobs when
an interactive login shell exits.
</p>
<p>If Bash is waiting for a command to complete and receives a signal
for which a trap has been set,
it will not execute the trap until the command completes.
If Bash is waiting for an asynchronous command via the <code class="code">wait</code> builtin,
and it receives a signal for which a trap has been set,
the <code class="code">wait</code> builtin will return immediately with an exit status
greater than 128, immediately after which the shell executes the trap.
</p>
<p>When job control is not enabled, and Bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals
such as <code class="code">SIGINT</code> (usually generated by &lsquo;<samp class="samp">^C</samp>&rsquo;) that users
commonly intend to send to that command.
This happens because the shell and the command are in the same process
group as the terminal, and &lsquo;<samp class="samp">^C</samp>&rsquo; sends <code class="code">SIGINT</code> to all processes
in that process group.
Since Bash does not enable job control by default when the
shell is not interactive,
this scenario is most common in non-interactive shells.
</p>
<p>When job control is enabled, and Bash is waiting for a foreground
command to complete, the shell does not receive keyboard-generated
signals, because it is not in the same process group as the terminal.
This scenario is most common in interactive shells, where Bash
attempts to enable job control by default.
See <a class="ref" href="#Job-Control">Job Control</a>, for a more in-depth discussion of process groups.
</p>
<p>When job control is not enabled, and Bash receives <code class="code">SIGINT</code>
while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the <code class="code">SIGINT</code>:
</p>
<ol class="enumerate">
<li> If the command terminates due to the <code class="code">SIGINT</code>, Bash concludes
that the user meant to send the <code class="code">SIGINT</code> to the shell as well,
and acts on the
<code class="code">SIGINT</code>
(e.g., by running a <code class="code">SIGINT</code> trap,
exiting a non-interactive shell,
or returning to the top level to read a new command).

</li><li> If the command does not terminate due to <code class="code">SIGINT</code>, the program
handled the <code class="code">SIGINT</code> itself and did not treat it as a fatal signal.
In that case, Bash does not treat <code class="code">SIGINT</code> as a fatal signal,
either, instead assuming that the <code class="code">SIGINT</code> was used as part of the
program&rsquo;s normal operation (e.g., <code class="command">emacs</code> uses it to abort editing
commands) or deliberately discarded.
However, Bash will run any
trap set on <code class="code">SIGINT</code>, as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
</li></ol>

<p>When job control is enabled, Bash does not receive keyboard-generated
signals such as <code class="code">SIGINT</code>
while it is waiting for a foreground command.
An interactive shell does not pay attention to the <code class="code">SIGINT</code>,
even if the foreground command terminates as a result, other than noting
its exit status.
If the shell is not interactive, and
the foreground command terminates due to the <code class="code">SIGINT</code>,
Bash pretends it received the <code class="code">SIGINT</code>
itself (scenario 1 above), for compatibility.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Shell-Scripts">
<div class="nav-panel">
<p>
Previous: <a href="#Executing-Commands" accesskey="p" rel="prev">Executing Commands</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Scripts-1"><span>3.8 Shell Scripts<a class="copiable-link" href="#Shell-Scripts-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-shell-script"></a>

<p>A shell script is a text file containing shell commands.
When such a file is used as the first non-option argument when
invoking Bash, and neither the <samp class="option">-c</samp> nor <samp class="option">-s</samp> option
is supplied (see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>), 
Bash reads and executes commands from the file, then exits.
This mode of operation creates a non-interactive shell.
If the filename does not contain any slashes, the shell first searches
for the file in the current directory, and looks in the directories in
<code class="env">$PATH</code> if not found there.
</p>
<p>Bash tries to determine whether the file is a text file or a binary,
and will not execute files it determines to be binaries.
</p>
<p>When Bash runs
a shell script, it sets the special parameter <code class="code">0</code> to the name
of the file, rather than the name of the shell, and the positional
parameters are set to the remaining arguments, if any are given.
If no additional arguments are supplied, the positional parameters
are unset.
</p>
<p>A shell script may be made executable by using the <code class="code">chmod</code> command
to turn on the execute bit.
When Bash finds such a file while searching the <code class="env">$PATH</code> for a command,
it creates a new instance of itself to execute it.
In other words, executing
</p><div class="example">
<pre class="example-preformatted">filename <var class="var">arguments</var>
</pre></div>
<p>is equivalent to executing
</p><div class="example">
<pre class="example-preformatted">bash filename <var class="var">arguments</var>
</pre></div>

<p>if <code class="code">filename</code> is an executable shell script.
This subshell reinitializes itself, so that the effect is as if a
new shell had been invoked to interpret the script, with the
exception that the locations of commands remembered by the parent
(see the description of <code class="code">hash</code> in <a class="ref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
are retained by the child.
</p>
<p>The <small class="sc">GNU</small> operating system,
and most versions of Unix,
make this a part of the operating system&rsquo;s command execution mechanism.
If the first line of a script begins with
the two characters &lsquo;<samp class="samp">#!</samp>&rsquo;, the remainder of the line specifies
an interpreter for the program and, depending on the operating system, one
or more optional arguments for that interpreter.
Thus, you can specify Bash, <code class="code">awk</code>, Perl, or some other
interpreter and write the rest of the script file in that language.
</p>
<p>The arguments to the interpreter
consist of one or more optional arguments following the interpreter
name on the first line of the script file, followed by the name of
the script file, followed by the rest of the arguments supplied to the
script.
The details of how the interpreter line is split into an interpreter name
and a set of arguments vary across systems.
Bash will perform this action on operating systems that do not handle it
themselves.
Note that some older versions of Unix limit the interpreter
name and a single argument to a maximum of 32 characters, so it&rsquo;s not
portable to assume that using more than one argument will work.
</p>
<p>Bash scripts often begin with <code class="code">#! /bin/bash</code> (assuming that
Bash has been installed in <samp class="file">/bin</samp>), since this ensures that
Bash will be used to interpret the script, even if it is executed
under another shell.
It&rsquo;s a common idiom to use <code class="code">env</code> to find
<code class="code">bash</code> even if it&rsquo;s been installed in another directory:
<code class="code">#!/usr/bin/env bash</code> will find the first occurrence of <code class="code">bash</code>
in <code class="env">$PATH</code>.
</p>
<hr>
</div>
</div>
<div class="chapter-level-extent" id="Shell-Builtin-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Variables" accesskey="n" rel="next">Shell Variables</a>, Previous: <a href="#Basic-Shell-Features" accesskey="p" rel="prev">Basic Shell Features</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Shell-Builtin-Commands-1"><span>4 Shell Builtin Commands<a class="copiable-link" href="#Shell-Builtin-Commands-1"> &para;</a></span></h2>


<p>Builtin commands are contained within the shell itself.
When the name of a builtin command is used as the first word of
a simple command (see <a class="pxref" href="#Simple-Commands">Simple Commands</a>), the shell executes
the command directly, without invoking another program.
Builtin commands are necessary to implement functionality impossible
or inconvenient to obtain with separate utilities.
</p>
<p>This section briefly describes the builtins which Bash inherits from
the Bourne Shell, as well as the builtin commands which are unique
to or have been extended in Bash.
</p>
<p>Several builtin commands are described in other chapters: builtin
commands which provide the Bash interface to the job control
facilities (see <a class="pxref" href="#Job-Control-Builtins">Job Control Builtins</a>), the directory stack
(see <a class="pxref" href="#Directory-Stack-Builtins">Directory Stack Builtins</a>), the command history
(see <a class="pxref" href="#Bash-History-Builtins">Bash History Builtins</a>), and the programmable completion
facilities (see <a class="pxref" href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a>).
</p>
<p>Many of the builtins have been extended by <small class="sc">POSIX</small> or Bash.
</p>
<p>Unless otherwise noted, each builtin command documented as accepting
options preceded by &lsquo;<samp class="samp">-</samp>&rsquo; accepts &lsquo;<samp class="samp">--</samp>&rsquo;
to signify the end of the options.
The <code class="code">:</code>, <code class="code">true</code>, <code class="code">false</code>, and <code class="code">test</code>/<code class="code">[</code>
builtins do not accept options and do not treat &lsquo;<samp class="samp">--</samp>&rsquo; specially.
The <code class="code">exit</code>, <code class="code">logout</code>, <code class="code">return</code>,
<code class="code">break</code>, <code class="code">continue</code>, <code class="code">let</code>,
and <code class="code">shift</code> builtins accept and process arguments beginning
with &lsquo;<samp class="samp">-</samp>&rsquo; without requiring &lsquo;<samp class="samp">--</samp>&rsquo;.
Other builtins that accept arguments but are not specified as accepting
options interpret arguments beginning with &lsquo;<samp class="samp">-</samp>&rsquo; as invalid options and
require &lsquo;<samp class="samp">--</samp>&rsquo; to prevent this interpretation.
</p>
<ul class="mini-toc">
<li><a href="#Bourne-Shell-Builtins" accesskey="1">Bourne Shell Builtins</a></li>
<li><a href="#Bash-Builtins" accesskey="2">Bash Builtin Commands</a></li>
<li><a href="#Modifying-Shell-Behavior" accesskey="3">Modifying Shell Behavior</a></li>
<li><a href="#Special-Builtins" accesskey="4">Special Builtins</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Bourne-Shell-Builtins">
<div class="nav-panel">
<p>
Next: <a href="#Bash-Builtins" accesskey="n" rel="next">Bash Builtin Commands</a>, Up: <a href="#Shell-Builtin-Commands" accesskey="u" rel="up">Shell Builtin Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bourne-Shell-Builtins-1"><span>4.1 Bourne Shell Builtins<a class="copiable-link" href="#Bourne-Shell-Builtins-1"> &para;</a></span></h3>

<p>The following shell builtin commands are inherited from the Bourne Shell.
These commands are implemented as specified by the <small class="sc">POSIX</small> standard.
</p>
<dl class="table">
<dt><a id="index-_003a"></a><span><code class="code">:    <span class="r">(a colon)</span></code><a class="copiable-link" href="#index-_003a"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">: [<var class="var">arguments</var>]
</pre></div>

<p>Do nothing beyond expanding <var class="var">arguments</var> and performing redirections.
The return status is zero.
</p>
</dd>
<dt><a id="index-_002e"></a><span><code class="code">.    <span class="r">(a period)</span></code><a class="copiable-link" href="#index-_002e"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">. [-p <var class="var">path</var>] <var class="var">filename</var> [<var class="var">arguments</var>]
</pre></div>

<p>The <code class="code">.</code> command reads and execute commands from the <var class="var">filename</var>
argument in the current shell context.
</p>
<p>If <var class="var">filename</var> does not contain a slash, <code class="code">.</code> searches for it.
If <samp class="option">-p</samp> is supplied, <code class="code">.</code> treats <var class="var">path</var>
as a colon-separated list of directories in which to find <var class="var">filename</var>;
otherwise, <code class="code">.</code> uses the directories in <code class="env">PATH</code> to find <var class="var">filename</var>.
<var class="var">filename</var> does not need to be executable.
When Bash is not in <small class="sc">POSIX</small> mode, it searches the current directory
if <var class="var">filename</var> is not found in <code class="env">$PATH</code>,
but does not search the current directory if <samp class="option">-p</samp> is supplied.
If the <code class="code">sourcepath</code> option (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>) is turned off,
<code class="code">.</code> does not search <code class="env">PATH</code>.
</p>
<p>If any <var class="var">arguments</var> are supplied, they become the positional
parameters when <var class="var">filename</var> is executed.
Otherwise the positional parameters are unchanged.
</p>
<p>If the <samp class="option">-T</samp> option is enabled, <code class="code">.</code> inherits any trap on
<code class="code">DEBUG</code>; if it is not, any <code class="code">DEBUG</code> trap string is saved and
restored around the call to <code class="code">.</code>, and <code class="code">.</code> unsets the
<code class="code">DEBUG</code> trap while it executes.
If <samp class="option">-T</samp> is not set, and the sourced file changes
the <code class="code">DEBUG</code> trap, the new value persists after <code class="code">.</code> completes.
The return status is the exit status of the last command executed
from <var class="var">filename</var>, or
zero if no commands are executed.
If <var class="var">filename</var> is not found, or cannot be read,
the return status is non-zero.
This builtin is equivalent to <code class="code">source</code>.
</p>
</dd>
<dt><a id="index-break"></a><span><code class="code">break</code><a class="copiable-link" href="#index-break"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">break [<var class="var">n</var>]
</pre></div>

<p>Exit from a <code class="code">for</code>, <code class="code">while</code>, <code class="code">until</code>, or <code class="code">select</code> loop.
If <var class="var">n</var> is supplied, <code class="code">break</code> exits the <var class="var">n</var>th enclosing loop.
<var class="var">n</var> must be greater than or equal to 1.
The return status is zero unless <var class="var">n</var> is not greater than or equal to 1.
</p>
</dd>
<dt><a id="index-cd"></a><span><code class="code">cd</code><a class="copiable-link" href="#index-cd"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">cd [-L] [-@] [<var class="var">directory</var>]
cd -P [-e] [-@] [<var class="var">directory</var>]
</pre></div>

<p>Change the current working directory to <var class="var">directory</var>.
If <var class="var">directory</var> is not supplied, the value of the <code class="env">HOME</code>
shell variable is used as <var class="var">directory</var>.
If the shell variable
<code class="env">CDPATH</code> exists,
and <var class="var">directory</var> does not begin with a slash,
<code class="code">cd</code> uses it as a search path:
<code class="code">cd</code> searches each directory name in <code class="env">CDPATH</code> for
<var class="var">directory</var>, with alternative directory names in <code class="env">CDPATH</code>
separated by a colon (&lsquo;<samp class="samp">:</samp>&rsquo;).
A null directory name in <code class="env">CDPATH</code> means the same thing as the
current directory.
</p>
<p>The <samp class="option">-P</samp> option means not to follow symbolic links: symbolic links
are resolved while <code class="code">cd</code> is traversing <var class="var">directory</var> and before
processing an instance of <samp class="file">..</samp> in <var class="var">directory</var>.
</p>
<p>By default, or when the <samp class="option">-L</samp> option is supplied, symbolic links
in <var class="var">directory</var> are resolved after <code class="code">cd</code> processes an instance
of <samp class="file">..</samp> in <var class="var">directory</var>.
</p>
<p>If <samp class="file">..</samp> appears in <var class="var">directory</var>, <code class="code">cd</code> processes it by removing the
immediately preceding pathname component, back to a slash or the beginning
of <var class="var">directory</var>,
and verifying that the portion of <var class="var">directory</var>
it has processed to
that point is still a valid directory name after removing the pathname
component.
If it is not a valid directory name, <code class="code">cd</code> returns a non-zero status.
</p>
<p>If the <samp class="option">-e</samp> option is supplied with <samp class="option">-P</samp>
and <code class="code">cd</code> cannot successfully determine the current working directory 
after a successful directory change, it returns a non-zero status.
</p>
<p>On systems that support it, the <samp class="option">-@</samp> option presents the extended
attributes associated with a file as a directory.              
</p>
<p>If <var class="var">directory</var> is &lsquo;<samp class="samp">-</samp>&rsquo;, it is converted to <code class="env">$OLDPWD</code>
before attempting the directory change.
</p>
<p>If <code class="code">cd</code> uses a non-empty directory name from <code class="env">CDPATH</code>, or if
&lsquo;<samp class="samp">-</samp>&rsquo; is the first argument, and the directory change is
successful, <code class="code">cd</code> writes the absolute pathname of the new
working directory to the standard output.
</p>
<p>If the directory change is successful, <code class="code">cd</code> sets the value of the
<code class="env">PWD</code> environment variable to the new directory name, and sets the
<code class="env">OLDPWD</code> environment variable to the value of the current working
directory before the change.
</p>
<p>The return status is zero if the directory is successfully changed,
non-zero otherwise.
</p>
</dd>
<dt><a id="index-continue"></a><span><code class="code">continue</code><a class="copiable-link" href="#index-continue"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">continue [<var class="var">n</var>]
</pre></div>

<p><code class="code">continue</code> resumes the next iteration of an enclosing <code class="code">for</code>,
<code class="code">while</code>, <code class="code">until</code>, or <code class="code">select</code> loop.
If <var class="var">n</var> is supplied, Bash resumes the execution of the <var class="var">n</var>th
enclosing loop.
<var class="var">n</var> must be greater than or equal to 1.
The return status is zero unless <var class="var">n</var> is not greater than or equal to 1.
</p>
</dd>
<dt><a id="index-eval"></a><span><code class="code">eval</code><a class="copiable-link" href="#index-eval"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">eval [<var class="var">arguments</var>]
</pre></div>

<p>The <var class="var">arguments</var> are concatenated together into a single command,
separated by spaces.
Bash then reads and executes this command and returns its exit status
as the exit status of <code class="code">eval</code>.
If there are no arguments or only empty arguments, the return status is
zero.
</p>
</dd>
<dt><a id="index-exec"></a><span><code class="code">exec</code><a class="copiable-link" href="#index-exec"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">exec [-cl] [-a <var class="var">name</var>] [<var class="var">command</var> [<var class="var">arguments</var>]]
</pre></div>

<p>If <var class="var">command</var>
is supplied, it replaces the shell without creating a new process.
<var class="var">command</var> cannot be a shell builtin or function.
The <var class="var">arguments</var> become the arguments to <var class="var">command</var>
If the <samp class="option">-l</samp> option is supplied, the shell places a dash at the
beginning of the zeroth argument passed to <var class="var">command</var>.
This is what the <code class="code">login</code> program does.
The <samp class="option">-c</samp> option causes <var class="var">command</var> to be executed with an empty
environment.
If <samp class="option">-a</samp> is supplied, the shell passes <var class="var">name</var> as the zeroth
argument to <var class="var">command</var>.
</p>
<p>If <var class="var">command</var>
cannot be executed for some reason, a non-interactive shell exits,
unless the <code class="code">execfail</code> shell option is enabled.
In that case, it returns a non-zero status.
An interactive shell returns a non-zero status if the file cannot be executed.
A subshell exits unconditionally if <code class="code">exec</code> fails.
</p>
<p>If <var class="var">command</var> is not specified, redirections may be used to affect
the current shell environment.
If there are no redirection errors, the return status is zero;
otherwise the return status is non-zero.
</p>
</dd>
<dt><a id="index-exit"></a><span><code class="code">exit</code><a class="copiable-link" href="#index-exit"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">exit [<var class="var">n</var>]
</pre></div>

<p>Exit the shell, returning a status of <var class="var">n</var> to the shell&rsquo;s parent.
If <var class="var">n</var> is omitted, the exit status is that of the last command executed.
Any trap on <code class="code">EXIT</code> is executed before the shell terminates.
</p>
</dd>
<dt><a id="index-export"></a><span><code class="code">export</code><a class="copiable-link" href="#index-export"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">export [-fn] [-p] [<var class="var">name</var>[=<var class="var">value</var>]]
</pre></div>

<p>Mark each <var class="var">name</var> to be passed to subsequently executed commands in the
environment.
If the <samp class="option">-f</samp> option is supplied, the <var class="var">name</var>s
refer to shell functions; otherwise the names refer to shell variables.
</p>
<p>The <samp class="option">-n</samp> option means to unexport each name: no longer mark
it for export.
If no <var class="var">name</var>s are supplied, or if only
the <samp class="option">-p</samp> option is given,
<code class="code">export</code> displays a list of names of all exported
variables on the standard output.
Using <samp class="option">-p</samp> and <samp class="option">-f</samp> together displays exported functions.
The <samp class="option">-p</samp> option displays output in a form that may be reused as input.
</p>
<p><code class="code">export</code> allows the value of a variable to be set at the same time
it is exported or unexported by following the variable name with
=<var class="var">value</var>.
This sets the value of the variable is to <var class="var">value</var> while modifying
the export attribute.
</p>
<p>The return status is zero unless an invalid option is supplied, one of
the names is not a valid shell variable name, or <samp class="option">-f</samp> is supplied
with a name that is not a shell function.
</p>
</dd>
<dt><a id="index-false"></a><span><code class="code">false</code><a class="copiable-link" href="#index-false"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">false
</pre></div>

<p>Does nothing; returns a non-zero status.
</p>
</dd>
<dt><a id="index-getopts"></a><span><code class="code">getopts</code><a class="copiable-link" href="#index-getopts"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">getopts <var class="var">optstring</var> <var class="var">name</var> [<var class="var">arg</var> ...]
</pre></div>

<p><code class="code">getopts</code> is used by shell scripts or functions to parse positional
parameters and obtain options and their arguments.
<var class="var">optstring</var> contains the option characters to be recognized; if a
character is followed by a colon, the option is expected to have an
argument, which should be separated from it by whitespace.
The colon (&lsquo;<samp class="samp">:</samp>&rsquo;) and question mark (&lsquo;<samp class="samp">?</samp>&rsquo;) may not be
used as option characters.
</p>
<p>Each time it is invoked, <code class="code">getopts</code>
places the next option in the shell variable <var class="var">name</var>, initializing
<var class="var">name</var> if it does not exist,
and the index of the next argument to be processed into the
variable <code class="env">OPTIND</code>.
<code class="env">OPTIND</code> is initialized to 1 each time the shell or a shell script
is invoked.
When an option requires an argument,
<code class="code">getopts</code> places that argument into the variable <code class="env">OPTARG</code>.
</p>
<p>The shell does not reset <code class="env">OPTIND</code> automatically; it must be manually
reset between multiple calls to <code class="code">getopts</code> within the same shell
invocation to use a new set of parameters.
</p>
<p>When it reaches the end of options, <code class="code">getopts</code> exits with a
return value greater than zero.
<code class="env">OPTIND</code> is set to the index of the first non-option argument,
and <var class="var">name</var> is set to &lsquo;<samp class="samp">?</samp>&rsquo;.
</p>
<p><code class="code">getopts</code>
normally parses the positional parameters, but if more arguments are
supplied as <var class="var">arg</var> values, <code class="code">getopts</code> parses those instead.
</p>
<p><code class="code">getopts</code> can report errors in two ways.
If the first character of
<var class="var">optstring</var> is a colon, <code class="code">getopts</code> uses <em class="emph">silent</em>
error reporting.
In normal operation, <code class="code">getopts</code> prints diagnostic messages
when it encounters invalid options or missing option arguments.
If the variable <code class="env">OPTERR</code>
is set to 0, <code class="code">getopts</code> does not display any error messages,
even if the first character of <code class="code">optstring</code> is not a colon.
</p>
<p>If <code class="code">getopts</code> detects an invalid option, it
places &lsquo;<samp class="samp">?</samp>&rsquo; into <var class="var">name</var> and, if not silent,
prints an error message and unsets <code class="env">OPTARG</code>.
If <code class="code">getopts</code> is silent, it assigns the option character found
to <code class="env">OPTARG</code> and does not print a diagnostic message.
</p>
<p>If a required argument is not found,
and <code class="code">getopts</code> is not silent,
it sets the value of <var class="var">name</var> to a question mark (&lsquo;<samp class="samp">?</samp>&rsquo;),
unsets <code class="code">OPTARG</code>, and prints a diagnostic message.
If <code class="code">getopts</code> is silent,
it sets the value of <var class="var">name</var> to a colon (&lsquo;<samp class="samp">:</samp>&rsquo;),
and sets <code class="env">OPTARG</code> to the option character found.
</p>
<p><code class="code">getopts</code> returns true if an option, specified or unspecified,
is found.
It returns false when it encounters the end of options or if an error occurs.
</p>
</dd>
<dt><a id="index-hash"></a><span><code class="code">hash</code><a class="copiable-link" href="#index-hash"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">hash [-r] [-p <var class="var">filename</var>] [-dt] [<var class="var">name</var>]
</pre></div>

<p>Each time <code class="code">hash</code> is invoked, it remembers the full filenames of the
commands specified as <var class="var">name</var> arguments,
so they need not be searched for on subsequent invocations.
The commands are found by searching through the directories listed in
<code class="env">$PATH</code>.
Any previously-remembered filename associated with <var class="var">name</var> is discarded.
The <samp class="option">-p</samp> option inhibits the path search, and <code class="code">hash</code> uses
<var class="var">filename</var> as the location of <var class="var">name</var>.
</p>
<p>The <samp class="option">-r</samp> option causes the shell to forget all remembered locations.
Assigning to the <code class="env">PATH</code> variable also clears all hashed filenames.
The <samp class="option">-d</samp> option causes the shell to forget the remembered location
of each <var class="var">name</var>.
</p>
<p>If the <samp class="option">-t</samp> option is supplied, <code class="code">hash</code> prints the full pathname
corresponding to each <var class="var">name</var>.
If multiple <var class="var">name</var> arguments are
supplied with <samp class="option">-t</samp>, <code class="code">hash</code> prints each <var class="var">name</var>
before the corresponding hashed full path.
The <samp class="option">-l</samp> option displays output in a format that may be reused as input.
</p>
<p>If no arguments are given, or if only <samp class="option">-l</samp> is supplied,
<code class="code">hash</code> prints information about remembered commands.
The <samp class="option">-t</samp>, <samp class="option">-d</samp>, and <samp class="option">-p</samp> options (the options that
act on the <var class="var">name</var> arguments) are mutually exclusive.
Only one will be active.
If more than one is supplied, <samp class="option">-t</samp> has higher priority than
<samp class="option">-p</samp>, and both have higher priority than <samp class="option">-d</samp>.
</p>
<p>The return status is zero unless a <var class="var">name</var> is not found or an invalid
option is supplied.
</p>
</dd>
<dt><a id="index-pwd"></a><span><code class="code">pwd</code><a class="copiable-link" href="#index-pwd"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">pwd [-LP]
</pre></div>

<p>Print the absolute pathname of the current working directory.
If the <samp class="option">-P</samp> option is supplied,
or the <samp class="option">-o physical</samp> option to the <code class="code">set</code> builtin
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>) is enabled,
the pathname printed will not
contain symbolic links.
If the <samp class="option">-L</samp> option is supplied, the pathname printed may contain
symbolic links.
The return status is zero unless an error is encountered while
determining the name of the current directory or an invalid option
is supplied.
</p>
</dd>
<dt><a id="index-readonly"></a><span><code class="code">readonly</code><a class="copiable-link" href="#index-readonly"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">readonly [-aAf] [-p] [<var class="var">name</var>[=<var class="var">value</var>]] ...
</pre></div>

<p>Mark each <var class="var">name</var> as readonly.
The values of these names may not be changed by subsequent assignment
or unset.
If the <samp class="option">-f</samp> option is supplied, each <var class="var">name</var> refers to a shell
function.
The <samp class="option">-a</samp> option means each <var class="var">name</var> refers to an indexed
array variable; the <samp class="option">-A</samp> option means each <var class="var">name</var> refers
to an associative array variable.
If both options are supplied, <samp class="option">-A</samp> takes precedence.
If no <var class="var">name</var> arguments are supplied, or if the <samp class="option">-p</samp>
option is supplied, print a list of all readonly names.
The other options may be used to restrict the output to a subset of
the set of readonly names.
The <samp class="option">-p</samp>
option displays output in a format that may be reused as input.
</p>
<p><code class="code">readonly</code> allows the value of a variable to be set at the same time
the readonly attribute is changed by following the variable name with
=<var class="var">value</var>.
This sets the value of the variable is to <var class="var">value</var> while modifying
the readonly attribute.
</p>
<p>The return status is zero unless an invalid option is supplied, one of
the <var class="var">name</var> arguments is not a valid shell variable or function name,
or the <samp class="option">-f</samp> option is supplied with a name that is not a shell function.
</p>
</dd>
<dt><a id="index-return"></a><span><code class="code">return</code><a class="copiable-link" href="#index-return"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">return [<var class="var">n</var>]
</pre></div>

<p>Stop executing a shell function or sourced file and return the value <var class="var">n</var>
to its caller.
If <var class="var">n</var> is not supplied, the return value is the exit status of the
last command executed.
If <code class="code">return</code> is executed by a trap handler, the last command used to
determine the status is the last command executed before the trap handler.
If <code class="code">return</code> is executed during a <code class="code">DEBUG</code> trap, the last command
used to determine the status is the last command executed by the trap
handler before <code class="code">return</code> was invoked.
</p>
<p>When <code class="code">return</code> is used to terminate execution of a script
being executed with the <code class="code">.</code> (<code class="code">source</code>) builtin, it
returns either <var class="var">n</var> or
the exit status of the last command executed within the script as the exit
status of the script.
If <var class="var">n</var> is supplied, the return value is its least significant
8 bits.
</p>
<p>Any command associated with the <code class="code">RETURN</code> trap is executed
before execution resumes after the function or script.
</p>
<p>The return status is non-zero if <code class="code">return</code> is supplied a non-numeric
argument or is used outside a function
and not during the execution of a script by <code class="code">.</code> or <code class="code">source</code>.
</p>
</dd>
<dt><a id="index-shift"></a><span><code class="code">shift</code><a class="copiable-link" href="#index-shift"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">shift [<var class="var">n</var>]
</pre></div>

<p>Shift the positional parameters to the left by <var class="var">n</var>:
the positional parameters from <var class="var">n</var>+1 &hellip; <code class="code">$#</code> are
renamed to <code class="code">$1</code> &hellip; <code class="code">$#</code>-<var class="var">n</var>.
Parameters represented by the numbers <code class="code">$#</code> down to <code class="code">$#</code>-<var class="var">n</var>+1
are unset.
<var class="var">n</var> must be a non-negative number less than or equal to <code class="code">$#</code>.
If <var class="var">n</var> is not supplied, it is assumed to be 1.
If <var class="var">n</var> is zero or greater than <code class="code">$#</code>, the positional parameters
are not changed.
The return status is zero unless <var class="var">n</var> is greater than <code class="code">$#</code> or
less than zero, non-zero otherwise.
</p>
</dd>
<dt><a class="index-entry-id" id="index-_005b"></a>
<a id="index-test"></a><span><code class="code">test</code><a class="copiable-link" href="#index-test"> &para;</a></span></dt>
<dt><code class="code">[</code></dt>
<dd><div class="example">
<pre class="example-preformatted">test <var class="var">expr</var>
</pre></div>

<p>Evaluate a conditional expression <var class="var">expr</var> and return a status of
0 (true) or 1 (false).
Each operator and operand must be a separate argument.
Expressions are composed of the primaries described below in
<a class="ref" href="#Bash-Conditional-Expressions">Bash Conditional Expressions</a>.
<code class="code">test</code> does not accept any options, nor does it accept and
ignore an argument of <samp class="option">--</samp> as signifying the end of options.
When using the <code class="code">[</code> form, the last argument to the command must
be a <code class="code">]</code>.
</p>
<p>Expressions may be combined using the following operators, listed in
decreasing order of precedence.
The evaluation depends on the number of arguments; see below.
<code class="code">test</code> uses operator precedence when there are five or more arguments.
</p>
<dl class="table">
<dt><code class="code">! <var class="var">expr</var></code></dt>
<dd><p>True if <var class="var">expr</var> is false.
</p>
</dd>
<dt><code class="code">( <var class="var">expr</var> )</code></dt>
<dd><p>Returns the value of <var class="var">expr</var>.
This may be used to override normal operator precedence.
</p>
</dd>
<dt><code class="code"><var class="var">expr1</var> -a <var class="var">expr2</var></code></dt>
<dd><p>True if both <var class="var">expr1</var> and <var class="var">expr2</var> are true.
</p>
</dd>
<dt><code class="code"><var class="var">expr1</var> -o <var class="var">expr2</var></code></dt>
<dd><p>True if either <var class="var">expr1</var> or <var class="var">expr2</var> is true.
</p></dd>
</dl>

<p>The <code class="code">test</code> and <code class="code">[</code> builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
</p>
<dl class="table">
<dt>0 arguments</dt>
<dd><p>The expression is false.
</p>
</dd>
<dt>1 argument</dt>
<dd><p>The expression is true if, and only if, the argument is not null.
</p>
</dd>
<dt>2 arguments</dt>
<dd><p>If the first argument is &lsquo;<samp class="samp">!</samp>&rsquo;, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators
(see <a class="pxref" href="#Bash-Conditional-Expressions">Bash Conditional Expressions</a>), the expression
is true if the unary test is true.
If the first argument is not a valid unary operator, the expression is
false.
</p>
</dd>
<dt>3 arguments</dt>
<dd><p>The following conditions are applied in the order listed.
</p>
<ol class="enumerate">
<li> If the second argument is one of the binary conditional
operators (see <a class="pxref" href="#Bash-Conditional-Expressions">Bash Conditional Expressions</a>), the
result of the expression is the result of the binary test using the
first and third arguments as operands.
The &lsquo;<samp class="samp">-a</samp>&rsquo; and &lsquo;<samp class="samp">-o</samp>&rsquo; operators are considered binary operators
when there are three arguments.
</li><li> If the first argument is &lsquo;<samp class="samp">!</samp>&rsquo;, the value is the negation of
the two-argument test using the second and third arguments.
</li><li> If the first argument is exactly &lsquo;<samp class="samp">(</samp>&rsquo; and the third argument is
exactly &lsquo;<samp class="samp">)</samp>&rsquo;, the result is the one-argument test of the second
argument.
</li><li> Otherwise, the expression is false.
</li></ol>

</dd>
<dt>4 arguments</dt>
<dd><p>The following conditions are applied in the order listed.
</p>
<ol class="enumerate">
<li> If the first argument is &lsquo;<samp class="samp">!</samp>&rsquo;, the result is the negation of
the three-argument expression composed of the remaining arguments.
</li><li> If the first argument is exactly &lsquo;<samp class="samp">(</samp>&rsquo; and the fourth argument is
exactly &lsquo;<samp class="samp">)</samp>&rsquo;, the result is the two-argument test of the second
and third arguments.
</li><li> Otherwise, the expression is parsed and evaluated according to
precedence using the rules listed above.
</li></ol>

</dd>
<dt>5 or more arguments</dt>
<dd><p>The expression is parsed and evaluated according to precedence
using the rules listed above.
</p></dd>
</dl>

<p>If the shell is in <small class="sc">POSIX</small> mode, or if the expression is part
of the <code class="code">[[</code> command,
the &lsquo;<samp class="samp">&lt;</samp>&rsquo; and &lsquo;<samp class="samp">&gt;</samp>&rsquo; operators sort using the current locale.
If the shell is not in <small class="sc">POSIX</small> mode, the <code class="code">test</code> and &lsquo;<samp class="samp">[</samp>&rsquo;
commands sort lexicographically using ASCII ordering.
</p>
<p>The historical operator-precedence parsing with 4 or more arguments can
lead to ambiguities when it encounters strings that look like primaries.
The
<small class="sc">POSIX</small>
standard has deprecated the <samp class="option">-a</samp> and <samp class="option">-o</samp>
primaries and enclosing expressions within parentheses.
Scripts should no longer use them.
It&rsquo;s much more reliable to restrict test invocations to a single primary,
and to replace uses of <samp class="option">-a</samp> and <samp class="option">-o</samp> with the shell&rsquo;s
<code class="code">&amp;&amp;</code> and <code class="code">||</code> list operators. For example, use
</p>
<div class="example">
<pre class="example-preformatted">test -n string1 &amp;&amp; test -n string2
</pre></div>

<p>instead of
</p>
<div class="example">
<pre class="example-preformatted">test -n string1 -a -n string2
</pre></div>

</dd>
<dt><a id="index-times"></a><span><code class="code">times</code><a class="copiable-link" href="#index-times"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">times
</pre></div>

<p>Print out the user and system times used by the shell and its children.
The return status is zero.
</p>
</dd>
<dt><a id="index-trap"></a><span><code class="code">trap</code><a class="copiable-link" href="#index-trap"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">trap [-lpP] [<var class="var">action</var>] [<var class="var">sigspec</var> ...]
</pre></div>

<p>The <var class="var">action</var> is a command that is read and executed when the
shell receives any of the signals <var class="var">sigspec</var>.
If <var class="var">action</var> is absent (and
there is a single <var class="var">sigspec</var>) or
equal to &lsquo;<samp class="samp">-</samp>&rsquo;, each specified <var class="var">sigspec</var>&rsquo;s disposition is reset
to the value it had when the shell was started.
If <var class="var">action</var> is the null string, then the signal specified by
each <var class="var">sigspec</var> is ignored by the shell and commands it invokes.
</p>
<p>If no arguments are supplied, <code class="code">trap</code> prints the actions
associated with each trapped signal
as a set of <code class="code">trap</code> commands that can be reused as shell input to
restore the current signal dispositions.
</p>
<p>If <var class="var">action</var> is not present and <samp class="option">-p</samp> has been supplied,
<code class="code">trap</code> displays the trap commands associated with each <var class="var">sigspec</var>,
or, if no <var class="var">sigspec</var>s are supplied, for all trapped signals,
as a set of <code class="code">trap</code> commands that can be reused as shell input to
restore the current signal dispositions.
The <samp class="option">-P</samp> option behaves similarly, but displays only the actions
associated with each <var class="var">sigspec</var> argument.
<samp class="option">-P</samp> requires at least one <var class="var">sigspec</var> argument.
The <samp class="option">-P</samp> or <samp class="option">-p</samp> options may be
used in a subshell environment (e.g., command substitution) and,
as long as they are used before <code class="code">trap</code> is used to change a
signal&rsquo;s handling, will display the state of its parent&rsquo;s traps.
</p>
<p>The <samp class="option">-l</samp> option prints a list of signal names
and their corresponding numbers.
Each <var class="var">sigspec</var> is either a signal name or a signal number.
Signal names are case insensitive and the <code class="code">SIG</code> prefix is optional.
If <samp class="option">-l</samp> is supplied with no <var class="var">sigspec</var> arguments, it prints a
list of valid signal names.
</p>
<p>If a <var class="var">sigspec</var>
is <code class="code">0</code> or <code class="code">EXIT</code>, <var class="var">action</var> is executed when the shell exits.
If a <var class="var">sigspec</var> is <code class="code">DEBUG</code>, <var class="var">action</var> is executed
before every simple command, <code class="code">for</code> command, <code class="code">case</code> command,
<code class="code">select</code> command, (( arithmetic command, [[ conditional command,
arithmetic <code class="code">for</code> command,
and before the first command executes in a shell function.
Refer to the description of the <code class="code">extdebug</code> shell option
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>) for details of its
effect on the <code class="code">DEBUG</code> trap.
If a <var class="var">sigspec</var> is <code class="code">RETURN</code>, <var class="var">action</var> is executed
each time a shell function or a script executed with the <code class="code">.</code> or
<code class="code">source</code> builtins finishes executing.
</p>
<p>If a <var class="var">sigspec</var> is <code class="code">ERR</code>, <var class="var">action</var> 
is executed whenever
a pipeline (which may consist of a single simple
command), a list, or a compound command returns a
non-zero exit status,
subject to the following conditions.
The <code class="code">ERR</code> trap is not executed if the failed command is part of the
command list immediately following an
<code class="code">until</code> or <code class="code">while</code> reserved word,
part of the test following the <code class="code">if</code> or <code class="code">elif</code> reserved words,
part of a command executed in a <code class="code">&amp;&amp;</code> or <code class="code">||</code> list
except the command following the final <code class="code">&amp;&amp;</code> or <code class="code">||</code>,
any command in a pipeline but the last,
(subject to the state of the <code class="code">pipefail</code> shell option),
or if the command&rsquo;s return
status is being inverted using <code class="code">!</code>.
These are the same conditions obeyed by the <code class="code">errexit</code> (<samp class="option">-e</samp>)
option.
</p>
<p>When the shell is not interactive,
signals ignored upon entry to a non-interactive shell cannot be trapped or
reset.
Interactive shells permit trapping signals ignored on entry.
Trapped signals that are not being ignored are reset to their original
values in a subshell or subshell environment when one is created.
</p>
<p>The return status is zero unless a <var class="var">sigspec</var> does not specify a
valid signal; non-zero otherwise.
</p>
</dd>
<dt><a id="index-true"></a><span><code class="code">true</code><a class="copiable-link" href="#index-true"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">true
</pre></div>

<p>Does nothing, returns a 0 status.
</p>
</dd>
<dt><a id="index-umask"></a><span><code class="code">umask</code><a class="copiable-link" href="#index-umask"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">umask [-p] [-S] [<var class="var">mode</var>]
</pre></div>

<p>Set the shell process&rsquo;s file creation mask to <var class="var">mode</var>.
If <var class="var">mode</var> begins with a digit, it is interpreted as an octal number;
if not, it is interpreted as a symbolic mode mask similar
to that accepted by the <code class="code">chmod</code> command.
If <var class="var">mode</var> is omitted, <code class="code">umask</code> prints the current value of the mask. 
If the <samp class="option">-S</samp>
option is supplied without a <var class="var">mode</var> argument, <code class="code">umask</code>
prints the mask in a symbolic format;
the default output is an octal number.
If the  <samp class="option">-p</samp> option is supplied, and <var class="var">mode</var>
is omitted, the output is in a form that may be reused as input.
The return status is zero if the mode is successfully changed or if
no <var class="var">mode</var> argument is supplied, and non-zero otherwise.
</p>
<p>Note that when the mode is interpreted as an octal number, each number
of the umask is subtracted from <code class="code">7</code>.  Thus, a umask of <code class="code">022</code>
results in permissions of <code class="code">755</code>.
</p>
</dd>
<dt><a id="index-unset"></a><span><code class="code">unset</code><a class="copiable-link" href="#index-unset"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">unset [-fnv] [<var class="var">name</var>]
</pre></div>

<p>Remove each variable or function <var class="var">name</var>.
If the <samp class="option">-v</samp> option is given, each
<var class="var">name</var> refers to a shell variable and that variable is removed.
If the <samp class="option">-f</samp> option is given, the <var class="var">name</var>s refer to shell
functions, and the function definition is removed.
If the <samp class="option">-n</samp> option is supplied, and <var class="var">name</var> is a variable with
the <code class="code">nameref</code> attribute, <var class="var">name</var> will be unset rather than the
variable it references.
<samp class="option">-n</samp> has no effect if the <samp class="option">-f</samp> option is supplied.
If no options are supplied, each <var class="var">name</var> refers to a variable; if
there is no variable by that name, a function with that name, if any, is
unset.
Readonly variables and functions may not be unset.
When variables or functions are removed, they are also removed
from the environment passed to subsequent commands.
Some shell variables may not be unset.
Some shell variables lose their special behavior if they are unset; such
behavior is noted in the description of the individual variables.
The return status is zero unless a <var class="var">name</var> is readonly or may not be unset.
</p></dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="Bash-Builtins">
<div class="nav-panel">
<p>
Next: <a href="#Modifying-Shell-Behavior" accesskey="n" rel="next">Modifying Shell Behavior</a>, Previous: <a href="#Bourne-Shell-Builtins" accesskey="p" rel="prev">Bourne Shell Builtins</a>, Up: <a href="#Shell-Builtin-Commands" accesskey="u" rel="up">Shell Builtin Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-Builtin-Commands"><span>4.2 Bash Builtin Commands<a class="copiable-link" href="#Bash-Builtin-Commands"> &para;</a></span></h3>

<p>This section describes builtin commands which are unique to
or have been extended in Bash.
Some of these commands are specified in the <small class="sc">POSIX</small> standard.
</p>
<dl class="table">
<dt><a id="index-alias"></a><span><code class="code">alias</code><a class="copiable-link" href="#index-alias"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">alias [-p] [<var class="var">name</var>[=<var class="var">value</var>] ...]
</pre></div>

<p>Without arguments or with the <samp class="option">-p</samp> option, <code class="code">alias</code> prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, define an alias for each <var class="var">name</var>
whose <var class="var">value</var> is given.
If no <var class="var">value</var> is given, print the name and value of the alias <var class="var">name</var>.
A trailing space in <var class="var">value</var> causes the next word to be
checked for alias substitution when the alias is expanded
during command parsing.
<code class="code">alias</code> returns true unless a <var class="var">name</var> is given
(without a corresponding =<var class="var">value</var>)
for which no alias has been defined.
Aliases are described in <a class="ref" href="#Aliases">Aliases</a>.
</p>
</dd>
<dt><a id="index-bind"></a><span><code class="code">bind</code><a class="copiable-link" href="#index-bind"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">bind [-m <var class="var">keymap</var>] [-lsvSVX]
bind [-m <var class="var">keymap</var>] [-q <var class="var">function</var>] [-u <var class="var">function</var>] [-r <var class="var">keyseq</var>]
bind [-m <var class="var">keymap</var>] -f <var class="var">filename</var>
bind [-m <var class="var">keymap</var>] -x <var class="var">keyseq[: ]shell-command</var>
bind [-m <var class="var">keymap</var>] <var class="var">keyseq:function-name</var>
bind [-m <var class="var">keymap</var>] <var class="var">keyseq:readline-command</var>
bind [-m <var class="var">keymap</var>] -p|-P [<var class="var">readline-command</var>]
bind <var class="var">readline-command-line</var>
</pre></div>

<p>Display current Readline (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>)
key and function bindings,
bind a key sequence to a Readline function or macro
or to a shell command,
or set a Readline variable.
Each non-option argument is a key binding or command as it would appear in a
Readline initialization file (see <a class="pxref" href="#Readline-Init-File">Readline Init File</a>),
but each binding or command must be passed as a separate argument;  e.g.,
&lsquo;<samp class="samp">&quot;\C-x\C-r&quot;:re-read-init-file</samp>&rsquo;.
</p>
<p>In the following descriptions, options that display output in a form
available to be re-read format their output
as commands that would appear in a Readline initialization file or
that would be supplied as individual arguments to a <code class="code">bind</code> command.
</p>
<p>Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-m <var class="var">keymap</var></code></dt>
<dd><p>Use <var class="var">keymap</var> as the keymap to be affected by
the subsequent bindings.  Acceptable <var class="var">keymap</var>
names are
<code class="code">emacs</code>,
<code class="code">emacs-standard</code>,
<code class="code">emacs-meta</code>,
<code class="code">emacs-ctlx</code>,
<code class="code">vi</code>,
<code class="code">vi-move</code>,
<code class="code">vi-command</code>, and
<code class="code">vi-insert</code>.
<code class="code">vi</code> is equivalent to <code class="code">vi-command</code>
(<code class="code">vi-move</code> is also a synonym);
<code class="code">emacs</code> is equivalent to <code class="code">emacs-standard</code>.
</p>
</dd>
<dt><code class="code">-l</code></dt>
<dd><p>List the names of all Readline functions.
</p>
</dd>
<dt><code class="code">-p</code></dt>
<dd><p>Display Readline function names and bindings
in such a way that they can be used as an argument to a subsequent
<code class="code">bind</code> command or in a Readline initialization file.
If arguments remain after option processing, <code class="code">bind</code> treats
them as readline command names and restricts output to those names.
</p>
</dd>
<dt><code class="code">-P</code></dt>
<dd><p>List current Readline function names and bindings.
If arguments remain after option processing, <code class="code">bind</code> treats
them as readline command names and restricts output to those names.
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>Display Readline key sequences bound to macros and the strings
they output in such a way that they can be used as
an argument to a subsequent <code class="code">bind</code> command
or in a Readline initialization file.
</p>
</dd>
<dt><code class="code">-S</code></dt>
<dd><p>Display Readline key sequences bound to macros and the strings they output.
</p>
</dd>
<dt><code class="code">-v</code></dt>
<dd><p>Display Readline variable names and values
in such a way that they can be used as
an argument to a subsequent <code class="code">bind</code> command
or in a Readline initialization file.
</p>
</dd>
<dt><code class="code">-V</code></dt>
<dd><p>List current Readline variable names and values.
</p>
</dd>
<dt><code class="code">-f <var class="var">filename</var></code></dt>
<dd><p>Read key bindings from <var class="var">filename</var>.
</p>
</dd>
<dt><code class="code">-q <var class="var">function</var></code></dt>
<dd><p>Display key sequences that invoke the named Readline <var class="var">function</var>.
</p>
</dd>
<dt><code class="code">-u <var class="var">function</var></code></dt>
<dd><p>Unbind all key sequences bound to the named Readline <var class="var">function</var>.
</p>
</dd>
<dt><code class="code">-r <var class="var">keyseq</var></code></dt>
<dd><p>Remove any current binding for <var class="var">keyseq</var>.
</p>
</dd>
<dt><code class="code">-x <var class="var">keyseq:shell-command</var></code></dt>
<dd><p>Cause <var class="var">shell-command</var> to be executed whenever <var class="var">keyseq</var> is
entered.
The separator between <var class="var">keyseq</var> and <var class="var">shell-command</var> is either
whitespace or a colon optionally followed by whitespace.
If the separator is whitespace, <var class="var">shell-command</var>
must be enclosed in double quotes and Readline expands any of its
special backslash-escapes in <var class="var">shell-command</var> before saving it.
If the separator is a colon, any enclosing double quotes are optional, and
Readline does not expand the command string before saving it.
Since the entire key binding expression must be a single argument, it
should be enclosed in single quotes.
When <var class="var">shell-command</var> is executed, the shell sets the
<code class="code">READLINE_LINE</code> variable to the contents of the Readline line
buffer
and the
<code class="code">READLINE_POINT</code> and <code class="code">READLINE_MARK</code>
variables to the current location of the insertion point and the saved
insertion point (the <var class="var">mark</var>), respectively.
The shell assigns any numeric argument the user supplied to the
<code class="code">READLINE_ARGUMENT</code> variable.
If there was no argument, that variable is not set.
If the executed command changes the value of any of
<code class="code">READLINE_LINE</code>, <code class="code">READLINE_POINT</code>, or <code class="code">READLINE_MARK</code>,
those new values will be reflected in the editing state.
</p>
</dd>
<dt><code class="code">-X</code></dt>
<dd><p>List all key sequences bound to shell commands and the associated commands
in a format that can be reused as
an argument to a subsequent <code class="code">bind</code> command.
</p></dd>
</dl>

<p>The return status is zero unless an invalid option is supplied or an
error occurs.
</p>
</dd>
<dt><a id="index-builtin"></a><span><code class="code">builtin</code><a class="copiable-link" href="#index-builtin"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">builtin [<var class="var">shell-builtin</var> [<var class="var">args</var>]]
</pre></div>

<p>Execute the specified shell builtin <var class="var">shell-builtin</var>,
passing it <var class="var">args</var>, and return its exit status.
This is useful when defining a shell function with the same
name as a shell builtin,
retaining the functionality of the builtin within the function.
The return status is non-zero if <var class="var">shell-builtin</var> is not a shell
builtin command.
</p>
</dd>
<dt><a id="index-caller"></a><span><code class="code">caller</code><a class="copiable-link" href="#index-caller"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">caller [<var class="var">expr</var>]
</pre></div>

<p>Returns the context of any active subroutine call (a shell function or
a script executed with the <code class="code">.</code> or <code class="code">source</code> builtins).
</p>
<p>Without <var class="var">expr</var>, <code class="code">caller</code> displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as <var class="var">expr</var>, <code class="code">caller</code>
displays the line number, subroutine name, and source file corresponding
to that position in the current execution call stack.
This extra information may be used, for example, to print a stack trace.
The current frame is frame 0.
</p>
<p>The return value is 0 unless the shell is not executing a subroutine
call or <var class="var">expr</var> does not correspond to a valid position in the
call stack.
</p>
</dd>
<dt><a id="index-command"></a><span><code class="code">command</code><a class="copiable-link" href="#index-command"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">command [-pVv] <var class="var">command</var> [<var class="var">arguments</var> ...]
</pre></div>

<p>The <code class="code">command</code> builtin runs <var class="var">command</var> with <var class="var">arguments</var>
ignoring any shell function named <var class="var">command</var>.
Only shell builtin commands or commands found by searching the
<code class="env">PATH</code> are executed.
If there is a shell function named <code class="code">ls</code>, running &lsquo;<samp class="samp">command ls</samp>&rsquo;
within the function will execute the external command <code class="code">ls</code>
instead of calling the function recursively.
The <samp class="option">-p</samp> option means to use a default value for <code class="env">PATH</code>
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if <var class="var">command</var> cannot be
found or an error occurred, and the exit status of <var class="var">command</var>
otherwise.
</p>
<p>If either the <samp class="option">-V</samp> or <samp class="option">-v</samp> option is supplied, <code class="code">command</code>
prints a description of <var class="var">command</var>.
The <samp class="option">-v</samp> option
displays a single word indicating the command or file name used to
invoke <var class="var">command</var>;
the <samp class="option">-V</samp> option produces a more verbose description.
In this case, the return status is
zero if <var class="var">command</var> is found, and non-zero if not.
</p>
</dd>
<dt><a id="index-declare"></a><span><code class="code">declare</code><a class="copiable-link" href="#index-declare"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">declare [-aAfFgiIlnrtux] [-p] [<var class="var">name</var>[=<var class="var">value</var>] ...]
</pre></div>

<p>Declare variables and give them attributes.
If no <var class="var">name</var>s are given, then display the values of variables or
shell functions instead. 
</p>
<p>The <samp class="option">-p</samp> option will display the attributes and values of each
<var class="var">name</var>.
When <samp class="option">-p</samp> is used with <var class="var">name</var> arguments, additional options,
other than <samp class="option">-f</samp> and <samp class="option">-F</samp>, are ignored.
</p>
<p>When <samp class="option">-p</samp> is supplied without <var class="var">name</var> arguments,
<code class="code">declare</code> will display the attributes and values
of all variables having the attributes specified by the additional options.
If no other options are supplied with <samp class="option">-p</samp>, <code class="code">declare</code> will
display the attributes and values of all shell variables.
The <samp class="option">-f</samp> option restricts the display to shell functions.
</p>
<p>The <samp class="option">-F</samp> option inhibits the display of function definitions;
only the function name and attributes are printed.
If the <code class="code">extdebug</code> shell option is enabled using <code class="code">shopt</code>
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), the source file name and line number where
each <var class="var">name</var> is defined are displayed as well.
<samp class="option">-F</samp> implies <samp class="option">-f</samp>.
</p>
<p>The <samp class="option">-g</samp> option forces variables to be created or modified at
the global scope, even when <code class="code">declare</code> is executed in a shell function.
It is ignored in when <code class="code">declare</code> is  not executed in a shell function.
</p>
<p>The <samp class="option">-I</samp> option causes local variables to inherit the attributes
(except the <code class="code">nameref</code> attribute)
and value of any existing variable with the same
<var class="var">name</var> at a surrounding scope.
If there is no existing variable, the local variable is initially unset.
</p>
<p>The following options can be used to restrict output to variables with
the specified attributes or to give variables attributes:
</p>
<dl class="table">
<dt><code class="code">-a</code></dt>
<dd><p>Each <var class="var">name</var> is an indexed array variable (see <a class="pxref" href="#Arrays">Arrays</a>).
</p>
</dd>
<dt><code class="code">-A</code></dt>
<dd><p>Each <var class="var">name</var> is an associative array variable (see <a class="pxref" href="#Arrays">Arrays</a>).
</p>
</dd>
<dt><code class="code">-f</code></dt>
<dd><p>Each <var class="var">name</var> refers to a shell function.
</p>
</dd>
<dt><code class="code">-i</code></dt>
<dd><p>The variable is to be treated as an integer;
arithmetic evaluation (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>) is
performed when the variable is assigned a value.
</p>
</dd>
<dt><code class="code">-l</code></dt>
<dd><p>When the variable is assigned a value, all upper-case characters are
converted to lower-case.
The upper-case attribute is disabled.
</p>
</dd>
<dt><code class="code">-n</code></dt>
<dd><p>Give each <var class="var">name</var> the <code class="code">nameref</code> attribute, making
it a name reference to another variable.
That other variable is defined by the value of <var class="var">name</var>.
All references, assignments, and attribute modifications
to <var class="var">name</var>, except for those using or changing the
<samp class="option">-n</samp> attribute itself, are performed on the variable referenced by
<var class="var">name</var>&rsquo;s value.
The nameref attribute cannot be applied to array variables.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>Make <var class="var">name</var>s readonly.
These names cannot then be assigned values
by subsequent assignment statements or unset.
</p>
</dd>
<dt><code class="code">-t</code></dt>
<dd><p>Give each <var class="var">name</var> the <code class="code">trace</code> attribute.
Traced functions inherit the <code class="code">DEBUG</code> and <code class="code">RETURN</code> traps from
the calling shell.
The trace attribute has no special meaning for variables.
</p>
</dd>
<dt><code class="code">-u</code></dt>
<dd><p>When the variable is assigned a value, all lower-case characters are
converted to upper-case.
The lower-case attribute is disabled.
</p>
</dd>
<dt><code class="code">-x</code></dt>
<dd><p>Mark each <var class="var">name</var> for export to subsequent commands via
the environment.
</p></dd>
</dl>

<p>Using &lsquo;<samp class="samp">+</samp>&rsquo; instead of &lsquo;<samp class="samp">-</samp>&rsquo; turns off the specified
attribute instead, with the exceptions that &lsquo;<samp class="samp">+a</samp>&rsquo; and &lsquo;<samp class="samp">+A</samp>&rsquo;
may not be used to destroy array variables and &lsquo;<samp class="samp">+r</samp>&rsquo; will not
remove the readonly attribute.
</p>
<p>When used in a function, <code class="code">declare</code> makes each <var class="var">name</var> local,
as with the <code class="code">local</code> command, unless the <samp class="option">-g</samp> option is supplied.
If a variable name is followed by =<var class="var">value</var>, the value of the variable
is set to <var class="var">value</var>.
</p>
<p>When using <samp class="option">-a</samp> or <samp class="option">-A</samp> and the compound assignment syntax to 
create array variables, additional attributes do not take effect until
subsequent assignments.
</p>
<p>The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using &lsquo;<samp class="samp">-f foo=bar</samp>&rsquo;,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see <a class="pxref" href="#Arrays">Arrays</a>),
one of the <var class="var">name</var>s is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with <samp class="option">-f</samp>.
</p>
</dd>
<dt><a id="index-echo"></a><span><code class="code">echo</code><a class="copiable-link" href="#index-echo"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">echo [-neE] [<var class="var">arg</var> ...]
</pre></div>

<p>Output the <var class="var">arg</var>s, separated by spaces, terminated with a
newline.
The return status is 0 unless a write error occurs.
If <samp class="option">-n</samp> is specified, the trailing newline is not printed.
</p>
<p>If the <samp class="option">-e</samp> option is given, <code class="code">echo</code> interprets the following
backslash-escaped characters.
The <samp class="option">-E</samp> option disables interpretation of these escape characters,
even on systems where they are interpreted by default.
The <code class="code">xpg_echo</code> shell option determines
whether or not <code class="code">echo</code> interprets any options and
expands these escape characters.
<code class="code">echo</code> does not interpret <samp class="option">--</samp> to mean the end of options.
</p>
<p><code class="code">echo</code> interprets the following escape sequences:
</p><dl class="table">
<dt><code class="code">\a</code></dt>
<dd><p>alert (bell)
</p></dd>
<dt><code class="code">\b</code></dt>
<dd><p>backspace
</p></dd>
<dt><code class="code">\c</code></dt>
<dd><p>suppress further output
</p></dd>
<dt><code class="code">\e</code></dt>
<dt><code class="code">\E</code></dt>
<dd><p>escape
</p></dd>
<dt><code class="code">\f</code></dt>
<dd><p>form feed
</p></dd>
<dt><code class="code">\n</code></dt>
<dd><p>new line
</p></dd>
<dt><code class="code">\r</code></dt>
<dd><p>carriage return
</p></dd>
<dt><code class="code">\t</code></dt>
<dd><p>horizontal tab
</p></dd>
<dt><code class="code">\v</code></dt>
<dd><p>vertical tab
</p></dd>
<dt><code class="code">\\</code></dt>
<dd><p>backslash
</p></dd>
<dt><code class="code">\0<var class="var">nnn</var></code></dt>
<dd><p>The eight-bit character whose value is the octal value <var class="var">nnn</var>
(zero to three octal digits).
</p></dd>
<dt><code class="code">\x<var class="var">HH</var></code></dt>
<dd><p>The eight-bit character whose value is the hexadecimal value <var class="var">HH</var>
(one or two hex digits).
</p></dd>
<dt><code class="code">\u<var class="var">HHHH</var></code></dt>
<dd><p>The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var class="var">HHHH</var> (one to four hex digits).
</p></dd>
<dt><code class="code">\U<var class="var">HHHHHHHH</var></code></dt>
<dd><p>The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var class="var">HHHHHHHH</var> (one to eight hex digits).
</p></dd>
</dl>

<p><code class="code">echo</code> writes any unrecognized backslash-escaped characters unchanged.
</p>
</dd>
<dt><a id="index-enable"></a><span><code class="code">enable</code><a class="copiable-link" href="#index-enable"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">enable [-a] [-dnps] [-f <var class="var">filename</var>] [<var class="var">name</var> ...]
</pre></div>

<p>Enable and disable builtin shell commands.
Disabling a builtin allows an executable file which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before files.
</p>
<p>If <samp class="option">-n</samp> is supplied, the <var class="var">name</var>s are disabled.
Otherwise <var class="var">name</var>s are enabled.
For example, to use the <code class="code">test</code> binary
found using <code class="env">$PATH</code> instead of the shell builtin version, type
&lsquo;<samp class="samp">enable -n test</samp>&rsquo;.
</p>
<p>If the <samp class="option">-p</samp> option is supplied, or no <var class="var">name</var> arguments are
supplied, print a list of shell builtins.
With no other arguments, the list consists of all enabled shell builtins.
The <samp class="option">-n</samp> option means to print only disabled builtins.
The <samp class="option">-a</samp> option means to list
each builtin with an indication of whether or not it is enabled. 
The <samp class="option">-s</samp> option means to
restrict <code class="code">enable</code> to the
<small class="sc">POSIX</small>
special builtins.
</p>
<p>The <samp class="option">-f</samp> option means to load the new builtin command <var class="var">name</var>
from shared object <var class="var">filename</var>, on systems that support dynamic loading.
If <var class="var">filename</var> does not contain a slash.
Bash will use the value of the <code class="env">BASH_LOADABLES_PATH</code> variable as a
colon-separated list of directories in which to search for <var class="var">filename</var>.
The default for <code class="env">BASH_LOADABLES_PATH</code> is system-dependent,
and may include &quot;.&quot; to force a search of the current directory.
The <samp class="option">-d</samp> option will delete a builtin loaded with <samp class="option">-f</samp>.
If <samp class="option">-s</samp> is used with <samp class="option">-f</samp>, the new builtin becomes a
<small class="sc">POSIX</small>
special builtin (see <a class="pxref" href="#Special-Builtins">Special Builtins</a>).
</p>
<p>If no options are supplied and a <var class="var">name</var> is not a shell builtin,
<code class="code">enable</code> will attempt to load <var class="var">name</var> from a shared object named
<var class="var">name</var>, as if the command were
&lsquo;<samp class="samp">enable -f <var class="var">name</var> <var class="var">name</var></samp>&rsquo;.
</p>
<p>The return status is zero unless a <var class="var">name</var> is not a shell builtin
or there is an error loading a new builtin from a shared object.
</p>
</dd>
<dt><a id="index-help"></a><span><code class="code">help</code><a class="copiable-link" href="#index-help"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">help [-dms] [<var class="var">pattern</var>]
</pre></div>

<p>Display helpful information about builtin commands.
If <var class="var">pattern</var> is specified, <code class="code">help</code> gives detailed help
on all commands matching <var class="var">pattern</var>
as described below;
otherwise it displays a list of
all builtins and shell compound commands.
</p>
<p>Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-d</code></dt>
<dd><p>Display a short description of each <var class="var">pattern</var>
</p></dd>
<dt><code class="code">-m</code></dt>
<dd><p>Display the description of each <var class="var">pattern</var> in a manpage-like format
</p></dd>
<dt><code class="code">-s</code></dt>
<dd><p>Display only a short usage synopsis for each <var class="var">pattern</var>
</p></dd>
</dl>

<p>If <var class="var">pattern</var> contains pattern matching characters
(see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>)
it&rsquo;s treated as a shell pattern and
<code class="code">help</code> prints the description of each
help topic matching <var class="var">pattern</var>.
</p>
<p>If not, and <var class="var">pattern</var> exactly matches the name of a help topic,
<code class="code">help</code> prints the description
associated with that topic.
Otherwise, <code class="code">help</code> performs prefix matching and
prints the descriptions of all matching help topics. 
</p>
<p>The return status is zero unless no command matches <var class="var">pattern</var>.
</p>
</dd>
<dt><a id="index-let"></a><span><code class="code">let</code><a class="copiable-link" href="#index-let"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">let <var class="var">expression</var> [<var class="var">expression</var> ...]
</pre></div>

<p>The <code class="code">let</code> builtin allows arithmetic to be performed on shell variables. 
Each <var class="var">expression</var> is evaluated as an arithmetic expression
according to the rules given below in <a class="ref" href="#Shell-Arithmetic">Shell Arithmetic</a>.
If the last <var class="var">expression</var> evaluates to 0,
<code class="code">let</code> returns 1;
otherwise <code class="code">let</code> returns 0.
</p>
</dd>
<dt><a id="index-local"></a><span><code class="code">local</code><a class="copiable-link" href="#index-local"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">local [<var class="var">option</var>] <var class="var">name</var>[=<var class="var">value</var>] ...
</pre></div>

<p>For each argument, create a local variable named <var class="var">name</var>,
and assign it <var class="var">value</var>.
The <var class="var">option</var> can be any of the options accepted by <code class="code">declare</code>.
<code class="code">local</code> can only be used within a function; it makes the variable
<var class="var">name</var> have a visible scope restricted to that function and its
children.
It is an error to use <code class="code">local</code> when not within a function.
</p>
<p>If <var class="var">name</var> is &lsquo;<samp class="samp">-</samp>&rsquo;, it makes the set of shell options
local to the  function in which <code class="code">local</code> is invoked:
any shell options changed using the <code class="code">set</code> builtin inside
the function after the call to <code class="code">local</code> are restored to their
original values when the function returns.
The restore is performed as if a series of <code class="code">set</code> commands were
executed to restore the values that were in place before the function.
</p>
<p>With no operands, <code class="code">local</code>
writes a list of local variables to the standard output.
</p>
<p>The return status is zero unless <code class="code">local</code> is used outside
a function, an invalid <var class="var">name</var> is supplied, or <var class="var">name</var> is a
readonly variable.
</p>
</dd>
<dt><a id="index-logout"></a><span><code class="code">logout</code><a class="copiable-link" href="#index-logout"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">logout [<var class="var">n</var>]
</pre></div>

<p>Exit a login shell, returning a status of <var class="var">n</var> to the shell&rsquo;s
parent.
</p>
</dd>
<dt><a id="index-mapfile"></a><span><code class="code">mapfile</code><a class="copiable-link" href="#index-mapfile"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">mapfile [-d <var class="var">delim</var>] [-n <var class="var">count</var>] [-O <var class="var">origin</var>] [-s <var class="var">count</var>]
    [-t] [-u <var class="var">fd</var>] [-C <var class="var">callback</var>] [-c <var class="var">quantum</var>] [<var class="var">array</var>]
</pre></div>

<p>Read lines from the standard input,
or from file descriptor <var class="var">fd</var> if the <samp class="option">-u</samp> option is supplied,
into the indexed array variable <var class="var">array</var>.
The variable <code class="code">MAPFILE</code> is the default <var class="var">array</var>.
Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-d</code></dt>
<dd><p>Use the first character of <var class="var">delim</var> to terminate each input line,
rather than newline.
If <var class="var">delim</var> is the empty string, <code class="code">mapfile</code> will terminate a line
when it reads a NUL character.
</p></dd>
<dt><code class="code">-n</code></dt>
<dd><p>Copy at most <var class="var">count</var> lines.
If <var class="var">count</var> is 0, copy all lines.
</p></dd>
<dt><code class="code">-O</code></dt>
<dd><p>Begin assigning to <var class="var">array</var> at index <var class="var">origin</var>.
The default index is 0.
</p></dd>
<dt><code class="code">-s</code></dt>
<dd><p>Discard the first <var class="var">count</var> lines read.
</p></dd>
<dt><code class="code">-t</code></dt>
<dd><p>Remove a trailing <var class="var">delim</var> (default newline) from each line read.
</p></dd>
<dt><code class="code">-u</code></dt>
<dd><p>Read lines from file descriptor <var class="var">fd</var> instead of the standard input.
</p></dd>
<dt><code class="code">-C</code></dt>
<dd><p>Evaluate <var class="var">callback</var> each time <var class="var">quantum</var> lines are read.
The <samp class="option">-c</samp> option specifies <var class="var">quantum</var>.
</p></dd>
<dt><code class="code">-c</code></dt>
<dd><p>Specify the number of lines read between each call to <var class="var">callback</var>.
</p></dd>
</dl>

<p>If <samp class="option">-C</samp> is specified without <samp class="option">-c</samp>,
the default quantum is 5000.
When <var class="var">callback</var>  is evaluated, it is supplied the index of the next
array element to be assigned and the line to be assigned to that element
as additional arguments.
<var class="var">callback</var> is evaluated after the line is read but before the
array element is assigned.
</p>
<p>If not supplied with an explicit origin, <code class="code">mapfile</code> will clear <var class="var">array</var>
before assigning to it.
</p>
<p><code class="code">mapfile</code> returns zero unless an invalid option or option
argument is supplied, <var class="var">array</var> is invalid or unassignable, or if
<var class="var">array</var> is not an indexed array.
</p>
</dd>
<dt><a id="index-printf"></a><span><code class="code">printf</code><a class="copiable-link" href="#index-printf"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">printf [-v <var class="var">var</var>] <var class="var">format</var> [<var class="var">arguments</var>]
</pre></div>

<p>Write the formatted <var class="var">arguments</var> to the standard output under the
control of the <var class="var">format</var>.
The <samp class="option">-v</samp> option assigns the output to the variable
<var class="var">var</var> rather than printing it to the standard output.
</p>
<p>The <var class="var">format</var> is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
<var class="var">argument</var>.
In addition to the standard <code class="code">printf(3)</code> format characters
<code class="code">cCsSndiouxXeEfFgGaA</code>,
<code class="code">printf</code> interprets the following additional format specifiers:
</p>
<dl class="table">
<dt><code class="code">%b</code></dt>
<dd><p>Causes <code class="code">printf</code> to expand backslash escape sequences in the
corresponding <var class="var">argument</var> in the same way as <code class="code">echo -e</code>
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p></dd>
<dt><code class="code">%q</code></dt>
<dd><p>Causes <code class="code">printf</code> to output the
corresponding <var class="var">argument</var> in a format that can be reused as shell input.
<code class="code">%q</code> and <code class="code">%Q</code>P use the ANSI-C quoting style (see <a class="pxref" href="#ANSI_002dC-Quoting">ANSI-C Quoting</a>)
if any characters
in the argument string require it, and backslash quoting otherwise.
If the format string uses the <code class="code">printf</code> <em class="emph">alternate form</em>, these two
formats quote the argument string using single quotes.
</p>
</dd>
<dt><code class="code">%Q</code></dt>
<dd><p>like <code class="code">%q</code>, but applies any supplied precision to the <var class="var">argument</var>
before quoting it.
</p>
</dd>
<dt><code class="code">%(<var class="var">datefmt</var>)T</code></dt>
<dd><p>Causes <code class="code">printf</code> to output the date-time string resulting from using
<var class="var">datefmt</var> as a format string for <code class="code">strftime</code>(3).
The corresponding <var class="var">argument</var> is an integer representing the number of
seconds since the epoch.
This format specifier recognizes Two special argument values:
-1 represents the current time,
and -2 represents the time the shell was invoked.
If no argument is specified, conversion behaves as if -1 had been supplied.
This is an exception to the usual <code class="code">printf</code> behavior.
</p></dd>
</dl>

<p>The %b, %q, and %T format specifiers all use the field width and precision
arguments from the format specification and write that many bytes from    
(or use that wide a field for) the expanded argument, which usually
contains more characters than the original.
</p>
<p>The %n format specifier accepts a corresponding argument that is treated
as a shell variable name.
</p>
<p>The %s and %c format specifiers accept an l (long) modifier, which forces
them to convert the argument string to a wide-character string and apply
any supplied field width and precision in terms of characters, not bytes.
The %S and %C format specifiers are equivalent to %ls and %lc, respectively.
</p>

<p>Arguments to non-string format specifiers are treated as C language constants,
except that a leading plus or minus sign is allowed, and if the leading
character is a single or double quote, the value is the numeric value of
the following character, using the current locale.
</p>
<p>The <var class="var">format</var> is reused as necessary to consume all of the <var class="var">arguments</var>.
If the <var class="var">format</var> requires more <var class="var">arguments</var> than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied.
The return value is zero on success,
non-zero if an invalid option is supplied or a write or assignment error
occurs.
</p>
</dd>
<dt><a id="index-read"></a><span><code class="code">read</code><a class="copiable-link" href="#index-read"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">read [-Eers] [-a <var class="var">aname</var>] [-d <var class="var">delim</var>] [-i <var class="var">text</var>] [-n <var class="var">nchars</var>]
    [-N <var class="var">nchars</var>] [-p <var class="var">prompt</var>] [-t <var class="var">timeout</var>] [-u <var class="var">fd</var>] [<var class="var">name</var> ...]
</pre></div>

<p>Read one line from the standard input, or from the file descriptor
<var class="var">fd</var> supplied as an argument to the <samp class="option">-u</samp> option,
split it into words as described above in <a class="ref" href="#Word-Splitting">Word Splitting</a>,
and assign the first word to the first <var class="var">name</var>,
the second word to the second <var class="var">name</var>, and so on.
If there are more words than names,
the remaining words and their intervening delimiters are assigned
to the last <var class="var">name</var>.
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in the value of the <code class="env">IFS</code> variable
are used to split the line into words using the same rules the shell
uses for expansion (described above in <a class="ref" href="#Word-Splitting">Word Splitting</a>).
The backslash character &lsquo;<samp class="samp">\</samp>&rsquo; removes any special
meaning for the next character read and is used for line continuation.
</p>
<p>Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-a <var class="var">aname</var></code></dt>
<dd><p>The words are assigned to sequential indices of the array variable
<var class="var">aname</var>,
starting at 0.
All elements are removed from <var class="var">aname</var> before the assignment.
Other <var class="var">name</var> arguments are ignored.
</p>
</dd>
<dt><code class="code">-d <var class="var">delim</var></code></dt>
<dd><p>The first character of <var class="var">delim</var> terminates the input line,
rather than newline.
If <var class="var">delim</var> is the empty string, <code class="code">read</code> will terminate a line
when it reads a NUL character.
</p>
</dd>
<dt><code class="code">-e</code></dt>
<dd><p>If the standard input is coming from a terminal,
<code class="code">read</code> uses
Readline (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>)
to obtain the line.
Readline uses the current (or default, if line editing was not previously
active) editing settings, but uses Readline&rsquo;s default filename completion.
</p>
</dd>
<dt><code class="code">-E</code></dt>
<dd><p>If the standard input is coming from a terminal,
<code class="code">read</code> uses
Readline (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>) to obtain the line.
Readline uses the current (or default, if line editing was not previously
active) editing settings, but uses Bash&rsquo;s default completion, including
programmable completion.
</p>
</dd>
<dt><code class="code">-i <var class="var">text</var></code></dt>
<dd><p>If Readline is being used to read the line, <code class="code">read</code> places <var class="var">text</var> into
the editing buffer before editing begins.
</p>
</dd>
<dt><code class="code">-n <var class="var">nchars</var></code></dt>
<dd><p><code class="code">read</code> returns after reading <var class="var">nchars</var> characters rather than
waiting for a complete line of input,
unless it encounters EOF or <code class="code">read</code> times out,
but honors a delimiter if it reads fewer
than <var class="var">nchars</var> characters before the delimiter.
</p>
</dd>
<dt><code class="code">-N <var class="var">nchars</var></code></dt>
<dd><p><code class="code">read</code> returns after reading exactly <var class="var">nchars</var> characters rather
than waiting for a complete line of input,
unless it encounters EOF or <code class="code">read</code> times out.
Delimiter characters in the input are
not treated specially and do not cause <code class="code">read</code> to return until
it has read <var class="var">nchars</var> characters.
The result is not split on the characters in <code class="code">IFS</code>; the intent is
that the variable is assigned exactly the characters read
(with the exception of backslash; see the <samp class="option">-r</samp> option below).
</p>
</dd>
<dt><code class="code">-p <var class="var">prompt</var></code></dt>
<dd><p>Display <var class="var">prompt</var>, without a trailing newline, before attempting
to read any input, but only if input is coming from a terminal.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>If this option is given, backslash does not act as an escape character.
The backslash is considered to be part of the line.
In particular, a backslash-newline pair may not then be used as a line
continuation.
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>Silent mode.
If input is coming from a terminal, characters are not echoed.
</p>
</dd>
<dt><code class="code">-t <var class="var">timeout</var></code></dt>
<dd><p>Cause <code class="code">read</code> to time out and return failure if it does not read
a complete line of input (or a specified number of characters)
within <var class="var">timeout</var> seconds.
<var class="var">timeout</var>  may be a decimal number with a fractional portion following
the decimal point.
This option is only effective if <code class="code">read</code> is reading input from a
terminal, pipe, or other special file; it has no effect when reading
from regular files.
If <code class="code">read</code> times out, it saves any partial input read into
the specified variable <var class="var">name</var>, and returns a status greater than 128.
If <var class="var">timeout</var> is 0, <code class="code">read</code> returns immediately, without trying to
read any data.
In this case, the exit status is 0 if input is available on the specified
file descriptor, or the read will return EOF, non-zero otherwise.
</p>
</dd>
<dt><code class="code">-u <var class="var">fd</var></code></dt>
<dd><p>Read input from file descriptor <var class="var">fd</var> instead of the standard input.
</p></dd>
</dl>

<p>Other than the case where <var class="var">delim</var> is the empty string, <code class="code">read</code>
ignores any NUL characters in the input.
</p>
<p>If no <var class="var">name</var>s are supplied, <code class="code">read</code> assigns the line read,
without the ending delimiter but otherwise unmodified,
to the variable <code class="env">REPLY</code>.
</p>
<p>The exit status is zero, unless end-of-file is encountered, <code class="code">read</code>
times out (in which case the status is greater than 128),
a variable assignment error (such as assigning to a readonly variable) occurs,
or an invalid file descriptor is supplied as the argument to <samp class="option">-u</samp>.
</p>
</dd>
<dt><a id="index-readarray"></a><span><code class="code">readarray</code><a class="copiable-link" href="#index-readarray"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">readarray [-d <var class="var">delim</var>] [-n <var class="var">count</var>] [-O <var class="var">origin</var>] [-s <var class="var">count</var>]
    [-t] [-u <var class="var">fd</var>] [-C <var class="var">callback</var>] [-c <var class="var">quantum</var>] [<var class="var">array</var>]
</pre></div>

<p>Read lines from the standard input into the indexed array variable <var class="var">array</var>,
or from file descriptor <var class="var">fd</var>
if the <samp class="option">-u</samp> option is supplied.
</p>
<p>A synonym for <code class="code">mapfile</code>.
</p>
</dd>
<dt><a id="index-source"></a><span><code class="code">source</code><a class="copiable-link" href="#index-source"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">source [-p <var class="var">path</var>] <var class="var">filename</var> [<var class="var">arguments</var>]
</pre></div>

<p>A synonym for <code class="code">.</code> (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
</p>
</dd>
<dt><a id="index-type"></a><span><code class="code">type</code><a class="copiable-link" href="#index-type"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">type [-afptP] [<var class="var">name</var> ...]
</pre></div>

<p>Indicate how each <var class="var">name</var> would be interpreted if used as a command
name.
</p>
<p>If the <samp class="option">-t</samp> option is used, <code class="code">type</code> prints a single word
which is one of &lsquo;<samp class="samp">alias</samp>&rsquo;, &lsquo;<samp class="samp">keyword</samp>&rsquo;, &lsquo;<samp class="samp">function</samp>&rsquo;,
&lsquo;<samp class="samp">builtin</samp>&rsquo;, or &lsquo;<samp class="samp">file</samp>&rsquo;,
if <var class="var">name</var> is an alias, shell reserved word, shell function,
shell builtin, or executable file, respectively.
If the <var class="var">name</var> is not found, <code class="code">type</code> prints nothing and
returns a failure status.
</p>
<p>If the <samp class="option">-p</samp> option is used, <code class="code">type</code> either returns the name
of the executable file that would be found by searching <code class="code">$PATH</code>
for <code class="code">name</code>,
or nothing if <samp class="option">-t</samp> would not return &lsquo;<samp class="samp">file</samp>&rsquo;.
</p>
<p>The <samp class="option">-P</samp> option forces a path search for each <var class="var">name</var>, even if
<samp class="option">-t</samp> would not return &lsquo;<samp class="samp">file</samp>&rsquo;.
</p>
<p>If a <var class="var">name</var> is present in the table of hashed commands,
options <samp class="option">-p</samp> and <samp class="option">-P</samp> print the hashed value, which is not
necessarily the file that appears first in <code class="code">$PATH</code>.
</p>
<p>If the <samp class="option">-a</samp> option is used, <code class="code">type</code> returns all of the places
that contain a command named <var class="var">name</var>.
This includes aliases, reserved words, functions, and builtins,
but the path search options (<samp class="option">-p</samp> and <samp class="option">-P</samp>) can be supplied
to restrict the output to executable files.
If <samp class="option">-a</samp> is supplied with <samp class="option">-p</samp>, <code class="code">type</code> does not look
in the table of hashed commands, and only performs a <code class="code">PATH</code>
search for <var class="var">name</var>.
</p>
<p>If the <samp class="option">-f</samp> option is used, <code class="code">type</code> does not attempt to find
shell functions, as with the <code class="code">command</code> builtin.
</p>
<p>The return status is zero if all of the <var class="var">name</var>s are found, non-zero
if any are not found.
</p>
</dd>
<dt><a id="index-typeset"></a><span><code class="code">typeset</code><a class="copiable-link" href="#index-typeset"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">typeset [-afFgrxilnrtux] [-p] [<var class="var">name</var>[=<var class="var">value</var>] ...]
</pre></div>

<p>The <code class="code">typeset</code> command is supplied for compatibility with the Korn
shell.
It is a synonym for the <code class="code">declare</code> builtin command.
</p>
</dd>
<dt><a id="index-ulimit"></a><span><code class="code">ulimit</code><a class="copiable-link" href="#index-ulimit"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [<var class="var">limit</var>]
</pre></div>

<p><code class="code">ulimit</code> provides control over the resources available to the
shell and to processes it starts, on systems that allow such control.
If an option is given, it is interpreted as follows:
</p>
<dl class="table">
<dt><code class="code">-S</code></dt>
<dd><p>Change and report the soft limit associated with a resource.
</p>
</dd>
<dt><code class="code">-H</code></dt>
<dd><p>Change and report the hard limit associated with a resource.
</p>
</dd>
<dt><code class="code">-a</code></dt>
<dd><p>Report all current limits; no limits are set.
</p>
</dd>
<dt><code class="code">-b</code></dt>
<dd><p>The maximum socket buffer size.
</p>
</dd>
<dt><code class="code">-c</code></dt>
<dd><p>The maximum size of core files created.
</p>
</dd>
<dt><code class="code">-d</code></dt>
<dd><p>The maximum size of a process&rsquo;s data segment.
</p>
</dd>
<dt><code class="code">-e</code></dt>
<dd><p>The maximum scheduling priority (&quot;nice&quot;).
</p>
</dd>
<dt><code class="code">-f</code></dt>
<dd><p>The maximum size of files written by the shell and its children.
</p>
</dd>
<dt><code class="code">-i</code></dt>
<dd><p>The maximum number of pending signals.
</p>
</dd>
<dt><code class="code">-k</code></dt>
<dd><p>The maximum number of kqueues that may be allocated.
</p>
</dd>
<dt><code class="code">-l</code></dt>
<dd><p>The maximum size that may be locked into memory.
</p>
</dd>
<dt><code class="code">-m</code></dt>
<dd><p>The maximum resident set size (many systems do not honor this limit).
</p>
</dd>
<dt><code class="code">-n</code></dt>
<dd><p>The maximum number of open file descriptors (most systems do not
allow this value to be set).
</p>
</dd>
<dt><code class="code">-p</code></dt>
<dd><p>The pipe buffer size.
</p>
</dd>
<dt><code class="code">-q</code></dt>
<dd><p>The maximum number of bytes in
<small class="sc">POSIX</small>
message queues.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>The maximum real-time scheduling priority.
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>The maximum stack size.
</p>
</dd>
<dt><code class="code">-t</code></dt>
<dd><p>The maximum amount of cpu time in seconds.
</p>
</dd>
<dt><code class="code">-u</code></dt>
<dd><p>The maximum number of processes available to a single user.
</p>
</dd>
<dt><code class="code">-v</code></dt>
<dd><p>The maximum amount of virtual memory available to the shell, and, on
some systems, to its children.
</p>
</dd>
<dt><code class="code">-x</code></dt>
<dd><p>The maximum number of file locks.
</p>
</dd>
<dt><code class="code">-P</code></dt>
<dd><p>The maximum number of pseudoterminals.
</p>
</dd>
<dt><code class="code">-R</code></dt>
<dd><p>The maximum time a real-time process can run before blocking, in microseconds.
</p>
</dd>
<dt><code class="code">-T</code></dt>
<dd><p>The maximum number of threads.
</p></dd>
</dl>

<p>If <var class="var">limit</var> is supplied, and the <samp class="option">-a</samp> option is not used,
<var class="var">limit</var> is the new value of the specified resource.
The special <var class="var">limit</var> values <code class="code">hard</code>, <code class="code">soft</code>, and
<code class="code">unlimited</code> stand for the current hard limit, the current soft limit,
and no limit, respectively.
A hard limit cannot be increased by a non-root user once it is set;
a soft limit may be increased up to the value of the hard limit.
Otherwise, <code class="code">ulimit</code> prints the current value of the soft limit
for the specified resource, unless the <samp class="option">-H</samp> option is supplied.
When more than one
resource is specified, the limit name and unit, if appropriate,
are printed before the value.
When setting new limits, if neither <samp class="option">-H</samp> nor <samp class="option">-S</samp> is supplied,
<code class="code">ulimit</code> sets both the hard and soft limits.
If no option is supplied, then <samp class="option">-f</samp> is assumed.
</p>
<p>Values are in 1024-byte increments, except for
<samp class="option">-t</samp>, which is in seconds;
<samp class="option">-R</samp>, which is in microseconds;
<samp class="option">-p</samp>, which is in units of 512-byte blocks;
<samp class="option">-P</samp>,
<samp class="option">-T</samp>,
<samp class="option">-b</samp>,
<samp class="option">-k</samp>,
<samp class="option">-n</samp> and <samp class="option">-u</samp>, which are unscaled values;
and, when in <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>),
<samp class="option">-c</samp> and <samp class="option">-f</samp>, which are in 512-byte increments.
</p>
<p>The return status is zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
</p>
</dd>
<dt><a id="index-unalias"></a><span><code class="code">unalias</code><a class="copiable-link" href="#index-unalias"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">unalias [-a] [<var class="var">name</var> ... ]
</pre></div>

<p>Remove each <var class="var">name</var> from the list of aliases.
If <samp class="option">-a</samp> is supplied, remove all aliases.
The return value is true unless a supplied <var class="var">name</var> is not a defined alias.
Aliases are described in <a class="ref" href="#Aliases">Aliases</a>.
</p></dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="Modifying-Shell-Behavior">
<div class="nav-panel">
<p>
Next: <a href="#Special-Builtins" accesskey="n" rel="next">Special Builtins</a>, Previous: <a href="#Bash-Builtins" accesskey="p" rel="prev">Bash Builtin Commands</a>, Up: <a href="#Shell-Builtin-Commands" accesskey="u" rel="up">Shell Builtin Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Modifying-Shell-Behavior-1"><span>4.3 Modifying Shell Behavior<a class="copiable-link" href="#Modifying-Shell-Behavior-1"> &para;</a></span></h3>
 

<ul class="mini-toc">
<li><a href="#The-Set-Builtin" accesskey="1">The Set Builtin</a></li>
<li><a href="#The-Shopt-Builtin" accesskey="2">The Shopt Builtin</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="The-Set-Builtin">
<div class="nav-panel">
<p>
Next: <a href="#The-Shopt-Builtin" accesskey="n" rel="next">The Shopt Builtin</a>, Up: <a href="#Modifying-Shell-Behavior" accesskey="u" rel="up">Modifying Shell Behavior</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="The-Set-Builtin-1"><span>4.3.1 The Set Builtin<a class="copiable-link" href="#The-Set-Builtin-1"> &para;</a></span></h4>

<p>This builtin is so complicated that it deserves its own section.  <code class="code">set</code>
allows you to change the values of shell options and set the positional
parameters, or to display the names and values of shell variables.
</p>
<dl class="table">
<dt><a id="index-set"></a><span><code class="code">set</code><a class="copiable-link" href="#index-set"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">set [-abefhkmnptuvxBCEHPT] [-o <var class="var">option-name</var>] [--] [-] [<var class="var">argument</var> ...]
set [+abefhkmnptuvxBCEHPT] [+o <var class="var">option-name</var>] [--] [-] [<var class="var">argument</var> ...]
set -o
set +o
</pre></div>

<p>If no options or arguments are supplied, <code class="code">set</code> displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input
for setting or resetting the currently-set variables.
Read-only variables cannot be reset.
In <small class="sc">POSIX</small> mode, only shell variables are listed.
</p>
<p>When options are supplied, they set or unset shell attributes.
Any arguments remaining after option processing replace the
positional parameters.
</p>
<p>Options, if specified, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-a</code></dt>
<dd><p>Each variable or function that is created or modified is given the
export attribute and marked for export to the environment of
subsequent commands.
</p>
</dd>
<dt><code class="code">-b</code></dt>
<dd><p>Cause the status of terminated background jobs to be reported
immediately, rather than before printing the next primary prompt
or, under some circumstances,  when a foreground command exits.
This is effective only when job control is enabled.
</p>
</dd>
<dt><code class="code">-e</code></dt>
<dd><p>Exit immediately if
a pipeline (see <a class="pxref" href="#Pipelines">Pipelines</a>), which may consist of a single simple command
(see <a class="pxref" href="#Simple-Commands">Simple Commands</a>),
a list (see <a class="pxref" href="#Lists">Lists of Commands</a>),
or a compound command (see <a class="pxref" href="#Compound-Commands">Compound Commands</a>)
returns a non-zero status.
The shell does not exit if the command that fails is part of the
command list immediately following a
<code class="code">while</code> or <code class="code">until</code> reserved word,
part of the test in an <code class="code">if</code> statement,
part of any command executed in a <code class="code">&amp;&amp;</code> or <code class="code">||</code> list except
the command following the final <code class="code">&amp;&amp;</code> or <code class="code">||</code>,
any command in a pipeline but the last
(subject to the state of the <code class="code">pipefail</code> shell option),
or if the command&rsquo;s return status is being inverted with <code class="code">!</code>.
If a compound command other than a subshell
returns a non-zero status because a command failed
while <samp class="option">-e</samp> was being ignored, the shell does not exit.
A trap on <code class="code">ERR</code>, if set, is executed before the shell exits.
</p>
<p>This option applies to the shell environment and each subshell environment
separately (see <a class="pxref" href="#Command-Execution-Environment">Command Execution Environment</a>), and may cause
subshells to exit before executing all the commands in the subshell.
</p>
<p>If a compound command or shell function executes in a context where
<samp class="option">-e</samp> is being ignored,
none of the commands executed within the compound command or function body
will be affected by the <samp class="option">-e</samp> setting, even if <samp class="option">-e</samp> is set
and a command returns a failure status.
If a compound command or shell function sets <samp class="option">-e</samp> while executing in
a context where <samp class="option">-e</samp> is ignored, that setting will not have any
effect until the compound command or the command containing the function
call completes.
</p>
</dd>
<dt><code class="code">-f</code></dt>
<dd><p>Disable filename expansion (globbing).
</p>
</dd>
<dt><code class="code">-h</code></dt>
<dd><p>Locate and remember (hash) commands as they are looked up for execution.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">-k</code></dt>
<dd><p>All arguments in the form of assignment statements are placed
in the environment for a command, not just those that precede
the command name.
</p>
</dd>
<dt><code class="code">-m</code></dt>
<dd><p>Job control is enabled (see <a class="pxref" href="#Job-Control">Job Control</a>).
All processes run in a separate process group.
When a background job completes, the shell prints a line
containing its exit status.
</p>
</dd>
<dt><code class="code">-n</code></dt>
<dd><p>Read commands but do not execute them.
This may be used to check a script for syntax errors.
This option is ignored by interactive shells.
</p>
</dd>
<dt><code class="code">-o <var class="var">option-name</var></code></dt>
<dd>
<p>Set the option corresponding to <var class="var">option-name</var>.
If <samp class="option">-o</samp> is supplied with no <var class="var">option-name</var>,
<code class="code">set</code> prints the current shell options settings.
If <samp class="option">+o</samp> is supplied with no <var class="var">option-name</var>,
<code class="code">set</code> prints a series of
<code class="code">set</code>
commands to recreate the current option settings
on the standard output.
Valid option names are:
</p>
<dl class="table">
<dt><code class="code">allexport</code></dt>
<dd><p>Same as <code class="code">-a</code>.
</p>
</dd>
<dt><code class="code">braceexpand</code></dt>
<dd><p>Same as <code class="code">-B</code>.
</p>
</dd>
<dt><code class="code">emacs</code></dt>
<dd><p>Use an <code class="code">emacs</code>-style line editing interface (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>).
This also affects the editing interface used for <code class="code">read -e</code>.
</p>
</dd>
<dt><code class="code">errexit</code></dt>
<dd><p>Same as <code class="code">-e</code>.
</p>
</dd>
<dt><code class="code">errtrace</code></dt>
<dd><p>Same as <code class="code">-E</code>.
</p>
</dd>
<dt><code class="code">functrace</code></dt>
<dd><p>Same as <code class="code">-T</code>.
</p>
</dd>
<dt><code class="code">hashall</code></dt>
<dd><p>Same as <code class="code">-h</code>.
</p>
</dd>
<dt><code class="code">histexpand</code></dt>
<dd><p>Same as <code class="code">-H</code>.
</p>
</dd>
<dt><code class="code">history</code></dt>
<dd><p>Enable command history, as described in <a class="ref" href="#Bash-History-Facilities">Bash History Facilities</a>.
This option is on by default in interactive shells.
</p>
</dd>
<dt><code class="code">ignoreeof</code></dt>
<dd><p>An interactive shell will not exit upon reading EOF.
</p>
</dd>
<dt><code class="code">keyword</code></dt>
<dd><p>Same as <code class="code">-k</code>.
</p>
</dd>
<dt><code class="code">monitor</code></dt>
<dd><p>Same as <code class="code">-m</code>.
</p>
</dd>
<dt><code class="code">noclobber</code></dt>
<dd><p>Same as <code class="code">-C</code>.
</p>
</dd>
<dt><code class="code">noexec</code></dt>
<dd><p>Same as <code class="code">-n</code>.
</p>
</dd>
<dt><code class="code">noglob</code></dt>
<dd><p>Same as <code class="code">-f</code>.
</p>
</dd>
<dt><code class="code">nolog</code></dt>
<dd><p>Currently ignored.
</p>
</dd>
<dt><code class="code">notify</code></dt>
<dd><p>Same as <code class="code">-b</code>.
</p>
</dd>
<dt><code class="code">nounset</code></dt>
<dd><p>Same as <code class="code">-u</code>.
</p>
</dd>
<dt><code class="code">onecmd</code></dt>
<dd><p>Same as <code class="code">-t</code>.
</p>
</dd>
<dt><code class="code">physical</code></dt>
<dd><p>Same as <code class="code">-P</code>.
</p>
</dd>
<dt><code class="code">pipefail</code></dt>
<dd><p>If set, the return value of a pipeline is the value of the last
(rightmost) command to exit with a non-zero status, or zero if all
commands in the pipeline exit successfully.
This option is disabled by default.
</p>
</dd>
<dt><code class="code">posix</code></dt>
<dd><p>Enable <small class="sc">POSIX</small> mode;
change the behavior of Bash where the default operation differs
from the <small class="sc">POSIX</small> standard to match the standard
(see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>).
This is intended to make Bash behave as a strict superset of that
standard.
</p>
</dd>
<dt><code class="code">privileged</code></dt>
<dd><p>Same as <code class="code">-p</code>.
</p>
</dd>
<dt><code class="code">verbose</code></dt>
<dd><p>Same as <code class="code">-v</code>.
</p>
</dd>
<dt><code class="code">vi</code></dt>
<dd><p>Use a <code class="code">vi</code>-style line editing interface.
This also affects the editing interface used for <code class="code">read -e</code>.
</p>
</dd>
<dt><code class="code">xtrace</code></dt>
<dd><p>Same as <code class="code">-x</code>.
</p></dd>
</dl>

</dd>
<dt><code class="code">-p</code></dt>
<dd><p>Turn on privileged mode.
In this mode, the <code class="env">$BASH_ENV</code> and <code class="env">$ENV</code> files are not
processed, shell functions are not inherited from the environment,
and the <code class="env">SHELLOPTS</code>, <code class="env">BASHOPTS</code>, <code class="env">CDPATH</code> and <code class="env">GLOBIGNORE</code>
variables, if they appear in the environment, are ignored.
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the <samp class="option">-p</samp> option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the <samp class="option">-p</samp> option is supplied at startup, the effective user id is
not reset.
Turning this option off causes the effective user
and group ids to be set to the real user and group ids.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>Enable restricted shell mode (see <a class="pxref" href="#The-Restricted-Shell">The Restricted Shell</a>).
This option cannot be unset once it has been set.
</p>
</dd>
<dt><code class="code">-t</code></dt>
<dd><p>Exit after reading and executing one command.
</p>
</dd>
<dt><code class="code">-u</code></dt>
<dd><p>Treat unset variables and parameters other than the special parameters
&lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
or array variables subscripted with &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
as an error when performing parameter expansion.
An error message will be written to the standard error, and a non-interactive
shell will exit.
</p>
</dd>
<dt><code class="code">-v</code></dt>
<dd><p>Print shell input lines to standard error as they are read.
</p>
</dd>
<dt><code class="code">-x</code></dt>
<dd><p>Print a trace of simple commands, <code class="code">for</code> commands, <code class="code">case</code>
commands, <code class="code">select</code> commands, and arithmetic <code class="code">for</code> commands
and their arguments or associated word lists to the standard error
after they are expanded and before they are executed.
The shell prints the expanded value of the <code class="env">PS4</code> variable before
the command and its expanded arguments.
</p>
</dd>
<dt><code class="code">-B</code></dt>
<dd><p>The shell will perform brace expansion (see <a class="pxref" href="#Brace-Expansion">Brace Expansion</a>).
This option is on by default.
</p>
</dd>
<dt><code class="code">-C</code></dt>
<dd><p>Prevent output redirection using &lsquo;<samp class="samp">&gt;</samp>&rsquo;, &lsquo;<samp class="samp">&gt;&amp;</samp>&rsquo;, and &lsquo;<samp class="samp">&lt;&gt;</samp>&rsquo;
from overwriting existing files.
Using the redirection operator &lsquo;<samp class="samp">&gt;|</samp>&rsquo; instead of &lsquo;<samp class="samp">&gt;</samp>&rsquo;
will override this and force the creation of an output file.
</p>
</dd>
<dt><code class="code">-E</code></dt>
<dd><p>If set, any trap on <code class="code">ERR</code> is inherited by shell functions, command
substitutions, and commands executed in a subshell environment.
The <code class="code">ERR</code> trap is normally not inherited in such cases.
</p>
</dd>
<dt><code class="code">-H</code></dt>
<dd><p>Enable &lsquo;<samp class="samp">!</samp>&rsquo; style history substitution (see <a class="pxref" href="#History-Interaction">History Expansion</a>).
This option is on by default for interactive shells.
</p>
</dd>
<dt><code class="code">-P</code></dt>
<dd><p>If set, Bash does not resolve symbolic links when executing commands
such as <code class="code">cd</code> which change the current directory.
It uses the physical directory structure instead.
By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.
</p>
<p>For example, if <samp class="file">/usr/sys</samp> is a symbolic link to <samp class="file">/usr/local/sys</samp>
then:
</p><div class="example">
<pre class="example-preformatted">$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
</pre></div>

<p>If <code class="code">set -P</code> is on, then:
</p><div class="example">
<pre class="example-preformatted">$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
</pre></div>

</dd>
<dt><code class="code">-T</code></dt>
<dd><p>If set, any traps on <code class="code">DEBUG</code> and <code class="code">RETURN</code> are inherited by
shell functions, command substitutions, and commands executed
in a subshell environment.
The <code class="code">DEBUG</code> and <code class="code">RETURN</code> traps are normally not inherited
in such cases.
</p>
</dd>
<dt><code class="code">--</code></dt>
<dd><p>If no arguments follow this option, unset the positional parameters.
Otherwise, the positional parameters are set to the
<var class="var">arguments</var>, even if some of them begin with a &lsquo;<samp class="samp">-</samp>&rsquo;.
</p>
</dd>
<dt><code class="code">-</code></dt>
<dd><p>Signal the end of options, and assign all remaining <var class="var">arguments</var>
to the positional parameters.
The <samp class="option">-x</samp> and <samp class="option">-v</samp> options are turned off.
If there are no arguments, the positional parameters remain unchanged.
</p></dd>
</dl>

<p>Using &lsquo;<samp class="samp">+</samp>&rsquo; rather than &lsquo;<samp class="samp">-</samp>&rsquo; causes these options to be
turned off.
The options can also be used upon invocation of the shell.
The current set of options may be found in <code class="code">$-</code>.
</p>
<p>The remaining N <var class="var">arguments</var> are positional parameters and are
assigned, in order, to <code class="code">$1</code>, <code class="code">$2</code>, &hellip;  <code class="code">$N</code>.
The special parameter <code class="code">#</code> is set to N.
</p>
<p>The return status is always zero unless an invalid option is supplied.
</p></dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="The-Shopt-Builtin">
<div class="nav-panel">
<p>
Previous: <a href="#The-Set-Builtin" accesskey="p" rel="prev">The Set Builtin</a>, Up: <a href="#Modifying-Shell-Behavior" accesskey="u" rel="up">Modifying Shell Behavior</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="The-Shopt-Builtin-1"><span>4.3.2 The Shopt Builtin<a class="copiable-link" href="#The-Shopt-Builtin-1"> &para;</a></span></h4>

<p>This builtin allows you to change additional optional shell behavior.
</p>
<dl class="table">
<dt><a id="index-shopt"></a><span><code class="code">shopt</code><a class="copiable-link" href="#index-shopt"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">shopt [-pqsu] [-o] [<var class="var">optname</var> ...]
</pre></div>

<p>Toggle the values of settings controlling optional shell behavior.
The settings can be either those listed below, or, if the
<samp class="option">-o</samp> option is used, those available with the <samp class="option">-o</samp>
option to the <code class="code">set</code> builtin command (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
</p>
<p>With no options, or with the <samp class="option">-p</samp> option, display a list of all
settable options, with an indication of whether or not each is set;
if any <var class="var">optname</var>s are supplied, the output is restricted to those options.
The <samp class="option">-p</samp> option displays output in a form that
may be reused as input.
</p>
<p>Other options have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-s</code></dt>
<dd><p>Enable (set) each <var class="var">optname</var>.
</p>
</dd>
<dt><code class="code">-u</code></dt>
<dd><p>Disable (unset) each <var class="var">optname</var>.
</p>
</dd>
<dt><code class="code">-q</code></dt>
<dd><p>Suppresses normal output; the return status
indicates whether the <var class="var">optname</var> is set or unset.
If multiple <var class="var">optname</var> arguments are supplied with <samp class="option">-q</samp>,
the return status is zero if all <var class="var">optname</var>s are enabled;
non-zero otherwise.
</p>
</dd>
<dt><code class="code">-o</code></dt>
<dd><p>Restricts the values of
<var class="var">optname</var> to be those defined for the <samp class="option">-o</samp> option to the
<code class="code">set</code> builtin (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
</p></dd>
</dl>

<p>If either <samp class="option">-s</samp> or <samp class="option">-u</samp>
is used with no <var class="var">optname</var> arguments, <code class="code">shopt</code> shows only
those options which are set or unset, respectively.
</p>
<p>Unless otherwise noted, the <code class="code">shopt</code> options are disabled (off)
by default.
</p>
<p>The return status when listing options is zero if all <var class="var">optname</var>s
are enabled, non-zero otherwise.  When setting or unsetting options,
the return status is zero unless an <var class="var">optname</var> is not a valid shell
option.
</p>
<p>The list of <code class="code">shopt</code> options is:
</p><dl class="table">
<dt><code class="code">array_expand_once</code></dt>
<dd><p>If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts
during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.
</p>
</dd>
<dt><code class="code">assoc_expand_once</code></dt>
<dd><p>Deprecated; a synonym for <code class="code">array_expand_once</code>. 
</p>
</dd>
<dt><code class="code">autocd</code></dt>
<dd><p>If set, a command name that is the name of a directory is executed as if
it were the argument to the <code class="code">cd</code> command.
This option is only used by interactive shells.
</p>
</dd>
<dt><code class="code">bash_source_fullpath</code></dt>
<dd><p>If set, filenames added to the <code class="code">BASH_SOURCE</code> array variable are
converted to full pathnames (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
</p>
</dd>
<dt><code class="code">cdable_vars</code></dt>
<dd><p>If this is set, an argument to the <code class="code">cd</code> builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.
</p>
</dd>
<dt><code class="code">cdspell</code></dt>
<dd><p>If set, the <code class="code">cd</code> command
attempts to correct
minor errors in the spelling of a directory component.
Minor errors include transposed characters,
a missing character, and one extra character.
If <code class="code">cd</code> corrects the directory name, it prints the corrected filename,
and the command proceeds.
This option is only used by interactive shells.
</p>
</dd>
<dt><code class="code">checkhash</code></dt>
<dd><p>If this is set, Bash checks that a command found in the hash
table exists before trying to execute it.
If a hashed command no longer exists, Bash performs a normal path search.
</p>
</dd>
<dt><code class="code">checkjobs</code></dt>
<dd><p>If set, Bash lists the status of any stopped and running jobs before
exiting an interactive shell.
If any jobs are running, Bash defers
the exit until a second exit is attempted without an
intervening command (see <a class="pxref" href="#Job-Control">Job Control</a>).
The shell always postpones exiting if any jobs are stopped.
</p>
</dd>
<dt><code class="code">checkwinsize</code></dt>
<dd><p>If set, Bash checks the window size after each external (non-builtin)
command and, if necessary, updates the values of    
<code class="env">LINES</code> and <code class="env">COLUMNS</code>,
using the file descriptor associated with stderr if it is a terminal.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">cmdhist</code></dt>
<dd><p>If set, Bash
attempts to save all lines of a multiple-line
command in the same history entry.
This allows easy re-editing of multi-line commands.
This option is enabled by default, but only has an effect if command
history is enabled (see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>).
</p>
</dd>
<dt><code class="code">compat31</code></dt>
<dt><code class="code">compat32</code></dt>
<dt><code class="code">compat40</code></dt>
<dt><code class="code">compat41</code></dt>
<dt><code class="code">compat42</code></dt>
<dt><code class="code">compat43</code></dt>
<dt><code class="code">compat44</code></dt>
<dd><p>These control aspects of the shell&rsquo;s compatibility mode
(see <a class="pxref" href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a>).
</p>
</dd>
<dt><code class="code">complete_fullquote</code></dt>
<dd><p>If set, Bash
quotes all shell metacharacters in filenames and directory names when
performing completion.
If not set, Bash
removes metacharacters such as the dollar sign from the set of
characters that will be quoted in completed filenames
when these metacharacters appear in shell variable references in words to be
completed.
This means that dollar signs in variable names that expand to directories
will not be quoted;
however, any dollar signs appearing in filenames will not be quoted, either.
This is active only when Bash is using backslashes to quote completed
filenames.
This variable is set by default, which is the default Bash behavior in
versions through 4.2.
</p>
</dd>
<dt><code class="code">direxpand</code></dt>
<dd><p>If set, Bash
replaces directory names with the results of word expansion when performing
filename completion.
This changes the contents of the Readline editing buffer.
If not set, Bash attempts to preserve what the user typed.
</p>
</dd>
<dt><code class="code">dirspell</code></dt>
<dd><p>If set, Bash
attempts spelling correction on directory names during word completion 
if the directory name initially supplied does not exist.
</p>
</dd>
<dt><code class="code">dotglob</code></dt>
<dd><p>If set, Bash includes filenames beginning with a &lsquo;<samp class="samp">.</samp>&rsquo; in
the results of filename expansion.
The filenames <samp class="file">.</samp> and <samp class="file">..</samp> must always be matched explicitly,
even if <code class="code">dotglob</code> is set.
</p>
</dd>
<dt><code class="code">execfail</code></dt>
<dd><p>If this is set, a non-interactive shell will not exit if
it cannot execute the file specified as an argument to the <code class="code">exec</code>
builtin.
An interactive shell does not exit if <code class="code">exec</code> fails.
</p>
</dd>
<dt><code class="code">expand_aliases</code></dt>
<dd><p>If set, aliases are expanded as described below under Aliases,
<a class="ref" href="#Aliases">Aliases</a>.
This option is enabled by default for interactive shells.
</p>
</dd>
<dt><code class="code">extdebug</code></dt>
<dd><p>If set at shell invocation, or in a shell startup file,
arrange to execute the debugger profile
before the shell starts, identical to the <samp class="option">--debugger</samp> option.
If set after invocation, behavior intended for use by debuggers is enabled:
</p>
<ol class="enumerate">
<li> The <samp class="option">-F</samp> option to the <code class="code">declare</code> builtin (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>)
displays the source file name and line number corresponding to each function
name supplied as an argument.

</li><li> If the command run by the <code class="code">DEBUG</code> trap returns a non-zero value, the
next command is skipped and not executed.

</li><li> If the command run by the <code class="code">DEBUG</code> trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell script
executed by the <code class="code">.</code> or <code class="code">source</code> builtins), the shell simulates
a call to <code class="code">return</code>.

</li><li> <code class="code">BASH_ARGC</code> and <code class="code">BASH_ARGV</code> are updated as described in their
descriptions (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).

</li><li> Function tracing is enabled: command substitution, shell functions, and
subshells invoked with <code class="code">( <var class="var">command</var> )</code> inherit the
<code class="code">DEBUG</code> and <code class="code">RETURN</code> traps.

</li><li> Error tracing is enabled: command substitution, shell functions, and
subshells invoked with <code class="code">( <var class="var">command</var> )</code> inherit the
<code class="code">ERR</code> trap.
</li></ol>

</dd>
<dt><code class="code">extglob</code></dt>
<dd><p>If set, enable the extended pattern matching features described above
(see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).
</p>
</dd>
<dt><code class="code">extquote</code></dt>
<dd><p>If set, <code class="code">$'<var class="var">string</var>'</code> and <code class="code">$&quot;<var class="var">string</var>&quot;</code> quoting is  
performed within <code class="code">${<var class="var">parameter</var>}</code> expansions                     
enclosed in double quotes.
This option is enabled by default. 
</p>
</dd>
<dt><code class="code">failglob</code></dt>
<dd><p>If set, patterns which fail to match filenames during filename expansion
result in an expansion error.
</p>
</dd>
<dt><code class="code">force_fignore</code></dt>
<dd><p>If set, the suffixes specified by the <code class="env">FIGNORE</code> shell variable
cause words to be ignored when performing word completion even if
the ignored words are the only possible completions.
See <a class="xref" href="#Bash-Variables">Bash Variables</a>, for a description of <code class="env">FIGNORE</code>.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">globasciiranges</code></dt>
<dd><p>If set, range expressions used in pattern matching bracket expressions
(see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>)
behave as if in the traditional C locale when performing
comparisons.
That is, pattern matching does not take
the current locale&rsquo;s collating sequence into account,
so &lsquo;<samp class="samp">b</samp>&rsquo; will not collate between &lsquo;<samp class="samp">A</samp>&rsquo; and &lsquo;<samp class="samp">B</samp>&rsquo;,
and upper-case and lower-case ASCII characters will collate together.   
</p>
</dd>
<dt><code class="code">globskipdots</code></dt>
<dd><p>If set, filename expansion will never match the filenames
<samp class="file">.</samp> and <samp class="file">..</samp>, even if the pattern begins with a &lsquo;<samp class="samp">.</samp>&rsquo;.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">globstar</code></dt>
<dd><p>If set, the pattern &lsquo;<samp class="samp">**</samp>&rsquo; used in a filename expansion context will
match all files and zero or more directories and subdirectories.
If the pattern is followed by a &lsquo;<samp class="samp">/</samp>&rsquo;, only directories and
subdirectories match.
</p>
</dd>
<dt><code class="code">gnu_errfmt</code></dt>
<dd><p>If set, shell error messages are written in the standard <small class="sc">GNU</small> error
message format.
</p>
</dd>
<dt><code class="code">histappend</code></dt>
<dd><p>If set, the history list is appended to the file named by the value
of the <code class="env">HISTFILE</code>
variable when the shell exits, rather than overwriting the file.
</p>
</dd>
<dt><code class="code">histreedit</code></dt>
<dd><p>If set, and Readline is being used,
the user is given the opportunity to re-edit a failed history substitution.
</p>
</dd>
<dt><code class="code">histverify</code></dt>
<dd><p>If set, and Readline
is being used, the results of history substitution are not immediately
passed to the shell parser.
Instead, the resulting line is loaded into the Readline editing buffer,
allowing further modification.
</p>
</dd>
<dt><code class="code">hostcomplete</code></dt>
<dd><p>If set, and Readline is being used, Bash will attempt to perform
hostname completion when a word containing a &lsquo;<samp class="samp">@</samp>&rsquo; is being
completed (see <a class="pxref" href="#Commands-For-Completion">Letting Readline Type For You</a>).
This option is enabled by default.
</p>
</dd>
<dt><code class="code">huponexit</code></dt>
<dd><p>If set, Bash will send <code class="code">SIGHUP</code> to all jobs when an interactive
login shell exits (see <a class="pxref" href="#Signals">Signals</a>).
</p>
</dd>
<dt><code class="code">inherit_errexit</code></dt>
<dd><p>If set, command substitution inherits the value of the <code class="code">errexit</code> option,
instead of unsetting it in the subshell environment.
This option is enabled when <small class="sc">POSIX</small> mode is enabled.
</p>
</dd>
<dt><code class="code">interactive_comments</code></dt>
<dd><p>In an interactive shell, a word beginning with &lsquo;<samp class="samp">#</samp>&rsquo;
causes that word and all remaining characters on that
line to be ignored, as in a non-interactive shell.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">lastpipe</code></dt>
<dd><p>If set, and job control is not active, the shell runs the last command of
a pipeline not executed in the background in the current shell environment.
</p>
</dd>
<dt><code class="code">lithist</code></dt>
<dd><p>If enabled, and the <code class="code">cmdhist</code>
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
</p>
</dd>
<dt><code class="code">localvar_inherit</code></dt>
<dd><p>If set, local variables inherit the value and attributes of a variable of
the same name that exists at a previous scope before any new value is
assigned.  The <code class="code">nameref</code> attribute is not inherited.
</p>
</dd>
<dt><code class="code">localvar_unset</code></dt>
<dd><p>If set, calling <code class="code">unset</code> on local variables in previous function scopes
marks them so subsequent lookups find them unset until that function
returns.
This is identical to the behavior of unsetting local variables at the
current function scope.
</p>
</dd>
<dt><code class="code">login_shell</code></dt>
<dd><p>The shell sets this option if it is started as a login shell
(see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>).
The value may not be changed.
</p>
</dd>
<dt><code class="code">mailwarn</code></dt>
<dd><p>If set, and a file that Bash is checking for mail has been
accessed since the last time it was checked, Bash displays the message
<code class="code">&quot;The mail in <var class="var">mailfile</var> has been read&quot;</code>.
</p>
</dd>
<dt><code class="code">no_empty_cmd_completion</code></dt>
<dd><p>If set, and Readline is being used, Bash does not search
the <code class="env">PATH</code>
for possible completions when completion is attempted on an empty line.
</p>
</dd>
<dt><code class="code">nocaseglob</code></dt>
<dd><p>If set, Bash matches filenames in a case-insensitive fashion when
performing filename expansion.
</p>
</dd>
<dt><code class="code">nocasematch</code></dt>
<dd><p>If set, Bash matches patterns in a case-insensitive fashion when
performing matching while executing <code class="code">case</code> or <code class="code">[[</code>
conditional commands (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>,
when performing pattern substitution word expansions,
or when filtering possible completions as part of programmable completion.
</p>
</dd>
<dt><code class="code">noexpand_translation</code></dt>
<dd><p>If set, Bash
encloses the translated results of $&quot;&hellip;&quot; quoting in single quotes
instead of double quotes.
If the string is not translated, this has no effect.
</p>
</dd>
<dt><code class="code">nullglob</code></dt>
<dd><p>If set, filename expansion patterns which match no files
(see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>)
expand to nothing and are removed, rather than expanding to themselves.
</p>
</dd>
<dt><code class="code">patsub_replacement</code></dt>
<dd><p>If set, Bash
expands occurrences of &lsquo;<samp class="samp">&amp;</samp>&rsquo; in the replacement string of pattern
substitution to the text matched by the pattern, as described
above (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).
This option is enabled by default.
</p>
</dd>
<dt><code class="code">progcomp</code></dt>
<dd><p>If set, enable the programmable completion facilities
(see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
This option is enabled by default.
</p>
</dd>
<dt><code class="code">progcomp_alias</code></dt>
<dd><p>If set, and programmable completion is enabled, Bash treats a command
name that doesn&rsquo;t have any completions as a possible alias and attempts
alias expansion.
If it has an alias, Bash attempts programmable
completion using the command word resulting from the expanded alias.
</p>
</dd>
<dt><code class="code">promptvars</code></dt>
<dd><p>If set, prompt strings undergo
parameter expansion, command substitution, arithmetic
expansion, and quote removal after being expanded
as described below (see <a class="pxref" href="#Controlling-the-Prompt">Controlling the Prompt</a>).
This option is enabled by default.
</p>
</dd>
<dt><code class="code">restricted_shell</code></dt>
<dd><p>The shell sets this option if it is started in restricted mode
(see <a class="pxref" href="#The-Restricted-Shell">The Restricted Shell</a>).
The value may not be changed.
This is not reset when the startup files are executed, allowing
the startup files to discover whether or not a shell is restricted.
</p>
</dd>
<dt><code class="code">shift_verbose</code></dt>
<dd><p>If this is set, the <code class="code">shift</code>
builtin prints an error message when the shift count exceeds the
number of positional parameters.
</p>
</dd>
<dt><code class="code">sourcepath</code></dt>
<dd><p>If set, the <code class="code">.</code> (<code class="code">source</code>) builtin uses the value of <code class="env">PATH</code>
to find the directory containing the file supplied as an argument
when the <samp class="option">-p</samp> option is not supplied.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">varredir_close</code></dt>
<dd><p>If set, the shell automatically closes file descriptors assigned using the
<code class="code">{varname}</code> redirection syntax (see <a class="pxref" href="#Redirections">Redirections</a>) instead of
leaving them open when the command completes.
</p>
</dd>
<dt><code class="code">xpg_echo</code></dt>
<dd><p>If set, the <code class="code">echo</code> builtin expands backslash-escape sequences
by default.
If the <code class="code">posix</code> shell option (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>) is also enabled,
<code class="code">echo</code> does not interpret any options.
</p>
</dd>
</dl>
</dd>
</dl>

<hr>
</div>
</div>
<div class="section-level-extent" id="Special-Builtins">
<div class="nav-panel">
<p>
Previous: <a href="#Modifying-Shell-Behavior" accesskey="p" rel="prev">Modifying Shell Behavior</a>, Up: <a href="#Shell-Builtin-Commands" accesskey="u" rel="up">Shell Builtin Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Special-Builtins-1"><span>4.4 Special Builtins<a class="copiable-link" href="#Special-Builtins-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-special-builtin-1"></a>

<p>For historical reasons, the <small class="sc">POSIX</small> standard has classified
several builtin commands as <em class="emph">special</em>.
When Bash is executing in <small class="sc">POSIX</small> mode, the special builtins
differ from other builtin commands in three respects:
</p>
<ol class="enumerate">
<li> Special builtins are found before shell functions during command lookup.

</li><li> If a special builtin returns an error status, a non-interactive shell exits.

</li><li> Assignment statements preceding the command stay in effect in the shell
environment after the command completes.
</li></ol>

<p>When Bash is not executing in <small class="sc">POSIX</small> mode, these builtins behave no
differently than the rest of the Bash builtin commands.
The Bash <small class="sc">POSIX</small> mode is described in <a class="ref" href="#Bash-POSIX-Mode">Bash and POSIX</a>. 
</p>
<p>These are the <small class="sc">POSIX</small> special builtins:
</p><div class="example">
<pre class="example-preformatted">break : . source continue eval exec exit export readonly return set<!-- /@w -->
shift times trap unset<!-- /@w -->
</pre></div>

<hr>
</div>
</div>
<div class="chapter-level-extent" id="Shell-Variables">
<div class="nav-panel">
<p>
Next: <a href="#Bash-Features" accesskey="n" rel="next">Bash Features</a>, Previous: <a href="#Shell-Builtin-Commands" accesskey="p" rel="prev">Shell Builtin Commands</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Shell-Variables-1"><span>5 Shell Variables<a class="copiable-link" href="#Shell-Variables-1"> &para;</a></span></h2>


<p>This chapter describes the shell variables that Bash uses.
Bash automatically assigns default values to a number of variables.
</p>
<ul class="mini-toc">
<li><a href="#Bourne-Shell-Variables" accesskey="1">Bourne Shell Variables</a></li>
<li><a href="#Bash-Variables" accesskey="2">Bash Variables</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Bourne-Shell-Variables">
<div class="nav-panel">
<p>
Next: <a href="#Bash-Variables" accesskey="n" rel="next">Bash Variables</a>, Up: <a href="#Shell-Variables" accesskey="u" rel="up">Shell Variables</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bourne-Shell-Variables-1"><span>5.1 Bourne Shell Variables<a class="copiable-link" href="#Bourne-Shell-Variables-1"> &para;</a></span></h3>

<p>Bash uses certain shell variables in the same way as the Bourne shell.
In some cases, Bash assigns a default value to the variable.
</p>
<dl class="vtable">
<dt><a id="index-CDPATH"></a><span><code class="code">CDPATH</code><a class="copiable-link" href="#index-CDPATH"> &para;</a></span></dt>
<dd><p>A colon-separated list of directories used as a search path for
the <code class="code">cd</code> builtin command.
</p>
</dd>
<dt><a id="index-HOME"></a><span><code class="code">HOME</code><a class="copiable-link" href="#index-HOME"> &para;</a></span></dt>
<dd><p>The current user&rsquo;s home directory; the default for the <code class="code">cd</code> builtin
command.
The value of this variable is also used by tilde expansion
(see <a class="pxref" href="#Tilde-Expansion">Tilde Expansion</a>).
</p>
</dd>
<dt><a id="index-IFS"></a><span><code class="code">IFS</code><a class="copiable-link" href="#index-IFS"> &para;</a></span></dt>
<dd><p>A list of characters that separate fields; used when the shell splits
words as part of expansion and by the <code class="code">read</code> builtin to split
lines into words.
See <a class="xref" href="#Word-Splitting">Word Splitting</a>, for a description of word splitting.
</p>
</dd>
<dt><a id="index-MAIL"></a><span><code class="code">MAIL</code><a class="copiable-link" href="#index-MAIL"> &para;</a></span></dt>
<dd><p>If the value is set to a filename or directory name
and the <code class="env">MAILPATH</code> variable
is not set, Bash informs the user of the arrival of mail in
the specified file or Maildir-format directory.
</p>
</dd>
<dt><a id="index-MAILPATH"></a><span><code class="code">MAILPATH</code><a class="copiable-link" href="#index-MAILPATH"> &para;</a></span></dt>
<dd><p>A colon-separated list of filenames which the shell periodically checks
for new mail.
Each list entry can specify the message that is printed when new mail
arrives in the mail file by separating the filename from the message with
a &lsquo;<samp class="samp">?</samp>&rsquo;.
When used in the text of the message, <code class="code">$_</code> expands to the name of
the current mail file.
</p>
</dd>
<dt><a id="index-OPTARG"></a><span><code class="code">OPTARG</code><a class="copiable-link" href="#index-OPTARG"> &para;</a></span></dt>
<dd><p>The value of the last option argument processed by the <code class="code">getopts</code> builtin.
</p>
</dd>
<dt><a id="index-OPTIND"></a><span><code class="code">OPTIND</code><a class="copiable-link" href="#index-OPTIND"> &para;</a></span></dt>
<dd><p>The index of the next argument to be processed by the <code class="code">getopts</code> builtin.
</p>
</dd>
<dt><a id="index-PATH"></a><span><code class="code">PATH</code><a class="copiable-link" href="#index-PATH"> &para;</a></span></dt>
<dd><p>A colon-separated list of directories in which the shell looks for
commands.
A zero-length (null) directory name in the value of <code class="code">PATH</code> indicates the
current directory.
A null directory name may appear as two adjacent colons, or as an initial
or trailing colon.
The default path is system-dependent, and is set by the administrator
who installs <code class="code">bash</code>.
A common value is
&quot;/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin&quot;.
</p>
</dd>
<dt><a id="index-PS1"></a><span><code class="code">PS1</code><a class="copiable-link" href="#index-PS1"> &para;</a></span></dt>
<dd><p>The primary prompt string.
The default value is &lsquo;<samp class="samp">\s-\v\$ </samp>&rsquo;.
See <a class="xref" href="#Controlling-the-Prompt">Controlling the Prompt</a>, for the complete list of escape
sequences that are expanded before <code class="env">PS1</code> is displayed.
</p>
</dd>
<dt><a id="index-PS2"></a><span><code class="code">PS2</code><a class="copiable-link" href="#index-PS2"> &para;</a></span></dt>
<dd><p>The secondary prompt string.
The default value is &lsquo;<samp class="samp">&gt; </samp>&rsquo;.
<code class="env">PS2</code> is expanded in the same way as <code class="env">PS1</code> before being
displayed.
</p>
</dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="Bash-Variables">
<div class="nav-panel">
<p>
Previous: <a href="#Bourne-Shell-Variables" accesskey="p" rel="prev">Bourne Shell Variables</a>, Up: <a href="#Shell-Variables" accesskey="u" rel="up">Shell Variables</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-Variables-1"><span>5.2 Bash Variables<a class="copiable-link" href="#Bash-Variables-1"> &para;</a></span></h3>

<p>These variables are set or used by Bash, but other shells
do not normally treat them specially.
</p>
<p>A few variables used by Bash are described in different chapters:
variables for controlling the job control facilities
(see <a class="pxref" href="#Job-Control-Variables">Job Control Variables</a>).
</p>
<dl class="vtable">
<dt><a id="index-_005f"></a><span><code class="code">_</code><a class="copiable-link" href="#index-_005f"> &para;</a></span></dt>
<dd><a class="index-entry-id" id="index-_0024_005f"></a>
<p>($_, an underscore.)
This has a number of meanings depending on context.
At shell startup, $_ set to the pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list.
Subsequently, it expands to the last argument to the previous simple
command executed in the foreground, after expansion. 
It is also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, $_ expands to the name of the mail file.
</p>
</dd>
<dt><a id="index-BASH"></a><span><code class="code">BASH</code><a class="copiable-link" href="#index-BASH"> &para;</a></span></dt>
<dd><p>The full pathname used to execute the current instance of Bash.
</p>
</dd>
<dt><a id="index-BASHOPTS"></a><span><code class="code">BASHOPTS</code><a class="copiable-link" href="#index-BASHOPTS"> &para;</a></span></dt>
<dd><p>A colon-separated list of enabled shell options.
Each word in the list is a valid argument for the <samp class="option">-s</samp> option
to the <code class="code">shopt</code> builtin command (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
The options appearing in <code class="env">BASHOPTS</code> are those reported
as &lsquo;<samp class="samp">on</samp>&rsquo; by &lsquo;<samp class="samp">shopt</samp>&rsquo;.
If this variable is in the environment when Bash
starts up, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
This variable is readonly.
</p>
</dd>
<dt><a id="index-BASHPID"></a><span><code class="code">BASHPID</code><a class="copiable-link" href="#index-BASHPID"> &para;</a></span></dt>
<dd><p>Expands to the process <small class="sc">ID</small> of the current Bash process.
This differs from <code class="code">$$</code> under certain circumstances, such as subshells
that do not require Bash to be re-initialized.
Assignments to <code class="env">BASHPID</code> have no effect.
If <code class="env">BASHPID</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fALIASES"></a><span><code class="code">BASH_ALIASES</code><a class="copiable-link" href="#index-BASH_005fALIASES"> &para;</a></span></dt>
<dd><p>An associative array variable whose members correspond to the internal
list of aliases as maintained by the <code class="code">alias</code> builtin.
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not cause aliases to be removed
from the alias list.
If <code class="env">BASH_ALIASES</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fARGC"></a><span><code class="code">BASH_ARGC</code><a class="copiable-link" href="#index-BASH_005fARGC"> &para;</a></span></dt>
<dd><p>An array variable whose values are the number of parameters in each
frame of the current Bash execution call stack.
The number of parameters to the current subroutine (shell function or
script executed with <code class="code">.</code> or <code class="code">source</code>) is at the top of the stack.
When a
subroutine is executed, the number of parameters passed is pushed onto
<code class="code">BASH_ARGC</code>.
The shell sets <code class="code">BASH_ARGC</code> only when in extended debugging mode
(see <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>
for a description of the <code class="code">extdebug</code> option to the <code class="code">shopt</code>
builtin).
Setting <code class="code">extdebug</code> after the shell has started to execute a subroutine,
or referencing this variable when <code class="code">extdebug</code> is not set,
may result in inconsistent values.
Assignments to <code class="env">BASH_ARGC</code> have no effect, and it may not be unset.
</p>
</dd>
<dt><a id="index-BASH_005fARGV"></a><span><code class="code">BASH_ARGV</code><a class="copiable-link" href="#index-BASH_005fARGV"> &para;</a></span></dt>
<dd><p>An array variable containing all of the parameters in the current Bash
execution call stack.
The final parameter of the last subroutine call is at the top of the stack;
the first parameter of the initial call is at the bottom.
When a subroutine is executed, the shell pushes the supplied parameters
onto <code class="code">BASH_ARGV</code>.
The shell sets <code class="code">BASH_ARGV</code> only when in extended debugging mode
(see <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>
for a description of the <code class="code">extdebug</code> option to the <code class="code">shopt</code>
builtin).
Setting <code class="code">extdebug</code> after the shell has started to execute a script,
or referencing this variable when <code class="code">extdebug</code> is not set,
may result in inconsistent values.
Assignments to <code class="env">BASH_ARGV</code> have no effect, and it may not be unset.
</p>
</dd>
<dt><a id="index-BASH_005fARGV0"></a><span><code class="code">BASH_ARGV0</code><a class="copiable-link" href="#index-BASH_005fARGV0"> &para;</a></span></dt>
<dd><p>When referenced, this variable expands to the name of the shell or shell
script (identical to <code class="code">$0</code>; See <a class="xref" href="#Special-Parameters">Special Parameters</a>,
for the description of special parameter 0).
Assigning a value to
<code class="code">BASH_ARGV0</code>
sets <code class="code">$0</code> to the same value.
If
<code class="env">BASH_ARGV0</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fCMDS"></a><span><code class="code">BASH_CMDS</code><a class="copiable-link" href="#index-BASH_005fCMDS"> &para;</a></span></dt>
<dd><p>An associative array variable whose members correspond to the internal
hash table of commands as maintained by the <code class="code">hash</code> builtin
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
Adding elements to this array makes them appear in the hash table; however,
unsetting array elements currently does not remove command names
from the hash table.
If <code class="env">BASH_CMDS</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fCOMMAND"></a><span><code class="code">BASH_COMMAND</code><a class="copiable-link" href="#index-BASH_005fCOMMAND"> &para;</a></span></dt>
<dd><p>Expands to the command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the trap.
If <code class="env">BASH_COMMAND</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fCOMPAT"></a><span><code class="code">BASH_COMPAT</code><a class="copiable-link" href="#index-BASH_005fCOMPAT"> &para;</a></span></dt>
<dd><p>The value is used to set the shell&rsquo;s compatibility level.
See <a class="xref" href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a>, for a description of the various
compatibility levels and their effects.
The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
corresponding to the desired compatibility level.
If <code class="env">BASH_COMPAT</code> is unset or set to the empty string, the compatibility
level is set to the default for the current version.
If <code class="env">BASH_COMPAT</code> is set to a value that is not one of the valid
compatibility levels, the shell prints an error message and sets the
compatibility level to the default for the current version.
A subset of the valid values correspond to the compatibility levels
described below (see <a class="pxref" href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a>).
For example, 4.2 and 42 are valid values that correspond
to the <code class="code">compat42</code> <code class="code">shopt</code> option
and set the compatibility level to 42.
The current version is also a valid value.
</p>
</dd>
<dt><a id="index-BASH_005fENV"></a><span><code class="code">BASH_ENV</code><a class="copiable-link" href="#index-BASH_005fENV"> &para;</a></span></dt>
<dd><p>If this variable is set when Bash is invoked to execute a shell
script, its value is expanded and used as the name of a startup file
to read before executing the script.
Bash does not use <code class="env">PATH</code> to search for the resultant filename.
See <a class="xref" href="#Bash-Startup-Files">Bash Startup Files</a>.
</p>
</dd>
<dt><a id="index-BASH_005fEXECUTION_005fSTRING"></a><span><code class="code">BASH_EXECUTION_STRING</code><a class="copiable-link" href="#index-BASH_005fEXECUTION_005fSTRING"> &para;</a></span></dt>
<dd><p>The command argument to the <samp class="option">-c</samp> invocation option.
</p>
</dd>
<dt><a id="index-BASH_005fLINENO"></a><span><code class="code">BASH_LINENO</code><a class="copiable-link" href="#index-BASH_005fLINENO"> &para;</a></span></dt>
<dd><p>An array variable whose members are the line numbers in source files
where each corresponding member of <code class="env">FUNCNAME</code> was invoked.
<code class="code">${BASH_LINENO[$i]}</code> is the line number in the source file
(<code class="code">${BASH_SOURCE[$i+1]}</code>) where
<code class="code">${FUNCNAME[$i]}</code> was called (or <code class="code">${BASH_LINENO[$i-1]}</code> if
referenced within another shell function). 
Use <code class="code">LINENO</code> to obtain the current line number.
Assignments to <code class="env">BASH_LINENO</code> have no effect, and it may not be unset.
</p>
</dd>
<dt><a id="index-BASH_005fLOADABLES_005fPATH"></a><span><code class="code">BASH_LOADABLES_PATH</code><a class="copiable-link" href="#index-BASH_005fLOADABLES_005fPATH"> &para;</a></span></dt>
<dd><p>A colon-separated list of directories in which the <code class="code">enable</code>
command looks for dynamically loadable builtins.
</p>
</dd>
<dt><a id="index-BASH_005fMONOSECONDS"></a><span><code class="code">BASH_MONOSECONDS</code><a class="copiable-link" href="#index-BASH_005fMONOSECONDS"> &para;</a></span></dt>
<dd><p>Each time this variable is referenced, it expands to the value returned
by the system&rsquo;s monotonic clock, if one is available.
If there is no monotonic clock, this is equivalent to <code class="env">EPOCHSECONDS</code>.
If <code class="env">BASH_MONOSECONDS</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fREMATCH"></a><span><code class="code">BASH_REMATCH</code><a class="copiable-link" href="#index-BASH_005fREMATCH"> &para;</a></span></dt>
<dd><p>An array variable whose members are assigned by the &lsquo;<samp class="samp">=~</samp>&rsquo; binary
operator to the <code class="code">[[</code> conditional command
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index <var class="var">n</var> is the portion of the
string matching the <var class="var">n</var>th parenthesized subexpression.
</p>
</dd>
<dt><a id="index-BASH_005fSOURCE"></a><span><code class="code">BASH_SOURCE</code><a class="copiable-link" href="#index-BASH_005fSOURCE"> &para;</a></span></dt>
<dd><p>An array variable whose members are the source filenames where the
corresponding shell function names in the <code class="code">FUNCNAME</code> array
variable are defined.
The shell function <code class="code">${FUNCNAME[$i]}</code> is defined in the file
<code class="code">${BASH_SOURCE[$i]}</code> and called from <code class="code">${BASH_SOURCE[$i+1]}</code>
Assignments to <code class="env">BASH_SOURCE</code> have no effect, and it may not be unset.
</p>
</dd>
<dt><a id="index-BASH_005fSUBSHELL"></a><span><code class="code">BASH_SUBSHELL</code><a class="copiable-link" href="#index-BASH_005fSUBSHELL"> &para;</a></span></dt>
<dd><p>Incremented by one within each subshell or subshell environment when
the shell begins executing in that environment.
The initial value is 0.
If <code class="env">BASH_SUBSHELL</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-BASH_005fTRAPSIG"></a><span><code class="code">BASH_TRAPSIG</code><a class="copiable-link" href="#index-BASH_005fTRAPSIG"> &para;</a></span></dt>
<dd><p>Set to the signal number corresponding to the trap action being executed
during its execution.
See the description of <code class="code">trap</code>
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
for information about signal numbers and trap execution.
</p>
</dd>
<dt><a id="index-BASH_005fVERSINFO"></a><span><code class="code">BASH_VERSINFO</code><a class="copiable-link" href="#index-BASH_005fVERSINFO"> &para;</a></span></dt>
<dd><p>A readonly array variable (see <a class="pxref" href="#Arrays">Arrays</a>)
whose members hold version information for this instance of Bash.
The values assigned to the array members are as follows:
</p>
<dl class="table">
<dt><code class="code">BASH_VERSINFO[0]</code></dt>
<dd><p>The major version number (the <em class="dfn">release</em>).
</p>
</dd>
<dt><code class="code">BASH_VERSINFO[1]</code></dt>
<dd><p>The minor version number (the <em class="dfn">version</em>).
</p>
</dd>
<dt><code class="code">BASH_VERSINFO[2]</code></dt>
<dd><p>The patch level.
</p>
</dd>
<dt><code class="code">BASH_VERSINFO[3]</code></dt>
<dd><p>The build version.
</p>
</dd>
<dt><code class="code">BASH_VERSINFO[4]</code></dt>
<dd><p>The release status (e.g., <code class="code">beta</code>).
</p>
</dd>
<dt><code class="code">BASH_VERSINFO[5]</code></dt>
<dd><p>The value of <code class="env">MACHTYPE</code>.
</p></dd>
</dl>

</dd>
<dt><a id="index-BASH_005fVERSION"></a><span><code class="code">BASH_VERSION</code><a class="copiable-link" href="#index-BASH_005fVERSION"> &para;</a></span></dt>
<dd><p>Expands to a string describing the version of this  instance  of
Bash (e.g., 5.2.37(3)-release).
</p>
</dd>
<dt><a id="index-BASH_005fXTRACEFD"></a><span><code class="code">BASH_XTRACEFD</code><a class="copiable-link" href="#index-BASH_005fXTRACEFD"> &para;</a></span></dt>
<dd><p>If set to an integer corresponding to a valid file descriptor,
Bash writes the trace output generated when
&lsquo;<samp class="samp">set -x</samp>&rsquo;
is enabled to that file descriptor,
instead of the standard error.
This allows tracing output to be separated from diagnostic and error
messages.
The file descriptor is closed when
<code class="code">BASH_XTRACEFD</code>
is unset or assigned a new value.
Unsetting
<code class="code">BASH_XTRACEFD</code>
or assigning it the empty string causes the
trace output to be sent to the standard error.
Note that setting
<code class="code">BASH_XTRACEFD</code>
to 2 (the standard error file
descriptor) and then unsetting it will result in the standard error
being closed.
</p>
</dd>
<dt><a id="index-CHILD_005fMAX"></a><span><code class="code">CHILD_MAX</code><a class="copiable-link" href="#index-CHILD_005fMAX"> &para;</a></span></dt>
<dd><p>Set the number of exited child status values for the shell to remember.
Bash will not allow this value to be decreased below a
<small class="sc">POSIX</small>-mandated minimum,
and there is a maximum value (currently 8192) that this may not exceed.
The minimum value is system-dependent.
</p>
</dd>
<dt><a id="index-COLUMNS"></a><span><code class="code">COLUMNS</code><a class="copiable-link" href="#index-COLUMNS"> &para;</a></span></dt>
<dd><p>Used by the <code class="code">select</code> command to determine the terminal width
when printing selection lists.
Automatically set if the <code class="code">checkwinsize</code> option is enabled
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), or in an interactive shell upon receipt of a
<code class="code">SIGWINCH</code>.
</p>
</dd>
<dt><a id="index-COMP_005fCWORD"></a><span><code class="code">COMP_CWORD</code><a class="copiable-link" href="#index-COMP_005fCWORD"> &para;</a></span></dt>
<dd><p>An index into <code class="env">${COMP_WORDS}</code> of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMP_005fKEY"></a><span><code class="code">COMP_KEY</code><a class="copiable-link" href="#index-COMP_005fKEY"> &para;</a></span></dt>
<dd><p>The key (or final key of a key sequence) used to invoke the current
completion function.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMP_005fLINE"></a><span><code class="code">COMP_LINE</code><a class="copiable-link" href="#index-COMP_005fLINE"> &para;</a></span></dt>
<dd><p>The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMP_005fPOINT"></a><span><code class="code">COMP_POINT</code><a class="copiable-link" href="#index-COMP_005fPOINT"> &para;</a></span></dt>
<dd><p>The index of the current cursor position relative to the beginning of
the current command.
If the current cursor position is at the end of the current command,
the value of this variable is equal to <code class="code">${#COMP_LINE}</code>.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMP_005fTYPE"></a><span><code class="code">COMP_TYPE</code><a class="copiable-link" href="#index-COMP_005fTYPE"> &para;</a></span></dt>
<dd><p>Set to an integer value corresponding to the type of attempted completion
that caused a completion function to be called:
<kbd class="key">TAB</kbd>, for normal completion,
&lsquo;<samp class="samp">?</samp>&rsquo;, for listing completions after successive tabs,
&lsquo;<samp class="samp">!</samp>&rsquo;, for listing alternatives on partial word completion,
&lsquo;<samp class="samp">@</samp>&rsquo;, to list completions if the word is not unmodified,
or
&lsquo;<samp class="samp">%</samp>&rsquo;, for menu completion.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMP_005fWORDBREAKS"></a><span><code class="code">COMP_WORDBREAKS</code><a class="copiable-link" href="#index-COMP_005fWORDBREAKS"> &para;</a></span></dt>
<dd><p>The set of characters that the Readline library treats as word
separators when performing word completion.
If <code class="env">COMP_WORDBREAKS</code>
is unset, it loses its special properties,
even if it is subsequently reset.
</p>
</dd>
<dt><a id="index-COMP_005fWORDS"></a><span><code class="code">COMP_WORDS</code><a class="copiable-link" href="#index-COMP_005fWORDS"> &para;</a></span></dt>
<dd><p>An array variable consisting of the individual
words in the current command line.
The line is split into words as Readline would split it, using
<code class="code">COMP_WORDBREAKS</code> as described above.
This variable is available only in shell functions invoked by the
programmable completion facilities (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
</dd>
<dt><a id="index-COMPREPLY"></a><span><code class="code">COMPREPLY</code><a class="copiable-link" href="#index-COMPREPLY"> &para;</a></span></dt>
<dd><p>An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
Each array element contains one possible completion.
</p>
</dd>
<dt><a id="index-COPROC"></a><span><code class="code">COPROC</code><a class="copiable-link" href="#index-COPROC"> &para;</a></span></dt>
<dd><p>An array variable created to hold the file descriptors
for output from and input to an unnamed coprocess (see <a class="pxref" href="#Coprocesses">Coprocesses</a>).
</p>
</dd>
<dt><a id="index-DIRSTACK"></a><span><code class="code">DIRSTACK</code><a class="copiable-link" href="#index-DIRSTACK"> &para;</a></span></dt>
<dd><p>An array variable containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
<code class="code">dirs</code> builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the <code class="code">pushd</code> and <code class="code">popd</code>
builtins must be used to add and remove directories.
Assigning to this variable does not change the current directory.
If <code class="env">DIRSTACK</code>
is unset, it loses its special properties, even if
it is subsequently reset.
</p>
</dd>
<dt><a id="index-EMACS"></a><span><code class="code">EMACS</code><a class="copiable-link" href="#index-EMACS"> &para;</a></span></dt>
<dd><p>If Bash finds this variable in the environment when the shell
starts, and its value is &lsquo;<samp class="samp">t</samp>&rsquo;, Bash assumes that the shell is
running in an Emacs shell buffer and disables line editing.
</p>
</dd>
<dt><a id="index-ENV"></a><span><code class="code">ENV</code><a class="copiable-link" href="#index-ENV"> &para;</a></span></dt>
<dd><p>Expanded and executed similarly to <code class="code">BASH_ENV</code>
(see <a class="pxref" href="#Bash-Startup-Files">Bash Startup Files</a>)
when an interactive shell is invoked in
<small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>).
</p>
</dd>
<dt><a id="index-EPOCHREALTIME"></a><span><code class="code">EPOCHREALTIME</code><a class="copiable-link" href="#index-EPOCHREALTIME"> &para;</a></span></dt>
<dd><p>Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch as a floating-point value with micro-second granularity
(see the documentation for the C library function <code class="code">time</code> for the
definition of Epoch).
Assignments to <code class="env">EPOCHREALTIME</code> are ignored.
If <code class="env">EPOCHREALTIME</code>
is unset, it loses its special properties, even if
it is subsequently reset.
</p>
</dd>
<dt><a id="index-EPOCHSECONDS"></a><span><code class="code">EPOCHSECONDS</code><a class="copiable-link" href="#index-EPOCHSECONDS"> &para;</a></span></dt>
<dd><p>Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see the documentation for the C library function
<code class="code">time</code> for the definition of Epoch).
Assignments to <code class="env">EPOCHSECONDS</code> are ignored.
If <code class="env">EPOCHSECONDS</code>
is unset, it loses its special properties, even if
it is subsequently reset.
</p>
</dd>
<dt><a id="index-EUID"></a><span><code class="code">EUID</code><a class="copiable-link" href="#index-EUID"> &para;</a></span></dt>
<dd><p>The numeric effective user id of the current user.
This variable is readonly.
</p>
</dd>
<dt><a id="index-EXECIGNORE"></a><span><code class="code">EXECIGNORE</code><a class="copiable-link" href="#index-EXECIGNORE"> &para;</a></span></dt>
<dd><p>A colon-separated list of shell patterns (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>)
defining the set of filenames to be ignored by command search using
<code class="code">PATH</code>.
Files whose full pathnames match one of these patterns are not considered
executable files for the purposes of completion and command execution
via <code class="code">PATH</code> lookup.
This does not affect the behavior of the <code class="code">[</code>, <code class="code">test</code>, and <code class="code">[[</code>
commands.
Full pathnames in the command hash table are not subject to <code class="code">EXECIGNORE</code>.
Use this variable to ignore shared library files that have the executable
bit set, but are not executable files.
The pattern matching honors the setting of the <code class="code">extglob</code> shell 
option.
</p>
</dd>
<dt><a id="index-FCEDIT"></a><span><code class="code">FCEDIT</code><a class="copiable-link" href="#index-FCEDIT"> &para;</a></span></dt>
<dd><p>The editor used as a default by the <code class="code">fc</code> builtin command.
</p>
</dd>
<dt><a id="index-FIGNORE"></a><span><code class="code">FIGNORE</code><a class="copiable-link" href="#index-FIGNORE"> &para;</a></span></dt>
<dd><p>A colon-separated list of suffixes to ignore when performing
filename completion.
A filename whose suffix matches one of the entries in 
<code class="env">FIGNORE</code>
is excluded from the list of matched filenames.  A sample
value is &lsquo;<samp class="samp">.o:~</samp>&rsquo;
</p>
</dd>
<dt><a id="index-FUNCNAME"></a><span><code class="code">FUNCNAME</code><a class="copiable-link" href="#index-FUNCNAME"> &para;</a></span></dt>
<dd><p>An array variable containing the names of all shell functions
currently in the execution call stack.
The element with index 0 is the name of any currently-executing
shell function.
The bottom-most element (the one with the highest index)
is <code class="code">&quot;main&quot;</code>.
This variable exists only when a shell function is executing.
Assignments to <code class="env">FUNCNAME</code> have no effect.
If <code class="env">FUNCNAME</code>
is unset, it loses its special properties, even if
it is subsequently reset.
</p>
<p>This variable can be used with <code class="code">BASH_LINENO</code> and <code class="code">BASH_SOURCE</code>.
Each element of <code class="code">FUNCNAME</code> has corresponding elements in
<code class="code">BASH_LINENO</code> and <code class="code">BASH_SOURCE</code> to describe the call stack.
For instance,
<code class="code">${FUNCNAME[$i]}</code>
was called from the file
<code class="code">${BASH_SOURCE[$i+1]}</code>
at line number
<code class="code">${BASH_LINENO[$i]}</code>.
The <code class="code">caller</code> builtin displays the current call stack using this
information.
</p>
</dd>
<dt><a id="index-FUNCNEST"></a><span><code class="code">FUNCNEST</code><a class="copiable-link" href="#index-FUNCNEST"> &para;</a></span></dt>
<dd><p>A numeric value greater than 0 defines a maximum function nesting level.
Function invocations that exceed this nesting level
cause the current command to abort.
</p>
</dd>
<dt><a id="index-GLOBIGNORE"></a><span><code class="code">GLOBIGNORE</code><a class="copiable-link" href="#index-GLOBIGNORE"> &para;</a></span></dt>
<dd><p>A colon-separated list of patterns defining the set of file names to
be ignored by filename expansion.
If a file name matched by a filename expansion pattern also matches one
of the patterns in <code class="env">GLOBIGNORE</code>, it is removed from the list
of matches.
The pattern matching honors the setting of the <code class="code">extglob</code> shell
option.
</p>
</dd>
<dt><a id="index-GLOBSORT"></a><span><code class="code">GLOBSORT</code><a class="copiable-link" href="#index-GLOBSORT"> &para;</a></span></dt>
<dd><p>Controls how the results of filename expansion are sorted.
The value of this variable specifies the sort criteria and sort order for
the results of filename expansion.
If this variable is unset or set to the null string, filename expansion
uses the historical behavior of sorting by name,
in ascending lexicographic order as determined by the
<code class="code">LC_COLLATE</code> shell variable.
</p>
<p>If set, a valid value begins with an optional &lsquo;<samp class="samp">+</samp>&rsquo;, which is ignored,
or &lsquo;<samp class="samp">-</samp>&rsquo;, which reverses the sort order from ascending to descending,
followed by a sort specifier.
The valid sort specifiers are
&lsquo;<samp class="samp">name</samp>&rsquo;,
&lsquo;<samp class="samp">numeric</samp>&rsquo;,
&lsquo;<samp class="samp">size</samp>&rsquo;,
&lsquo;<samp class="samp">mtime</samp>&rsquo;,
&lsquo;<samp class="samp">atime</samp>&rsquo;,
&lsquo;<samp class="samp">ctime</samp>&rsquo;,
and
&lsquo;<samp class="samp">blocks</samp>&rsquo;,
which sort the files on name,
names in numeric rather than lexicographic order,
file size, modification time, access time,
inode change time, and number of blocks, respectively.
If any of the non-name keys compare as equal (e.g., if two files are
the same size), sorting uses the name as a secondary sort key.
</p>
<p>For example, a value of <code class="code">-mtime</code> sorts the results in descending
order by modification time (newest first).
</p>
<p>The &lsquo;<samp class="samp">numeric</samp>&rsquo; specifier treats names consisting solely of digits as
numbers and sorts them using their numeric value
(so &ldquo;2&rdquo; sorts before &ldquo;10&rdquo;, for example).
When using &lsquo;<samp class="samp">numeric</samp>&rsquo;, names containing non-digits sort after all
the all-digit names and are sorted by name using the traditional behavior.
</p>
<p>A sort specifier of &lsquo;<samp class="samp">nosort</samp>&rsquo; disables sorting completely;
Bash returns the results
in the order they are read from the file system,
ignoring any leading &lsquo;<samp class="samp">-</samp>&rsquo;.
</p>
<p>If the sort specifier is missing, it defaults to <var class="var">name</var>,
so a value of &lsquo;<samp class="samp">+</samp>&rsquo; is equivalent to the null string,
and a value of &lsquo;<samp class="samp">-</samp>&rsquo; sorts by name in descending order.
</p>
<p>Any invalid value restores the historical sorting behavior.
</p>
</dd>
<dt><a id="index-GROUPS"></a><span><code class="code">GROUPS</code><a class="copiable-link" href="#index-GROUPS"> &para;</a></span></dt>
<dd><p>An array variable containing the list of groups of which the current    
user is a member.
Assignments to <code class="env">GROUPS</code> have no effect.
If <code class="env">GROUPS</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-histchars"></a><span><code class="code">histchars</code><a class="copiable-link" href="#index-histchars"> &para;</a></span></dt>
<dd><p>The two or three characters which control history expansion,
quick substitution, and tokenization
(see <a class="pxref" href="#History-Interaction">History Expansion</a>).
The first character is the <em class="dfn">history expansion</em> character,
the character which begins a history expansion, normally
&lsquo;<samp class="samp">!</samp>&rsquo;.
The second character is the
<em class="dfn">quick substitution</em>
character, normally
&lsquo;<samp class="samp">^</samp>&rsquo;.
When it appears as the first character on the line,
history substitution repeats the previous command,
replacing one string with another.
The optional third character is the
<em class="dfn">history comment</em>
character, normally &lsquo;<samp class="samp">#</samp>&rsquo;,
which indicates
that the remainder of the line is a comment
when it appears as the first character of a word.
The history comment character disables history substitution
for the remaining words on the line.
It does not necessarily cause the shell parser to treat the rest of the
line as a comment.
</p>
</dd>
<dt><a id="index-HISTCMD"></a><span><code class="code">HISTCMD</code><a class="copiable-link" href="#index-HISTCMD"> &para;</a></span></dt>
<dd><p>The history number, or index in the history list, of the current
command.
Assignments to <code class="env">HISTCMD</code>
have no effect.
If <code class="env">HISTCMD</code>
is unset, it loses its special properties,
even if it is subsequently reset.
</p>
</dd>
<dt><a id="index-HISTCONTROL"></a><span><code class="code">HISTCONTROL</code><a class="copiable-link" href="#index-HISTCONTROL"> &para;</a></span></dt>
<dd><p>A colon-separated list of values controlling how commands are saved on
the history list.
If the list of values includes &lsquo;<samp class="samp">ignorespace</samp>&rsquo;, lines which begin
with a space character are not saved in the history list.
A value of &lsquo;<samp class="samp">ignoredups</samp>&rsquo; causes lines which match the previous
history entry not to be saved.
A value of &lsquo;<samp class="samp">ignoreboth</samp>&rsquo; is shorthand for
&lsquo;<samp class="samp">ignorespace</samp>&rsquo; and &lsquo;<samp class="samp">ignoredups</samp>&rsquo;.
A value of &lsquo;<samp class="samp">erasedups</samp>&rsquo; causes all previous lines matching the
current line to be removed from the history list before that line
is saved.
Any value not in the above list is ignored.
If <code class="env">HISTCONTROL</code> is unset, or does not include a valid value,
Bash saves all lines read by the shell parser on the history list, 
subject to the value of <code class="env">HISTIGNORE</code>.
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
<code class="env">HISTCONTROL</code>.
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
</p>
</dd>
<dt><a id="index-HISTFILE"></a><span><code class="code">HISTFILE</code><a class="copiable-link" href="#index-HISTFILE"> &para;</a></span></dt>
<dd><p>The name of the file to which the command history is saved.
Bash assigns a default value of <samp class="file">~/.bash_history</samp>.
If <code class="env">HISTFILE</code> is unset or null,
the shell does not save the command history when it exits.
</p>
</dd>
<dt><a id="index-HISTFILESIZE"></a><span><code class="code">HISTFILESIZE</code><a class="copiable-link" href="#index-HISTFILESIZE"> &para;</a></span></dt>
<dd><p>The maximum number of lines contained in the history file.
When this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than
the number of history entries
that total no more than that number of lines
by removing the oldest entries.
If the history list contains multi-line entries,
the history file may contain more lines than this maximum
to avoid leaving partial history entries.
The history file is also truncated to this size after
writing it when a shell exits or by the <code class="code">history</code> builtin.
If the value is 0, the history file is truncated to zero size.
Non-numeric values and numeric values less than zero inhibit truncation.
The shell sets the default value to the value of <code class="env">HISTSIZE</code>
after reading any startup files.
</p>
</dd>
<dt><a id="index-HISTIGNORE"></a><span><code class="code">HISTIGNORE</code><a class="copiable-link" href="#index-HISTIGNORE"> &para;</a></span></dt>
<dd><p>A colon-separated list of patterns used to decide which command
lines should be saved on the history list.
If a command line matches one of the patterns in the value of
<code class="code">HISTIGNORE</code>,
it is not saved on the history list.
Each pattern is anchored at the
beginning of the line and must match the complete line
(Bash does not implicitly append a
&lsquo;<samp class="samp">*</samp>&rsquo;).
Each pattern is tested against the line
after the checks specified by
<code class="env">HISTCONTROL</code>
are applied.
In addition to the normal shell pattern matching characters,
&lsquo;<samp class="samp">&amp;</samp>&rsquo;
matches the previous history line.
A backslash escapes the
&lsquo;<samp class="samp">&amp;</samp>&rsquo;;
the backslash is removed before attempting a match.
If the first line of a multi-line compound command was saved,
the second and subsequent lines are not tested,
and are added to the history regardless of the value of
<code class="env">HISTIGNORE</code>.
If the first line was not saved, the second and subsequent lines of
the command are not saved either.
The pattern matching honors the setting of the <code class="code">extglob</code> shell
option.
</p>
<p><code class="env">HISTIGNORE</code> subsumes some of the function of <code class="env">HISTCONTROL</code>.
A pattern of &lsquo;<samp class="samp">&amp;</samp>&rsquo; is identical to <code class="code">ignoredups</code>, and a
pattern of &lsquo;<samp class="samp">[ ]*</samp>&rsquo; is identical to <code class="code">ignorespace</code>. 
Combining these two patterns, separating them with a colon,
provides the functionality of <code class="code">ignoreboth</code>. 
</p>
</dd>
<dt><a id="index-HISTSIZE"></a><span><code class="code">HISTSIZE</code><a class="copiable-link" href="#index-HISTSIZE"> &para;</a></span></dt>
<dd><p>The maximum number of commands to remember on the history list.
If the value is 0, commands are not saved in the history list.
Numeric values less than zero result in every command being saved
on the history list (there is no limit).
The shell sets the default value to 500 after reading any startup files.
</p>
</dd>
<dt><a id="index-HISTTIMEFORMAT"></a><span><code class="code">HISTTIMEFORMAT</code><a class="copiable-link" href="#index-HISTTIMEFORMAT"> &para;</a></span></dt>
<dd><p>If this variable is set and not null, its value is used as a format string
for <code class="code">strftime</code>(3) to print the time stamp associated with each history
entry displayed by the <code class="code">history</code> builtin.
If this variable is set, the shell writes time stamps to the history file so
they may be preserved across shell sessions.
This uses the history comment character to distinguish timestamps from
other history lines.
</p>
</dd>
<dt><a id="index-HOSTFILE"></a><span><code class="code">HOSTFILE</code><a class="copiable-link" href="#index-HOSTFILE"> &para;</a></span></dt>
<dd><p>Contains the name of a file in the same format as <samp class="file">/etc/hosts</samp> that
should be read when the shell needs to complete a hostname.
The list of possible hostname completions may be changed while the shell
is running;
the next time hostname completion is attempted after the
value is changed, Bash adds the contents of the new file to the
existing list.
If <code class="env">HOSTFILE</code>
is set, but has no value, or does not name a readable file,
Bash attempts to read 
<samp class="file">/etc/hosts</samp>
to obtain the list of possible hostname completions.
When <code class="env">HOSTFILE</code> is unset,
Bash clears the hostname list.
</p>
</dd>
<dt><a id="index-HOSTNAME"></a><span><code class="code">HOSTNAME</code><a class="copiable-link" href="#index-HOSTNAME"> &para;</a></span></dt>
<dd><p>The name of the current host.
</p>
</dd>
<dt><a id="index-HOSTTYPE"></a><span><code class="code">HOSTTYPE</code><a class="copiable-link" href="#index-HOSTTYPE"> &para;</a></span></dt>
<dd><p>A string describing the machine Bash is running on.
</p>
</dd>
<dt><a id="index-IGNOREEOF"></a><span><code class="code">IGNOREEOF</code><a class="copiable-link" href="#index-IGNOREEOF"> &para;</a></span></dt>
<dd><p>Controls the action of the shell on receipt of an <code class="code">EOF</code> character
as the sole input.
If set, the value is the number
of consecutive <code class="code">EOF</code> characters that can be read as the
first character on an input line before Bash exits.
If the variable is set but does not have a numeric value,
or the value is null, then the default is 10.
If the variable is unset, then <code class="code">EOF</code> signifies the end of 
input to the shell.
This is only in effect for interactive shells.
</p>
</dd>
<dt><a id="index-INPUTRC"></a><span><code class="code">INPUTRC</code><a class="copiable-link" href="#index-INPUTRC"> &para;</a></span></dt>
<dd><p>The name of the Readline initialization file, overriding the default
of <samp class="file">~/.inputrc</samp>.
</p>
</dd>
<dt><a id="index-INSIDE_005fEMACS"></a><span><code class="code">INSIDE_EMACS</code><a class="copiable-link" href="#index-INSIDE_005fEMACS"> &para;</a></span></dt>
<dd><p>If Bash finds this variable in the environment when the shell
starts, it assumes that the shell is running in an Emacs shell buffer
and may disable line editing depending on the value of <code class="env">TERM</code>.
</p>
</dd>
<dt><a id="index-LANG-1"></a><span><code class="code">LANG</code><a class="copiable-link" href="#index-LANG-1"> &para;</a></span></dt>
<dd><p>Used to determine the locale category for any category not specifically
selected with a variable starting with <code class="code">LC_</code>.
</p>
</dd>
<dt><a id="index-LC_005fALL"></a><span><code class="code">LC_ALL</code><a class="copiable-link" href="#index-LC_005fALL"> &para;</a></span></dt>
<dd><p>This variable overrides the value of <code class="env">LANG</code> and any other
<code class="code">LC_</code> variable specifying a locale category.
</p>
</dd>
<dt><a id="index-LC_005fCOLLATE"></a><span><code class="code">LC_COLLATE</code><a class="copiable-link" href="#index-LC_005fCOLLATE"> &para;</a></span></dt>
<dd><p>This variable determines the collation order used when sorting the
results of filename expansion, and
determines the behavior of range expressions, equivalence classes,
and collating sequences within filename expansion and pattern matching
(see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>).
</p>
</dd>
<dt><a id="index-LC_005fCTYPE"></a><span><code class="code">LC_CTYPE</code><a class="copiable-link" href="#index-LC_005fCTYPE"> &para;</a></span></dt>
<dd><p>This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>).
</p>
</dd>
<dt><a id="index-LC_005fMESSAGES-1"></a><span><code class="code">LC_MESSAGES</code><a class="copiable-link" href="#index-LC_005fMESSAGES-1"> &para;</a></span></dt>
<dd><p>This variable determines the locale used to translate double-quoted
strings preceded by a &lsquo;<samp class="samp">$</samp>&rsquo; (see <a class="pxref" href="#Locale-Translation">Locale-Specific Translation</a>).
</p>
</dd>
<dt><a id="index-LC_005fNUMERIC"></a><span><code class="code">LC_NUMERIC</code><a class="copiable-link" href="#index-LC_005fNUMERIC"> &para;</a></span></dt>
<dd><p>This variable determines the locale category used for number formatting.
</p>
</dd>
<dt><a id="index-LC_005fTIME"></a><span><code class="code">LC_TIME</code><a class="copiable-link" href="#index-LC_005fTIME"> &para;</a></span></dt>
<dd><p>This variable determines the locale category used for data and time
formatting.
</p>
</dd>
<dt><a id="index-LINENO"></a><span><code class="code">LINENO</code><a class="copiable-link" href="#index-LINENO"> &para;</a></span></dt>
<dd><p>The line number in the script or shell function currently executing.
Line numbers start with 1.
When not in a script or function, the value is not
guaranteed to be meaningful.
If <code class="env">LINENO</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-LINES"></a><span><code class="code">LINES</code><a class="copiable-link" href="#index-LINES"> &para;</a></span></dt>
<dd><p>Used by the <code class="code">select</code> command to determine the column length
for printing selection lists.
Automatically set if the <code class="code">checkwinsize</code> option is enabled
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), or in an interactive shell upon receipt of a
<code class="code">SIGWINCH</code>.
</p>
</dd>
<dt><a id="index-MACHTYPE"></a><span><code class="code">MACHTYPE</code><a class="copiable-link" href="#index-MACHTYPE"> &para;</a></span></dt>
<dd><p>A string that fully describes the system type on which Bash
is executing, in the standard <small class="sc">GNU</small> <var class="var">cpu-company-system</var> format.
</p>
</dd>
<dt><a id="index-MAILCHECK"></a><span><code class="code">MAILCHECK</code><a class="copiable-link" href="#index-MAILCHECK"> &para;</a></span></dt>
<dd><p>How often (in seconds) that the shell should check for mail in the
files specified in the <code class="env">MAILPATH</code> or <code class="env">MAIL</code> variables.
The default is 60 seconds.  When it is time to check
for mail, the shell does so before displaying the primary prompt.
If this variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.
</p>
</dd>
<dt><a id="index-MAPFILE"></a><span><code class="code">MAPFILE</code><a class="copiable-link" href="#index-MAPFILE"> &para;</a></span></dt>
<dd><p>An array variable created to hold the text read by the
<code class="code">mapfile</code> builtin when no variable name is supplied.
</p>
</dd>
<dt><a id="index-OLDPWD"></a><span><code class="code">OLDPWD</code><a class="copiable-link" href="#index-OLDPWD"> &para;</a></span></dt>
<dd><p>The previous working directory as set by the <code class="code">cd</code> builtin.
</p>
</dd>
<dt><a id="index-OPTERR"></a><span><code class="code">OPTERR</code><a class="copiable-link" href="#index-OPTERR"> &para;</a></span></dt>
<dd><p>If set to the value 1, Bash displays error messages
generated by the <code class="code">getopts</code> builtin command.
<code class="env">OPTERR</code>
is initialized to 1 each time the shell is invoked.
</p>
</dd>
<dt><a id="index-OSTYPE"></a><span><code class="code">OSTYPE</code><a class="copiable-link" href="#index-OSTYPE"> &para;</a></span></dt>
<dd><p>A string describing the operating system Bash is running on.
</p>
</dd>
<dt><a id="index-PIPESTATUS"></a><span><code class="code">PIPESTATUS</code><a class="copiable-link" href="#index-PIPESTATUS"> &para;</a></span></dt>
<dd><p>An array variable (see <a class="pxref" href="#Arrays">Arrays</a>)
containing a list of exit status values from the commands
in the most-recently-executed foreground pipeline, which may
consist of only a simple command
(see <a class="pxref" href="#Shell-Commands">Shell Commands</a>).
Bash sets
<code class="code">PIPESTATUS</code>
after executing multi-element pipelines,
timed and negated pipelines,
simple commands,
subshells created with the &lsquo;<samp class="samp">(</samp>&rsquo; operator,
the
<code class="code">[[</code>
and
<code class="code">((</code>
compound commands, and
after error conditions that result in the
shell aborting command execution.
</p>
</dd>
<dt><a id="index-POSIXLY_005fCORRECT"></a><span><code class="code">POSIXLY_CORRECT</code><a class="copiable-link" href="#index-POSIXLY_005fCORRECT"> &para;</a></span></dt>
<dd><p>If this variable is in the environment when Bash starts, the shell
enters <small class="sc">POSIX</small> mode (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>) before reading the
startup files, as if the <samp class="option">--posix</samp> invocation option had been supplied.
If it is set while the shell is running, Bash enables <small class="sc">POSIX</small> mode,
as if the command
</p><div class="example">
<pre class="example-preformatted"><code class="code">set -o posix</code>
</pre></div>
<p>had been executed.
When the shell enters <small class="sc">POSIX</small> mode, it sets this variable if it was
not already set.
</p>
</dd>
<dt><a id="index-PPID"></a><span><code class="code">PPID</code><a class="copiable-link" href="#index-PPID"> &para;</a></span></dt>
<dd><p>The process <small class="sc">ID</small> of the shell&rsquo;s parent process.
This variable is readonly.
</p>
</dd>
<dt><a id="index-PROMPT_005fCOMMAND"></a><span><code class="code">PROMPT_COMMAND</code><a class="copiable-link" href="#index-PROMPT_005fCOMMAND"> &para;</a></span></dt>
<dd><p>If this variable is set, and is an array,
the value of each set element is interpreted as a command to execute
before printing the primary prompt (<code class="env">$PS1</code>).
If this is set but not an array variable,
its value is used as a command to execute instead.
</p>
</dd>
<dt><a id="index-PROMPT_005fDIRTRIM"></a><span><code class="code">PROMPT_DIRTRIM</code><a class="copiable-link" href="#index-PROMPT_005fDIRTRIM"> &para;</a></span></dt>
<dd><p>If set to a number greater than zero, the value is used as the number of
trailing directory components to retain when expanding the <code class="code">\w</code> and
<code class="code">\W</code> prompt string escapes (see <a class="pxref" href="#Controlling-the-Prompt">Controlling the Prompt</a>).
Characters removed are replaced with an ellipsis.
</p>
</dd>
<dt><a id="index-PS0"></a><span><code class="code">PS0</code><a class="copiable-link" href="#index-PS0"> &para;</a></span></dt>
<dd><p>The value of this parameter is expanded like <code class="env">PS1</code>
and displayed by interactive shells after reading a command
and before the command is executed.
</p>
</dd>
<dt><a id="index-PS3"></a><span><code class="code">PS3</code><a class="copiable-link" href="#index-PS3"> &para;</a></span></dt>
<dd><p>The value of this variable is used as the prompt for the
<code class="code">select</code> command.  If this variable is not set, the
<code class="code">select</code> command prompts with &lsquo;<samp class="samp">#? </samp>&rsquo;
</p>
</dd>
<dt><a id="index-PS4"></a><span><code class="code">PS4</code><a class="copiable-link" href="#index-PS4"> &para;</a></span></dt>
<dd><p>The value of this parameter is expanded like <code class="env">PS1</code>
and the expanded value is the prompt printed before the command line
is echoed when the <samp class="option">-x</samp> option is set (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
The first character of the expanded value is replicated multiple times,
as necessary, to indicate multiple levels of indirection.
The default is &lsquo;<samp class="samp">+ </samp>&rsquo;.
</p>
</dd>
<dt><a id="index-PWD"></a><span><code class="code">PWD</code><a class="copiable-link" href="#index-PWD"> &para;</a></span></dt>
<dd><p>The current working directory as set by the <code class="code">cd</code> builtin.
</p>
</dd>
<dt><a id="index-RANDOM"></a><span><code class="code">RANDOM</code><a class="copiable-link" href="#index-RANDOM"> &para;</a></span></dt>
<dd><p>Each time this parameter is referenced, it expands to a random integer
between 0 and 32767.
Assigning a value to
<code class="env">RANDOM</code>
initializes (seeds) the sequence of random numbers.
Seeding the random number generator with the same constant value
produces the same sequence of values.
If <code class="env">RANDOM</code>
is unset, it loses its special properties, even if it is
subsequently reset.
</p>
</dd>
<dt><a id="index-READLINE_005fARGUMENT"></a><span><code class="code">READLINE_ARGUMENT</code><a class="copiable-link" href="#index-READLINE_005fARGUMENT"> &para;</a></span></dt>
<dd><p>Any numeric argument given to a Readline
command that was defined using
&lsquo;<samp class="samp">bind -x</samp>&rsquo; (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>
when it was invoked.
</p>
</dd>
<dt><a id="index-READLINE_005fLINE"></a><span><code class="code">READLINE_LINE</code><a class="copiable-link" href="#index-READLINE_005fLINE"> &para;</a></span></dt>
<dd><p>The contents of the Readline
line buffer, for use with
&lsquo;<samp class="samp">bind -x</samp>&rsquo; (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p>
</dd>
<dt><a id="index-READLINE_005fMARK"></a><span><code class="code">READLINE_MARK</code><a class="copiable-link" href="#index-READLINE_005fMARK"> &para;</a></span></dt>
<dd><p>The position of the <em class="dfn">mark</em> (saved insertion point) in the Readline
line buffer, for use
with &lsquo;<samp class="samp">bind -x</samp>&rsquo; (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
The characters between the insertion point and the mark are often
called the <em class="dfn">region</em>.
</p>
</dd>
<dt><a id="index-READLINE_005fPOINT"></a><span><code class="code">READLINE_POINT</code><a class="copiable-link" href="#index-READLINE_005fPOINT"> &para;</a></span></dt>
<dd><p>The position of the insertion point in the Readline
line buffer, for use with
&lsquo;<samp class="samp">bind -x</samp>&rsquo; (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p>
</dd>
<dt><a id="index-REPLY"></a><span><code class="code">REPLY</code><a class="copiable-link" href="#index-REPLY"> &para;</a></span></dt>
<dd><p>The default variable for the <code class="code">read</code> builtin;
set to the line read when <code class="code">read</code> is not supplied a variable name
argument.
</p>
</dd>
<dt><a id="index-SECONDS"></a><span><code class="code">SECONDS</code><a class="copiable-link" href="#index-SECONDS"> &para;</a></span></dt>
<dd><p>This variable expands to the number of seconds since the shell was started. 
Assignment to this variable resets the count to the value assigned, and the
expanded value becomes the value assigned plus the number of seconds
since the assignment.
The number of seconds at shell invocation and the current time are always
determined by querying the system clock at one-second resolution.
If <code class="env">SECONDS</code>
is unset, it loses its special properties,
even if it is subsequently reset.
</p>
</dd>
<dt><a id="index-SHELL"></a><span><code class="code">SHELL</code><a class="copiable-link" href="#index-SHELL"> &para;</a></span></dt>
<dd><p>This environment variable expands to the full pathname to the shell.
If it is not set when the shell starts,
Bash assigns to it the full pathname of the current user&rsquo;s login shell.
</p>
</dd>
<dt><a id="index-SHELLOPTS"></a><span><code class="code">SHELLOPTS</code><a class="copiable-link" href="#index-SHELLOPTS"> &para;</a></span></dt>
<dd><p>A colon-separated list of enabled shell options.
Each word in the list is a valid argument for the <samp class="option">-o</samp> option
to the <code class="code">set</code> builtin command (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
The options appearing in <code class="env">SHELLOPTS</code> are those reported
as &lsquo;<samp class="samp">on</samp>&rsquo; by &lsquo;<samp class="samp">set -o</samp>&rsquo;.
If this variable is in the environment when Bash
starts up, the shell enables each option in the list before
reading any startup files.
If this variable is exported, child shells will enable each option
in the list.
This variable is readonly.
</p>
</dd>
<dt><a id="index-SHLVL"></a><span><code class="code">SHLVL</code><a class="copiable-link" href="#index-SHLVL"> &para;</a></span></dt>
<dd><p>Incremented by one each time a new instance of Bash is started.
This is intended to be a count of how deeply your Bash shells are nested.
</p>
</dd>
<dt><a id="index-SRANDOM"></a><span><code class="code">SRANDOM</code><a class="copiable-link" href="#index-SRANDOM"> &para;</a></span></dt>
<dd><p>This variable expands to a 32-bit pseudo-random number each time it is
referenced.
The random number generator is not linear on systems that
support <samp class="file">/dev/urandom</samp> or <code class="code">arc4random</code>, so each returned number
has no relationship to the numbers preceding it.
The random number generator cannot be seeded, so assignments to this
variable have no effect.
If <code class="env">SRANDOM</code>
is unset, it loses its special properties,
even if it is subsequently reset.
</p>
</dd>
<dt><a id="index-TIMEFORMAT"></a><span><code class="code">TIMEFORMAT</code><a class="copiable-link" href="#index-TIMEFORMAT"> &para;</a></span></dt>
<dd><p>The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the <code class="code">time</code>
reserved word should be displayed.
The &lsquo;<samp class="samp">%</samp>&rsquo; character introduces an
escape sequence that is expanded to a time value or other
information.
The escape sequences and their meanings are as
follows; the brackets denote optional portions. 
</p>
<dl class="table">
<dt><code class="code">%%</code></dt>
<dd><p>A literal &lsquo;<samp class="samp">%</samp>&rsquo;.
</p>
</dd>
<dt><code class="code">%[<var class="var">p</var>][l]R</code></dt>
<dd><p>The elapsed time in seconds. 
</p>
</dd>
<dt><code class="code">%[<var class="var">p</var>][l]U</code></dt>
<dd><p>The number of CPU seconds spent in user mode. 
</p>
</dd>
<dt><code class="code">%[<var class="var">p</var>][l]S</code></dt>
<dd><p>The number of CPU seconds spent in system mode. 
</p>
</dd>
<dt><code class="code">%P</code></dt>
<dd><p>The CPU percentage, computed as (%U + %S) / %R. 
</p></dd>
</dl>

<p>The optional <var class="var">p</var> is a digit specifying the precision, the number of
fractional digits after a decimal point.
A value of 0 causes no decimal point or fraction to be output.
<code class="code">time</code> prints at most six digits after the decimal point;
values of <var class="var">p</var> greater than 6 are changed to 6.
If <var class="var">p</var> is not specified,
<code class="code">time</code> prints three digits after the decimal point.
</p>
<p>The optional <code class="code">l</code> specifies a longer format, including minutes, of
the form <var class="var">MM</var>m<var class="var">SS</var>.<var class="var">FF</var>s.
The value of <var class="var">p</var> determines whether or not the fraction is included. 
</p>
<p>If this variable is not set, Bash acts as if it had the value
</p><div class="example">
<pre class="example-preformatted"><code class="code">$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'</code>
</pre></div>
<p>If the value is null, Bash does not display any timing information.
A trailing newline is added when the format string is displayed.
</p>
</dd>
<dt><a id="index-TMOUT"></a><span><code class="code">TMOUT</code><a class="copiable-link" href="#index-TMOUT"> &para;</a></span></dt>
<dd><p>If set to a value greater than zero, the <code class="code">read</code> builtin uses the
value as its
default timeout (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
The <code class="code">select</code> command (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>) terminates
if input does not arrive after <code class="code">TMOUT</code> seconds when input is coming
from a terminal.
</p>
<p>In an interactive shell, the value is interpreted as
the number of seconds to wait for a line of input after issuing
the primary prompt.
Bash terminates after waiting for that number of seconds if a
complete line of input does not arrive.
</p>
</dd>
<dt><a id="index-TMPDIR"></a><span><code class="code">TMPDIR</code><a class="copiable-link" href="#index-TMPDIR"> &para;</a></span></dt>
<dd><p>If set, Bash uses its value as the name of a directory in which
Bash creates temporary files for the shell&rsquo;s use.
</p>
</dd>
<dt><a id="index-UID"></a><span><code class="code">UID</code><a class="copiable-link" href="#index-UID"> &para;</a></span></dt>
<dd><p>The numeric real user id of the current user.  This variable is readonly.
</p>
</dd>
</dl>

<hr>
</div>
</div>
<div class="chapter-level-extent" id="Bash-Features">
<div class="nav-panel">
<p>
Next: <a href="#Job-Control" accesskey="n" rel="next">Job Control</a>, Previous: <a href="#Shell-Variables" accesskey="p" rel="prev">Shell Variables</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Bash-Features-2"><span>6 Bash Features<a class="copiable-link" href="#Bash-Features-2"> &para;</a></span></h2>

<p>This chapter describes features unique to Bash.
</p>

<ul class="mini-toc">
<li><a href="#Invoking-Bash" accesskey="1">Invoking Bash</a></li>
<li><a href="#Bash-Startup-Files" accesskey="2">Bash Startup Files</a></li>
<li><a href="#Interactive-Shells" accesskey="3">Interactive Shells</a></li>
<li><a href="#Bash-Conditional-Expressions" accesskey="4">Bash Conditional Expressions</a></li>
<li><a href="#Shell-Arithmetic" accesskey="5">Shell Arithmetic</a></li>
<li><a href="#Aliases" accesskey="6">Aliases</a></li>
<li><a href="#Arrays" accesskey="7">Arrays</a></li>
<li><a href="#The-Directory-Stack" accesskey="8">The Directory Stack</a></li>
<li><a href="#Controlling-the-Prompt" accesskey="9">Controlling the Prompt</a></li>
<li><a href="#The-Restricted-Shell">The Restricted Shell</a></li>
<li><a href="#Bash-POSIX-Mode">Bash and POSIX</a></li>
<li><a href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Invoking-Bash">
<div class="nav-panel">
<p>
Next: <a href="#Bash-Startup-Files" accesskey="n" rel="next">Bash Startup Files</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Invoking-Bash-1"><span>6.1 Invoking Bash<a class="copiable-link" href="#Invoking-Bash-1"> &para;</a></span></h3>

<div class="example">
<pre class="example-preformatted">bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o <var class="var">option</var>]
    [-O <var class="var">shopt_option</var>] [<var class="var">argument</var> ...]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o <var class="var">option</var>]
    [-O <var class="var">shopt_option</var>] -c <var class="var">string</var> [<var class="var">argument</var> ...]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o <var class="var">option</var>]
    [-O <var class="var">shopt_option</var>] [<var class="var">argument</var> ...]
</pre></div>

<p>All of the single-character options used with the <code class="code">set</code> builtin
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>) can be used as options when the shell is invoked.
In addition, there are several multi-character
options that you can use.
These options must appear on the command
line before the single-character options to be recognized. 
</p>
<dl class="table">
<dt><code class="code">--debugger</code></dt>
<dd><p>Arrange for the debugger profile to be executed before the shell starts.
Turns on extended debugging mode (see <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>
for a description of the <code class="code">extdebug</code> option to the <code class="code">shopt</code>
builtin).
</p>
</dd>
<dt><code class="code">--dump-po-strings</code></dt>
<dd><p>Print a list of all double-quoted strings preceded by &lsquo;<samp class="samp">$</samp>&rsquo;
on the standard output
in the <small class="sc">GNU</small> <code class="code">gettext</code> PO (portable object) file format.
Equivalent to <samp class="option">-D</samp> except for the output format.
</p>
</dd>
<dt><code class="code">--dump-strings</code></dt>
<dd><p>Equivalent to <samp class="option">-D</samp>.
</p>
</dd>
<dt><code class="code">--help</code></dt>
<dd><p>Display a usage message on standard output and exit successfully.
</p>
</dd>
<dt><code class="code">--init-file <var class="var">filename</var></code></dt>
<dt><code class="code">--rcfile <var class="var">filename</var></code></dt>
<dd><p>Execute commands from <var class="var">filename</var> (instead of <samp class="file">~/.bashrc</samp>)
in an interactive shell.
</p>
</dd>
<dt><code class="code">--login</code></dt>
<dd><p>Equivalent to <samp class="option">-l</samp>.
</p>
</dd>
<dt><code class="code">--noediting</code></dt>
<dd><p>Do not use the <small class="sc">GNU</small> Readline library (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>)
to read  command lines when the shell is interactive.
</p>
</dd>
<dt><code class="code">--noprofile</code></dt>
<dd><p>Don&rsquo;t load the system-wide startup file <samp class="file">/etc/profile</samp>
or any of the personal initialization files
<samp class="file">~/.bash_profile</samp>, <samp class="file">~/.bash_login</samp>, or <samp class="file">~/.profile</samp>
when Bash is invoked as a login shell.
</p>
</dd>
<dt><code class="code">--norc</code></dt>
<dd><p>Don&rsquo;t read the <samp class="file">~/.bashrc</samp> initialization file in an
interactive shell.
This is on by default if the shell is invoked as <code class="code">sh</code>.
</p>
</dd>
<dt><code class="code">--posix</code></dt>
<dd><p>Enable <small class="sc">POSIX</small> mode;
change the behavior of Bash
where the default operation differs from the
<small class="sc">POSIX</small>
standard to match the standard.
This is intended to make Bash behave as a strict superset of that
standard.
See <a class="xref" href="#Bash-POSIX-Mode">Bash and POSIX</a>, for a description of the Bash <small class="sc">POSIX</small> mode.
</p>
</dd>
<dt><code class="code">--restricted</code></dt>
<dd><p>Equivalent to <samp class="option">-r</samp>.
Make the shell a restricted shell (see <a class="pxref" href="#The-Restricted-Shell">The Restricted Shell</a>).
</p>
</dd>
<dt><code class="code">--verbose</code></dt>
<dd><p>Equivalent to <samp class="option">-v</samp>.
Print shell input lines as they&rsquo;re read.
</p>
</dd>
<dt><code class="code">--version</code></dt>
<dd><p>Show version information for this instance of
Bash on the standard output and exit successfully.
</p></dd>
</dl>

<p>There are several single-character options that may be supplied at
invocation which are not available with the <code class="code">set</code> builtin.
</p>
<dl class="table">
<dt><code class="code">-c</code></dt>
<dd><p>Read and execute commands from the first non-option argument
<var class="var">command_string</var>, then exit. 
If there are arguments after the <var class="var">command_string</var>,
the first argument is assigned to <code class="code">$0</code>
and any remaining arguments are assigned to the positional parameters.
The assignment to <code class="code">$0</code> sets the name of the shell, which is used
in warning and error messages.
</p>
</dd>
<dt><code class="code">-i</code></dt>
<dd><p>Force the shell to run interactively.
Interactive shells are described in <a class="ref" href="#Interactive-Shells">Interactive Shells</a>.
</p>
</dd>
<dt><code class="code">-l</code></dt>
<dd><p>Make this shell act as if it had been directly invoked by login.
When the shell is interactive, this is equivalent to starting a
login shell with &lsquo;<samp class="samp">exec -l bash</samp>&rsquo;.
When the shell is not interactive, it will read and execute the login
shell startup files.
&lsquo;<samp class="samp">exec bash -l</samp>&rsquo; or &lsquo;<samp class="samp">exec bash --login</samp>&rsquo;
will replace the current shell with a Bash login shell.
See <a class="xref" href="#Bash-Startup-Files">Bash Startup Files</a>, for a description of the special behavior
of a login shell.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>Make the shell a restricted shell (see <a class="pxref" href="#The-Restricted-Shell">The Restricted Shell</a>).
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>If this option is present, or if no arguments remain after option
processing, then Bash reads commands from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell or when reading input
through a pipe.
</p>
</dd>
<dt><code class="code">-D</code></dt>
<dd><p>Print a list of all double-quoted strings preceded by &lsquo;<samp class="samp">$</samp>&rsquo;
on the standard output.
These are the strings that
are subject to language translation when the current locale
is not <code class="code">C</code> or <code class="code">POSIX</code> (see <a class="pxref" href="#Locale-Translation">Locale-Specific Translation</a>).
This implies the <samp class="option">-n</samp> option; no commands will be executed.
</p>
</dd>
<dt><code class="code">[-+]O [<var class="var">shopt_option</var>]</code></dt>
<dd><p><var class="var">shopt_option</var> is one of the shell options accepted by the
<code class="code">shopt</code> builtin (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
If <var class="var">shopt_option</var> is present, <samp class="option">-O</samp> sets the value of that option;
<samp class="option">+O</samp> unsets it.  
If <var class="var">shopt_option</var> is not supplied, Bash
prints the names and values of the shell
options accepted by <code class="code">shopt</code> on the standard output.
If the invocation option is <samp class="option">+O</samp>, the output is displayed in a format
that may be reused as input.
</p>
</dd>
<dt><code class="code">--</code></dt>
<dd><p>A <code class="code">--</code> signals the end of options and disables further option
processing.
Any arguments after the <code class="code">--</code>
are treated as a shell script filename (see <a class="pxref" href="#Shell-Scripts">Shell Scripts</a>)
and arguments passed to that script.
</p>
</dd>
<dt><code class="code">-</code></dt>
<dd><p>Equivalent to <code class="code">--</code>.
</p>
</dd>
</dl>

<a class="index-entry-id" id="index-login-shell"></a>
<p>A <em class="dfn">login shell</em> is one whose first character of argument zero is
&lsquo;<samp class="samp">-</samp>&rsquo;, or one invoked with the <samp class="option">--login</samp> option.
</p>
<a class="index-entry-id" id="index-interactive-shell"></a>
<p>An <em class="dfn">interactive shell</em> is one started without non-option arguments,
unless <samp class="option">-s</samp> is specified,
without specifying the <samp class="option">-c</samp> option,
and whose standard input and standard error are both
connected to terminals (as determined by
<i class="i">isatty(3)</i>),
or one started with the <samp class="option">-i</samp> option.
See <a class="xref" href="#Interactive-Shells">Interactive Shells</a>, for more information.
</p>
<p>If arguments remain after option processing, and neither the
<samp class="option">-c</samp> nor the <samp class="option">-s</samp>
option has been supplied, the first argument is treated as
the name of a file containing shell commands (see <a class="pxref" href="#Shell-Scripts">Shell Scripts</a>).
When Bash is invoked in this fashion, <code class="code">$0</code>
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.   
Bash&rsquo;s exit status is the exit status of the last command executed
in the script.
If no commands are executed, the exit status is 0.
Bash
first attempts to open the file in the current directory,
and, if no file is found, searches the directories in
<code class="env">PATH</code>
for the script.
</p>
<hr>
</div>
<div class="section-level-extent" id="Bash-Startup-Files">
<div class="nav-panel">
<p>
Next: <a href="#Interactive-Shells" accesskey="n" rel="next">Interactive Shells</a>, Previous: <a href="#Invoking-Bash" accesskey="p" rel="prev">Invoking Bash</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-Startup-Files-1"><span>6.2 Bash Startup Files<a class="copiable-link" href="#Bash-Startup-Files-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-startup-files"></a>

<p>This section describes how Bash executes its startup files.
If any of the files exist but cannot be read, Bash reports an error.
Tildes are expanded in filenames as described above under
Tilde Expansion (see <a class="pxref" href="#Tilde-Expansion">Tilde Expansion</a>).
</p>
<p>Interactive shells are described in <a class="ref" href="#Interactive-Shells">Interactive Shells</a>.
</p>
<h4 class="subsubheading" id="Invoked-as-an-interactive-login-shell_002c-or-with-_002d_002dlogin"><span>Invoked as an interactive login shell, or with <samp class="option">--login</samp><a class="copiable-link" href="#Invoked-as-an-interactive-login-shell_002c-or-with-_002d_002dlogin"> &para;</a></span></h4>

<p>When Bash is invoked as an interactive login shell, or as a
non-interactive shell with the <samp class="option">--login</samp> option, it first reads and
executes commands from the file <samp class="file">/etc/profile</samp>, if that file exists.
After reading that file, it looks for <samp class="file">~/.bash_profile</samp>,
<samp class="file">~/.bash_login</samp>, and <samp class="file">~/.profile</samp>, in that order, and reads
and executes commands from the first one that exists and is readable.
The
<samp class="option">--noprofile</samp>
option inhibits this behavior.
</p>
<p>When an interactive login shell exits,
or a non-interactive login shell executes the <code class="code">exit</code> builtin command,
Bash reads and executes commands from
the file <samp class="file">~/.bash_logout</samp>, if it exists.
</p>
<h4 class="subsubheading" id="Invoked-as-an-interactive-non_002dlogin-shell"><span>Invoked as an interactive non-login shell<a class="copiable-link" href="#Invoked-as-an-interactive-non_002dlogin-shell"> &para;</a></span></h4>

<p>When Bash runs as an interactive shell that is not a login shell, it
reads and executes commands from <samp class="file">~/.bashrc</samp>, if that file exists.
The <samp class="option">--norc</samp> option inhibits this behavior.
The <samp class="option">--rcfile <var class="var">file</var></samp> option
causes Bash to
use <var class="var">file</var> instead of <samp class="file">~/.bashrc</samp>.
</p>
<p>So, typically, your <samp class="file">~/.bash_profile</samp> contains the line
</p><div class="example">
<pre class="example-preformatted"><code class="code">if [ -f ~/.bashrc ]; then . ~/.bashrc; fi</code>
</pre></div>
<p>after (or before) any login-specific initializations.
</p>
<h4 class="subsubheading" id="Invoked-non_002dinteractively"><span>Invoked non-interactively<a class="copiable-link" href="#Invoked-non_002dinteractively"> &para;</a></span></h4>

<p>When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable <code class="env">BASH_ENV</code> in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute.
Bash behaves as if the following command were executed:
</p><div class="example">
<pre class="example-preformatted"><code class="code">if [ -n &quot;$BASH_ENV&quot; ]; then . &quot;$BASH_ENV&quot;; fi</code>
</pre></div>
<p>but does not the value of the <code class="env">PATH</code> variable to search for the
filename.
</p>
<p>As noted above, if a non-interactive shell is invoked with the
<samp class="option">--login</samp> option,
Bash attempts to read and execute commands from the
login shell startup files. 
</p>
<h4 class="subsubheading" id="Invoked-with-name-sh"><span>Invoked with name <code class="code">sh</code><a class="copiable-link" href="#Invoked-with-name-sh"> &para;</a></span></h4>

<p>If Bash is invoked with the name <code class="command">sh</code>,
it tries to mimic the startup behavior of historical versions of
<code class="command">sh</code> as closely as possible,
while conforming to the
<small class="sc">POSIX</small>
standard as well.
</p>
<p>When invoked as an interactive login shell, or as a non-interactive
shell with the <samp class="option">--login</samp> option, it first attempts to read
and execute commands from
<samp class="file">/etc/profile</samp> and <samp class="file">~/.profile</samp>,
in that order.
The <samp class="option">--noprofile</samp> option inhibits this behavior.
</p>
<p>When invoked as an interactive shell with the name <code class="command">sh</code>, Bash
looks for the variable <code class="env">ENV</code>,
expands its value if it is defined, and uses the
expanded value as the name of a file to read and execute.
Since a shell invoked as <code class="command">sh</code>
does not attempt to read and execute commands from any other startup
files, the
<samp class="option">--rcfile</samp>
option has no effect.
</p>
<p>A non-interactive shell invoked with the name <code class="command">sh</code>
does not attempt to read any other startup files.
</p>
<p>When invoked as <code class="command">sh</code>, Bash enters <small class="sc">POSIX</small> mode after reading
the startup files.
</p>
<h4 class="subsubheading" id="Invoked-in-posix-mode"><span>Invoked in <small class="sc">POSIX</small> mode<a class="copiable-link" href="#Invoked-in-posix-mode"> &para;</a></span></h4>

<p>When Bash is started in <small class="sc">POSIX</small> mode, as with the
<samp class="option">--posix</samp> command line option, it follows the
<small class="sc">POSIX</small>
standard for startup files.
In this mode, interactive shells expand the <code class="env">ENV</code> variable
and read and execute commands from the file whose name is the
expanded value.
No other startup files are read.
</p>
<h4 class="subsubheading" id="Invoked-by-remote-shell-daemon"><span>Invoked by remote shell daemon<a class="copiable-link" href="#Invoked-by-remote-shell-daemon"> &para;</a></span></h4>

<p>Bash
attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by
the historical and rarely-seen remote shell daemon, usually <code class="code">rshd</code>,
or the secure shell daemon <code class="code">sshd</code>.
If
Bash
determines it is being run non-interactively in this fashion,
it reads and executes commands from
<samp class="file">~/.bashrc</samp>,
if that file exists and is readable.
Bash does not read this file if invoked as <code class="code">sh</code>.
The
<samp class="option">--norc</samp>
option inhibits this behavior, and the
<samp class="option">--rcfile</samp>
option makes Bash use a different file instead of
<samp class="file">~/.bashrc</samp>,
but neither
<code class="code">rshd</code> nor <code class="code">sshd</code> generally invoke the shell with those
options or allow them to be specified.
</p>
<h4 class="subsubheading" id="Invoked-with-unequal-effective-and-real-uid_002fgids"><span>Invoked with unequal effective and real <small class="sc">UID/GID</small>s<a class="copiable-link" href="#Invoked-with-unequal-effective-and-real-uid_002fgids"> &para;</a></span></h4>

<p>If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the <samp class="option">-p</samp> option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the <code class="env">SHELLOPTS</code>, <code class="env">BASHOPTS</code>, <code class="env">CDPATH</code>, and <code class="env">GLOBIGNORE</code>
variables, if they appear in the environment, are ignored, and the effective
user id is set to the real user id.
If the <samp class="option">-p</samp> option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.
</p>
<hr>
</div>
<div class="section-level-extent" id="Interactive-Shells">
<div class="nav-panel">
<p>
Next: <a href="#Bash-Conditional-Expressions" accesskey="n" rel="next">Bash Conditional Expressions</a>, Previous: <a href="#Bash-Startup-Files" accesskey="p" rel="prev">Bash Startup Files</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Interactive-Shells-1"><span>6.3 Interactive Shells<a class="copiable-link" href="#Interactive-Shells-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-interactive-shell-1"></a>
<a class="index-entry-id" id="index-shell_002c-interactive"></a>


<ul class="mini-toc">
<li><a href="#What-is-an-Interactive-Shell_003f" accesskey="1">What is an Interactive Shell?</a></li>
<li><a href="#Is-this-Shell-Interactive_003f" accesskey="2">Is this Shell Interactive?</a></li>
<li><a href="#Interactive-Shell-Behavior" accesskey="3">Interactive Shell Behavior</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="What-is-an-Interactive-Shell_003f">
<div class="nav-panel">
<p>
Next: <a href="#Is-this-Shell-Interactive_003f" accesskey="n" rel="next">Is this Shell Interactive?</a>, Up: <a href="#Interactive-Shells" accesskey="u" rel="up">Interactive Shells</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="What-is-an-Interactive-Shell_003f-1"><span>6.3.1 What is an Interactive Shell?<a class="copiable-link" href="#What-is-an-Interactive-Shell_003f-1"> &para;</a></span></h4>

<p>An interactive shell
is one started without non-option arguments
(unless <samp class="option">-s</samp> is specified)
and without specifying the <samp class="option">-c</samp> option,
whose input and error output are both
connected to terminals (as determined by <code class="code">isatty(3)</code>),
or one started with the <samp class="option">-i</samp> option.
</p>
<p>An interactive shell generally reads from and writes to a user&rsquo;s
terminal.
</p>
<p>The <samp class="option">-s</samp> invocation option may be used to set the positional parameters
when an interactive shell starts.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Is-this-Shell-Interactive_003f">
<div class="nav-panel">
<p>
Next: <a href="#Interactive-Shell-Behavior" accesskey="n" rel="next">Interactive Shell Behavior</a>, Previous: <a href="#What-is-an-Interactive-Shell_003f" accesskey="p" rel="prev">What is an Interactive Shell?</a>, Up: <a href="#Interactive-Shells" accesskey="u" rel="up">Interactive Shells</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Is-this-Shell-Interactive_003f-1"><span>6.3.2 Is this Shell Interactive?<a class="copiable-link" href="#Is-this-Shell-Interactive_003f-1"> &para;</a></span></h4>

<p>To determine within a startup script whether or not Bash is
running interactively,
test the value of the &lsquo;<samp class="samp">-</samp>&rsquo; special parameter.
It contains <code class="code">i</code> when the shell is interactive.
For example:
</p>
<div class="example">
<pre class="example-preformatted">case &quot;$-&quot; in
*i*)	echo This shell is interactive ;;
*)	echo This shell is not interactive ;;
esac
</pre></div>

<p>Alternatively, startup scripts may examine the variable
<code class="env">PS1</code>; it is unset in non-interactive shells, and set in
interactive shells.
Thus:
</p>
<div class="example">
<pre class="example-preformatted">if [ -z &quot;$PS1&quot; ]; then
        echo This shell is not interactive
else
        echo This shell is interactive
fi
</pre></div>

<hr>
</div>
<div class="subsection-level-extent" id="Interactive-Shell-Behavior">
<div class="nav-panel">
<p>
Previous: <a href="#Is-this-Shell-Interactive_003f" accesskey="p" rel="prev">Is this Shell Interactive?</a>, Up: <a href="#Interactive-Shells" accesskey="u" rel="up">Interactive Shells</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Interactive-Shell-Behavior-1"><span>6.3.3 Interactive Shell Behavior<a class="copiable-link" href="#Interactive-Shell-Behavior-1"> &para;</a></span></h4>

<p>When the shell is running interactively, it changes its behavior in
several ways.
</p>
<ol class="enumerate">
<li> Bash reads and executes startup files as described in <a class="ref" href="#Bash-Startup-Files">Bash Startup Files</a>.

</li><li> Job Control (see <a class="pxref" href="#Job-Control">Job Control</a>) is enabled by default.
When job control is in effect, Bash ignores the keyboard-generated
job control signals <code class="code">SIGTTIN</code>, <code class="code">SIGTTOU</code>, and <code class="code">SIGTSTP</code>.

</li><li> Bash executes the values of the set elements of the <code class="env">PROMPT_COMMAND</code>
array variable as commands before printing the primary prompt, <code class="env">$PS1</code>
(see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).

</li><li> Bash expands and displays <code class="env">PS1</code> before reading the first line
of a command, and expands and displays <code class="env">PS2</code> before reading the
second and subsequent lines of a multi-line command.
Bash expands and displays <code class="env">PS0</code> after it reads a command but before
executing it.
See <a class="ref" href="#Controlling-the-Prompt">Controlling the Prompt</a>, for a complete list of prompt
string escape sequences.

</li><li> Bash uses Readline (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>) to read commands from
the user&rsquo;s terminal.

</li><li> Bash inspects the value of the <code class="code">ignoreeof</code> option to <code class="code">set -o</code>
instead of exiting immediately when it receives an <code class="code">EOF</code> on its
standard input when reading a command (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li> Bash enables Command history (see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>)
and history expansion (see <a class="pxref" href="#History-Interaction">History Expansion</a>)
by default.
When a shell with history enabled exits,
Bash saves the command history to the file named by <code class="env">$HISTFILE</code>.

</li><li> Alias expansion (see <a class="pxref" href="#Aliases">Aliases</a>) is performed by default.

</li><li> In the absence of any traps, Bash ignores <code class="code">SIGTERM</code>
(see <a class="pxref" href="#Signals">Signals</a>).

</li><li> In the absence of any traps, <code class="code">SIGINT</code> is caught and handled
(see <a class="pxref" href="#Signals">Signals</a>).
<code class="code">SIGINT</code> will interrupt some shell builtins.

</li><li> An interactive login shell sends a <code class="code">SIGHUP</code> to all jobs on exit
if the <code class="code">huponexit</code> shell option has been enabled (see <a class="pxref" href="#Signals">Signals</a>).

</li><li> The <samp class="option">-n</samp> option has no effect, whether at invocation or when
using &lsquo;<samp class="samp">set -n</samp>&rsquo; (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li> Bash will check for mail periodically, depending on the values of the
<code class="env">MAIL</code>, <code class="env">MAILPATH</code>, and <code class="env">MAILCHECK</code> shell variables
(see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).

</li><li> The shell will not exit on
expansion errors due to references to unbound shell variables after
&lsquo;<samp class="samp">set -u</samp>&rsquo; has been enabled (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li> The shell will not exit on
expansion errors caused by <var class="var">var</var> being unset or null in
<code class="code">${<var class="var">var</var>:?<var class="var">word</var>}</code> expansions
(see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li> Redirection errors encountered by shell builtins will not cause the
shell to exit.

</li><li> When running in <small class="sc">POSIX</small> mode, a special builtin returning an error
status will not cause the shell to exit (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>).

</li><li> A failed <code class="code">exec</code> will not cause the shell to exit
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).

</li><li> Parser syntax errors will not cause the shell to exit.

</li><li> If the <code class="code">cdspell</code> shell option is enabled, the shell will attempt
simple spelling correction for directory arguments to the <code class="code">cd</code>
builtin (see the description of the <code class="code">cdspell</code>
option to the <code class="code">shopt</code> builtin in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
The <code class="code">cdspell</code> option is only effective in interactive shells.

</li><li> The shell will check the value of the <code class="env">TMOUT</code> variable and exit
if a command is not read within the specified number of seconds after
printing <code class="env">$PS1</code> (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).

</li></ol>

<hr>
</div>
</div>
<div class="section-level-extent" id="Bash-Conditional-Expressions">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Arithmetic" accesskey="n" rel="next">Shell Arithmetic</a>, Previous: <a href="#Interactive-Shells" accesskey="p" rel="prev">Interactive Shells</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-Conditional-Expressions-1"><span>6.4 Bash Conditional Expressions<a class="copiable-link" href="#Bash-Conditional-Expressions-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-expressions_002c-conditional"></a>

<p>Conditional expressions are used by the <code class="code">[[</code> compound command
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>)
and the <code class="code">test</code> and <code class="code">[</code> builtin commands
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).
The <code class="code">test</code>
and <code class="code">[</code> commands determine their behavior based on the number
of arguments; see the descriptions of those commands for any other
command-specific actions.
</p>
<p>Expressions may be unary or binary,
and are formed from the primaries listed below.
Unary expressions are often used to examine the status of a file
or shell variable.
Binary operators are used for string, numeric, and file attribute
comparisons.
</p>
<p>Bash handles several filenames specially when they are used in
expressions.
If the operating system on which Bash is running provides these
special files, Bash uses them; otherwise it emulates them
internally with this behavior:
If the <var class="var">file</var> argument to one of the primaries is of the form
<samp class="file">/dev/fd/<var class="var">N</var></samp>, then Bash checks file descriptor <var class="var">N</var>.
If the <var class="var">file</var> argument to one of the primaries is one of
<samp class="file">/dev/stdin</samp>, <samp class="file">/dev/stdout</samp>, or <samp class="file">/dev/stderr</samp>,
Bash checks file descriptor 0, 1, or 2, respectively.
</p>
<p>When used with <code class="code">[[</code>, the &lsquo;<samp class="samp">&lt;</samp>&rsquo; and &lsquo;<samp class="samp">&gt;</samp>&rsquo; operators sort
lexicographically using the current locale.
The <code class="code">test</code> command uses ASCII ordering.
</p>
<p>Unless otherwise specified, primaries that operate on files follow symbolic
links and operate on the target of the link, rather than the link itself.
</p>
<dl class="table">
<dt><code class="code">-a <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists.
</p>
</dd>
<dt><code class="code">-b <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a block special file.
</p>
</dd>
<dt><code class="code">-c <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a character special file.
</p>
</dd>
<dt><code class="code">-d <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a directory.
</p>
</dd>
<dt><code class="code">-e <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists.
</p>
</dd>
<dt><code class="code">-f <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a regular file.
</p>
</dd>
<dt><code class="code">-g <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and its set-group-id bit is set.
</p>
</dd>
<dt><code class="code">-h <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a symbolic link.
</p>
</dd>
<dt><code class="code">-k <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and its &quot;sticky&quot; bit is set.
</p>
</dd>
<dt><code class="code">-p <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a named pipe (FIFO).
</p>
</dd>
<dt><code class="code">-r <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is readable.
</p>
</dd>
<dt><code class="code">-s <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and has a size greater than zero.
</p>
</dd>
<dt><code class="code">-t <var class="var">fd</var></code></dt>
<dd><p>True if file descriptor <var class="var">fd</var> is open and refers to a terminal.
</p>
</dd>
<dt><code class="code">-u <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and its set-user-id bit is set.
</p>
</dd>
<dt><code class="code">-w <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is writable.
</p>
</dd>
<dt><code class="code">-x <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is executable.
</p>
</dd>
<dt><code class="code">-G <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is owned by the effective group id.
</p>
</dd>
<dt><code class="code">-L <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a symbolic link.
</p>
</dd>
<dt><code class="code">-N <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and has been modified since it was last
accessed.
</p>
</dd>
<dt><code class="code">-O <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is owned by the effective user id.
</p>
</dd>
<dt><code class="code">-S <var class="var">file</var></code></dt>
<dd><p>True if <var class="var">file</var> exists and is a socket.
</p>
</dd>
<dt><code class="code"><var class="var">file1</var> -ef <var class="var">file2</var></code></dt>
<dd><p>True if <var class="var">file1</var> and <var class="var">file2</var> refer to the same device and
inode numbers.
</p>
</dd>
<dt><code class="code"><var class="var">file1</var> -nt <var class="var">file2</var></code></dt>
<dd><p>True if <var class="var">file1</var> is newer (according to modification date)
than <var class="var">file2</var>, or if <var class="var">file1</var> exists and <var class="var">file2</var> does not.
</p>
</dd>
<dt><code class="code"><var class="var">file1</var> -ot <var class="var">file2</var></code></dt>
<dd><p>True if <var class="var">file1</var> is older than <var class="var">file2</var>,
or if <var class="var">file2</var> exists and <var class="var">file1</var> does not.
</p>
</dd>
<dt><code class="code">-o <var class="var">optname</var></code></dt>
<dd><p>True if the shell option <var class="var">optname</var> is enabled.
The list of options appears in the description of the <samp class="option">-o</samp>
option to the <code class="code">set</code> builtin (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
</p>
</dd>
<dt><code class="code">-v <var class="var">varname</var></code></dt>
<dd><p>True if the shell variable <var class="var">varname</var> is set (has been assigned a value).
If <var class="var">varname</var> is an indexed
array variable name subscripted by &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
this returns true if the array has any set elements.
If <var class="var">varname</var> is an associative
array variable name subscripted by &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
this returns true if an element with that key is set.
</p>
</dd>
<dt><code class="code">-R <var class="var">varname</var></code></dt>
<dd><p>True if the shell variable <var class="var">varname</var> is set and is a name reference.
</p>
</dd>
<dt><code class="code">-z <var class="var">string</var></code></dt>
<dd><p>True if the length of <var class="var">string</var> is zero.
</p>
</dd>
<dt><code class="code">-n <var class="var">string</var></code></dt>
<dt><code class="code"><var class="var">string</var></code></dt>
<dd><p>True if the length of <var class="var">string</var> is non-zero.
</p>
</dd>
<dt><code class="code"><var class="var">string1</var> == <var class="var">string2</var></code></dt>
<dt><code class="code"><var class="var">string1</var> = <var class="var">string2</var></code></dt>
<dd><p>True if the strings are equal.
When used with the <code class="code">[[</code> command, this performs pattern matching as
described above (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
</p>
<p>&lsquo;<samp class="samp">=</samp>&rsquo; should be used with the <code class="code">test</code> command for
<small class="sc">POSIX</small>
conformance.
</p>
</dd>
<dt><code class="code"><var class="var">string1</var> != <var class="var">string2</var></code></dt>
<dd><p>True if the strings are not equal.
</p>
</dd>
<dt><code class="code"><var class="var">string1</var> &lt; <var class="var">string2</var></code></dt>
<dd><p>True if <var class="var">string1</var> sorts before <var class="var">string2</var> lexicographically.
</p>
</dd>
<dt><code class="code"><var class="var">string1</var> &gt; <var class="var">string2</var></code></dt>
<dd><p>True if <var class="var">string1</var> sorts after <var class="var">string2</var> lexicographically.
</p>
</dd>
<dt><code class="code"><var class="var">arg1</var> OP <var class="var">arg2</var></code></dt>
<dd><p><code class="code">OP</code> is one of 
&lsquo;<samp class="samp">-eq</samp>&rsquo;, &lsquo;<samp class="samp">-ne</samp>&rsquo;, &lsquo;<samp class="samp">-lt</samp>&rsquo;, &lsquo;<samp class="samp">-le</samp>&rsquo;, &lsquo;<samp class="samp">-gt</samp>&rsquo;, or &lsquo;<samp class="samp">-ge</samp>&rsquo;.
These arithmetic binary operators return true if <var class="var">arg1</var>
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to <var class="var">arg2</var>,
respectively.  <var class="var">Arg1</var> and <var class="var">arg2</var>
may be positive or negative integers.
When used with the <code class="code">[[</code> command, <var class="var">arg1</var> and <var class="var">arg2</var>
are evaluated as arithmetic expressions (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).
Since the expansions the <code class="code">[[</code> command performs on
<var class="var">arg1</var> and <var class="var">arg2</var>
can potentially result in empty strings,
arithmetic expression evaluation treats
those as expressions that evaluate to 0.
</p></dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="Shell-Arithmetic">
<div class="nav-panel">
<p>
Next: <a href="#Aliases" accesskey="n" rel="next">Aliases</a>, Previous: <a href="#Bash-Conditional-Expressions" accesskey="p" rel="prev">Bash Conditional Expressions</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Arithmetic-1"><span>6.5 Shell Arithmetic<a class="copiable-link" href="#Shell-Arithmetic-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-arithmetic_002c-shell"></a>
<a class="index-entry-id" id="index-shell-arithmetic"></a>
<a class="index-entry-id" id="index-expressions_002c-arithmetic"></a>
<a class="index-entry-id" id="index-evaluation_002c-arithmetic"></a>
<a class="index-entry-id" id="index-arithmetic-evaluation"></a>

<p>The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by using the <code class="code">((</code> compound command,
the <code class="code">let</code> and <code class="code">declare</code> builtins,
the arithmetic <code class="code">for</code> command,
the <code class="code">[[</code> conditional command,
or the <samp class="option">-i</samp> option to the <code class="code">declare</code> builtin.
</p>
<p>Evaluation is done in the largest fixed-width integers available,
with no check for overflow,
though division by 0 is trapped and flagged as an error.
The operators and their precedence, associativity, and values
are the same as in the C language.
The following list of operators is grouped into levels of
equal-precedence operators.
The levels are listed in order of decreasing precedence. 
</p>
<dl class="table">
<dd><a class="index-entry-id" id="index-arithmetic-operators"></a>
<a class="index-entry-id" id="index-unary-arithmetic-operators"></a>
<a class="index-entry-id" id="index-binary-arithmetic-operators"></a>
<a class="index-entry-id" id="index-conditional-arithmetic-operator"></a>
<a class="index-entry-id" id="index-bitwise-arithmetic-operators"></a>

</dd>
<dt><code class="code"><var class="var">id</var>++ <var class="var">id</var>--</code></dt>
<dd><p>variable post-increment and post-decrement 
</p>
</dd>
<dt><code class="code">++<var class="var">id</var> --<var class="var">id</var></code></dt>
<dd><p>variable pre-increment and pre-decrement
</p>
</dd>
<dt><code class="code">- +</code></dt>
<dd><p>unary minus and plus
</p>
</dd>
<dt><code class="code">! ~</code></dt>
<dd><p>logical and bitwise negation
</p>
</dd>
<dt><code class="code">**</code></dt>
<dd><p>exponentiation
</p>
</dd>
<dt><code class="code">* / %</code></dt>
<dd><p>multiplication, division, remainder
</p>
</dd>
<dt><code class="code">+ -</code></dt>
<dd><p>addition, subtraction
</p>
</dd>
<dt><code class="code">&lt;&lt; &gt;&gt;</code></dt>
<dd><p>left and right bitwise shifts
</p>
</dd>
<dt><code class="code">&lt;= &gt;= &lt; &gt;</code></dt>
<dd><p>comparison
</p>
</dd>
<dt><code class="code">== !=</code></dt>
<dd><p>equality and inequality
</p>
</dd>
<dt><code class="code">&amp;</code></dt>
<dd><p>bitwise AND
</p>
</dd>
<dt><code class="code">^</code></dt>
<dd><p>bitwise exclusive OR
</p>
</dd>
<dt><code class="code">|</code></dt>
<dd><p>bitwise OR
</p>
</dd>
<dt><code class="code">&amp;&amp;</code></dt>
<dd><p>logical AND
</p>
</dd>
<dt><code class="code">||</code></dt>
<dd><p>logical OR
</p>
</dd>
<dt><code class="code">expr ? if-true-expr : if-false-expr</code></dt>
<dd><p>conditional operator
</p>
</dd>
<dt><code class="code">= *= /= %= += -= &lt;&lt;= &gt;&gt;= &amp;= ^= |=</code></dt>
<dd><p>assignment
</p>
</dd>
<dt><code class="code">expr1 , expr2</code></dt>
<dd><p>comma
</p></dd>
</dl>

<p>Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. 
Within an expression, shell variables may also be referenced by name
without using the parameter expansion syntax.
This means you can use <var class="var">x</var>,
where <var class="var">x</var> is a shell variable name, in an arithmetic expression,
and the shell will evaluate its value as an expression and use the
result.
A shell variable that is null or unset evaluates to 0 when referenced
by name in an expression.
</p>
<p>The value of a variable is evaluated as an arithmetic expression
when it is referenced, or when a variable which has been given the  
<code class="code">integer</code> attribute using &lsquo;<samp class="samp">declare -i</samp>&rsquo; is assigned a value.
A null value evaluates to 0.
A shell variable need not have its <code class="code">integer</code> attribute turned on
to be used in an expression.
</p>
<p>Integer constants follow the C language definition, without suffixes or
character constants.
Constants with a leading 0 are interpreted as octal numbers.
A leading &lsquo;<samp class="samp">0x</samp>&rsquo; or &lsquo;<samp class="samp">0X</samp>&rsquo; denotes hexadecimal.  Otherwise,
numbers take the form [<var class="var">base</var><code class="code">#</code>]<var class="var">n</var>, where the optional <var class="var">base</var>
is a decimal number between 2 and 64 representing the arithmetic
base, and <var class="var">n</var> is a number in that base.
If <var class="var">base</var><code class="code">#</code> is omitted, then base 10 is used.
When specifying <var class="var">n</var>,
if a non-digit is required,
the digits greater than 9 are represented by the lowercase letters,
the uppercase letters, &lsquo;<samp class="samp">@</samp>&rsquo;, and &lsquo;<samp class="samp">_</samp>&rsquo;, in that order.
If <var class="var">base</var> is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between 10
and 35.
</p>
<p>Operators are evaluated in precedence order.
Sub-expressions in parentheses are evaluated first
and may override the precedence rules above.
</p>
<hr>
</div>
<div class="section-level-extent" id="Aliases">
<div class="nav-panel">
<p>
Next: <a href="#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="#Shell-Arithmetic" accesskey="p" rel="prev">Shell Arithmetic</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Aliases-1"><span>6.6 Aliases<a class="copiable-link" href="#Aliases-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-alias-expansion"></a>

<p><em class="dfn">Aliases</em> allow a string to be substituted for a word that is in
a position in the input where it can be the first word of a simple
command.
Aliases have names and corresponding values that are set
and unset using the <code class="code">alias</code> and <code class="code">unalias</code> builtin commands
(see <a class="pxref" href="#Shell-Builtin-Commands">Shell Builtin Commands</a>).
</p>
<p>If the shell reads an unquoted word in the right position, it checks
the word to see if it matches an alias name.
If it matches, the shell
replaces the word with the alias value, and reads that value as if it
had been read instead of the word.
The shell doesn&rsquo;t look at any characters following the word before
attempting alias substitution. 
</p>
<p>The characters &lsquo;<samp class="samp">/</samp>&rsquo;, &lsquo;<samp class="samp">$</samp>&rsquo;, &lsquo;<samp class="samp">`</samp>&rsquo;, &lsquo;<samp class="samp">=</samp>&rsquo; and any of the
shell metacharacters or quoting characters listed above may not appear
in an alias name.
The replacement text may contain any valid
shell input, including shell metacharacters.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time.
This means that one may alias <code class="code">ls</code> to <code class="code">&quot;ls -F&quot;</code>,
for instance, and Bash does not try to recursively expand the
replacement text.
</p>
<p>If the last character of the alias value is a
<code class="code">blank</code>, then the shell checks
the next command word following the alias for alias expansion.
</p>
<p>Aliases are created and listed with the <code class="code">alias</code>
command, and removed with the <code class="code">unalias</code> command.
</p>
<p>There is no mechanism for using arguments in the replacement text,
as in <code class="code">csh</code>.
If arguments are needed, use a shell function
(see <a class="pxref" href="#Shell-Functions">Shell Functions</a>) instead.
</p>
<p>Aliases are not expanded when the shell is not interactive,
unless the <code class="code">expand_aliases</code> shell option is set using
<code class="code">shopt</code> (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
</p>
<p>The rules concerning the definition and use of aliases are
somewhat confusing.
Bash always reads at least one complete line of input,
and all lines that make up a compound command,
before executing any of the commands on that line or the compound command.
Aliases are expanded when a command is read, not when it is executed.
Therefore, an
alias definition appearing on the same line as another
command does not take effect until the shell reads the next line of input,
and an alias definition in a compound command does not take
effect until the shell parses and executes the entire compound command.
The commands following the alias definition
on that line,
or in the rest of a compound command,
are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a command.
As a consequence, aliases
defined in a function are not available until after that
function is executed.
To be safe, always put
alias definitions on a separate line, and do not use <code class="code">alias</code>
in compound commands.
</p>
<p>For almost every purpose, shell functions are preferable to aliases.
</p>
<hr>
</div>
<div class="section-level-extent" id="Arrays">
<div class="nav-panel">
<p>
Next: <a href="#The-Directory-Stack" accesskey="n" rel="next">The Directory Stack</a>, Previous: <a href="#Aliases" accesskey="p" rel="prev">Aliases</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Arrays-1"><span>6.7 Arrays<a class="copiable-link" href="#Arrays-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-arrays"></a>

<p>Bash
provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the
<code class="code">declare</code>
builtin explicitly declares an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using arithmetic expressions
that must expand to an integer (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>))
and are zero-based;
associative arrays use arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.
</p>
<p>The shell performs
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on indexed array subscripts.
Since this
can potentially result in empty strings,
subscript indexing treats
those as expressions that evaluate to 0.
</p>
<p>The shell performs
tilde expansion,
parameter and variable expansion, arithmetic expansion,
command substitution, and quote removal
on associative array subscripts.
Empty strings cannot be used as associative array keys.
</p>
<p>Bash automatically creates an indexed array
if any variable is assigned to using the syntax
</p><div class="example">
<pre class="example-preformatted"><var class="var">name</var>[<var class="var">subscript</var>]=<var class="var">value</var>
</pre></div>

<p>The <var class="var">subscript</var>
is treated as an arithmetic expression that must evaluate to a number
greater than or equal to zero.
To explicitly declare an indexed array, use
</p><div class="example">
<pre class="example-preformatted">declare -a <var class="var">name</var>
</pre></div>
<p>(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
The syntax
</p><div class="example">
<pre class="example-preformatted">declare -a <var class="var">name</var>[<var class="var">subscript</var>]
</pre></div>
<p>is also accepted; the <var class="var">subscript</var> is ignored.
</p>
<p>Associative arrays are created using
</p><div class="example">
<pre class="example-preformatted">declare -A <var class="var">name</var>
</pre></div>

<p>Attributes may be specified for an array variable using the
<code class="code">declare</code> and <code class="code">readonly</code> builtins.
Each attribute applies to all members of an array.
</p>
<p>Arrays are assigned using compound assignments of the form
</p><div class="example">
<pre class="example-preformatted"><var class="var">name</var>=(<var class="var">value1</var> <var class="var">value2</var> ... )
</pre></div>
<p>where each
<var class="var">value</var> may be of the form <code class="code">[<var class="var">subscript</var>]=</code><var class="var">string</var>.
Indexed array assignments do not require anything but <var class="var">string</var>.
</p>
<p>Each <var class="var">value</var> in the list undergoes the shell expansions
described above (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>),
but <var class="var">value</var>s that are valid variable assignments
including the brackets and subscript do not undergo
brace expansion and word splitting, as with individual
variable assignments.
</p>
<p>When assigning to indexed arrays, if
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one.
Indexing starts at zero.
</p>
<p>When assigning to an associative array, the words in a compound assignment
may be either assignment statements, for which the subscript is required,
or a list of words that is interpreted as a sequence of alternating keys
and values:
<var class="var">name</var>=(<var class="var">key1</var> <var class="var">value1</var> <var class="var">key2</var> <var class="var">value2</var> &hellip; ).
These are treated identically to
<var class="var">name</var>=( [<var class="var">key1</var>]=<var class="var">value1</var> [<var class="var">key2</var>]=<var class="var">value2</var> &hellip; ).
The first word in the list determines how the remaining words
are interpreted; all assignments in a list must be of the same type.
When using key/value pairs, the keys may not be missing or empty;
a final missing value is treated like the empty string.
</p>
<p>This syntax is also accepted by the <code class="code">declare</code>
builtin.
Individual array elements may be assigned to using the
<code class="code"><var class="var">name</var>[<var class="var">subscript</var>]=<var class="var">value</var></code> syntax introduced above.
</p>
<p>When assigning to an indexed array, if <var class="var">name</var>
is subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
<var class="var">name</var>, so negative indices count back from the end of the
array, and an index of -1 references the last element.
</p>
<p>The &lsquo;<samp class="samp">+=</samp>&rsquo;
operator appends to an array variable when assigning
using the compound assignment syntax; see
<a class="ref" href="#Shell-Parameters">Shell Parameters</a> above.
</p>
<p>An array element is referenced using
<code class="code">${<var class="var">name</var>[<var class="var">subscript</var>]}</code>.
The braces are required to avoid
conflicts with the shell&rsquo;s filename expansion operators.
If the <var class="var">subscript</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the word expands to all members
of the array <var class="var">name</var>, unless otherwise noted in the description of a
builtin or word expansion.
These subscripts differ only when the word
appears within double quotes.
If the word is double-quoted,
<code class="code">${<var class="var">name</var>[*]}</code> expands to a single word with
the value of each array member separated by the first character of the
<code class="env">IFS</code> variable, and <code class="code">${<var class="var">name</var>[@]}</code> expands each element of
<var class="var">name</var> to a separate word.
When there are no array members, <code class="code">${<var class="var">name</var>[@]}</code> expands to
nothing.
If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the beginning part of the
expansion of the original word,
and the expansion of the last parameter is joined with the last part of the
expansion of the original word.
This is analogous to the
expansion of the special parameters &lsquo;<samp class="samp">@</samp>&rsquo; and &lsquo;<samp class="samp">*</samp>&rsquo;. 
</p>
<p><code class="code">${#<var class="var">name</var>[<var class="var">subscript</var>]}</code> expands to the length of
<code class="code">${<var class="var">name</var>[<var class="var">subscript</var>]}</code>.
If <var class="var">subscript</var> is &lsquo;<samp class="samp">@</samp>&rsquo; or &lsquo;<samp class="samp">*</samp>&rsquo;,
the expansion is the number of elements in the array. 
</p>
<p>If the <var class="var">subscript</var>
used to reference an element of an indexed array
evaluates to a number less than zero, it is
interpreted as relative to one greater than the maximum index of the array,
so negative indices count back from the end of the array,
and an index of -1 refers to the last element.
</p>
<p>Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0.
Any reference to a variable using a valid subscript is valid;
Bash creates an array if necessary.
</p>
<p>An array variable is considered set if a subscript has been assigned a
value.
The null string is a valid value.
</p>
<p>It is possible to obtain the keys (indices) of an array as well as the values.
${!<var class="var">name</var>[@]} and ${!<var class="var">name</var>[*]} expand to the indices
assigned in array variable <var class="var">name</var>.
The treatment when in double quotes is similar to the expansion of the
special parameters &lsquo;<samp class="samp">@</samp>&rsquo; and &lsquo;<samp class="samp">*</samp>&rsquo; within double quotes.
</p>
<p>The <code class="code">unset</code> builtin is used to destroy arrays.
<code class="code">unset <var class="var">name</var>[<var class="var">subscript</var>]</code>
unsets the array element at index <var class="var">subscript</var>.
Negative subscripts to indexed arrays are interpreted as described above.
Unsetting the last element of an array variable does not unset the variable.
<code class="code">unset <var class="var">name</var></code>, where <var class="var">name</var> is an array, removes the
entire array.
<code class="code">unset <var class="var">name</var>[<var class="var">subscript</var>]</code> behaves differently
depending on the array type when
<var class="var">subscript</var> is &lsquo;<samp class="samp">*</samp>&rsquo; or &lsquo;<samp class="samp">@</samp>&rsquo;.
When <var class="var">name</var> is an associative array, it removes the element with key
&lsquo;<samp class="samp">*</samp>&rsquo; or &lsquo;<samp class="samp">@</samp>&rsquo;.
If <var class="var">name</var> is an indexed array, <code class="code">unset</code> removes all of the elements,
but does not remove the array itself.
</p>
<p>When using a variable name with a subscript as an argument to a command,
such as with <code class="code">unset</code>, without using the word expansion syntax
described above (e.g., unset a[4]),
the argument is subject to the shell&rsquo;s filename expansion.
Quote the argument if pathname expansion is not desired
(e.g., unset &rsquo;a[4]&rsquo;).
</p>
<p>The <code class="code">declare</code>, <code class="code">local</code>, and <code class="code">readonly</code>
builtins each accept a <samp class="option">-a</samp> option to specify an indexed
array and a <samp class="option">-A</samp> option to specify an associative array.
If both options are supplied, <samp class="option">-A</samp> takes precedence.
The <code class="code">read</code> builtin accepts a <samp class="option">-a</samp>
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements.  The <code class="code">set</code> and <code class="code">declare</code>
builtins display array values in a way that allows them to be
reused as input.
Other builtins accept array name arguments as well
(e.g., <code class="code">mapfile</code>); see the descriptions
of individual builtins for details.
The shell provides a number of builtin array variables.
</p>
<hr>
</div>
<div class="section-level-extent" id="The-Directory-Stack">
<div class="nav-panel">
<p>
Next: <a href="#Controlling-the-Prompt" accesskey="n" rel="next">Controlling the Prompt</a>, Previous: <a href="#Arrays" accesskey="p" rel="prev">Arrays</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="The-Directory-Stack-1"><span>6.8 The Directory Stack<a class="copiable-link" href="#The-Directory-Stack-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-directory-stack"></a>


<p>The directory stack is a list of recently-visited directories.  The
<code class="code">pushd</code> builtin adds directories to the stack as it changes
the current directory, and the <code class="code">popd</code> builtin removes specified
directories from the stack and changes the current directory to
the directory removed.  The <code class="code">dirs</code> builtin displays the contents
of the directory stack.  The current directory is always the &quot;top&quot;
of the directory stack.
</p>
<p>The contents of the directory stack are also visible
as the value of the <code class="env">DIRSTACK</code> shell variable.
</p>
<ul class="mini-toc">
<li><a href="#Directory-Stack-Builtins" accesskey="1">Directory Stack Builtins</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Directory-Stack-Builtins">
<div class="nav-panel">
<p>
Up: <a href="#The-Directory-Stack" accesskey="u" rel="up">The Directory Stack</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Directory-Stack-Builtins-1"><span>6.8.1 Directory Stack Builtins<a class="copiable-link" href="#Directory-Stack-Builtins-1"> &para;</a></span></h4>

<dl class="table">
<dt><a id="index-dirs"></a><span><code class="code">dirs</code><a class="copiable-link" href="#index-dirs"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">dirs [-clpv] [+<var class="var">N</var> | -<var class="var">N</var>]
</pre></div>

<p>Without options, display the list of currently remembered directories.
Directories are added to the list with the <code class="code">pushd</code> command;
the <code class="code">popd</code> command removes directories from the list.
The current directory is always the first directory in the stack.
</p>
<p>Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-c</code></dt>
<dd><p>Clears the directory stack by deleting all of the elements.
</p></dd>
<dt><code class="code">-l</code></dt>
<dd><p>Produces a listing using full pathnames;
the default listing format uses a tilde to denote the home directory.
</p></dd>
<dt><code class="code">-p</code></dt>
<dd><p>Causes <code class="code">dirs</code> to print the directory stack with one entry per
line.
</p></dd>
<dt><code class="code">-v</code></dt>
<dd><p>Causes <code class="code">dirs</code> to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
</p></dd>
<dt><code class="code">+<var class="var">N</var></code></dt>
<dd><p>Displays the <var class="var">N</var>th directory (counting from the left of the
list printed by <code class="code">dirs</code> when invoked without options), starting
with zero.
</p></dd>
<dt><code class="code">-<var class="var">N</var></code></dt>
<dd><p>Displays the <var class="var">N</var>th directory (counting from the right of the
list printed by <code class="code">dirs</code> when invoked without options), starting
with zero.
</p></dd>
</dl>

</dd>
<dt><a id="index-popd"></a><span><code class="code">popd</code><a class="copiable-link" href="#index-popd"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">popd [-n] [+<var class="var">N</var> | -<var class="var">N</var>]
</pre></div>

<p>Remove elements from the directory stack.
The elements are numbered from 0 starting at the first directory
listed by <code class="code">dirs</code>;
that is, <code class="code">popd</code> is equivalent to <code class="code">popd +0</code>.
</p>
<p>When no arguments are given, <code class="code">popd</code> removes the top directory
from the stack and changes to the new top directory.
</p>
<p>Arguments, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-n</code></dt>
<dd><p>Suppress the normal change of directory when removing directories
from the stack, only manipulate the stack.
</p></dd>
<dt><code class="code">+<var class="var">N</var></code></dt>
<dd><p>Remove the <var class="var">N</var>th directory (counting from the left of the
list printed by <code class="code">dirs</code>), starting with zero, from the stack.
</p></dd>
<dt><code class="code">-<var class="var">N</var></code></dt>
<dd><p>Remove the <var class="var">N</var>th directory (counting from the right of the
list printed by <code class="code">dirs</code>), starting with zero, from the stack.
</p></dd>
</dl>

<p>If the top element of the directory stack is modified, and
the <samp class="option">-n</samp> option was not supplied, <code class="code">popd</code> uses the <code class="code">cd</code>
builtin to change to the directory at the top of the stack.
If the <code class="code">cd</code> fails, <code class="code">popd</code> returns a non-zero value.
</p>
<p>Otherwise, <code class="code">popd</code> returns an unsuccessful status if
an invalid option is specified, the directory stack
is empty, or <var class="var">N</var> specifies a non-existent directory stack entry.
</p>
<p>If the <code class="code">popd</code> command is successful,
Bash runs <code class="code">dirs</code> to show the final contents of the directory stack,
and the return status is 0.
</p>
</dd>
<dt><a id="index-pushd"></a><span><code class="code">pushd</code><a class="copiable-link" href="#index-pushd"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">pushd [-n] [<var class="var">+N</var> | <var class="var">-N</var> | <var class="var">dir</var>]
</pre></div>

<p>Add a directory to the top of the directory stack, or rotate
the stack, making the new top of the stack the current working
directory.
With no arguments, <code class="code">pushd</code> exchanges the top two elements
of the directory stack.
</p>
<p>Arguments, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-n</code></dt>
<dd><p>Suppress the normal change of directory when rotating or
adding directories to the stack, only manipulate the stack.
</p></dd>
<dt><code class="code">+<var class="var">N</var></code></dt>
<dd><p>Rotate the stack so that
the <var class="var">N</var>th directory (counting from the left of the
list printed by <code class="code">dirs</code>, starting with zero) is at the top.
</p></dd>
<dt><code class="code">-<var class="var">N</var></code></dt>
<dd><p>Rotate the stack so that
the <var class="var">N</var>th directory (counting from the right of the
list printed by <code class="code">dirs</code>, starting with zero) is at the top.
</p></dd>
<dt><code class="code"><var class="var">dir</var></code></dt>
<dd><p>Make <var class="var">dir</var> be the top of the stack.
</p></dd>
</dl>

<p>After the stack has been modified, if the <samp class="option">-n</samp> option was not
supplied, <code class="code">pushd</code> uses the <code class="code">cd</code> builtin to change to the
directory at the top of the stack.
If the <code class="code">cd</code> fails, <code class="code">pushd</code> returns a non-zero value.
</p>
<p>Otherwise, if no arguments are supplied, <code class="code">pushd</code> returns zero
unless the directory stack is empty.
When rotating the directory stack, <code class="code">pushd</code> returns zero unless
the directory stack is empty or <var class="var">N</var> specifies a non-existent
directory stack element.
</p>
<p>If the <code class="code">pushd</code> command is successful,
Bash runs <code class="code">dirs</code> to show the final contents of the directory stack.
</p>
</dd>
</dl>

<hr>
</div>
</div>
<div class="section-level-extent" id="Controlling-the-Prompt">
<div class="nav-panel">
<p>
Next: <a href="#The-Restricted-Shell" accesskey="n" rel="next">The Restricted Shell</a>, Previous: <a href="#The-Directory-Stack" accesskey="p" rel="prev">The Directory Stack</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Controlling-the-Prompt-1"><span>6.9 Controlling the Prompt<a class="copiable-link" href="#Controlling-the-Prompt-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-prompting"></a>

<p>In addition, the following table describes the special characters which
can appear in the prompt variables <code class="env">PS0</code>, <code class="env">PS1</code>, <code class="env">PS2</code>, and
<code class="env">PS4</code>:
</p>
<dl class="table">
<dt><code class="code">\a</code></dt>
<dd><p>A bell character.
</p></dd>
<dt><code class="code">\d</code></dt>
<dd><p>The date, in &quot;Weekday Month Date&quot; format (e.g., &quot;Tue May 26&quot;).
</p></dd>
<dt><code class="code">\D{<var class="var">format</var>}</code></dt>
<dd><p>The <var class="var">format</var> is passed to <code class="code">strftime</code>(3) and the result is inserted
into the prompt string; an empty <var class="var">format</var> results in a locale-specific
time representation.
The braces are required.
</p></dd>
<dt><code class="code">\e</code></dt>
<dd><p>An escape character.
</p></dd>
<dt><code class="code">\h</code></dt>
<dd><p>The hostname, up to the first &lsquo;<samp class="samp">.</samp>&rsquo;.
</p></dd>
<dt><code class="code">\H</code></dt>
<dd><p>The hostname.
</p></dd>
<dt><code class="code">\j</code></dt>
<dd><p>The number of jobs currently managed by the shell.
</p></dd>
<dt><code class="code">\l</code></dt>
<dd><p>The basename of the shell&rsquo;s terminal device name (e.g., &quot;ttys0&quot;).
</p></dd>
<dt><code class="code">\n</code></dt>
<dd><p>A newline.
</p></dd>
<dt><code class="code">\r</code></dt>
<dd><p>A carriage return.
</p></dd>
<dt><code class="code">\s</code></dt>
<dd><p>The name of the shell: the basename of <code class="code">$0</code> (the portion
following the final slash).
</p></dd>
<dt><code class="code">\t</code></dt>
<dd><p>The time, in 24-hour HH:MM:SS format.
</p></dd>
<dt><code class="code">\T</code></dt>
<dd><p>The time, in 12-hour HH:MM:SS format.
</p></dd>
<dt><code class="code">\@</code></dt>
<dd><p>The time, in 12-hour am/pm format.
</p></dd>
<dt><code class="code">\A</code></dt>
<dd><p>The time, in 24-hour HH:MM format.
</p></dd>
<dt><code class="code">\u</code></dt>
<dd><p>The username of the current user.
</p></dd>
<dt><code class="code">\v</code></dt>
<dd><p>The Bash version (e.g., 2.00).
</p></dd>
<dt><code class="code">\V</code></dt>
<dd><p>The Bash release, version + patchlevel (e.g., 2.00.0).
</p></dd>
<dt><code class="code">\w</code></dt>
<dd><p>The value of the <code class="code">PWD</code> shell variable (<code class="env">$PWD</code>),
with <code class="env">$HOME</code> abbreviated with a tilde
(uses the <code class="env">$PROMPT_DIRTRIM</code> variable).
</p></dd>
<dt><code class="code">\W</code></dt>
<dd><p>The basename of <code class="env">$PWD</code>, with <code class="env">$HOME</code> abbreviated with a tilde.
</p></dd>
<dt><code class="code">\!</code></dt>
<dd><p>The history number of this command.
</p></dd>
<dt><code class="code">\#</code></dt>
<dd><p>The command number of this command.
</p></dd>
<dt><code class="code">\$</code></dt>
<dd><p>If the effective uid is 0, <code class="code">#</code>, otherwise <code class="code">$</code>.
</p></dd>
<dt><code class="code">\<var class="var">nnn</var></code></dt>
<dd><p>The character whose ASCII code is the octal value <var class="var">nnn</var>.
</p></dd>
<dt><code class="code">\\</code></dt>
<dd><p>A backslash.
</p></dd>
<dt><code class="code">\[</code></dt>
<dd><p>Begin a sequence of non-printing characters.
Thiss could be used to
embed a terminal control sequence into the prompt.
</p></dd>
<dt><code class="code">\]</code></dt>
<dd><p>End a sequence of non-printing characters.
</p></dd>
</dl>

<p>The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>), while the command number is
the position in the sequence of commands executed during the current
shell session.
</p>
<p>After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
<code class="code">promptvars</code> shell option (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
This can have unwanted side effects if escaped portions of the string
appear within command substitution or contain characters special to
word expansion.
</p>
<hr>
</div>
<div class="section-level-extent" id="The-Restricted-Shell">
<div class="nav-panel">
<p>
Next: <a href="#Bash-POSIX-Mode" accesskey="n" rel="next">Bash and POSIX</a>, Previous: <a href="#Controlling-the-Prompt" accesskey="p" rel="prev">Controlling the Prompt</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="The-Restricted-Shell-1"><span>6.10 The Restricted Shell<a class="copiable-link" href="#The-Restricted-Shell-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-restricted-shell"></a>

<p>If Bash is started with the name <code class="code">rbash</code>, or the
<samp class="option">--restricted</samp>
or
<samp class="option">-r</samp>
option is supplied at invocation, the shell becomes <var class="var">restricted</var>.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to <code class="code">bash</code>
with the exception that the following are disallowed or not performed:
</p>
<ul class="itemize mark-bullet">
<li>Changing directories with the <code class="code">cd</code> builtin.
</li><li>Setting or unsetting the values of the <code class="env">SHELL</code>, <code class="env">PATH</code>,
<code class="env">HISTFILE</code>,
<code class="env">ENV</code>, or <code class="env">BASH_ENV</code> variables.
</li><li>Specifying command names containing slashes.
</li><li>Specifying a filename containing a slash as an argument to the <code class="code">.</code>
builtin command.
</li><li>Using the <samp class="option">-p</samp> option to the <code class="code">.</code> builtin command
to specify a search path.
</li><li>Specifying a filename containing a slash as an argument to the <code class="code">history</code>
builtin command.
</li><li>Specifying a filename containing a slash as an argument to the <samp class="option">-p</samp>
option to the <code class="code">hash</code> builtin command.
</li><li>Importing function definitions from the shell environment at startup.
</li><li>Parsing the value of <code class="env">SHELLOPTS</code> from the shell environment at startup.
</li><li>Redirecting output using the &lsquo;<samp class="samp">&gt;</samp>&rsquo;, &lsquo;<samp class="samp">&gt;|</samp>&rsquo;, &lsquo;<samp class="samp">&lt;&gt;</samp>&rsquo;, &lsquo;<samp class="samp">&gt;&amp;</samp>&rsquo;,
&lsquo;<samp class="samp">&amp;&gt;</samp>&rsquo;, and &lsquo;<samp class="samp">&gt;&gt;</samp>&rsquo; redirection operators.
</li><li>Using the <code class="code">exec</code> builtin to replace the shell with another command.
</li><li>Adding or deleting builtin commands with the
<samp class="option">-f</samp> and <samp class="option">-d</samp> options to the <code class="code">enable</code> builtin.
</li><li>Using the <code class="code">enable</code> builtin command to enable disabled shell builtins.
</li><li>Specifying the <samp class="option">-p</samp> option to the <code class="code">command</code> builtin.
</li><li>Turning off restricted mode with &lsquo;<samp class="samp">set +r</samp>&rsquo; or &lsquo;<samp class="samp">shopt -u restricted_shell</samp>&rsquo;.
</li></ul>

<p>These restrictions are enforced after any startup files are read.
</p>
<p>When a command that is found to be a shell script is executed
(see <a class="pxref" href="#Shell-Scripts">Shell Scripts</a>), <code class="code">rbash</code> turns off any restrictions in
the shell spawned to execute the script.
</p>
<p>The restricted shell mode is only one component of a useful restricted
environment. It should be accompanied by setting <code class="env">PATH</code> to a value
that allows execution of only a few verified commands (commands that
allow shell escapes are particularly vulnerable), changing the current
directory to a non-writable directory other than <code class="env">$HOME</code> after login,
not allowing the restricted shell to execute shell scripts, and cleaning
the environment of variables that cause some commands to modify their
behavior (e.g., <code class="env">VISUAL</code> or <code class="env">PAGER</code>).
</p>
<p>Modern systems provide more secure ways to implement a restricted environment,
such as <code class="code">jails</code>, <code class="code">zones</code>, or <code class="code">containers</code>.
</p>

<hr>
</div>
<div class="section-level-extent" id="Bash-POSIX-Mode">
<div class="nav-panel">
<p>
Next: <a href="#Shell-Compatibility-Mode" accesskey="n" rel="next">Shell Compatibility Mode</a>, Previous: <a href="#The-Restricted-Shell" accesskey="p" rel="prev">The Restricted Shell</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-and-POSIX"><span>6.11 Bash and POSIX<a class="copiable-link" href="#Bash-and-POSIX"> &para;</a></span></h3>

<ul class="mini-toc">
<li><a href="#What-is-POSIX_003f" accesskey="1">What is POSIX?</a></li>
<li><a href="#Bash-POSIX-Mode-1" accesskey="2">Bash POSIX Mode</a></li>
</ul>
<div class="subsection-level-extent" id="What-is-POSIX_003f">
<h4 class="subsection"><span>6.11.1 What is POSIX?<a class="copiable-link" href="#What-is-POSIX_003f"> &para;</a></span></h4>
<a class="index-entry-id" id="index-POSIX-description"></a>

<p><small class="sc">POSIX</small>
is the name for a family of standards based on Unix.
A number of Unix services, tools, and functions are part of the standard,
ranging from the basic system calls and C library functions to common
applications and tools to system administration and management.
</p>
<p>The <small class="sc">POSIX</small> Shell and Utilities standard was originally developed by
IEEE Working Group 1003.2 (POSIX.2).
The first edition of the 1003.2 standard was published in 1992.
It was merged with the original IEEE 1003.1 Working Group and is
currently maintained by the Austin Group (a joint working group of the
IEEE, The Open Group and ISO/IEC SC22/WG15).
Today the Shell and Utilities are a volume within the set of documents that
make up IEEE Std 1003.1-2024, and thus the former POSIX.2 (from 1992)
is now part of the current unified <small class="sc">POSIX</small> standard.
</p>
<p>The Shell and Utilities volume concentrates on the command
interpreter interface and utility programs commonly executed from
the command line or by other programs.
The standard is freely available on the web at
<a class="url" href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html">https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html</a>.
</p>
<p>Bash is concerned with the aspects of the shell&rsquo;s behavior defined
by the <small class="sc">POSIX</small> Shell and Utilities volume.
The shell command
language has of course been standardized, including the basic flow
control and program execution constructs, I/O redirection and
pipelines, argument handling, variable expansion, and quoting.
</p>
<p>The <i class="i">special</i> builtins, which must be implemented as part of the
shell to provide the desired functionality, are specified as
being part of the shell; examples of these are <code class="code">eval</code> and
<code class="code">export</code>.
Other utilities appear in the sections of <small class="sc">POSIX</small> not
devoted to the shell which are commonly (and in some cases must
be) implemented as builtin commands, such as
<code class="code">read</code> and <code class="code">test</code>.
<small class="sc">POSIX</small> also specifies aspects of the shell&rsquo;s interactive
behavior, including job control and command
line editing.
Only vi-style line editing commands have been standardized;
emacs editing commands were left out due to objections.
</p>
</div>
<div class="subsection-level-extent" id="Bash-POSIX-Mode-1">
<h4 class="subsection"><span>6.11.2 Bash POSIX Mode<a class="copiable-link" href="#Bash-POSIX-Mode-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-POSIX-Mode"></a>

<p>Although Bash is an implementation of the <small class="sc">POSIX</small> shell
specification, there are areas where the Bash default behavior
differs from the specification.
The Bash <em class="dfn">posix mode</em> changes the Bash
behavior in these areas so that it conforms more strictly
to the standard.
</p>
<p>Starting Bash with the <samp class="option">--posix</samp> command-line option or executing
&lsquo;<samp class="samp">set -o posix</samp>&rsquo; while Bash is running will cause Bash to conform more
closely to the <small class="sc">POSIX</small> standard by changing the behavior to
match that specified by <small class="sc">POSIX</small> in areas where the Bash default differs.
</p>
<p>When invoked as <code class="code">sh</code>, Bash enters <small class="sc">POSIX</small> mode after reading the
startup files.
</p>
<p>The following list is what&rsquo;s changed when <small class="sc">POSIX</small> mode is in effect:
</p>
<ol class="enumerate">
<li> Bash ensures that the <code class="env">POSIXLY_CORRECT</code> variable is set.

</li><li> Bash reads and executes the <small class="sc">POSIX</small> startup files
(<code class="env">$ENV</code>) rather than
the normal Bash files (see <a class="pxref" href="#Bash-Startup-Files">Bash Startup Files</a>.

</li><li> Alias expansion is always enabled, even in non-interactive shells.

</li><li> Reserved words appearing in a context where reserved words are recognized
do not undergo alias expansion.

</li><li> Alias expansion is performed when initially parsing a command substitution.
The default (non-posix)
mode generally defers it, when enabled, until the command
substitution is executed.
This means that command substitution will not
expand aliases that are defined after the command substitution is initially
parsed (e.g., as part of a function definition).

</li><li> The <code class="code">time</code> reserved word may be used by itself as a simple command.
When used in this way, it displays timing statistics for the shell
and its completed children.
The <code class="env">TIMEFORMAT</code> variable controls the format of the timing information.

</li><li> The parser does not recognize <code class="code">time</code> as a reserved word if the next
token begins with a &lsquo;<samp class="samp">-</samp>&rsquo;.

</li><li> When parsing and expanding a ${&hellip;} expansion that appears within
double quotes, single quotes are no longer special and cannot be used to
quote a closing brace or other special character, unless the operator is
one of those defined to perform pattern removal.
In this case, they do not have to appear as matched pairs.


</li><li> Redirection operators do not perform filename expansion on the word
in a redirection unless the shell is interactive.

</li><li> Redirection operators do not perform word splitting on the word in a
redirection.

</li><li> Function names may not be the same as one of the <small class="sc">POSIX</small> special
builtins.

</li><li> Tilde expansion is only performed on assignments preceding a command
name, rather than on all assignment statements on the line.

</li><li> While variable indirection is available, it may not be applied to the
&lsquo;<samp class="samp">#</samp>&rsquo; and &lsquo;<samp class="samp">?</samp>&rsquo; special parameters.

</li><li> Expanding the &lsquo;<samp class="samp">*</samp>&rsquo; special parameter in a pattern context where the
expansion is double-quoted does not treat the <code class="code">$*</code> as if it were
double-quoted.

</li><li> A double quote character (&lsquo;<samp class="samp">&quot;</samp>&rsquo;) is treated specially when it appears
in a backquoted command substitution in the body of a here-document that
undergoes expansion.
That means, for example, that a backslash preceding a double quote
character will escape it and the backslash will be removed.

</li><li> Command substitutions don&rsquo;t set the &lsquo;<samp class="samp">?</samp>&rsquo; special parameter.
The exit
status of a simple command without a command word is still the exit status
of the last command substitution that occurred while evaluating the variable
assignments and redirections in that command, but that does not happen until
after all of the assignments and redirections.

</li><li> Literal tildes that appear as the first character in elements of
the <code class="env">PATH</code> variable are not expanded as described above
under <a class="ref" href="#Tilde-Expansion">Tilde Expansion</a>.

</li><li> Command lookup finds <small class="sc">POSIX</small> special builtins before shell functions,
including output printed by the <code class="code">type</code> and <code class="code">command</code> builtins.

</li><li> Even if a shell function whose name contains a slash was defined before
entering <small class="sc">POSIX</small> mode, the shell will not execute a function whose name
contains one or more slashes.

</li><li> When a command in the hash table no longer exists, Bash will re-search
<code class="env">$PATH</code> to find the new location.
This is also available with &lsquo;<samp class="samp">shopt -s checkhash</samp>&rsquo;.

</li><li> Bash will not insert a command without the execute bit set into the
command hash table, even if it returns it as a (last-ditch) result
from a <code class="env">$PATH</code> search.

</li><li> The message printed by the job control code and builtins when a job
exits with a non-zero status is &lsquo;Done(status)&rsquo;.

</li><li> The message printed by the job control code and builtins when a job
is stopped is &lsquo;Stopped(<var class="var">signame</var>)&rsquo;, where <var class="var">signame</var> is, for
example, <code class="code">SIGTSTP</code>.

</li><li> If the shell is interactive, Bash does not perform job notifications
between executing commands in lists separated by &lsquo;<samp class="samp">;</samp>&rsquo; or newline.
Non-interactive shells print status messages after a foreground job
in a list completes.

</li><li> If the shell is interactive, Bash waits until the next prompt before
printing the status of a background job that changes status or a foreground
job that terminates due to a signal.
Non-interactive shells print status messages after a foreground job
completes.

</li><li> Bash permanently removes jobs from the jobs table after notifying the
user of their termination via the <code class="code">wait</code> or <code class="code">jobs</code> builtins.
It removes the job from the jobs list after notifying the user of its
termination, but the status is still available via <code class="code">wait</code>, as long
as <code class="code">wait</code> is supplied a <small class="sc">PID</small> argument.

</li><li> The <code class="code">vi</code> editing mode will invoke the <code class="code">vi</code> editor directly when
the &lsquo;<samp class="samp">v</samp>&rsquo; command is run, instead of checking <code class="code">$VISUAL</code> and
<code class="code">$EDITOR</code>.

</li><li> Prompt expansion enables the
<small class="sc">POSIX</small> <code class="env">PS1</code> and <code class="env">PS2</code> expansions of &lsquo;<samp class="samp">!</samp>&rsquo; to
the history number and &lsquo;<samp class="samp">!!</samp>&rsquo; to &lsquo;<samp class="samp">!</samp>&rsquo;,
and Bash performs parameter expansion on the values of <code class="env">PS1</code> and
<code class="env">PS2</code> regardless of the setting of the <code class="code">promptvars</code> option.

</li><li> The default history file is <samp class="file">~/.sh_history</samp> (this is the
default value the shell assigns to <code class="env">$HISTFILE</code>).

</li><li> The &lsquo;<samp class="samp">!</samp>&rsquo; character does not introduce history expansion within a
double-quoted string, even if the <code class="code">histexpand</code> option is enabled.

</li><li> When printing shell function definitions (e.g., by <code class="code">type</code>), Bash does
not print the <code class="code">function</code> reserved word unless necessary.

</li><li> Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.

</li><li> Non-interactive shells exit if a parameter expansion error occurs.

</li><li> If a <small class="sc">POSIX</small> special builtin returns an error status, a
non-interactive shell exits.
The fatal errors are those listed in
the <small class="sc">POSIX</small> standard, and include things like passing incorrect options,
redirection errors, variable assignment errors for assignments preceding
the command name, and so on.

</li><li> A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
statements.
A variable assignment error occurs, for example, when trying to assign
a value to a readonly variable.

</li><li> A non-interactive shell exits with an error status if a variable
assignment error occurs in an assignment statement preceding a special
builtin, but not with any other simple command.
For any other simple
command, the shell aborts execution of that command, and execution continues
at the top level (&quot;the shell shall not perform any further processing of the
command in which the error occurred&quot;).

</li><li> A non-interactive shell exits with an error status if the iteration
variable in a <code class="code">for</code> statement or the selection variable in a
<code class="code">select</code> statement is a readonly variable or has an invalid name.

</li><li> Non-interactive shells exit if <var class="var">filename</var> in <code class="code">.</code> <var class="var">filename</var>
is not found.

</li><li> Non-interactive shells exit if there is a syntax error in a script read
with the <code class="code">.</code> or <code class="code">source</code> builtins, or in a string processed by
the <code class="code">eval</code> builtin.

</li><li> Non-interactive shells exit
if the <code class="code">export</code>, <code class="code">readonly</code> or <code class="code">unset</code>
builtin commands get an argument
that is not a valid identifier, and they are not operating on shell
functions.
These errors force an exit because these are special builtins.

</li><li> Assignment statements preceding <small class="sc">POSIX</small> special builtins
persist in the shell environment after the builtin completes.

</li><li> The <code class="code">command</code> builtin does not prevent builtins that take assignment
statements as arguments from expanding them as assignment statements;
when not in <small class="sc">POSIX</small> mode, declaration commands lose their assignment
statement expansion properties when preceded by <code class="code">command</code>.

</li><li> Enabling <small class="sc">POSIX</small> mode has the effect of setting the
<code class="code">inherit_errexit</code> option, so
subshells spawned to execute command substitutions inherit the value of
the <samp class="option">-e</samp> option from the parent shell.
When the <code class="code">inherit_errexit</code> option is not enabled,
Bash clears the <samp class="option">-e</samp> option in such subshells.

</li><li> Enabling <small class="sc">POSIX</small> mode has the effect of setting the
<code class="code">shift_verbose</code> option, so numeric arguments to <code class="code">shift</code>
that exceed the number of positional parameters will result in an
error message.

</li><li> Enabling <small class="sc">POSIX</small> mode has the effect of setting the
<code class="code">interactive_comments</code> option (see <a class="pxref" href="#Comments">Comments</a>).

</li><li> The <code class="code">.</code> and <code class="code">source</code> builtins do not search the current directory
for the filename argument if it is not found by searching <code class="env">PATH</code>.

</li><li> When the <code class="code">alias</code> builtin displays alias definitions, it does not
display them with a leading &lsquo;<samp class="samp">alias </samp>&rsquo; unless the <samp class="option">-p</samp> option
is supplied.

</li><li> The <code class="code">bg</code> builtin uses the required format to describe each job placed
in the background, which does not include an indication of whether the job
is the current or previous job.

</li><li> When the <code class="code">cd</code> builtin is invoked in logical mode, and the pathname
constructed from <code class="code">$PWD</code> and the directory name supplied as an argument
does not refer to an existing directory, <code class="code">cd</code> will fail instead of
falling back to physical mode.

</li><li> When the <code class="code">cd</code> builtin cannot change a directory because the
length of the pathname 
constructed from <code class="code">$PWD</code> and the directory name supplied as an argument
exceeds <code class="code">PATH_MAX</code> when canonicalized, <code class="code">cd</code> will
attempt to use the supplied directory name.

</li><li> When the <code class="code">xpg_echo</code> option is enabled, Bash does not attempt to
interpret any arguments to <code class="code">echo</code> as options.
<code class="code">echo</code> displays each argument after converting escape sequences.

</li><li> The <code class="code">export</code> and <code class="code">readonly</code> builtin commands display their
output in the format required by <small class="sc">POSIX</small>.

</li><li> When listing the history, the <code class="code">fc</code> builtin does not include an
indication of whether or not a history entry has been modified.

</li><li> The default editor used by <code class="code">fc</code> is <code class="code">ed</code>.

</li><li> <code class="code">fc</code> treats extra arguments as an error instead of ignoring them.

</li><li> If there are too many arguments supplied to <code class="code">fc -s</code>, <code class="code">fc</code> prints
an error message and returns failure.

</li><li> The output of &lsquo;<samp class="samp">kill -l</samp>&rsquo; prints all the signal names on a single line,
separated by spaces, without the &lsquo;<samp class="samp">SIG</samp>&rsquo; prefix.

</li><li> The <code class="code">kill</code> builtin does not accept signal names with a &lsquo;<samp class="samp">SIG</samp>&rsquo;
prefix.

</li><li> The <code class="code">kill</code> builtin returns a failure status if any of the pid or job
arguments are invalid or if sending the specified signal to any of them
fails.
In default mode, <code class="code">kill</code> returns success if the signal was
successfully sent to any of the specified processes.

</li><li> The <code class="code">printf</code> builtin uses <code class="code">double</code> (via <code class="code">strtod</code>) to convert
arguments corresponding to floating point conversion specifiers, instead of
<code class="code">long double</code> if it&rsquo;s available.
The &lsquo;<samp class="samp">L</samp>&rsquo; length modifier forces
<code class="code">printf</code> to use <code class="code">long double</code> if it&rsquo;s available.

</li><li> The <code class="code">pwd</code> builtin verifies that the value it prints is the same as the
current directory, even if it is not asked to check the file system with the
<samp class="option">-P</samp> option.

</li><li> The <code class="code">read</code> builtin may be interrupted by a signal for which a trap
has been set.
If Bash receives a trapped signal while executing <code class="code">read</code>, the trap
handler executes and <code class="code">read</code> returns an exit status greater than 128.

</li><li> When the <code class="code">set</code> builtin is invoked without options, it does not display
shell function names and definitions.

</li><li> When the <code class="code">set</code> builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.

</li><li> The <code class="code">test</code> builtin compares strings using the current locale when
evaluating the &lsquo;<samp class="samp">&lt;</samp>&rsquo; and &lsquo;<samp class="samp">&gt;</samp>&rsquo; binary operators.

</li><li> The <code class="code">test</code> builtin&rsquo;s <samp class="option">-t</samp> unary primary requires an argument.
Historical versions of <code class="code">test</code> made the argument optional in certain
cases, and Bash attempts to accommodate those for backwards compatibility.

</li><li> The <code class="code">trap</code> builtin displays signal names without the leading
<code class="code">SIG</code>.

</li><li> The <code class="code">trap</code> builtin doesn&rsquo;t check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of digits and
is a valid signal number.
If users want to reset the handler for a given
signal to the original disposition, they should use &lsquo;<samp class="samp">-</samp>&rsquo; as the
first argument.

</li><li> <code class="code">trap -p</code> without arguments displays signals whose dispositions are
set to SIG_DFL and those that were ignored when the shell started, not
just trapped signals.

</li><li> The <code class="code">type</code> and <code class="code">command</code> builtins will not report a non-executable
file as having been found, though the shell will attempt to execute such a
file if it is the only so-named file found in <code class="code">$PATH</code>.

</li><li> The <code class="code">ulimit</code> builtin uses a block size of 512 bytes for the <samp class="option">-c</samp>
and <samp class="option">-f</samp> options.

</li><li> The <code class="code">unset</code> builtin with the <samp class="option">-v</samp> option specified returns a
fatal error if it attempts to unset a <code class="code">readonly</code> or <code class="code">non-unsettable</code>
variable,
which causes a non-interactive shell to exit.

</li><li> When asked to unset a variable that appears in an assignment statement
preceding the command, the <code class="code">unset</code> builtin attempts to unset a variable
of the same name in the current or previous scope as well.
This implements the required &quot;if an assigned variable is further modified
by the utility, the modifications made by the utility shall persist&quot; behavior.

</li><li> The arrival of <code class="code">SIGCHLD</code>  when a trap is set on <code class="code">SIGCHLD</code> does
not interrupt the <code class="code">wait</code> builtin and cause it to return immediately.
The trap command is run once for each child that exits.

</li><li> Bash removes an exited background process&rsquo;s status from the list of such
statuses after the <code class="code">wait</code> builtin returns it.

</li></ol>

<p>There is additional <small class="sc">POSIX</small> behavior that Bash does not implement by
default even when in <small class="sc">POSIX</small> mode.
Specifically:
</p>
<ol class="enumerate">
<li> <small class="sc">POSIX</small> requires that word splitting be byte-oriented.
That is, each <em class="emph">byte</em> in the value of <code class="env">IFS</code> potentially splits a
word, even if that byte is part of a multibyte character in <code class="env">IFS</code>
or part of multibyte character in the word.
Bash allows multibyte characters in the value of <code class="env">IFS</code>, treating
a valid multibyte character as a single delimiter, and will not
split a valid multibyte character even if one of the bytes composing that
character appears in <code class="env">IFS</code>.
This is <small class="sc">POSIX</small> interpretation 1560, further modified by issue 1924.

</li><li> The <code class="code">fc</code> builtin checks <code class="code">$EDITOR</code> as a program to edit history
entries if <code class="code">FCEDIT</code> is unset, rather than defaulting directly to
<code class="code">ed</code>.
<code class="code">fc</code> uses <code class="code">ed</code> if <code class="code">EDITOR</code> is unset.


</li><li> As noted above, Bash requires the <code class="code">xpg_echo</code> option to be enabled for
the <code class="code">echo</code> builtin to be fully conformant.

</li></ol>

<p>Bash can be configured to be <small class="sc">POSIX</small>-conformant by default, by specifying
the <samp class="option">--enable-strict-posix-default</samp> to <code class="code">configure</code> when building
(see <a class="pxref" href="#Optional-Features">Optional Features</a>).
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Shell-Compatibility-Mode">
<div class="nav-panel">
<p>
Previous: <a href="#Bash-POSIX-Mode" accesskey="p" rel="prev">Bash and POSIX</a>, Up: <a href="#Bash-Features" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Shell-Compatibility-Mode-1"><span>6.12 Shell Compatibility Mode<a class="copiable-link" href="#Shell-Compatibility-Mode-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-Compatibility-Level"></a>
<a class="index-entry-id" id="index-Compatibility-Mode"></a>

<p>Bash-4.0 introduced the concept of a <em class="dfn">shell compatibility level</em>,
specified as a set of options to the shopt builtin
(<code class="code">compat31</code>,
<code class="code">compat32</code>,
<code class="code">compat40</code>,
<code class="code">compat41</code>,
and so on).
There is only one current
compatibility level &ndash; each option is mutually exclusive.
The compatibility level is intended to allow users to select behavior
from previous versions that is incompatible with newer versions
while they migrate scripts to use current features and behavior.
It&rsquo;s intended to be a temporary solution.
</p>
<p>This section does not mention behavior that is standard for a particular
version (e.g., setting <code class="code">compat32</code> means that quoting the right hand
side of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and subsequent versions). 
</p>
<p>If a user enables, say, <code class="code">compat32</code>, it may affect the behavior of other
compatibility levels up to and including the current compatibility level.
The idea is that each compatibility level controls behavior that changed
in that version of Bash,
but that behavior may have been present in earlier versions.
For instance, the change to use locale-based comparisons with the <code class="code">[[</code>
command came in bash-4.1, and earlier versions used ASCII-based comparisons,
so enabling <code class="code">compat32</code> will enable ASCII-based comparisons as well.
That granularity may not be sufficient for
all uses, and as a result users should employ compatibility levels carefully.
Read the documentation for a particular feature to find out the
current behavior.
</p>
<p>Bash-4.3 introduced a new shell variable: <code class="env">BASH_COMPAT</code>.
The value assigned
to this variable (a decimal version number like 4.2, or an integer
corresponding to the <code class="code">compat</code><var class="var">NN</var> option, like 42) determines the
compatibility level.
</p>
<p>Starting with bash-4.4, Bash began deprecating older compatibility
levels.
Eventually, the options will be removed in favor of <code class="env">BASH_COMPAT</code>.
</p>
<p>Bash-5.0 was the final version for which there was an individual shopt
option for the previous version.
<code class="env">BASH_COMPAT</code> is the only mechanism to control the compatibility level
in versions newer than bash-5.0.
</p>
<p>The following table describes the behavior changes controlled by each
compatibility level setting.
The <code class="code">compat</code><var class="var">NN</var> tag is used as shorthand for setting the
compatibility level
to <var class="var">NN</var> using one of the following mechanisms.
For versions prior to bash-5.0, the compatibility level may be set using
the corresponding <code class="code">compat</code><var class="var">NN</var> shopt option.
For bash-4.3 and later versions, the <code class="env">BASH_COMPAT</code> variable is preferred,
and it is required for bash-5.1 and later versions.
</p>
<dl class="table">
<dt><code class="code">compat31</code></dt>
<dd><ul class="itemize mark-bullet">
<li>Quoting the rhs of the <code class="code">[[</code> command&rsquo;s regexp matching operator (=~)
has no special effect
</li></ul>

</dd>
<dt><code class="code">compat40</code></dt>
<dd><ul class="itemize mark-bullet">
<li>The &lsquo;<samp class="samp">&lt;</samp>&rsquo; and &lsquo;<samp class="samp">&gt;</samp>&rsquo; operators to the <code class="code">[[</code> command do not
consider the current locale when comparing strings; they use ASCII
ordering.
Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
bash-4.1 and later use the current locale&rsquo;s collation sequence and
strcoll(3).
</li></ul>

</dd>
<dt><code class="code">compat41</code></dt>
<dd><ul class="itemize mark-bullet">
<li>In <small class="sc">POSIX</small> mode, <code class="code">time</code> may be followed by options and still be
recognized as a reserved word (this is
<small class="sc">POSIX</small>
interpretation 267).
</li><li>In <small class="sc">POSIX</small> mode, the parser requires that an even number of single
quotes occur in the <var class="var">word</var> portion of a double-quoted ${&hellip;}
parameter expansion and treats them specially, so that characters within
the single quotes are considered quoted
(this is
<small class="sc">POSIX</small>
interpretation 221).
</li></ul>

</dd>
<dt><code class="code">compat42</code></dt>
<dd><ul class="itemize mark-bullet">
<li>The replacement string in double-quoted pattern substitution does not
undergo quote removal, as it does in versions after bash-4.2.
</li><li>In <small class="sc">POSIX</small> mode, single quotes are considered special when expanding
the <var class="var">word</var> portion of a double-quoted ${&hellip;} parameter expansion
and can be used to quote a closing brace or other special character
(this is part of
<small class="sc">POSIX</small>
interpretation 221);
in later versions, single quotes
are not special within double-quoted word expansions.
</li></ul>

</dd>
<dt><code class="code">compat43</code></dt>
<dd><ul class="itemize mark-bullet">
<li>Word expansion errors are considered non-fatal errors that cause the
current command to fail, even in <small class="sc">POSIX</small> mode
(the default behavior is to make them fatal errors that cause the shell
to exit).
</li><li>When executing a shell function, the loop state (while/until/etc.)
is not reset, so <code class="code">break</code> or <code class="code">continue</code> in that function will break
or continue loops in the calling context.
Bash-4.4 and later reset the loop state to prevent this.
</li></ul>

</dd>
<dt><code class="code">compat44</code></dt>
<dd><ul class="itemize mark-bullet">
<li>The shell sets up the values used by <code class="env">BASH_ARGV</code> and <code class="env">BASH_ARGC</code>
so they can expand to the shell&rsquo;s positional parameters even if extended
debugging mode is not enabled.
</li><li>A subshell inherits loops from its parent context, so <code class="code">break</code>
or <code class="code">continue</code> will cause the subshell to exit.
Bash-5.0 and later reset the loop state to prevent the exit.
</li><li>Variable assignments preceding builtins like <code class="code">export</code> and <code class="code">readonly</code>
that set attributes continue to affect variables with the same
name in the calling environment even if the shell is not in
<small class="sc">POSIX</small>
mode.
</li></ul>

</dd>
<dt><code class="code">compat50 (set using BASH_COMPAT)</code></dt>
<dd><ul class="itemize mark-bullet">
<li>Bash-5.1 changed the way <code class="code">$RANDOM</code> is generated to introduce slightly
more randomness.
If the shell compatibility level is set to 50 or lower, it reverts to
the method from bash-5.0 and previous versions,
so seeding the random number generator by assigning a value to
<code class="env">RANDOM</code> will produce the same sequence as in bash-5.0.
</li><li>If the command hash table is empty, Bash versions prior to bash-5.1
printed an informational message to that effect, even when producing
output that can be reused as input.
Bash-5.1 suppresses that message when the <samp class="option">-l</samp> option is supplied.
</li></ul>

</dd>
<dt><code class="code">compat51 (set using BASH_COMPAT)</code></dt>
<dd><ul class="itemize mark-bullet">
<li>The <code class="code">unset</code> builtin will unset the array <code class="code">a</code> given an argument like
&lsquo;<samp class="samp">a[@]</samp>&rsquo;.
Bash-5.2 will unset an element with key &lsquo;<samp class="samp">@</samp>&rsquo; (associative arrays)
or remove all the elements without unsetting the array (indexed arrays).
</li><li>Arithmetic commands ( ((&hellip;)) ) and the expressions in an arithmetic for
statement can be expanded more than once.
</li><li>Expressions used as arguments to arithmetic operators in the <code class="code">[[</code>
conditional command can be expanded more than once.
</li><li>The expressions in substring parameter brace expansion can be
expanded more than once.
</li><li>The expressions in the $(( &hellip; )) word expansion can be expanded
more than once.
</li><li>Arithmetic expressions used as indexed array subscripts can be
expanded more than once.
</li><li><code class="code">test -v</code>, when given an argument of &lsquo;<samp class="samp">A[@]</samp>&rsquo;, where <var class="var">A</var> is
an existing associative array, will return true if the array has any set
elements.
Bash-5.2 will look for and report on a key named &lsquo;<samp class="samp">@</samp>&rsquo;.
</li><li>the ${<var class="var">parameter</var>[:]=<var class="var">value</var>} word expansion will return
<var class="var">value</var>, before any variable-specific transformations have been
performed (e.g., converting to lowercase).
Bash-5.2 will return the final value assigned to the variable.
</li><li>Parsing command substitutions will behave as if extended globbing
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
is enabled, so that parsing a command substitution containing an extglob
pattern (say, as part of a shell function) will not fail.
This assumes the intent is to enable extglob before the command is executed
and word expansions are performed.
It will fail at word expansion time if extglob hasn&rsquo;t been
enabled by the time the command is executed.
</li></ul>

</dd>
<dt><code class="code">compat52 (set using BASH_COMPAT)</code></dt>
<dd><ul class="itemize mark-bullet">
<li>The <code class="code">test</code> builtin uses its historical algorithm to parse parenthesized
subexpressions when given five or more arguments.
</li><li>If the <samp class="option">-p</samp> or <samp class="option">-P</samp> option is supplied to the <code class="code">bind</code> builtin,
<code class="code">bind</code> treats any arguments remaining after option processing
as bindable command names, and
displays any key sequences bound to those commands, instead of treating
the arguments as key sequences to bind.
</li><li>Interactive shells will notify the user of completed jobs while sourcing a
script.
Newer versions defer notification until script execution completes.
</li></ul>

</dd>
</dl>

<hr>
</div>
</div>
<div class="chapter-level-extent" id="Job-Control">
<div class="nav-panel">
<p>
Next: <a href="#Command-Line-Editing" accesskey="n" rel="next">Command Line Editing</a>, Previous: <a href="#Bash-Features" accesskey="p" rel="prev">Bash Features</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Job-Control-1"><span>7 Job Control<a class="copiable-link" href="#Job-Control-1"> &para;</a></span></h2>

<p>This chapter discusses what job control is, how it works, and how
Bash allows you to access its facilities.
</p>

<ul class="mini-toc">
<li><a href="#Job-Control-Basics" accesskey="1">Job Control Basics</a></li>
<li><a href="#Job-Control-Builtins" accesskey="2">Job Control Builtins</a></li>
<li><a href="#Job-Control-Variables" accesskey="3">Job Control Variables</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Job-Control-Basics">
<div class="nav-panel">
<p>
Next: <a href="#Job-Control-Builtins" accesskey="n" rel="next">Job Control Builtins</a>, Up: <a href="#Job-Control" accesskey="u" rel="up">Job Control</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Job-Control-Basics-1"><span>7.1 Job Control Basics<a class="copiable-link" href="#Job-Control-Basics-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-job-control-1"></a>
<a class="index-entry-id" id="index-foreground"></a>
<a class="index-entry-id" id="index-background"></a>
<a class="index-entry-id" id="index-suspending-jobs"></a>

<p>Job control
refers to the ability to selectively stop (suspend)
the execution of processes and continue (resume)
their execution at a later point.
A user typically employs
this facility via an interactive interface supplied jointly
by the operating system kernel&rsquo;s terminal driver and Bash.
</p>
<p>The shell associates a <var class="var">job</var> with each pipeline.
It keeps a
table of currently executing jobs, which the
<code class="code">jobs</code> command will display.
Each job has a <em class="dfn">job number</em>, which <code class="code">jobs</code> displays between brackets.
Job numbers start at 1.
When Bash starts a job asynchronously, it prints a line that looks
like:
</p><div class="example">
<pre class="example-preformatted">[1] 25647
</pre></div>
<p>indicating that this job is job number 1 and that the process <small class="sc">ID</small>
of the last process in the pipeline associated with this job is
25647.
All of the processes in a single pipeline are members of
the same job.
Bash uses the <var class="var">job</var> abstraction as the basis for job control. 
</p>
<p>To facilitate the implementation of the user interface to job control,
each process has a <em class="dfn">process group <small class="sc">ID</small></em>, and
the operating system maintains the notion of a current terminal
process group <small class="sc">ID</small>.
This terminal process group <small class="sc">ID</small> is associated with the
<em class="dfn">controlling terminal</em>.
</p>
<p>Processes that have the same process group ID are said to be part of
the same <em class="dfn">process group</em>.
Members of the foreground process group (processes whose
process group <small class="sc">ID</small> is equal to the current terminal process group
<small class="sc">ID</small>) receive keyboard-generated signals such as <code class="code">SIGINT</code>. 
Processes in the foreground process group are said to be
foreground processes.
Background processes
are those whose process group <small class="sc">ID</small> differs from the
controlling terminal&rsquo;s;
such processes are immune to keyboard-generated signals.
Only foreground processes are allowed to read from or,
if the user so specifies with
<code class="code">stty tostop</code>,
write to the controlling terminal.
The system sends a
<code class="code">SIGTTIN</code> (<code class="code">SIGTTOU</code>)
signal to background processes which attempt to
read from (write to when <code class="code">tostop</code> is in effect)
the terminal,
which, unless caught, suspends the process. 
</p>
<p>If the operating system on which Bash is running supports
job control, Bash contains facilities to use it.
Typing the
<em class="dfn">suspend</em> character (typically &lsquo;<samp class="samp">^Z</samp>&rsquo;, Control-Z) while a
process is running stops that process
and returns control to Bash.
Typing the <em class="dfn">delayed suspend</em> character
(typically &lsquo;<samp class="samp">^Y</samp>&rsquo;, Control-Y) causes the process to stop
when it attempts to read input from the terminal,
and returns control to Bash.
The user then manipulates the state of
this job, using
the <code class="code">bg</code> command to continue it in the background,
the <code class="code">fg</code> command to continue it in the foreground, or
the <code class="code">kill</code> command to kill it.
The suspend character
takes effect immediately, and has the additional side effect of
discarding any pending output and typeahead.
If you want to force a background process to stop, or stop a process
that&rsquo;s not associated with your terminal session,
send it the <code class="code">SIGSTOP</code> signal using <code class="code">kill</code>.
</p>
<p>There are a number of ways to refer to a job in the shell.
The &lsquo;<samp class="samp">%</samp>&rsquo; character introduces a <em class="dfn">job specification</em> (jobspec).
</p>
<p>Job number <code class="code">n</code> may be referred to as &lsquo;<samp class="samp">%n</samp>&rsquo;.
A job may also be referred to
using a prefix of the name used to start it,
or using a substring that appears in its command line.
For example, &lsquo;<samp class="samp">%ce</samp>&rsquo; refers
to a job whose command name begins with &lsquo;<samp class="samp">ce</samp>&rsquo;.
Using &lsquo;<samp class="samp">%?ce</samp>&rsquo;, on the
other hand, refers to any job containing the string &lsquo;<samp class="samp">ce</samp>&rsquo; in
its command line.
If the prefix or substring matches more than one job,
Bash reports an error.
</p>
<p>The symbols &lsquo;<samp class="samp">%%</samp>&rsquo; and  &lsquo;<samp class="samp">%+</samp>&rsquo; refer to the shell&rsquo;s notion of the
<em class="dfn">current job</em>.
A single &lsquo;<samp class="samp">%</samp>&rsquo; (with no accompanying job specification) also refers
to the current job.
&lsquo;<samp class="samp">%-</samp>&rsquo; refers to the <em class="dfn">previous job</em>.
When a job starts in the background,
a job stops while in the foreground,
or a job is resumed in the background,
it becomes the current job.
The job that was the current job becomes the previous job.
When the current job terminates, the previous job becomes the
current job.
If there is only a single job, &lsquo;<samp class="samp">%+</samp>&rsquo; and &lsquo;<samp class="samp">%-</samp>&rsquo; can both be used
to refer to that job.
In output pertaining to jobs (e.g., the output of the <code class="code">jobs</code>
command), the current job is always marked with a &lsquo;<samp class="samp">+</samp>&rsquo;, and the
previous job with a &lsquo;<samp class="samp">-</samp>&rsquo;. 
</p>
<p>Simply naming a job can be used to bring it into the foreground:
&lsquo;<samp class="samp">%1</samp>&rsquo; is a synonym for &lsquo;<samp class="samp">fg %1</samp>&rsquo;, bringing job 1 from the
background into the foreground.
Similarly, &lsquo;<samp class="samp">%1 &amp;</samp>&rsquo; resumes
job 1 in the background, equivalent to &lsquo;<samp class="samp">bg %1</samp>&rsquo;.
</p>
<p>The shell learns immediately whenever a job changes state. 
Normally, Bash waits until it is about to print a prompt before
notifying the user about
changes in a job&rsquo;s status so as to not interrupt
any other output,
though it will notify of changes in a job&rsquo;s status after a
foreground command in
a list completes, before executing the next command in the list.
If the <samp class="option">-b</samp> option to the <code class="code">set</code> builtin is enabled,
Bash reports status changes immediately (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
Bash executes any trap on <code class="code">SIGCHLD</code>
for each child process that terminates.
</p>
<p>When a job terminates and Bash notifies the user about it,
Bash removes the job from the jobs table.
It will not appear in <code class="code">jobs</code> output, but <code class="code">wait</code> will
report its exit status, as long as it&rsquo;s supplied the process ID
associated with the job as an argument.
When the table is empty, job numbers start over at 1.
</p>
<p>If a user attempts to exit
Bash while jobs are stopped, (or running, if
the <code class="code">checkjobs</code> option is enabled &ndash; see <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), the
shell prints a warning message, and if the <code class="code">checkjobs</code> option is
enabled, lists the jobs and their statuses.
The <code class="code">jobs</code> command may then be used to inspect their status.
If the user immediately attempts to exit again,
without an intervening command,
Bash does not print another warning, and
terminates any stopped jobs.
</p>
<p>When the shell is waiting for a job or process using the <code class="code">wait</code>
builtin, and job control is enabled, <code class="code">wait</code> will return when the
job changes state.
The <samp class="option">-f</samp> option causes <code class="code">wait</code> to wait
until the job or process terminates before returning.
</p>
<hr>
</div>
<div class="section-level-extent" id="Job-Control-Builtins">
<div class="nav-panel">
<p>
Next: <a href="#Job-Control-Variables" accesskey="n" rel="next">Job Control Variables</a>, Previous: <a href="#Job-Control-Basics" accesskey="p" rel="prev">Job Control Basics</a>, Up: <a href="#Job-Control" accesskey="u" rel="up">Job Control</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Job-Control-Builtins-1"><span>7.2 Job Control Builtins<a class="copiable-link" href="#Job-Control-Builtins-1"> &para;</a></span></h3>

<dl class="table">
<dt><a id="index-bg"></a><span><code class="code">bg</code><a class="copiable-link" href="#index-bg"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">bg [<var class="var">jobspec</var> ...]
</pre></div>

<p>Resume each suspended job <var class="var">jobspec</var> in the background, as if it
had been started with &lsquo;<samp class="samp">&amp;</samp>&rsquo;.
If <var class="var">jobspec</var> is not supplied, the shell uses its
notion of the current job.
<code class="code">bg</code> returns  zero unless it is run when job control is not
enabled, or, when run with job control enabled, any
<var class="var">jobspec</var> was not found or specifies a job
that was started without job control.
</p>
</dd>
<dt><a id="index-fg"></a><span><code class="code">fg</code><a class="copiable-link" href="#index-fg"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">fg [<var class="var">jobspec</var>]
</pre></div>

<p>Resume the job <var class="var">jobspec</var> in the foreground and make it the current job.
If <var class="var">jobspec</var> is not supplied, <code class="code">fg</code> resumes the current job.
The return status is that of the command placed into the foreground,
or non-zero if run when job control is disabled or, when run with
job control enabled, <var class="var">jobspec</var> does not specify a valid job or
<var class="var">jobspec</var> specifies a job that was started without job control.
</p>
</dd>
<dt><a id="index-jobs"></a><span><code class="code">jobs</code><a class="copiable-link" href="#index-jobs"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">jobs [-lnprs] [<var class="var">jobspec</var>]
jobs -x <var class="var">command</var> [<var class="var">arguments</var>]
</pre></div>

<p>The first form lists the active jobs.
The options have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-l</code></dt>
<dd><p>List process <small class="sc">ID</small>s in addition to the normal information.
</p>
</dd>
<dt><code class="code">-n</code></dt>
<dd><p>Display information only about jobs that have changed status since
the user was last notified of their status.
</p>
</dd>
<dt><code class="code">-p</code></dt>
<dd><p>List only the process <small class="sc">ID</small> of the job&rsquo;s process group leader.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>Display only running jobs.
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>Display only stopped jobs.
</p></dd>
</dl>

<p>If <var class="var">jobspec</var> is supplied,
<code class="code">jobs</code> restricts output to information about that job. 
If <var class="var">jobspec</var> is not supplied, <code class="code">jobs</code> lists the status of all jobs.
The return status is zero unless an invalid option is encountered
or an invalid
<var class="var">jobspec</var>
is supplied.
</p>
<p>If the <samp class="option">-x</samp> option is supplied, <code class="code">jobs</code> replaces any
<var class="var">jobspec</var> found in <var class="var">command</var> or <var class="var">arguments</var> with the
corresponding process group <small class="sc">ID</small>, and executes <var class="var">command</var>,
passing it <var class="var">argument</var>s, returning its exit status. 
</p>
</dd>
<dt><a id="index-kill"></a><span><code class="code">kill</code><a class="copiable-link" href="#index-kill"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">kill [-s <var class="var">sigspec</var>] [-n <var class="var">signum</var>] [-<var class="var">sigspec</var>] <var class="var">id</var> [...]
kill -l|-L [<var class="var">exit_status</var>]
</pre></div>

<p>Send a signal specified by <var class="var">sigspec</var> or <var class="var">signum</var> to the processes
named by each <var class="var">id</var>.
Each <var class="var">id</var> may be a
job specification <var class="var">jobspec</var> or process <small class="sc">ID</small> <var class="var">pid</var>.
<var class="var">sigspec</var> is either a case-insensitive signal name such as
<code class="code">SIGINT</code> (with or without the <code class="code">SIG</code> prefix)
or a signal number; <var class="var">signum</var> is a signal number.
If <var class="var">sigspec</var> and <var class="var">signum</var> are not present, <code class="code">kill</code>
sends <code class="code">SIGTERM</code>.
</p>
<p>The <samp class="option">-l</samp> option lists the signal names.
If any arguments are supplied when <samp class="option">-l</samp> is supplied,
<code class="code">kill</code> lists the names of the signals corresponding to the arguments,
and the return status is zero.
<var class="var">exit_status</var> is a number specifying a signal number or the exit
status of a process terminated by a signal;
if it is supplied, <code class="code">kill</code> prints the name of the signal that caused
the process to terminate.
<code class="code">kill</code> assumes that process exit statuses are greater than 128;
anything less than that is a signal number.
The <samp class="option">-L</samp> option is equivalent to <samp class="option">-l</samp>.
</p>
<p>The return status is zero if at least one signal was successfully sent,
or non-zero if an error occurs or an invalid option is encountered.
</p>
</dd>
<dt><a id="index-wait"></a><span><code class="code">wait</code><a class="copiable-link" href="#index-wait"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">wait [-fn] [-p <var class="var">varname</var>] [<var class="var">id</var> ...]
</pre></div>

<p>Wait until the child process specified by each <var class="var">id</var> exits and
return the exit status of the last <var class="var">id</var>.
Each <var class="var">id</var> may be a process <small class="sc">ID</small> <var class="var">pid</var>
or a job specification <var class="var">jobspec</var>;
if a jobspec is supplied, <code class="code">wait</code> waits for all processes in the job.
</p>
<p>If no options or <var class="var">id</var>s are supplied,
<code class="code">wait</code> waits for all running background jobs and
the last-executed process substitution,
if its process id is the same as <var class="var">$!</var>,
and the return status is zero.
</p>
<p>If the <samp class="option">-n</samp> option is supplied, <code class="code">wait</code> waits for any one of
the <var class="var">id</var>s or,
if no <var class="var">id</var>s are supplied, any job or process substitution,
to complete and returns its exit status.
If none of the supplied <var class="var">id</var>s is a child of the shell,
or if no arguments are supplied and the shell has no unwaited-for children,
the exit status is 127.
</p>
<p>If the <samp class="option">-p</samp> option is supplied, <code class="code">wait</code> assigns
the process or job identifier of the job
for which the exit status is returned to the
variable <var class="var">varname</var> named by the option argument.
The variable,
which cannot be readonly,
will be unset initially, before any assignment.
This is useful only when used with the <samp class="option">-n</samp> option.
</p>
<p>Supplying the <samp class="option">-f</samp> option, when job control is enabled,
forces <code class="code">wait</code> to wait for each <var class="var">id</var> to terminate before
returning its status, instead of returning when it changes status.
</p>
<p>If none of the <var class="var">id</var>s specify one of the shell&rsquo;s an active child
processes, the return status is 127.
If <code class="code">wait</code> is interrupted by a signal,
any <var class="var">varname</var> will remain unset,
and the return status will be greater
than 128, as described above (see <a class="pxref" href="#Signals">Signals</a>).
Otherwise, the return status is the exit  status of the last <var class="var">id</var>.
</p>
</dd>
<dt><a id="index-disown"></a><span><code class="code">disown</code><a class="copiable-link" href="#index-disown"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">disown [-ar] [-h] [<var class="var">id</var> ...]
</pre></div>

<p>Without options, remove each <var class="var">id</var> from the table of
active jobs.
Each <var class="var">id</var> may be a job specification <var class="var">jobspec</var>
or a process <small class="sc">ID</small> <var class="var">pid</var>;
if <var class="var">id</var> is a <var class="var">pid</var>,
<code class="code">disown</code> uses the job containing <var class="var">pid</var> as <var class="var">jobspec</var>.
</p>
<p>If the <samp class="option">-h</samp> option is supplied,
<code class="code">disown</code> does not remove the jobs corresponding to each <code class="code">id</code>
from the jobs table,
but rather marks them so the shell does not send
<code class="code">SIGHUP</code>
to the job if the shell receives a
<code class="code">SIGHUP</code>.
</p>
<p>If no <var class="var">id</var> is supplied, the <samp class="option">-a</samp> option means to remove or
mark all jobs; the <samp class="option">-r</samp> option without an <var class="var">id</var>
argument removes or marks running jobs.
If no <var class="var">id</var> is supplied,
and neither the <samp class="option">-a</samp> nor the <samp class="option">-r</samp> option is supplied,
<code class="code">disown</code> removes or marks the current job.
</p>
<p>The return value is 0 unless an <var class="var">id</var> does not specify a valid job.
</p>
</dd>
<dt><a id="index-suspend"></a><span><code class="code">suspend</code><a class="copiable-link" href="#index-suspend"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">suspend [-f]
</pre></div>

<p>Suspend the execution of this shell until it receives a
<code class="code">SIGCONT</code> signal.
A login shell, or a shell without job control enabled,
cannot be suspended; the <samp class="option">-f</samp>
option will override this and force the suspension.
The return status is 0 unless the shell is a login shell
or job control is not enabled
and
<samp class="option">-f</samp>
is not supplied.
</p>
</dd>
</dl>

<p>When job control is not active, the <code class="code">kill</code> and <code class="code">wait</code>
builtins do not accept <var class="var">jobspec</var> arguments.
They must be supplied process <small class="sc">ID</small>s.
</p>
<hr>
</div>
<div class="section-level-extent" id="Job-Control-Variables">
<div class="nav-panel">
<p>
Previous: <a href="#Job-Control-Builtins" accesskey="p" rel="prev">Job Control Builtins</a>, Up: <a href="#Job-Control" accesskey="u" rel="up">Job Control</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Job-Control-Variables-1"><span>7.3 Job Control Variables<a class="copiable-link" href="#Job-Control-Variables-1"> &para;</a></span></h3>

<dl class="vtable">
<dt><a id="index-auto_005fresume"></a><span><code class="code">auto_resume</code><a class="copiable-link" href="#index-auto_005fresume"> &para;</a></span></dt>
<dd><p>This variable controls how the shell interacts with the user and
job control.
If this variable exists then simple commands
consisting of only a single word,
without redirections, are treated as candidates for resumption
of an existing job.
There is no ambiguity allowed; if there is more than one job
beginning with or containing the word, then
this selects the most recently accessed job.
The name of a stopped job, in this context, is the command line
used to start it, as displayed by <code class="code">jobs</code>.
If this variable is set to the value &lsquo;<samp class="samp">exact</samp>&rsquo;,
the word must match the name of a stopped job exactly;
if set to &lsquo;<samp class="samp">substring</samp>&rsquo;,
the word needs to match a substring of the name of a stopped job.
The &lsquo;<samp class="samp">substring</samp>&rsquo; value provides functionality
analogous to the &lsquo;<samp class="samp">%?string</samp>&rsquo; job <small class="sc">ID</small> (see <a class="pxref" href="#Job-Control-Basics">Job Control Basics</a>).
If set to any other value (e.g., &lsquo;<samp class="samp">prefix</samp>&rsquo;),
the word must be a prefix of a stopped job&rsquo;s name;
this provides functionality analogous to the &lsquo;<samp class="samp">%string</samp>&rsquo; job <small class="sc">ID</small>.
</p>
</dd>
</dl>

<a class="index-entry-id" id="index-Readline_002c-how-to-use"></a>




<hr>
</div>
</div>
<div class="chapter-level-extent" id="Command-Line-Editing">
<div class="nav-panel">
<p>
Next: <a href="#Using-History-Interactively" accesskey="n" rel="next">Using History Interactively</a>, Previous: <a href="#Job-Control" accesskey="p" rel="prev">Job Control</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Command-Line-Editing-1"><span>8 Command Line Editing<a class="copiable-link" href="#Command-Line-Editing-1"> &para;</a></span></h2>

<p>This chapter describes the basic features of the <small class="sc">GNU</small>
command line editing interface.
Command line editing is provided by the Readline library, which is
used by several different programs, including Bash.
Command line editing is enabled by default when using an interactive shell,
unless the <samp class="option">--noediting</samp> option is supplied at shell invocation.
Line editing is also used when using the <samp class="option">-e</samp> option to the
<code class="code">read</code> builtin command (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
By default, the line editing commands are similar to those of Emacs;
a vi-style line editing interface is also available.
Line editing can be enabled at any time using the <samp class="option">-o emacs</samp> or
<samp class="option">-o vi</samp> options to the <code class="code">set</code> builtin command
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>), or disabled using the <samp class="option">+o emacs</samp> or 
<samp class="option">+o vi</samp> options to <code class="code">set</code>.
</p>

<ul class="mini-toc">
<li><a href="#Introduction-and-Notation" accesskey="1">Introduction to Line Editing</a></li>
<li><a href="#Readline-Interaction" accesskey="2">Readline Interaction</a></li>
<li><a href="#Readline-Init-File" accesskey="3">Readline Init File</a></li>
<li><a href="#Bindable-Readline-Commands" accesskey="4">Bindable Readline Commands</a></li>
<li><a href="#Readline-vi-Mode" accesskey="5">Readline vi Mode</a></li>
<li><a href="#Programmable-Completion" accesskey="6">Programmable Completion</a></li>
<li><a href="#Programmable-Completion-Builtins" accesskey="7">Programmable Completion Builtins</a></li>
<li><a href="#A-Programmable-Completion-Example" accesskey="8">A Programmable Completion Example</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Introduction-and-Notation">
<div class="nav-panel">
<p>
Next: <a href="#Readline-Interaction" accesskey="n" rel="next">Readline Interaction</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Introduction-to-Line-Editing"><span>8.1 Introduction to Line Editing<a class="copiable-link" href="#Introduction-to-Line-Editing"> &para;</a></span></h3>

<p>The following paragraphs use Emacs style to
describe the notation used to represent keystrokes.
</p>
<p>The text <kbd class="kbd">C-k</kbd> is read as &lsquo;Control-K&rsquo; and describes the character
produced when the <kbd class="key">k</kbd> key is pressed while the Control key
is depressed.
</p>
<p>The text <kbd class="kbd">M-k</kbd> is read as &lsquo;Meta-K&rsquo; and describes the character
produced when the Meta key (if you have one) is depressed, and the <kbd class="key">k</kbd>
key is pressed (a <em class="dfn">meta character</em>), then both are released.
The Meta key is labeled <kbd class="key">ALT</kbd> or <kbd class="key">Option</kbd> on many keyboards.
On keyboards with two keys labeled <kbd class="key">ALT</kbd> (usually to either side of
the space bar), the <kbd class="key">ALT</kbd> on the left side is generally set to
work as a Meta key.
One of the <kbd class="key">ALT</kbd> keys may also be configured
as some other modifier, such as a
Compose key for typing accented characters.
</p>
<p>On some keyboards, the Meta key modifier produces characters with
the eighth bit (0200) set.
You can use the <code class="code">enable-meta-key</code> variable
to control whether or not it does this, if the keyboard allows it.
On many others, the terminal or terminal emulator converts the metafied
key to a key sequence beginning with <kbd class="key">ESC</kbd> as described in the
next paragraph.
</p>
<p>If you do not have a Meta or <kbd class="key">ALT</kbd> key, or another key working as
a Meta key, you can generally achieve the latter effect by typing <kbd class="key">ESC</kbd>
<em class="emph">first</em>, and then typing <kbd class="key">k</kbd>.
The <kbd class="key">ESC</kbd> character is known as the <em class="dfn">meta prefix</em>).
</p>
<p>Either process is known as <em class="dfn">metafying</em> the <kbd class="key">k</kbd> key.
</p>
<p>If your Meta key produces a key sequence with the <kbd class="key">ESC</kbd> meta prefix,
you can make <kbd class="kbd">M-key</kbd> key bindings you specify
(see <code class="code">Key Bindings</code> in <a class="ref" href="#Readline-Init-File-Syntax">Readline Init File Syntax</a>)
do the same thing by setting the <code class="code">force-meta-prefix</code> variable.
</p>
<p>The text <kbd class="kbd">M-C-k</kbd> is read as &lsquo;Meta-Control-k&rsquo; and describes the
character produced by metafying <kbd class="kbd">C-k</kbd>.
</p>
<p>In addition, several keys have their own names.
Specifically,
<kbd class="key">DEL</kbd>, <kbd class="key">ESC</kbd>, <kbd class="key">LFD</kbd>, <kbd class="key">SPC</kbd>, <kbd class="key">RET</kbd>, and <kbd class="key">TAB</kbd> all
stand for themselves when seen in this text, or in an init file
(see <a class="pxref" href="#Readline-Init-File">Readline Init File</a>).
If your keyboard lacks a <kbd class="key">LFD</kbd> key, typing <kbd class="key">C-j</kbd> will
output the appropriate character.
The <kbd class="key">RET</kbd> key may be labeled <kbd class="key">Return</kbd> or <kbd class="key">Enter</kbd> on
some keyboards.
</p>
<hr>
</div>
<div class="section-level-extent" id="Readline-Interaction">
<div class="nav-panel">
<p>
Next: <a href="#Readline-Init-File" accesskey="n" rel="next">Readline Init File</a>, Previous: <a href="#Introduction-and-Notation" accesskey="p" rel="prev">Introduction to Line Editing</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Readline-Interaction-1"><span>8.2 Readline Interaction<a class="copiable-link" href="#Readline-Interaction-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-interaction_002c-readline"></a>

<p>Often during an interactive session you type in a long line of text,
only to notice that the first word on the line is misspelled.
The Readline library gives you a set of commands for manipulating the text
as you type it in, allowing you to just fix your typo, and not forcing
you to retype the majority of the line.
Using these editing commands,
you move the cursor to the place that needs correction, and delete or
insert the text of the corrections.
Then, when you are satisfied with the line, you simply press <kbd class="key">RET</kbd>.
You do not have to be at the
end of the line to press <kbd class="key">RET</kbd>; the entire line is accepted
regardless of the location of the cursor within the line.
</p>

<ul class="mini-toc">
<li><a href="#Readline-Bare-Essentials" accesskey="1">Readline Bare Essentials</a></li>
<li><a href="#Readline-Movement-Commands" accesskey="2">Readline Movement Commands</a></li>
<li><a href="#Readline-Killing-Commands" accesskey="3">Readline Killing Commands</a></li>
<li><a href="#Readline-Arguments" accesskey="4">Readline Arguments</a></li>
<li><a href="#Searching" accesskey="5">Searching for Commands in the History</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Readline-Bare-Essentials">
<div class="nav-panel">
<p>
Next: <a href="#Readline-Movement-Commands" accesskey="n" rel="next">Readline Movement Commands</a>, Up: <a href="#Readline-Interaction" accesskey="u" rel="up">Readline Interaction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Readline-Bare-Essentials-1"><span>8.2.1 Readline Bare Essentials<a class="copiable-link" href="#Readline-Bare-Essentials-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-notation_002c-readline"></a>
<a class="index-entry-id" id="index-command-editing"></a>
<a class="index-entry-id" id="index-editing-command-lines"></a>

<p>In order to enter characters into the line, simply type them.
The typed
character appears where the cursor was, and then the cursor moves one
space to the right.
If you mistype a character, you can use your
erase character to back up and delete the mistyped character.
</p>
<p>Sometimes you may mistype a character, and
not notice the error until you have typed several other characters.
In that case, you can type <kbd class="kbd">C-b</kbd> to move the cursor to the left,
and then correct your mistake.
Afterwards, you can move the cursor to the right with <kbd class="kbd">C-f</kbd>.
</p>
<p>When you add text in the middle of a line, you will notice that characters
to the right of the cursor are &lsquo;pushed over&rsquo; to make room for the text
that you have inserted.
Likewise, when you delete text behind the cursor,
characters to the right of the cursor are &lsquo;pulled back&rsquo; to fill in the
blank space created by the removal of the text.
These are the bare
essentials for editing the text of an input line:
</p>
<dl class="table">
<dt><kbd class="kbd">C-b</kbd></dt>
<dd><p>Move back one character.
</p></dd>
<dt><kbd class="kbd">C-f</kbd></dt>
<dd><p>Move forward one character.
</p></dd>
<dt><kbd class="key">DEL</kbd> or <kbd class="key">Backspace</kbd></dt>
<dd><p>Delete the character to the left of the cursor.
</p></dd>
<dt><kbd class="kbd">C-d</kbd></dt>
<dd><p>Delete the character underneath the cursor.
</p></dd>
<dt>Printing&nbsp;characters<!-- /@w --></dt>
<dd><p>Insert the character into the line at the cursor.
</p></dd>
<dt><kbd class="kbd">C-_</kbd> or <kbd class="kbd">C-x C-u</kbd></dt>
<dd><p>Undo the last editing command.
You can undo all the way back to an empty line.
</p></dd>
</dl>

<p>Depending on your configuration, the <kbd class="key">Backspace</kbd> key might be set to
delete the character to the left of the cursor and the <kbd class="key">DEL</kbd> key set
to delete the character underneath the cursor, like <kbd class="kbd">C-d</kbd>, rather
than the character to the left of the cursor.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Readline-Movement-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Readline-Killing-Commands" accesskey="n" rel="next">Readline Killing Commands</a>, Previous: <a href="#Readline-Bare-Essentials" accesskey="p" rel="prev">Readline Bare Essentials</a>, Up: <a href="#Readline-Interaction" accesskey="u" rel="up">Readline Interaction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Readline-Movement-Commands-1"><span>8.2.2 Readline Movement Commands<a class="copiable-link" href="#Readline-Movement-Commands-1"> &para;</a></span></h4>

<p>The above table describes the most basic keystrokes that you need
in order to do editing of the input line.
For your convenience, many other commands are available in
addition to <kbd class="kbd">C-b</kbd>, <kbd class="kbd">C-f</kbd>, <kbd class="kbd">C-d</kbd>, and <kbd class="key">DEL</kbd>.
Here are some commands for moving more rapidly within the line.
</p>
<dl class="table">
<dt><kbd class="kbd">C-a</kbd></dt>
<dd><p>Move to the start of the line.
</p></dd>
<dt><kbd class="kbd">C-e</kbd></dt>
<dd><p>Move to the end of the line.
</p></dd>
<dt><kbd class="kbd">M-f</kbd></dt>
<dd><p>Move forward a word, where a word is composed of letters and digits.
</p></dd>
<dt><kbd class="kbd">M-b</kbd></dt>
<dd><p>Move backward a word.
</p></dd>
<dt><kbd class="kbd">C-l</kbd></dt>
<dd><p>Clear the screen, reprinting the current line at the top.
</p></dd>
</dl>

<p>Notice how <kbd class="kbd">C-f</kbd> moves forward a character, while <kbd class="kbd">M-f</kbd> moves
forward a word.
It is a loose convention that control keystrokes
operate on characters while meta keystrokes operate on words.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Readline-Killing-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Readline-Arguments" accesskey="n" rel="next">Readline Arguments</a>, Previous: <a href="#Readline-Movement-Commands" accesskey="p" rel="prev">Readline Movement Commands</a>, Up: <a href="#Readline-Interaction" accesskey="u" rel="up">Readline Interaction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Readline-Killing-Commands-1"><span>8.2.3 Readline Killing Commands<a class="copiable-link" href="#Readline-Killing-Commands-1"> &para;</a></span></h4>

<a class="index-entry-id" id="index-killing-text"></a>
<a class="index-entry-id" id="index-yanking-text"></a>

<p><em class="dfn">Killing</em> text means to delete the text from the line, but to save
it away for later use, usually by <em class="dfn">yanking</em> (re-inserting)
it back into the line.
(&lsquo;Cut&rsquo; and &lsquo;paste&rsquo; are more recent jargon for &lsquo;kill&rsquo; and &lsquo;yank&rsquo;.)
</p>
<p>If the description for a command says that it &lsquo;kills&rsquo; text, then you can
be sure that you can get the text back in a different (or the same)
place later.
</p>
<p>When you use a kill command, the text is saved in a <em class="dfn">kill-ring</em>.
Any number of consecutive kills save all of the killed text together, so
that when you yank it back, you get it all.
The kill ring is not line specific; the text that you killed on a previously
typed line is available to be yanked back later, when you are typing
another line.
<a class="index-entry-id" id="index-kill-ring"></a>
</p>
<p>Here is the list of commands for killing text.
</p>
<dl class="table">
<dt><kbd class="kbd">C-k</kbd></dt>
<dd><p>Kill the text from the current cursor position to the end of the line.
</p>
</dd>
<dt><kbd class="kbd">M-d</kbd></dt>
<dd><p>Kill from the cursor to the end of the current word, or, if between
words, to the end of the next word.
Word boundaries are the same as those used by <kbd class="kbd">M-f</kbd>.
</p>
</dd>
<dt><kbd class="kbd">M-<kbd class="key">DEL</kbd></kbd></dt>
<dd><p>Kill from the cursor to the start of the current word, or, if between
words, to the start of the previous word.
Word boundaries are the same as those used by <kbd class="kbd">M-b</kbd>.
</p>
</dd>
<dt><kbd class="kbd">C-w</kbd></dt>
<dd><p>Kill from the cursor to the previous whitespace.
This is different than
<kbd class="kbd">M-<kbd class="key">DEL</kbd></kbd> because the word boundaries differ.
</p>
</dd>
</dl>

<p>Here is how to <em class="dfn">yank</em> the text back into the line.  Yanking
means to copy the most-recently-killed text from the kill buffer
into the line at the current cursor position.
</p>
<dl class="table">
<dt><kbd class="kbd">C-y</kbd></dt>
<dd><p>Yank the most recently killed text back into the buffer at the cursor.
</p>
</dd>
<dt><kbd class="kbd">M-y</kbd></dt>
<dd><p>Rotate the kill-ring, and yank the new top.
You can only do this if the prior command is <kbd class="kbd">C-y</kbd> or <kbd class="kbd">M-y</kbd>.
</p></dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Readline-Arguments">
<div class="nav-panel">
<p>
Next: <a href="#Searching" accesskey="n" rel="next">Searching for Commands in the History</a>, Previous: <a href="#Readline-Killing-Commands" accesskey="p" rel="prev">Readline Killing Commands</a>, Up: <a href="#Readline-Interaction" accesskey="u" rel="up">Readline Interaction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Readline-Arguments-1"><span>8.2.4 Readline Arguments<a class="copiable-link" href="#Readline-Arguments-1"> &para;</a></span></h4>

<p>You can pass numeric arguments to Readline commands.
Sometimes the
argument acts as a repeat count, other times it is the <i class="i">sign</i> of the
argument that is significant.
If you pass a negative argument to a
command which normally acts in a forward direction, that command will
act in a backward direction.
For example, to kill text back to the
start of the line, you might type &lsquo;<samp class="samp">M-- C-k</samp>&rsquo;.
</p>
<p>The general way to pass numeric arguments to a command is to type meta
digits before the command.
If the first &lsquo;digit&rsquo; typed is a minus
sign (&lsquo;<samp class="samp">-</samp>&rsquo;), then the sign of the argument will be negative.
Once you have typed one meta digit to get the argument started, you can
type the remainder of the digits, and then the command.
For example, to give
the <kbd class="kbd">C-d</kbd> command an argument of 10, you could type &lsquo;<samp class="samp">M-1 0 C-d</samp>&rsquo;,
which will delete the next ten characters on the input line.
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Searching">
<div class="nav-panel">
<p>
Previous: <a href="#Readline-Arguments" accesskey="p" rel="prev">Readline Arguments</a>, Up: <a href="#Readline-Interaction" accesskey="u" rel="up">Readline Interaction</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Searching-for-Commands-in-the-History"><span>8.2.5 Searching for Commands in the History<a class="copiable-link" href="#Searching-for-Commands-in-the-History"> &para;</a></span></h4>

<p>Readline provides commands for searching through the command history
(see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>)
for lines containing a specified string.
There are two search modes:  <em class="dfn">incremental</em> and <em class="dfn">non-incremental</em>.
</p>
<p>Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, Readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
When using emacs editing mode, type <kbd class="kbd">C-r</kbd>
to search backward in the history for a particular string.
Typing <kbd class="kbd">C-s</kbd> searches forward through the history.
The characters present in the value of the <code class="code">isearch-terminators</code> variable
are used to terminate an incremental search.
If that variable has not been assigned a value, the <kbd class="key">ESC</kbd> and
<kbd class="kbd">C-j</kbd> characters terminate an incremental search.
<kbd class="kbd">C-g</kbd> aborts an incremental search and restores the original line.
When the search is terminated, the history entry containing the
search string becomes the current line.
</p>
<p>To find other matching entries in the history list, type <kbd class="kbd">C-r</kbd> or
<kbd class="kbd">C-s</kbd> as appropriate.
This searches backward or forward in the history for the next
entry matching the search string typed so far.
Any other key sequence bound to a Readline command terminates
the search and executes that command.
For instance, a <kbd class="key">RET</kbd> terminates the search and accepts
the line, thereby executing the command from the history list.
A movement command will terminate the search, make the last line found
the current line, and begin editing.
</p>
<p>Readline remembers the last incremental search string.
If two <kbd class="kbd">C-r</kbd>s are typed without any intervening characters defining
a new search string, Readline uses any remembered search string.
</p>
<p>Non-incremental searches read the entire search string before starting
to search for matching history entries.
The search string may be typed by the user or be part of the contents of
the current line.
</p>
<hr>
</div>
</div>
<div class="section-level-extent" id="Readline-Init-File">
<div class="nav-panel">
<p>
Next: <a href="#Bindable-Readline-Commands" accesskey="n" rel="next">Bindable Readline Commands</a>, Previous: <a href="#Readline-Interaction" accesskey="p" rel="prev">Readline Interaction</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Readline-Init-File-1"><span>8.3 Readline Init File<a class="copiable-link" href="#Readline-Init-File-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-initialization-file_002c-readline"></a>

<p>Although the Readline library comes with a set of Emacs-like
keybindings installed by default, it is possible to use a different set
of keybindings.
Any user can customize programs that use Readline by putting
commands in an <em class="dfn">inputrc</em> file, conventionally in their home directory.
The name of this file is taken from the value of the
shell variable <code class="env">INPUTRC</code>.
If that variable is unset, the default is <samp class="file">~/.inputrc</samp>.
If that file does not exist or cannot be read, Readline looks for
<samp class="file">/etc/inputrc</samp>.
The <code class="code">bind</code><!-- /@w --> builtin command can also be used to set Readline
keybindings and variables.
See <a class="xref" href="#Bash-Builtins">Bash Builtin Commands</a>.
</p>
<p>When a program that uses the Readline library starts up, Readline reads
the init file and sets any variables and key bindings it contains.
</p>
<p>In addition, the <code class="code">C-x C-r</code> command re-reads this init file, thus
incorporating any changes that you might have made to it.
</p>

<ul class="mini-toc">
<li><a href="#Readline-Init-File-Syntax" accesskey="1">Readline Init File Syntax</a></li>
<li><a href="#Conditional-Init-Constructs" accesskey="2">Conditional Init Constructs</a></li>
<li><a href="#Sample-Init-File" accesskey="3">Sample Init File</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Readline-Init-File-Syntax">
<div class="nav-panel">
<p>
Next: <a href="#Conditional-Init-Constructs" accesskey="n" rel="next">Conditional Init Constructs</a>, Up: <a href="#Readline-Init-File" accesskey="u" rel="up">Readline Init File</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Readline-Init-File-Syntax-1"><span>8.3.1 Readline Init File Syntax<a class="copiable-link" href="#Readline-Init-File-Syntax-1"> &para;</a></span></h4>

<p>There are only a few basic constructs allowed in the
Readline init file.
Blank lines are ignored.
Lines beginning with a &lsquo;<samp class="samp">#</samp>&rsquo; are comments.
Lines beginning with a &lsquo;<samp class="samp">$</samp>&rsquo; indicate conditional
constructs (see <a class="pxref" href="#Conditional-Init-Constructs">Conditional Init Constructs</a>).
Other lines denote variable settings and key bindings.
</p>
<dl class="table">
<dt>Variable Settings</dt>
<dd><p>You can modify the run-time behavior of Readline by
altering the values of variables in Readline
using the <code class="code">set</code> command within the init file.
The syntax is simple:
</p>
<div class="example">
<pre class="example-preformatted">set <var class="var">variable</var> <var class="var">value</var>
</pre></div>

<p>Here, for example, is how to
change from the default Emacs-like key binding to use
<code class="code">vi</code> line editing commands:
</p>
<div class="example">
<pre class="example-preformatted">set editing-mode vi
</pre></div>

<p>Variable names and values, where appropriate, are recognized without
regard to case.
Unrecognized variable names are ignored.
</p>
<p>Boolean variables (those that can be set to on or off) are set to on if
the value is null or empty, <var class="var">on</var> (case-insensitive), or 1.
Any other value results in the variable being set to off.
</p>
<p>The <code class="code">bind&nbsp;-V</code><!-- /@w --> command lists the current Readline variable names
and values.  See <a class="xref" href="#Bash-Builtins">Bash Builtin Commands</a>.
</p>
<p>A great deal of run-time behavior is changeable with the following
variables.
</p>
<a class="index-entry-id" id="index-variables_002c-readline"></a>
<dl class="table">
<dt><a id="index-active_002dregion_002dstart_002dcolor"></a><span><code class="code">active-region-start-color</code><a class="copiable-link" href="#index-active_002dregion_002dstart_002dcolor"> &para;</a></span></dt>
<dd><p>A string variable that controls the text color and background when displaying
the text in the active region (see the description of
<code class="code">enable-active-region</code> below).
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal before displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that puts the terminal in standout mode,
as obtained from the terminal&rsquo;s terminfo description.
A sample value might be &lsquo;<samp class="samp">\e[01;33m</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-active_002dregion_002dend_002dcolor"></a><span><code class="code">active-region-end-color</code><a class="copiable-link" href="#index-active_002dregion_002dend_002dcolor"> &para;</a></span></dt>
<dd><p>A string variable that &ldquo;undoes&rdquo;
the effects of <code class="code">active-region-start-color</code>
and restores &ldquo;normal&rdquo;
terminal display appearance after displaying text in the active region.
This string must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences.
It is output to the terminal after displaying the text in the active region.
This variable is reset to the default value whenever the terminal type changes.
The default value is the string that restores the terminal from standout mode,
as obtained from the terminal&rsquo;s terminfo description.
A sample value might be &lsquo;<samp class="samp">\e[0m</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-bell_002dstyle"></a><span><code class="code">bell-style</code><a class="copiable-link" href="#index-bell_002dstyle"> &para;</a></span></dt>
<dd><p>Controls what happens when Readline wants to ring the terminal bell.
If set to &lsquo;<samp class="samp">none</samp>&rsquo;, Readline never rings the bell.
If set to &lsquo;<samp class="samp">visible</samp>&rsquo;, Readline uses a visible bell if one is available.
If set to &lsquo;<samp class="samp">audible</samp>&rsquo; (the default), Readline attempts to ring
the terminal&rsquo;s bell.
</p>
</dd>
<dt><a id="index-bind_002dtty_002dspecial_002dchars"></a><span><code class="code">bind-tty-special-chars</code><a class="copiable-link" href="#index-bind_002dtty_002dspecial_002dchars"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo; (the default), Readline attempts to bind the control
characters that are
treated specially by the kernel&rsquo;s terminal driver to their
Readline equivalents.
These override the default Readline bindings described here.
Type &lsquo;<samp class="samp">stty -a</samp>&rsquo; at a Bash prompt to see your current terminal settings,
including the special control characters (usually <code class="code">cchars</code>).
</p>
</dd>
<dt><a id="index-blink_002dmatching_002dparen"></a><span><code class="code">blink-matching-paren</code><a class="copiable-link" href="#index-blink_002dmatching_002dparen"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-colored_002dcompletion_002dprefix"></a><span><code class="code">colored-completion-prefix</code><a class="copiable-link" href="#index-colored_002dcompletion_002dprefix"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, when listing completions, Readline displays the
common prefix of the set of possible completions using a different color.
The color definitions are taken from the value of the <code class="env">LS_COLORS</code>
environment variable.
If there is a color definition in <code class="env">LS_COLORS</code> for the custom suffix
&lsquo;<samp class="samp">readline-colored-completion-prefix</samp>&rsquo;, Readline uses this color for
the common prefix instead of its default.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-colored_002dstats"></a><span><code class="code">colored-stats</code><a class="copiable-link" href="#index-colored_002dstats"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline displays possible completions using different
colors to indicate their file type.
The color definitions are taken from the value of the <code class="env">LS_COLORS</code>
environment variable.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-comment_002dbegin"></a><span><code class="code">comment-begin</code><a class="copiable-link" href="#index-comment_002dbegin"> &para;</a></span></dt>
<dd><p>The string to insert at the beginning of the line by the
<code class="code">insert-comment</code> command.
The default value is <code class="code">&quot;#&quot;</code>.
</p>
</dd>
<dt><a id="index-completion_002ddisplay_002dwidth"></a><span><code class="code">completion-display-width</code><a class="copiable-link" href="#index-completion_002ddisplay_002dwidth"> &para;</a></span></dt>
<dd><p>The number of screen columns used to display possible matches
when performing completion.
The value is ignored if it is less than 0 or greater than the terminal
screen width.
A value of 0 causes matches to be displayed one per line.
The default value is -1.
</p>
</dd>
<dt><a id="index-completion_002dignore_002dcase"></a><span><code class="code">completion-ignore-case</code><a class="copiable-link" href="#index-completion_002dignore_002dcase"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline performs filename matching and completion
in a case-insensitive fashion.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-completion_002dmap_002dcase"></a><span><code class="code">completion-map-case</code><a class="copiable-link" href="#index-completion_002dmap_002dcase"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, and <var class="var">completion-ignore-case</var> is enabled, Readline
treats hyphens (&lsquo;<samp class="samp">-</samp>&rsquo;) and underscores (&lsquo;<samp class="samp">_</samp>&rsquo;) as equivalent when
performing case-insensitive filename matching and completion.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-completion_002dprefix_002ddisplay_002dlength"></a><span><code class="code">completion-prefix-display-length</code><a class="copiable-link" href="#index-completion_002dprefix_002ddisplay_002dlength"> &para;</a></span></dt>
<dd><p>The maximum
length in characters of the common prefix of a list of possible
completions that is displayed without modification.
When set to a value greater than zero, Readline
replaces common prefixes longer than this value
with an ellipsis when displaying possible completions.
If a completion begins with a period,
and Readline is completing filenames,
it uses three underscores instead of an ellipsis.
</p>
</dd>
<dt><a id="index-completion_002dquery_002ditems"></a><span><code class="code">completion-query-items</code><a class="copiable-link" href="#index-completion_002dquery_002ditems"> &para;</a></span></dt>
<dd><p>The number of possible completions that determines when the user is asked
whether the list of possibilities should be displayed.
If the number of possible completions is greater than
or equal to this value,
Readline asks whether or not the user wishes to view them;
otherwise, Readline simply lists the completions.
This variable must be set to an integer value greater than or equal to zero.
A zero value means Readline should never ask; negative
values are treated as zero.
The default limit is <code class="code">100</code>.
</p>
</dd>
<dt><a id="index-convert_002dmeta"></a><span><code class="code">convert-meta</code><a class="copiable-link" href="#index-convert_002dmeta"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline converts characters it reads
that have the eighth bit set to an <small class="sc">ASCII</small> key sequence by
clearing the eighth bit and prefixing an <kbd class="key">ESC</kbd> character,
converting them to a meta-prefixed key sequence.
The default value is &lsquo;<samp class="samp">on</samp>&rsquo;, but Readline sets it to &lsquo;<samp class="samp">off</samp>&rsquo;
if the locale contains
characters whose encodings may include bytes with the eighth bit set.
This variable is dependent on the <code class="code">LC_CTYPE</code> locale category, and
may change if the locale changes.
This variable also affects key bindings;
see the description of <code class="code">force-meta-prefix</code> below.
</p>
</dd>
<dt><a id="index-disable_002dcompletion"></a><span><code class="code">disable-completion</code><a class="copiable-link" href="#index-disable_002dcompletion"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">On</samp>&rsquo;, Readline inhibits word completion.
Completion characters are inserted into the line as if they
had been mapped to <code class="code">self-insert</code>.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-echo_002dcontrol_002dcharacters"></a><span><code class="code">echo-control-characters</code><a class="copiable-link" href="#index-echo_002dcontrol_002dcharacters"> &para;</a></span></dt>
<dd><p>When set to &lsquo;<samp class="samp">on</samp>&rsquo;, on operating systems that indicate they support it,
Readline echoes a character corresponding to a signal generated from the
keyboard.
The default is &lsquo;<samp class="samp">on</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-editing_002dmode"></a><span><code class="code">editing-mode</code><a class="copiable-link" href="#index-editing_002dmode"> &para;</a></span></dt>
<dd><p>The <code class="code">editing-mode</code> variable controls the default set of
key bindings.
By default, Readline starts up in emacs editing mode, where
the keystrokes are most similar to Emacs.
This variable can be set to either &lsquo;<samp class="samp">emacs</samp>&rsquo; or &lsquo;<samp class="samp">vi</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-emacs_002dmode_002dstring"></a><span><code class="code">emacs-mode-string</code><a class="copiable-link" href="#index-emacs_002dmode_002dstring"> &para;</a></span></dt>
<dd><p>If the <var class="var">show-mode-in-prompt</var> variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when emacs editing mode is active.
The value is expanded like a
key binding, so the standard set of meta- and control- prefixes and
backslash escape sequences is available.
The &lsquo;<samp class="samp">\1</samp>&rsquo; and &lsquo;<samp class="samp">\2</samp>&rsquo; escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is &lsquo;<samp class="samp">@</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-enable_002dactive_002dregion-The"></a><span><code class="code">enable-active-region</code><a class="copiable-link" href="#index-enable_002dactive_002dregion-The"> &para;</a></span></dt>
<dd><p><em class="dfn">point</em> is the current cursor position, and <em class="dfn">mark</em> refers to a
saved cursor position (see <a class="pxref" href="#Commands-For-Moving">Commands For Moving</a>).
The text between the point and mark is referred to as the <em class="dfn">region</em>.
When this variable is set to &lsquo;<samp class="samp">On</samp>&rsquo;, Readline allows certain commands
to designate the region as <em class="dfn">active</em>.
When the region is active, Readline highlights the text in the region using
the value of the <code class="code">active-region-start-color</code>, which defaults to the
string that enables the terminal&rsquo;s standout mode. 
The active region shows the text inserted by bracketed-paste and any
matching text found by incremental and non-incremental history searches. 
The default is &lsquo;<samp class="samp">On</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-enable_002dbracketed_002dpaste"></a><span><code class="code">enable-bracketed-paste</code><a class="copiable-link" href="#index-enable_002dbracketed_002dpaste"> &para;</a></span></dt>
<dd><p>When set to &lsquo;<samp class="samp">On</samp>&rsquo;, Readline configures the terminal to insert each
paste into the editing buffer as a single string of characters, instead
of treating each character as if it had been read from the keyboard.
This is called putting the terminal into <em class="dfn">bracketed paste mode</em>;
it prevents Readline from executing any editing commands bound
to key sequences appearing in the pasted text.
The default is &lsquo;<samp class="samp">On</samp>&rsquo;. 
</p>
</dd>
<dt><a id="index-enable_002dkeypad"></a><span><code class="code">enable-keypad</code><a class="copiable-link" href="#index-enable_002dkeypad"> &para;</a></span></dt>
<dd><p>When set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline tries to enable the application
keypad when it is called.
Some systems need this to enable the arrow keys.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-enable_002dmeta_002dkey"></a><span><code class="code">enable-meta-key</code><a class="copiable-link" href="#index-enable_002dmeta_002dkey"> &para;</a></span></dt>
<dd><p>When set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline tries to enable any meta
modifier key the terminal claims to support when it is called.
On many terminals, the Meta key is used to send eight-bit characters;
this variable checks for the terminal capability that indicates the
terminal can enable and disable a mode that sets the eighth bit of a
character (0200) if the Meta key is held down when the character is
typed (a meta character).
The default is &lsquo;<samp class="samp">on</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-expand_002dtilde"></a><span><code class="code">expand-tilde</code><a class="copiable-link" href="#index-expand_002dtilde"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline attempts tilde expansion when it
attempts word completion.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-force_002dmeta_002dprefix"></a><span><code class="code">force-meta-prefix</code><a class="copiable-link" href="#index-force_002dmeta_002dprefix"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline modifies its behavior when binding key
sequences containing <kbd class="kbd">\M-</kbd> or <code class="code">Meta-</code>
(see <code class="code">Key Bindings</code> in <a class="ref" href="#Readline-Init-File-Syntax">Readline Init File Syntax</a>)
by converting a key sequence of the form
<kbd class="kbd">\M-</kbd><var class="var">C</var> or <code class="code">Meta-</code><var class="var">C</var> to the two-character sequence
<kbd class="kbd">ESC</kbd> <var class="var">C</var> (adding the meta prefix).
If <code class="code">force-meta-prefix</code> is set to &lsquo;<samp class="samp">off</samp>&rsquo; (the default),
Readline uses the value of the <code class="code">convert-meta</code> variable to determine
whether to perform this conversion:
if <code class="code">convert-meta</code> is &lsquo;<samp class="samp">on</samp>&rsquo;,
Readline performs the conversion described above;
if it is &lsquo;<samp class="samp">off</samp>&rsquo;, Readline converts <var class="var">C</var> to a meta character by
setting the eighth bit (0200).
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-history_002dpreserve_002dpoint"></a><span><code class="code">history-preserve-point</code><a class="copiable-link" href="#index-history_002dpreserve_002dpoint"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, the history code attempts to place the point (the
current cursor position) at the
same location on each history line retrieved with <code class="code">previous-history</code>
or <code class="code">next-history</code>.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-history_002dsize"></a><span><code class="code">history-size</code><a class="copiable-link" href="#index-history_002dsize"> &para;</a></span></dt>
<dd><p>Set the maximum number of history entries saved in the history list.
If set to zero, any existing history entries are deleted and no new entries
are saved.
If set to a value less than zero, the number of history entries is not
limited.
By default, Bash sets the maximum number of history entries to
the value of the <code class="code">HISTSIZE</code> shell variable.
If you try to set <var class="var">history-size</var> to a non-numeric value,
the maximum number of history entries will be set to 500.
</p>
</dd>
<dt><a id="index-horizontal_002dscroll_002dmode"></a><span><code class="code">horizontal-scroll-mode</code><a class="copiable-link" href="#index-horizontal_002dscroll_002dmode"> &para;</a></span></dt>
<dd><p>Setting this variable to &lsquo;<samp class="samp">on</samp>&rsquo; means that the text of the lines
being edited will scroll horizontally on a single screen line when
the lines are longer than the width of the screen, instead of wrapping
onto a new screen line.
This variable is automatically set to &lsquo;<samp class="samp">on</samp>&rsquo; for terminals of height 1.
By default, this variable is set to &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a class="index-entry-id" id="index-meta_002dflag"></a>
<a id="index-input_002dmeta"></a><span><code class="code">input-meta</code><a class="copiable-link" href="#index-input_002dmeta"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline enables eight-bit input (that is, it
does not clear the eighth bit in the characters it reads),
regardless of what the terminal claims it can support.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;, but Readline sets it to &lsquo;<samp class="samp">on</samp>&rsquo;
if the locale contains characters whose encodings may include bytes
with the eighth bit set.
This variable is dependent on the <code class="code">LC_CTYPE</code> locale category, and
its value may change if the locale changes.
The name <code class="code">meta-flag</code> is a synonym for <code class="code">input-meta</code>.
</p>
</dd>
<dt><a id="index-isearch_002dterminators"></a><span><code class="code">isearch-terminators</code><a class="copiable-link" href="#index-isearch_002dterminators"> &para;</a></span></dt>
<dd><p>The string of characters that should terminate an incremental search without
subsequently executing the character as a command (see <a class="pxref" href="#Searching">Searching for Commands in the History</a>).
If this variable has not been given a value, the characters <kbd class="key">ESC</kbd> and
<kbd class="kbd">C-j</kbd> terminate an incremental search.
</p>
</dd>
<dt><a id="index-keymap"></a><span><code class="code">keymap</code><a class="copiable-link" href="#index-keymap"> &para;</a></span></dt>
<dd><p>Sets Readline&rsquo;s idea of the current keymap for key binding commands.
Built-in <code class="code">keymap</code> names are
<code class="code">emacs</code>,
<code class="code">emacs-standard</code>,
<code class="code">emacs-meta</code>,
<code class="code">emacs-ctlx</code>,
<code class="code">vi</code>,
<code class="code">vi-move</code>,
<code class="code">vi-command</code>, and
<code class="code">vi-insert</code>.
<code class="code">vi</code> is equivalent to <code class="code">vi-command</code> (<code class="code">vi-move</code> is also a
synonym); <code class="code">emacs</code> is equivalent to <code class="code">emacs-standard</code>.
Applications may add additional names.
The default value is <code class="code">emacs</code>;
the value of the <code class="code">editing-mode</code> variable also affects the
default keymap.
</p>
</dd>
<dt><code class="code">keyseq-timeout</code></dt>
<dd><p>Specifies the duration Readline will wait for a character when
reading an ambiguous key sequence
(one that can form a complete key sequence using the input read so far,
or can take additional input to complete a longer key sequence).
If Readline doesn&rsquo;t receive any input within the timeout, it uses the
shorter but complete key sequence.
Readline uses this value to determine whether or not input is
available on the current input source (<code class="code">rl_instream</code> by default).
The value is specified in milliseconds, so a value of 1000 means that
Readline will wait one second for additional input.
If this variable is set to a value less than or equal to zero, or to a
non-numeric value, Readline waits until another key is pressed to
decide which key sequence to complete.
The default value is <code class="code">500</code>.
</p>
</dd>
<dt><code class="code">mark-directories</code></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, completed directory names have a slash appended.
The default is &lsquo;<samp class="samp">on</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-mark_002dmodified_002dlines"></a><span><code class="code">mark-modified-lines</code><a class="copiable-link" href="#index-mark_002dmodified_002dlines"> &para;</a></span></dt>
<dd><p>When this variable is set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline displays an
asterisk (&lsquo;<samp class="samp">*</samp>&rsquo;) at the start of history lines which have been modified.
This variable is &lsquo;<samp class="samp">off</samp>&rsquo; by default.
</p>
</dd>
<dt><a id="index-mark_002dsymlinked_002ddirectories"></a><span><code class="code">mark-symlinked-directories</code><a class="copiable-link" href="#index-mark_002dsymlinked_002ddirectories"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, completed names which are symbolic links to directories
have a slash appended, subject to the value of <code class="code">mark-directories</code>.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-match_002dhidden_002dfiles"></a><span><code class="code">match-hidden-files</code><a class="copiable-link" href="#index-match_002dhidden_002dfiles"> &para;</a></span></dt>
<dd><p>This variable, when set to &lsquo;<samp class="samp">on</samp>&rsquo;, forces Readline to match files whose
names begin with a &lsquo;<samp class="samp">.</samp>&rsquo; (hidden files) when performing filename
completion.
If set to &lsquo;<samp class="samp">off</samp>&rsquo;, the user must include the leading &lsquo;<samp class="samp">.</samp>&rsquo;
in the filename to be completed.
This variable is &lsquo;<samp class="samp">on</samp>&rsquo; by default.
</p>
</dd>
<dt><a id="index-menu_002dcomplete_002ddisplay_002dprefix"></a><span><code class="code">menu-complete-display-prefix</code><a class="copiable-link" href="#index-menu_002dcomplete_002ddisplay_002dprefix"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling through
the list.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-output_002dmeta"></a><span><code class="code">output-meta</code><a class="copiable-link" href="#index-output_002dmeta"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline displays characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;, but Readline sets it to &lsquo;<samp class="samp">on</samp>&rsquo;
if the locale contains characters whose encodings may include
bytes with the eighth bit set.
This variable is dependent on the <code class="code">LC_CTYPE</code> locale category, and
its value may change if the locale changes.
</p>
</dd>
<dt><a id="index-page_002dcompletions"></a><span><code class="code">page-completions</code><a class="copiable-link" href="#index-page_002dcompletions"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline uses an internal pager resembling
<i class="i">more</i>(1)
to display a screenful of possible completions at a time.
This variable is &lsquo;<samp class="samp">on</samp>&rsquo; by default.
</p>
</dd>
<dt><code class="code">prefer-visible-bell</code></dt>
<dd><p>See <code class="code">bell-style</code>.
</p>
</dd>
<dt><code class="code">print-completions-horizontally</code></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline displays completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-revert_002dall_002dat_002dnewline"></a><span><code class="code">revert-all-at-newline</code><a class="copiable-link" href="#index-revert_002dall_002dat_002dnewline"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline will undo all changes to history lines
before returning when executing <code class="code">accept-line</code>.
By default,
history lines may be modified and retain individual undo lists across
calls to <code class="code">readline()</code>.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-search_002dignore_002dcase"></a><span><code class="code">search-ignore-case</code><a class="copiable-link" href="#index-search_002dignore_002dcase"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, Readline performs incremental and non-incremental
history list searches in a case-insensitive fashion.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-show_002dall_002dif_002dambiguous"></a><span><code class="code">show-all-if-ambiguous</code><a class="copiable-link" href="#index-show_002dall_002dif_002dambiguous"> &para;</a></span></dt>
<dd><p>This alters the default behavior of the completion functions.
If set to &lsquo;<samp class="samp">on</samp>&rsquo;, 
words which have more than one possible completion cause the
matches to be listed immediately instead of ringing the bell.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-show_002dall_002dif_002dunmodified"></a><span><code class="code">show-all-if-unmodified</code><a class="copiable-link" href="#index-show_002dall_002dif_002dunmodified"> &para;</a></span></dt>
<dd><p>This alters the default behavior of the completion functions in
a fashion similar to <var class="var">show-all-if-ambiguous</var>.
If set to &lsquo;<samp class="samp">on</samp>&rsquo;, 
words which have more than one possible completion without any
possible partial completion (the possible completions don&rsquo;t share
a common prefix) cause the matches to be listed immediately instead
of ringing the bell.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-show_002dmode_002din_002dprompt"></a><span><code class="code">show-mode-in-prompt</code><a class="copiable-link" href="#index-show_002dmode_002din_002dprompt"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi insertion.
The mode strings are user-settable (e.g., <var class="var">emacs-mode-string</var>).
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-skip_002dcompleted_002dtext"></a><span><code class="code">skip-completed-text</code><a class="copiable-link" href="#index-skip_002dcompleted_002dtext"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, this alters the default completion behavior when
inserting a single match into the line.
It&rsquo;s only active when performing completion in the middle of a word.
If enabled, Readline does not insert characters from the completion
that match characters after point in the word being completed,
so portions of the word following the cursor are not duplicated.
For instance, if this is enabled, attempting completion when the cursor
is after the first &lsquo;<samp class="samp">e</samp>&rsquo; in &lsquo;<samp class="samp">Makefile</samp>&rsquo; will result in
&lsquo;<samp class="samp">Makefile</samp>&rsquo; rather than &lsquo;<samp class="samp">Makefilefile</samp>&rsquo;,
assuming there is a single possible completion.
The default value is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-vi_002dcmd_002dmode_002dstring"></a><span><code class="code">vi-cmd-mode-string</code><a class="copiable-link" href="#index-vi_002dcmd_002dmode_002dstring"> &para;</a></span></dt>
<dd><p>If the <var class="var">show-mode-in-prompt</var> variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in command mode.
The value is expanded like a key binding, so the standard set of
meta- and control- prefixes and backslash escape sequences is available.
The &lsquo;<samp class="samp">\1</samp>&rsquo; and &lsquo;<samp class="samp">\2</samp>&rsquo; escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is &lsquo;<samp class="samp">(cmd)</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-vi_002dins_002dmode_002dstring"></a><span><code class="code">vi-ins-mode-string</code><a class="copiable-link" href="#index-vi_002dins_002dmode_002dstring"> &para;</a></span></dt>
<dd><p>If the <var class="var">show-mode-in-prompt</var> variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in insertion mode.
The value is expanded like a key binding, so the standard set of
meta- and control- prefixes and backslash escape sequences is available.
The &lsquo;<samp class="samp">\1</samp>&rsquo; and &lsquo;<samp class="samp">\2</samp>&rsquo; escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is &lsquo;<samp class="samp">(ins)</samp>&rsquo;.
</p>
</dd>
<dt><a id="index-visible_002dstats"></a><span><code class="code">visible-stats</code><a class="copiable-link" href="#index-visible_002dstats"> &para;</a></span></dt>
<dd><p>If set to &lsquo;<samp class="samp">on</samp>&rsquo;, a character denoting a file&rsquo;s type
is appended to the filename when listing possible
completions.
The default is &lsquo;<samp class="samp">off</samp>&rsquo;.
</p>
</dd>
</dl>

</dd>
<dt>Key Bindings</dt>
<dd><p>The syntax for controlling key bindings in the init file is simple.
First you need to find the name of the command that you
want to change.
The following sections contain tables of the command
name, the default keybinding, if any, and a short description of what
the command does.
</p>
<p>Once you know the name of the command, simply place on a line
in the init file the name of the key
you wish to bind the command to, a colon, and then the name of the
command.
There can be no space between the key name and the colon &ndash; that will be
interpreted as part of the key name.
The name of the key can be expressed in different ways, depending on
what you find most comfortable.
</p>
<p>In addition to command names, Readline allows keys to be bound
to a string that is inserted when the key is pressed (a <var class="var">macro</var>).
The difference between a macro and a command is that a macro is
enclosed in single or double quotes.
</p>
<p>The <code class="code">bind&nbsp;-p</code><!-- /@w --> command displays Readline function names and
bindings in a format that can be put directly into an initialization file.
See <a class="xref" href="#Bash-Builtins">Bash Builtin Commands</a>.
</p>
<dl class="table">
<dt><var class="var">keyname</var>:&nbsp;<var class="var">function-name</var>&nbsp;or&nbsp;<var class="var">macro</var><!-- /@w --></dt>
<dd><p><var class="var">keyname</var> is the name of a key spelled out in English.
For example:
</p><div class="example">
<pre class="example-preformatted">Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: &quot;&gt; output&quot;
</pre></div>

<p>In the example above, <kbd class="kbd">C-u</kbd> is bound to the function
<code class="code">universal-argument</code>,
<kbd class="kbd">M-DEL</kbd> is bound to the function <code class="code">backward-kill-word</code>, and
<kbd class="kbd">C-o</kbd> is bound to run the macro
expressed on the right hand side (that is, to insert the text
&lsquo;<samp class="samp">&gt; output</samp>&rsquo; into the line).
</p>
<p>This key binding syntax recognizes a number of symbolic character names:
<var class="var">DEL</var>,
<var class="var">ESC</var>,
<var class="var">ESCAPE</var>,
<var class="var">LFD</var>,
<var class="var">NEWLINE</var>,
<var class="var">RET</var>,
<var class="var">RETURN</var>,
<var class="var">RUBOUT</var>
(a destructive backspace),
<var class="var">SPACE</var>,
<var class="var">SPC</var>,
and
<var class="var">TAB</var>.
</p>
</dd>
<dt>&quot;<var class="var">keyseq</var>&quot;:&nbsp;<var class="var">function-name</var>&nbsp;or&nbsp;<var class="var">macro</var><!-- /@w --></dt>
<dd><p><var class="var">keyseq</var> differs from <var class="var">keyname</var> above in that strings
denoting an entire key sequence can be specified, by placing
the key sequence in double quotes.
Some <small class="sc">GNU</small> Emacs style key escapes can be used,
as in the following example, but none of the
special character names are recognized.
</p>
<div class="example">
<pre class="example-preformatted">&quot;\C-u&quot;: universal-argument
&quot;\C-x\C-r&quot;: re-read-init-file
&quot;\e[11~&quot;: &quot;Function Key 1&quot;
</pre></div>

<p>In the above example, <kbd class="kbd">C-u</kbd> is again bound to the function
<code class="code">universal-argument</code> (just as it was in the first example),
&lsquo;<samp class="samp"><kbd class="kbd">C-x</kbd> <kbd class="kbd">C-r</kbd></samp>&rsquo; is bound to the function <code class="code">re-read-init-file</code>,
and &lsquo;<samp class="samp"><kbd class="key">ESC</kbd> <kbd class="key">[</kbd> <kbd class="key">1</kbd> <kbd class="key">1</kbd> <kbd class="key">~</kbd></samp>&rsquo; is bound to insert
the text &lsquo;<samp class="samp">Function Key 1</samp>&rsquo;.
</p>
</dd>
</dl>

<p>The following <small class="sc">GNU</small> Emacs style escape sequences are available when
specifying key sequences:
</p>
<dl class="table">
<dt><code class="code"><kbd class="kbd">\C-</kbd></code></dt>
<dd><p>A control prefix.
</p></dd>
<dt><code class="code"><kbd class="kbd">\M-</kbd></code></dt>
<dd><p>Adding the meta prefix or converting the following character to a meta
character, as described above under <code class="code">force-meta-prefix</code>
(see <code class="code">Variable Settings</code> in <a class="ref" href="#Readline-Init-File-Syntax">Readline Init File Syntax</a>).
</p></dd>
<dt><code class="code"><kbd class="kbd">\e</kbd></code></dt>
<dd><p>An escape character.
</p></dd>
<dt><code class="code"><kbd class="kbd">\\</kbd></code></dt>
<dd><p>Backslash.
</p></dd>
<dt><code class="code"><kbd class="kbd">\&quot;</kbd></code></dt>
<dd><p><kbd class="key">&quot;</kbd>, a double quotation mark.
</p></dd>
<dt><code class="code"><kbd class="kbd">\'</kbd></code></dt>
<dd><p><kbd class="key">'</kbd>, a single quote or apostrophe.
</p></dd>
</dl>

<p>In addition to the <small class="sc">GNU</small> Emacs style escape sequences, a second
set of backslash escapes is available:
</p>
<dl class="table">
<dt><code class="code">\a</code></dt>
<dd><p>alert (bell)
</p></dd>
<dt><code class="code">\b</code></dt>
<dd><p>backspace
</p></dd>
<dt><code class="code">\d</code></dt>
<dd><p>delete
</p></dd>
<dt><code class="code">\f</code></dt>
<dd><p>form feed
</p></dd>
<dt><code class="code">\n</code></dt>
<dd><p>newline
</p></dd>
<dt><code class="code">\r</code></dt>
<dd><p>carriage return
</p></dd>
<dt><code class="code">\t</code></dt>
<dd><p>horizontal tab
</p></dd>
<dt><code class="code">\v</code></dt>
<dd><p>vertical tab
</p></dd>
<dt><code class="code">\<var class="var">nnn</var></code></dt>
<dd><p>The eight-bit character whose value is the octal value <var class="var">nnn</var>
(one to three digits).
</p></dd>
<dt><code class="code">\x<var class="var">HH</var></code></dt>
<dd><p>The eight-bit character whose value is the hexadecimal value <var class="var">HH</var>
(one or two hex digits).
</p></dd>
</dl>

<p>When entering the text of a macro, single or double quotes must
be used to indicate a macro definition.
Unquoted text is assumed to be a function name.
The backslash escapes described above are expanded
in the macro body.
Backslash will quote any other character in the macro text,
including &lsquo;<samp class="samp">&quot;</samp>&rsquo; and &lsquo;<samp class="samp">'</samp>&rsquo;.
For example, the following binding will make &lsquo;<samp class="samp"><kbd class="kbd">C-x</kbd> \</samp>&rsquo;
insert a single &lsquo;<samp class="samp">\</samp>&rsquo; into the line:
</p><div class="example">
<pre class="example-preformatted">&quot;\C-x\\&quot;: &quot;\\&quot;
</pre></div>

</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Conditional-Init-Constructs">
<div class="nav-panel">
<p>
Next: <a href="#Sample-Init-File" accesskey="n" rel="next">Sample Init File</a>, Previous: <a href="#Readline-Init-File-Syntax" accesskey="p" rel="prev">Readline Init File Syntax</a>, Up: <a href="#Readline-Init-File" accesskey="u" rel="up">Readline Init File</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Conditional-Init-Constructs-1"><span>8.3.2 Conditional Init Constructs<a class="copiable-link" href="#Conditional-Init-Constructs-1"> &para;</a></span></h4>

<p>Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key
bindings and variable settings to be performed as the result
of tests.
There are four parser directives available.
</p>
<dl class="table">
<dt><code class="code">$if</code></dt>
<dd><p>The <code class="code">$if</code> construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
Readline.
The text of the test, after any comparison operator,
extends to the end of the line;
unless otherwise noted, no characters are required to isolate it.
</p>
<dl class="table">
<dt><code class="code">mode</code></dt>
<dd><p>The <code class="code">mode=</code> form of the <code class="code">$if</code> directive is used to test
whether Readline is in <code class="code">emacs</code> or <code class="code">vi</code> mode.
This may be used in conjunction
with the &lsquo;<samp class="samp">set keymap</samp>&rsquo; command, for instance, to set bindings in
the <code class="code">emacs-standard</code> and <code class="code">emacs-ctlx</code> keymaps only if
Readline is starting out in <code class="code">emacs</code> mode.
</p>
</dd>
<dt><code class="code">term</code></dt>
<dd><p>The <code class="code">term=</code> form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by the
terminal&rsquo;s function keys.
The word on the right side of the
&lsquo;<samp class="samp">=</samp>&rsquo;
is tested against both the full name of the terminal and the portion
of the terminal name before the first &lsquo;<samp class="samp">-</samp>&rsquo;.
This allows <code class="code">xterm</code> to match both <code class="code">xterm</code> and
<code class="code">xterm-256color</code>, for instance.
</p>
</dd>
<dt><code class="code">version</code></dt>
<dd><p>The <code class="code">version</code> test may be used to perform comparisons against
specific Readline versions.
The <code class="code">version</code> expands to the current Readline version.
The set of comparison operators includes
&lsquo;<samp class="samp">=</samp>&rsquo; (and &lsquo;<samp class="samp">==</samp>&rsquo;), &lsquo;<samp class="samp">!=</samp>&rsquo;, &lsquo;<samp class="samp">&lt;=</samp>&rsquo;, &lsquo;<samp class="samp">&gt;=</samp>&rsquo;, &lsquo;<samp class="samp">&lt;</samp>&rsquo;,
and &lsquo;<samp class="samp">&gt;</samp>&rsquo;.
The version number supplied on the right side of the operator consists
of a major version number, an optional decimal point, and an optional
minor version (e.g., &lsquo;<samp class="samp">7.1</samp>&rsquo;).
If the minor version is omitted, it
defaults to &lsquo;<samp class="samp">0</samp>&rsquo;.
The operator may be separated from the string <code class="code">version</code> and
from the version number argument by whitespace.
The following example sets a variable if the Readline version being used
is 7.0 or newer:
</p><div class="example">
<pre class="example-preformatted">$if version &gt;= 7.0
set show-mode-in-prompt on
$endif
</pre></div>

</dd>
<dt><code class="code">application</code></dt>
<dd><p>The <var class="var">application</var> construct is used to include
application-specific settings.
Each program using the Readline
library sets the <var class="var">application name</var>, and you can test for
a particular value. 
This could be used to bind key sequences to functions useful for
a specific program.
For instance, the following command adds a
key sequence that quotes the current or previous word in Bash:
</p><div class="example">
<pre class="example-preformatted">$if Bash
# Quote the current or previous word
&quot;\C-xq&quot;: &quot;\eb\&quot;\ef\&quot;&quot;
$endif
</pre></div>

</dd>
<dt><code class="code">variable</code></dt>
<dd><p>The <var class="var">variable</var> construct provides simple equality tests for Readline
variables and values.
The permitted comparison operators are &lsquo;<samp class="samp">=</samp>&rsquo;, &lsquo;<samp class="samp">==</samp>&rsquo;, and &lsquo;<samp class="samp">!=</samp>&rsquo;.
The variable name must be separated from the comparison operator by
whitespace; the operator may be separated from the value on the right hand
side by whitespace.
String and boolean variables may be tested.
Boolean variables must be
tested against the values <var class="var">on</var> and <var class="var">off</var>.
The following example is equivalent to the <code class="code">mode=emacs</code> test described
above:
</p><div class="example">
<pre class="example-preformatted">$if editing-mode == emacs
set show-mode-in-prompt on
$endif
</pre></div>
</dd>
</dl>

</dd>
<dt><code class="code">$else</code></dt>
<dd><p>Commands in this branch of the <code class="code">$if</code> directive are executed if
the test fails.
</p>
</dd>
<dt><code class="code">$endif</code></dt>
<dd><p>This command, as seen in the previous example, terminates an
<code class="code">$if</code> command.
</p>
</dd>
<dt><code class="code">$include</code></dt>
<dd><p>This directive takes a single filename as an argument and reads commands
and key bindings from that file.
For example, the following directive reads from <samp class="file">/etc/inputrc</samp>:
</p><div class="example">
<pre class="example-preformatted">$include /etc/inputrc
</pre></div>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Sample-Init-File">
<div class="nav-panel">
<p>
Previous: <a href="#Conditional-Init-Constructs" accesskey="p" rel="prev">Conditional Init Constructs</a>, Up: <a href="#Readline-Init-File" accesskey="u" rel="up">Readline Init File</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Sample-Init-File-1"><span>8.3.3 Sample Init File<a class="copiable-link" href="#Sample-Init-File-1"> &para;</a></span></h4>

<p>Here is an example of an <var class="var">inputrc</var> file.  This illustrates key
binding, variable assignment, and conditional syntax.
</p>
<div class="example">
<pre class="example-preformatted"># This file controls the behavior of line input editing for
# programs that use the GNU Readline library.  Existing
# programs include FTP, Bash, and GDB.
#
# You can re-read the inputrc file with C-x C-r.
# Lines beginning with '#' are comments.
#
# First, include any system-wide bindings and variable
# assignments from /etc/Inputrc
$include /etc/Inputrc

#
# Set various bindings for emacs mode.

set editing-mode emacs 

$if mode=emacs

Meta-Control-h:	backward-kill-word	Text after the function name is ignored

#
# Arrow keys in keypad mode
#
#&quot;\M-OD&quot;:        backward-char
#&quot;\M-OC&quot;:        forward-char
#&quot;\M-OA&quot;:        previous-history
#&quot;\M-OB&quot;:        next-history
#
# Arrow keys in ANSI mode
#
&quot;\M-[D&quot;:        backward-char
&quot;\M-[C&quot;:        forward-char
&quot;\M-[A&quot;:        previous-history
&quot;\M-[B&quot;:        next-history
#
# Arrow keys in 8 bit keypad mode
#
#&quot;\M-\C-OD&quot;:       backward-char
#&quot;\M-\C-OC&quot;:       forward-char
#&quot;\M-\C-OA&quot;:       previous-history
#&quot;\M-\C-OB&quot;:       next-history
#
# Arrow keys in 8 bit ANSI mode
#
#&quot;\M-\C-[D&quot;:       backward-char
#&quot;\M-\C-[C&quot;:       forward-char
#&quot;\M-\C-[A&quot;:       previous-history
#&quot;\M-\C-[B&quot;:       next-history

C-q: quoted-insert

$endif

# An old-style binding.  This happens to be the default.
TAB: complete

# Macros that are convenient for shell interaction
$if Bash
# edit the path
&quot;\C-xp&quot;: &quot;PATH=${PATH}\e\C-e\C-a\ef\C-f&quot;
# prepare to type a quoted word --
# insert open and close double quotes
# and move to just after the open quote
&quot;\C-x\&quot;&quot;: &quot;\&quot;\&quot;\C-b&quot;
# insert a backslash (testing backslash escapes
# in sequences and macros)
&quot;\C-x\\&quot;: &quot;\\&quot;
# Quote the current or previous word
&quot;\C-xq&quot;: &quot;\eb\&quot;\ef\&quot;&quot;
# Add a binding to refresh the line, which is unbound
&quot;\C-xr&quot;: redraw-current-line
# Edit variable on current line.
&quot;\M-\C-v&quot;: &quot;\C-a\C-k$\C-y\M-\C-e\C-a\C-y=&quot;
$endif

# use a visible bell if one is available
set bell-style visible

# don't strip characters to 7 bits when reading
set input-meta on

# allow iso-latin1 characters to be inserted rather
# than converted to prefix-meta sequences
set convert-meta off

# display characters with the eighth bit set directly
# rather than as meta-prefixed characters
set output-meta on

# if there are 150 or more possible completions for a word,
# ask whether or not the user wants to see all of them
set completion-query-items 150

# For FTP
$if Ftp
&quot;\C-xg&quot;: &quot;get \M-?&quot;
&quot;\C-xt&quot;: &quot;put \M-?&quot;
&quot;\M-.&quot;: yank-last-arg
$endif
</pre></div>

<hr>
</div>
</div>
<div class="section-level-extent" id="Bindable-Readline-Commands">
<div class="nav-panel">
<p>
Next: <a href="#Readline-vi-Mode" accesskey="n" rel="next">Readline vi Mode</a>, Previous: <a href="#Readline-Init-File" accesskey="p" rel="prev">Readline Init File</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bindable-Readline-Commands-1"><span>8.4 Bindable Readline Commands<a class="copiable-link" href="#Bindable-Readline-Commands-1"> &para;</a></span></h3>


<p>This section describes Readline commands that may be bound to key
sequences.
You can list your key bindings by executing
<code class="code">bind&nbsp;-P</code><!-- /@w --> or, for a more terse format, suitable for an
<var class="var">inputrc</var> file, <code class="code">bind&nbsp;-p</code><!-- /@w -->.  (See <a class="xref" href="#Bash-Builtins">Bash Builtin Commands</a>.)
Command names without an accompanying key sequence are unbound by default.
</p>
<p>In the following descriptions, <em class="dfn">point</em> refers to the current cursor
position, and <em class="dfn">mark</em> refers to a cursor position saved by the
<code class="code">set-mark</code> command.
The text between the point and mark is referred to as the <em class="dfn">region</em>.
Readline
has the concept of an <em class="emph">active region</em>:
when the region is active,
Readline redisplay highlights the region using the
value of the
<code class="code">active-region-start-color</code>
variable.
The <code class="code">enable-active-region</code> variable turns this on and off.
Several commands set the region to active; those are noted below.
</p>
<ul class="mini-toc">
<li><a href="#Commands-For-Moving" accesskey="1">Commands For Moving</a></li>
<li><a href="#Commands-For-History" accesskey="2">Commands For Manipulating The History</a></li>
<li><a href="#Commands-For-Text" accesskey="3">Commands For Changing Text</a></li>
<li><a href="#Commands-For-Killing" accesskey="4">Killing And Yanking</a></li>
<li><a href="#Numeric-Arguments" accesskey="5">Specifying Numeric Arguments</a></li>
<li><a href="#Commands-For-Completion" accesskey="6">Letting Readline Type For You</a></li>
<li><a href="#Keyboard-Macros" accesskey="7">Keyboard Macros</a></li>
<li><a href="#Miscellaneous-Commands" accesskey="8">Some Miscellaneous Commands</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Commands-For-Moving">
<div class="nav-panel">
<p>
Next: <a href="#Commands-For-History" accesskey="n" rel="next">Commands For Manipulating The History</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Commands-For-Moving-1"><span>8.4.1 Commands For Moving<a class="copiable-link" href="#Commands-For-Moving-1"> &para;</a></span></h4>
<dl class="ftable">
<dt><a id="index-beginning_002dof_002dline-_0028C_002da_0029"></a><span><code class="code">beginning-of-line (C-a)</code><a class="copiable-link" href="#index-beginning_002dof_002dline-_0028C_002da_0029"> &para;</a></span></dt>
<dd><p>Move to the start of the current line.
This may also be bound to the Home key on some keyboards.
</p>
</dd>
<dt><a id="index-end_002dof_002dline-_0028C_002de_0029"></a><span><code class="code">end-of-line (C-e)</code><a class="copiable-link" href="#index-end_002dof_002dline-_0028C_002de_0029"> &para;</a></span></dt>
<dd><p>Move to the end of the line.
This may also be bound to the End key on some keyboards.
</p>
</dd>
<dt><a id="index-forward_002dchar-_0028C_002df_0029"></a><span><code class="code">forward-char (C-f)</code><a class="copiable-link" href="#index-forward_002dchar-_0028C_002df_0029"> &para;</a></span></dt>
<dd><p>Move forward a character.
This may also be bound to the right arrow key on some keyboards.
</p>
</dd>
<dt><a id="index-backward_002dchar-_0028C_002db_0029"></a><span><code class="code">backward-char (C-b)</code><a class="copiable-link" href="#index-backward_002dchar-_0028C_002db_0029"> &para;</a></span></dt>
<dd><p>Move back a character.
This may also be bound to the left arrow key on some keyboards.
</p>
</dd>
<dt><a id="index-forward_002dword-_0028M_002df_0029"></a><span><code class="code">forward-word (M-f)</code><a class="copiable-link" href="#index-forward_002dword-_0028M_002df_0029"> &para;</a></span></dt>
<dd><p>Move forward to the end of the next word.
Words are composed of letters and digits.
</p>
</dd>
<dt><a id="index-backward_002dword-_0028M_002db_0029"></a><span><code class="code">backward-word (M-b)</code><a class="copiable-link" href="#index-backward_002dword-_0028M_002db_0029"> &para;</a></span></dt>
<dd><p>Move back to the start of the current or previous word.
Words are composed of letters and digits.
</p>
</dd>
<dt><a id="index-shell_002dforward_002dword-_0028M_002dC_002df_0029"></a><span><code class="code">shell-forward-word (M-C-f)</code><a class="copiable-link" href="#index-shell_002dforward_002dword-_0028M_002dC_002df_0029"> &para;</a></span></dt>
<dd><p>Move forward to the end of the next word.
Words are delimited by non-quoted shell metacharacters.
</p>
</dd>
<dt><a id="index-shell_002dbackward_002dword-_0028M_002dC_002db_0029"></a><span><code class="code">shell-backward-word (M-C-b)</code><a class="copiable-link" href="#index-shell_002dbackward_002dword-_0028M_002dC_002db_0029"> &para;</a></span></dt>
<dd><p>Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.
</p>
</dd>
<dt><a id="index-previous_002dscreen_002dline-_0028_0029"></a><span><code class="code">previous-screen-line ()</code><a class="copiable-link" href="#index-previous_002dscreen_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Attempt to move point to the same physical screen column on the previous
physical screen line.
This will not have the desired effect if the current
Readline line does not take up more than one physical line or if point is not
greater than the length of the prompt plus the screen width.
</p>
</dd>
<dt><a id="index-next_002dscreen_002dline-_0028_0029"></a><span><code class="code">next-screen-line ()</code><a class="copiable-link" href="#index-next_002dscreen_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Attempt to move point to the same physical screen column on the next
physical screen line.
This will not have the desired effect if the current
Readline line does not take up more than one physical line or if the length
of the current Readline line is not greater than the length of the prompt
plus the screen width.
</p>
</dd>
<dt><a id="index-clear_002ddisplay-_0028M_002dC_002dl_0029"></a><span><code class="code">clear-display (M-C-l)</code><a class="copiable-link" href="#index-clear_002ddisplay-_0028M_002dC_002dl_0029"> &para;</a></span></dt>
<dd><p>Clear the screen and, if possible, the terminal&rsquo;s scrollback buffer,
then redraw the current line,
leaving the current line at the top of the screen.
</p>
</dd>
<dt><a id="index-clear_002dscreen-_0028C_002dl_0029"></a><span><code class="code">clear-screen (C-l)</code><a class="copiable-link" href="#index-clear_002dscreen-_0028C_002dl_0029"> &para;</a></span></dt>
<dd><p>Clear the screen,
then redraw the current line,
leaving the current line at the top of the screen.
If given a numeric argument, this refreshes the current line
without clearing the screen.
</p>
</dd>
<dt><a id="index-redraw_002dcurrent_002dline-_0028_0029"></a><span><code class="code">redraw-current-line ()</code><a class="copiable-link" href="#index-redraw_002dcurrent_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Refresh the current line.  By default, this is unbound.
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Commands-For-History">
<div class="nav-panel">
<p>
Next: <a href="#Commands-For-Text" accesskey="n" rel="next">Commands For Changing Text</a>, Previous: <a href="#Commands-For-Moving" accesskey="p" rel="prev">Commands For Moving</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Commands-For-Manipulating-The-History"><span>8.4.2 Commands For Manipulating The History<a class="copiable-link" href="#Commands-For-Manipulating-The-History"> &para;</a></span></h4>

<dl class="ftable">
<dt><a id="index-accept_002dline-_0028Newline-or-Return_0029"></a><span><code class="code">accept-line (Newline or Return)</code><a class="copiable-link" href="#index-accept_002dline-_0028Newline-or-Return_0029"> &para;</a></span></dt>
<dd><p>Accept the line regardless of where the cursor is.
If this line is
non-empty, add it to the history list according to the setting of
the <code class="env">HISTCONTROL</code> and <code class="env">HISTIGNORE</code> variables.
If this line is a modified history line, then restore the history line
to its original state.
</p>
</dd>
<dt><a id="index-previous_002dhistory-_0028C_002dp_0029"></a><span><code class="code">previous-history (C-p)</code><a class="copiable-link" href="#index-previous_002dhistory-_0028C_002dp_0029"> &para;</a></span></dt>
<dd><p>Move &lsquo;back&rsquo; through the history list, fetching the previous command.
This may also be bound to the up arrow key on some keyboards.
</p>
</dd>
<dt><a id="index-next_002dhistory-_0028C_002dn_0029"></a><span><code class="code">next-history (C-n)</code><a class="copiable-link" href="#index-next_002dhistory-_0028C_002dn_0029"> &para;</a></span></dt>
<dd><p>Move &lsquo;forward&rsquo; through the history list, fetching the next command.
This may also be bound to the down arrow key on some keyboards.
</p>
</dd>
<dt><a id="index-beginning_002dof_002dhistory-_0028M_002d_003c_0029"></a><span><code class="code">beginning-of-history (M-&lt;)</code><a class="copiable-link" href="#index-beginning_002dof_002dhistory-_0028M_002d_003c_0029"> &para;</a></span></dt>
<dd><p>Move to the first line in the history.
</p>
</dd>
<dt><a id="index-end_002dof_002dhistory-_0028M_002d_003e_0029"></a><span><code class="code">end-of-history (M-&gt;)</code><a class="copiable-link" href="#index-end_002dof_002dhistory-_0028M_002d_003e_0029"> &para;</a></span></dt>
<dd><p>Move to the end of the input history, i.e., the line currently
being entered.
</p>
</dd>
<dt><a id="index-reverse_002dsearch_002dhistory-_0028C_002dr_0029"></a><span><code class="code">reverse-search-history (C-r)</code><a class="copiable-link" href="#index-reverse_002dsearch_002dhistory-_0028C_002dr_0029"> &para;</a></span></dt>
<dd><p>Search backward starting at the current line and moving &lsquo;up&rsquo; through
the history as necessary.
This is an incremental search.
This command sets the region to the matched text and activates the region.
</p>
</dd>
<dt><a id="index-forward_002dsearch_002dhistory-_0028C_002ds_0029"></a><span><code class="code">forward-search-history (C-s)</code><a class="copiable-link" href="#index-forward_002dsearch_002dhistory-_0028C_002ds_0029"> &para;</a></span></dt>
<dd><p>Search forward starting at the current line and moving &lsquo;down&rsquo; through
the history as necessary.
This is an incremental search.
This command sets the region to the matched text and activates the region.
</p>
</dd>
<dt><a id="index-non_002dincremental_002dreverse_002dsearch_002dhistory-_0028M_002dp_0029"></a><span><code class="code">non-incremental-reverse-search-history (M-p)</code><a class="copiable-link" href="#index-non_002dincremental_002dreverse_002dsearch_002dhistory-_0028M_002dp_0029"> &para;</a></span></dt>
<dd><p>Search backward starting at the current line and moving &lsquo;up&rsquo;
through the history as necessary using a non-incremental search
for a string supplied by the user.
The search string may match anywhere in a history line.
</p>
</dd>
<dt><a id="index-non_002dincremental_002dforward_002dsearch_002dhistory-_0028M_002dn_0029"></a><span><code class="code">non-incremental-forward-search-history (M-n)</code><a class="copiable-link" href="#index-non_002dincremental_002dforward_002dsearch_002dhistory-_0028M_002dn_0029"> &para;</a></span></dt>
<dd><p>Search forward starting at the current line and moving &lsquo;down&rsquo;
through the history as necessary using a non-incremental search
for a string supplied by the user.
The search string may match anywhere in a history line.
</p>
</dd>
<dt><a id="index-history_002dsearch_002dbackward-_0028_0029"></a><span><code class="code">history-search-backward ()</code><a class="copiable-link" href="#index-history_002dsearch_002dbackward-_0028_0029"> &para;</a></span></dt>
<dd><p>Search backward through the history for the string of characters
between the start of the current line and the point.
The search string must match at the beginning of a history line.
This is a non-incremental search.
By default, this command is unbound, but may be bound to the Page Down
key on some keyboards.
</p>
</dd>
<dt><a id="index-history_002dsearch_002dforward-_0028_0029"></a><span><code class="code">history-search-forward ()</code><a class="copiable-link" href="#index-history_002dsearch_002dforward-_0028_0029"> &para;</a></span></dt>
<dd><p>Search forward through the history for the string of characters
between the start of the current line and the point.
The search string must match at the beginning of a history line.
This is a non-incremental search.
By default, this command is unbound, but may be bound to the Page Up
key on some keyboards.
</p>
</dd>
<dt><a id="index-history_002dsubstring_002dsearch_002dbackward-_0028_0029"></a><span><code class="code">history-substring-search-backward ()</code><a class="copiable-link" href="#index-history_002dsubstring_002dsearch_002dbackward-_0028_0029"> &para;</a></span></dt>
<dd><p>Search backward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
This is a non-incremental search.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-history_002dsubstring_002dsearch_002dforward-_0028_0029"></a><span><code class="code">history-substring-search-forward ()</code><a class="copiable-link" href="#index-history_002dsubstring_002dsearch_002dforward-_0028_0029"> &para;</a></span></dt>
<dd><p>Search forward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
This is a non-incremental search.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-yank_002dnth_002darg-_0028M_002dC_002dy_0029"></a><span><code class="code">yank-nth-arg (M-C-y)</code><a class="copiable-link" href="#index-yank_002dnth_002darg-_0028M_002dC_002dy_0029"> &para;</a></span></dt>
<dd><p>Insert the first argument to the previous command (usually
the second word on the previous line) at point.
With an argument <var class="var">n</var>,
insert the <var class="var">n</var>th word from the previous command (the words
in the previous command begin with word 0).
A negative argument inserts the <var class="var">n</var>th word from the end of
the previous command.
Once the argument <var class="var">n</var> is computed,
this uses the history expansion facilities to extract the
<var class="var">n</var>th word, as if the
&lsquo;<samp class="samp">!<var class="var">n</var></samp>&rsquo; history expansion had been specified.
</p>
</dd>
<dt><a id="index-yank_002dlast_002darg-_0028M_002d_002e-or-M_002d_005f_0029"></a><span><code class="code">yank-last-arg (M-. or M-_)</code><a class="copiable-link" href="#index-yank_002dlast_002darg-_0028M_002d_002e-or-M_002d_005f_0029"> &para;</a></span></dt>
<dd><p>Insert last argument to the previous command (the last word of the
previous history entry).
With a numeric argument, behave exactly like <code class="code">yank-nth-arg</code>.
Successive calls to <code class="code">yank-last-arg</code> move back through the history
list, inserting the last word (or the word specified by the argument to
the first call) of each line in turn.
Any numeric argument supplied to these successive calls determines
the direction to move through the history.
A negative argument switches the direction through the history
(back or forward).
This uses the history expansion facilities to extract the
last  word, as if the
&lsquo;<samp class="samp">!$</samp>&rsquo; history expansion had been specified.
</p>
</dd>
<dt><a id="index-operate_002dand_002dget_002dnext-_0028C_002do_0029"></a><span><code class="code">operate-and-get-next (C-o)</code><a class="copiable-link" href="#index-operate_002dand_002dget_002dnext-_0028C_002do_0029"> &para;</a></span></dt>
<dd><p>Accept the current line for return to the calling application as if a
newline had been entered,
and fetch the next line relative to the current line from the history
for editing.
A numeric argument, if supplied, specifies the history entry
to use instead of the current line.
</p>
</dd>
<dt><a id="index-fetch_002dhistory-_0028_0029"></a><span><code class="code">fetch-history ()</code><a class="copiable-link" href="#index-fetch_002dhistory-_0028_0029"> &para;</a></span></dt>
<dd><p>With a numeric argument, fetch that entry from the history list
and make it the current line.
Without an argument, move back to the first entry in the history list.
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Commands-For-Text">
<div class="nav-panel">
<p>
Next: <a href="#Commands-For-Killing" accesskey="n" rel="next">Killing And Yanking</a>, Previous: <a href="#Commands-For-History" accesskey="p" rel="prev">Commands For Manipulating The History</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Commands-For-Changing-Text"><span>8.4.3 Commands For Changing Text<a class="copiable-link" href="#Commands-For-Changing-Text"> &para;</a></span></h4>

<dl class="ftable">
<dt><a id="index-end_002dof_002dfile-_0028usually-C_002dd_0029"></a><span><code class="code"><i class="i">end-of-file</i> (usually C-d)</code><a class="copiable-link" href="#index-end_002dof_002dfile-_0028usually-C_002dd_0029"> &para;</a></span></dt>
<dd><p>The character indicating end-of-file as set, for example, by
<code class="code">stty</code>.
If this character is read when there are no characters
on the line, and point is at the beginning of the line, Readline
interprets it as the end of input and returns <small class="sc">EOF</small>.
</p>
</dd>
<dt><a id="index-delete_002dchar-_0028C_002dd_0029"></a><span><code class="code">delete-char (C-d)</code><a class="copiable-link" href="#index-delete_002dchar-_0028C_002dd_0029"> &para;</a></span></dt>
<dd><p>Delete the character at point.
If this function is bound to the
same character as the tty <small class="sc">EOF</small> character, as <kbd class="kbd">C-d</kbd>
commonly is, see above for the effects.
This may also be bound to the Delete key on some keyboards.
</p>
</dd>
<dt><a id="index-backward_002ddelete_002dchar-_0028Rubout_0029"></a><span><code class="code">backward-delete-char (Rubout)</code><a class="copiable-link" href="#index-backward_002ddelete_002dchar-_0028Rubout_0029"> &para;</a></span></dt>
<dd><p>Delete the character behind the cursor.
A numeric argument means
to kill the characters, saving them on the kill ring,
instead of deleting them.
</p>
</dd>
<dt><a id="index-forward_002dbackward_002ddelete_002dchar-_0028_0029"></a><span><code class="code">forward-backward-delete-char ()</code><a class="copiable-link" href="#index-forward_002dbackward_002ddelete_002dchar-_0028_0029"> &para;</a></span></dt>
<dd><p>Delete the character under the cursor, unless the cursor is at the
end of the line, in which case the character behind the cursor is
deleted.
By default, this is not bound to a key.
</p>
</dd>
<dt><a id="index-quoted_002dinsert-_0028C_002dq-or-C_002dv_0029"></a><span><code class="code">quoted-insert (C-q or C-v)</code><a class="copiable-link" href="#index-quoted_002dinsert-_0028C_002dq-or-C_002dv_0029"> &para;</a></span></dt>
<dd><p>Add the next character typed to the line verbatim.
This is how to insert key sequences like <kbd class="kbd">C-q</kbd>, for example.
</p>

</dd>
<dt><a id="index-self_002dinsert-_0028a_002c-b_002c-A_002c-1_002c-_0021_002c-_002e_002e_002e_0029"></a><span><code class="code">self-insert (a, b, A, 1, !, &hellip;)</code><a class="copiable-link" href="#index-self_002dinsert-_0028a_002c-b_002c-A_002c-1_002c-_0021_002c-_002e_002e_002e_0029"> &para;</a></span></dt>
<dd><p>Insert the character typed.
</p>
</dd>
<dt><a id="index-bracketed_002dpaste_002dbegin-_0028_0029"></a><span><code class="code">bracketed-paste-begin ()</code><a class="copiable-link" href="#index-bracketed_002dpaste_002dbegin-_0028_0029"> &para;</a></span></dt>
<dd><p>This function is intended to be bound to the &quot;bracketed paste&quot; escape
sequence sent by some terminals, and such a binding is assigned by default.
It allows Readline to insert the pasted text as a single unit without treating
each character as if it had been read from the keyboard.
The characters
are inserted as if each one was bound to <code class="code">self-insert</code> instead of
executing any editing commands.
</p>
<p>Bracketed paste sets the region (the characters between point and the mark)
to the inserted text.
It sets the <em class="emph">active region</em>.
</p>
</dd>
<dt><a id="index-transpose_002dchars-_0028C_002dt_0029"></a><span><code class="code">transpose-chars (C-t)</code><a class="copiable-link" href="#index-transpose_002dchars-_0028C_002dt_0029"> &para;</a></span></dt>
<dd><p>Drag the character before the cursor forward over
the character at the cursor, moving the
cursor forward as well.
If the insertion point
is at the end of the line, then this
transposes the last two characters of the line.
Negative arguments have no effect.
</p>
</dd>
<dt><a id="index-transpose_002dwords-_0028M_002dt_0029"></a><span><code class="code">transpose-words (M-t)</code><a class="copiable-link" href="#index-transpose_002dwords-_0028M_002dt_0029"> &para;</a></span></dt>
<dd><p>Drag the word before point past the word after point,
moving point past that word as well.
If the insertion point is at the end of the line, this transposes
the last two words on the line.
</p>
</dd>
<dt><a id="index-shell_002dtranspose_002dwords-_0028M_002dC_002dt_0029"></a><span><code class="code">shell-transpose-words (M-C-t)</code><a class="copiable-link" href="#index-shell_002dtranspose_002dwords-_0028M_002dC_002dt_0029"> &para;</a></span></dt>
<dd><p>Drag the word before point past the word after point,
moving point past that word as well.
If the insertion point is at the end of the line, this transposes
the last two words on the line.
Word boundaries are the same as <code class="code">shell-forward-word</code> and
<code class="code">shell-backward-word</code>.
</p>
</dd>
<dt><a id="index-upcase_002dword-_0028M_002du_0029"></a><span><code class="code">upcase-word (M-u)</code><a class="copiable-link" href="#index-upcase_002dword-_0028M_002du_0029"> &para;</a></span></dt>
<dd><p>Uppercase the current (or following) word.
With a negative argument,
uppercase the previous word, but do not move the cursor.
</p>
</dd>
<dt><a id="index-downcase_002dword-_0028M_002dl_0029"></a><span><code class="code">downcase-word (M-l)</code><a class="copiable-link" href="#index-downcase_002dword-_0028M_002dl_0029"> &para;</a></span></dt>
<dd><p>Lowercase the current (or following) word.
With a negative argument,
lowercase the previous word, but do not move the cursor.
</p>
</dd>
<dt><a id="index-capitalize_002dword-_0028M_002dc_0029"></a><span><code class="code">capitalize-word (M-c)</code><a class="copiable-link" href="#index-capitalize_002dword-_0028M_002dc_0029"> &para;</a></span></dt>
<dd><p>Capitalize the current (or following) word.
With a negative argument,
capitalize the previous word, but do not move the cursor.
</p>
</dd>
<dt><a id="index-overwrite_002dmode-_0028_0029"></a><span><code class="code">overwrite-mode ()</code><a class="copiable-link" href="#index-overwrite_002dmode-_0028_0029"> &para;</a></span></dt>
<dd><p>Toggle overwrite mode.
With an explicit positive numeric argument, switches to overwrite mode.
With an explicit non-positive numeric argument, switches to insert mode.
This command affects only <code class="code">emacs</code> mode;
<code class="code">vi</code> mode does overwrite differently.
Each call to <code class="code">readline()</code> starts in insert mode.
</p>
<p>In overwrite mode, characters bound to <code class="code">self-insert</code> replace
the text at point rather than pushing the text to the right.
Characters bound to <code class="code">backward-delete-char</code> replace the character
before point with a space.
</p>
<p>By default, this command is unbound, but may be bound to the Insert
key on some keyboards.
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Commands-For-Killing">
<div class="nav-panel">
<p>
Next: <a href="#Numeric-Arguments" accesskey="n" rel="next">Specifying Numeric Arguments</a>, Previous: <a href="#Commands-For-Text" accesskey="p" rel="prev">Commands For Changing Text</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Killing-And-Yanking"><span>8.4.4 Killing And Yanking<a class="copiable-link" href="#Killing-And-Yanking"> &para;</a></span></h4>

<dl class="ftable">
<dt><a id="index-kill_002dline-_0028C_002dk_0029"></a><span><code class="code">kill-line (C-k)</code><a class="copiable-link" href="#index-kill_002dline-_0028C_002dk_0029"> &para;</a></span></dt>
<dd><p>Kill the text from point to the end of the current line.
With a negative numeric argument, kill backward from the cursor to the
beginning of the line.
</p>
</dd>
<dt><a id="index-backward_002dkill_002dline-_0028C_002dx-Rubout_0029"></a><span><code class="code">backward-kill-line (C-x Rubout)</code><a class="copiable-link" href="#index-backward_002dkill_002dline-_0028C_002dx-Rubout_0029"> &para;</a></span></dt>
<dd><p>Kill backward from the cursor to the beginning of the current line.
With a negative numeric argument, kill forward from the cursor to the
end of the line.
</p>
</dd>
<dt><a id="index-unix_002dline_002ddiscard-_0028C_002du_0029"></a><span><code class="code">unix-line-discard (C-u)</code><a class="copiable-link" href="#index-unix_002dline_002ddiscard-_0028C_002du_0029"> &para;</a></span></dt>
<dd><p>Kill backward from the cursor to the beginning of the current line.
</p>
</dd>
<dt><a id="index-kill_002dwhole_002dline-_0028_0029"></a><span><code class="code">kill-whole-line ()</code><a class="copiable-link" href="#index-kill_002dwhole_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Kill all characters on the current line, no matter where point is.
By default, this is unbound.
</p>
</dd>
<dt><a id="index-kill_002dword-_0028M_002dd_0029"></a><span><code class="code">kill-word (M-d)</code><a class="copiable-link" href="#index-kill_002dword-_0028M_002dd_0029"> &para;</a></span></dt>
<dd><p>Kill from point to the end of the current word, or if between
words, to the end of the next word.
Word boundaries are the same as <code class="code">forward-word</code>.
</p>
</dd>
<dt><a id="index-backward_002dkill_002dword-_0028M_002dDEL_0029"></a><span><code class="code">backward-kill-word (M-<kbd class="key">DEL</kbd>)</code><a class="copiable-link" href="#index-backward_002dkill_002dword-_0028M_002dDEL_0029"> &para;</a></span></dt>
<dd><p>Kill the word behind point.
Word boundaries are the same as <code class="code">backward-word</code>.
</p>
</dd>
<dt><a id="index-shell_002dkill_002dword-_0028M_002dC_002dd_0029"></a><span><code class="code">shell-kill-word (M-C-d)</code><a class="copiable-link" href="#index-shell_002dkill_002dword-_0028M_002dC_002dd_0029"> &para;</a></span></dt>
<dd><p>Kill from point to the end of the current word, or if between
words, to the end of the next word.
Word boundaries are the same as <code class="code">shell-forward-word</code>.
</p>
</dd>
<dt><a id="index-shell_002dbackward_002dkill_002dword-_0028_0029"></a><span><code class="code">shell-backward-kill-word ()</code><a class="copiable-link" href="#index-shell_002dbackward_002dkill_002dword-_0028_0029"> &para;</a></span></dt>
<dd><p>Kill the word behind point.
Word boundaries are the same as <code class="code">shell-backward-word</code>.
</p>
</dd>
<dt><a id="index-unix_002dword_002drubout-_0028C_002dw_0029"></a><span><code class="code">unix-word-rubout (C-w)</code><a class="copiable-link" href="#index-unix_002dword_002drubout-_0028C_002dw_0029"> &para;</a></span></dt>
<dd><p>Kill the word behind point, using white space as a word boundary,
saving the killed text on the kill-ring.
</p>
</dd>
<dt><a id="index-unix_002dfilename_002drubout-_0028_0029"></a><span><code class="code">unix-filename-rubout ()</code><a class="copiable-link" href="#index-unix_002dfilename_002drubout-_0028_0029"> &para;</a></span></dt>
<dd><p>Kill the word behind point, using white space and the slash character
as the word boundaries,
saving the killed text on the kill-ring.
</p>
</dd>
<dt><a id="index-delete_002dhorizontal_002dspace-_0028_0029"></a><span><code class="code">delete-horizontal-space ()</code><a class="copiable-link" href="#index-delete_002dhorizontal_002dspace-_0028_0029"> &para;</a></span></dt>
<dd><p>Delete all spaces and tabs around point.
By default, this is unbound.
</p>
</dd>
<dt><a id="index-kill_002dregion-_0028_0029"></a><span><code class="code">kill-region ()</code><a class="copiable-link" href="#index-kill_002dregion-_0028_0029"> &para;</a></span></dt>
<dd><p>Kill the text in the current region.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-copy_002dregion_002das_002dkill-_0028_0029"></a><span><code class="code">copy-region-as-kill ()</code><a class="copiable-link" href="#index-copy_002dregion_002das_002dkill-_0028_0029"> &para;</a></span></dt>
<dd><p>Copy the text in the region to the kill buffer, so it can be yanked
right away.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-copy_002dbackward_002dword-_0028_0029"></a><span><code class="code">copy-backward-word ()</code><a class="copiable-link" href="#index-copy_002dbackward_002dword-_0028_0029"> &para;</a></span></dt>
<dd><p>Copy the word before point to the kill buffer.
The word boundaries are the same as <code class="code">backward-word</code>.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-copy_002dforward_002dword-_0028_0029"></a><span><code class="code">copy-forward-word ()</code><a class="copiable-link" href="#index-copy_002dforward_002dword-_0028_0029"> &para;</a></span></dt>
<dd><p>Copy the word following point to the kill buffer.
The word boundaries are the same as <code class="code">forward-word</code>.
By default, this command is unbound.
</p>
</dd>
<dt><a id="index-yank-_0028C_002dy_0029"></a><span><code class="code">yank (C-y)</code><a class="copiable-link" href="#index-yank-_0028C_002dy_0029"> &para;</a></span></dt>
<dd><p>Yank the top of the kill ring into the buffer at point.
</p>
</dd>
<dt><a id="index-yank_002dpop-_0028M_002dy_0029"></a><span><code class="code">yank-pop (M-y)</code><a class="copiable-link" href="#index-yank_002dpop-_0028M_002dy_0029"> &para;</a></span></dt>
<dd><p>Rotate the kill-ring, and yank the new top.
You can only do this if
the prior command is <code class="code">yank</code> or <code class="code">yank-pop</code>.
</p></dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Numeric-Arguments">
<div class="nav-panel">
<p>
Next: <a href="#Commands-For-Completion" accesskey="n" rel="next">Letting Readline Type For You</a>, Previous: <a href="#Commands-For-Killing" accesskey="p" rel="prev">Killing And Yanking</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Specifying-Numeric-Arguments"><span>8.4.5 Specifying Numeric Arguments<a class="copiable-link" href="#Specifying-Numeric-Arguments"> &para;</a></span></h4>
<dl class="ftable">
<dt><a id="index-digit_002dargument-_0028M_002d0_002c-M_002d1_002c-_002e_002e_002e-M_002d_002d_0029"></a><span><code class="code">digit-argument (<kbd class="kbd">M-0</kbd>, <kbd class="kbd">M-1</kbd>, &hellip; <kbd class="kbd">M--</kbd>)</code><a class="copiable-link" href="#index-digit_002dargument-_0028M_002d0_002c-M_002d1_002c-_002e_002e_002e-M_002d_002d_0029"> &para;</a></span></dt>
<dd><p>Add this digit to the argument already accumulating, or start a new
argument.
<kbd class="kbd">M--</kbd> starts a negative argument.
</p>
</dd>
<dt><a id="index-universal_002dargument-_0028_0029"></a><span><code class="code">universal-argument ()</code><a class="copiable-link" href="#index-universal_002dargument-_0028_0029"> &para;</a></span></dt>
<dd><p>This is another way to specify an argument.
If this command is followed by one or more digits, optionally with a
leading minus sign, those digits define the argument.
If the command is followed by digits, executing <code class="code">universal-argument</code>
again ends the numeric argument, but is otherwise ignored.
As a special case, if this command is immediately followed by a
character that is neither a digit nor minus sign, the argument count
for the next command is multiplied by four.
The argument count is initially one, so executing this function the
first time makes the argument count four, a second time makes the
argument count sixteen, and so on.
By default, this is not bound to a key.
</p></dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Commands-For-Completion">
<div class="nav-panel">
<p>
Next: <a href="#Keyboard-Macros" accesskey="n" rel="next">Keyboard Macros</a>, Previous: <a href="#Numeric-Arguments" accesskey="p" rel="prev">Specifying Numeric Arguments</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Letting-Readline-Type-For-You"><span>8.4.6 Letting Readline Type For You<a class="copiable-link" href="#Letting-Readline-Type-For-You"> &para;</a></span></h4>

<dl class="ftable">
<dt><a id="index-complete-_0028TAB_0029"></a><span><code class="code">complete (<kbd class="key">TAB</kbd>)</code><a class="copiable-link" href="#index-complete-_0028TAB_0029"> &para;</a></span></dt>
<dd><p>Attempt to perform completion on the text before point.
The actual completion performed is application-specific.
Bash attempts completion by first checking for any programmable
completions for the command word (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>),
otherwise treating the text as a
variable (if the text begins with &lsquo;<samp class="samp">$</samp>&rsquo;),
username (if the text begins with &lsquo;<samp class="samp">~</samp>&rsquo;),
hostname (if the text begins with &lsquo;<samp class="samp">@</samp>&rsquo;), or
command (including aliases, functions, and builtins) in turn.
If none  of these produces a match, it falls back to filename completion.
</p>
</dd>
<dt><a id="index-possible_002dcompletions-_0028M_002d_003f_0029"></a><span><code class="code">possible-completions (M-?)</code><a class="copiable-link" href="#index-possible_002dcompletions-_0028M_002d_003f_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point.
When displaying completions, Readline sets the number of columns used
for display to the value of <code class="code">completion-display-width</code>, the value of
the environment variable <code class="env">COLUMNS</code>, or the screen width, in that order.
</p>
</dd>
<dt><a id="index-insert_002dcompletions-_0028M_002d_002a_0029"></a><span><code class="code">insert-completions (M-*)</code><a class="copiable-link" href="#index-insert_002dcompletions-_0028M_002d_002a_0029"> &para;</a></span></dt>
<dd><p>Insert all completions of the text before point that would have
been generated by <code class="code">possible-completions</code>,
separated by a space.
</p>
</dd>
<dt><a id="index-menu_002dcomplete-_0028_0029"></a><span><code class="code">menu-complete ()</code><a class="copiable-link" href="#index-menu_002dcomplete-_0028_0029"> &para;</a></span></dt>
<dd><p>Similar to <code class="code">complete</code>, but replaces the word to be completed
with a single match from the list of possible completions.
Repeatedly executing <code class="code">menu-complete</code> steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions,
<code class="code">menu-complete</code> rings the bell
(subject to the setting of <code class="code">bell-style</code>)
and restores the original text.
An argument of <var class="var">n</var> moves <var class="var">n</var> positions forward in the list
of matches; a negative argument moves backward through the list.
This command is intended to be bound to <kbd class="key">TAB</kbd>, but is unbound
by default.
</p>
</dd>
<dt><a id="index-menu_002dcomplete_002dbackward-_0028_0029"></a><span><code class="code">menu-complete-backward ()</code><a class="copiable-link" href="#index-menu_002dcomplete_002dbackward-_0028_0029"> &para;</a></span></dt>
<dd><p>Identical to <code class="code">menu-complete</code>, but moves backward through the list
of possible completions, as if <code class="code">menu-complete</code> had been given a
negative argument.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-export_002dcompletions-_0028_0029"></a><span><code class="code">export-completions ()</code><a class="copiable-link" href="#index-export_002dcompletions-_0028_0029"> &para;</a></span></dt>
<dd><p>Perform completion on the word before point as described above
and write the list of possible completions to Readline&rsquo;s output stream
using the following format, writing information on separate lines:
</p>
<ul class="itemize mark-bullet">
<li>the number of matches <var class="var">N</var>;
</li><li>the word being completed;
</li><li><var class="var">S</var>:<var class="var">E</var>,
where <var class="var">S</var> and <var class="var">E</var> are the start and end offsets of the word
in the Readline line buffer; then
</li><li>each match, one per line
</li></ul>

<p>If there are no matches, the first line will be &ldquo;0&rdquo;,
and this command does not print any output after the <var class="var">S</var>:<var class="var">E</var>.
If there is only a single match, this prints a single line containing it.
If there is more than one match, this prints the common prefix of the
matches, which may be empty, on the first line after the <var class="var">S</var>:<var class="var">E</var>,
then the matches on subsequent lines.
In this case, <var class="var">N</var> will include the first line with the common prefix.
</p>
<p>The user or application
should be able to accommodate the possibility of a blank line.
The intent is that the user or application reads <var class="var">N</var> lines after
the line containing <var class="var">S</var>:<var class="var">E</var> to obtain the match list.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-delete_002dchar_002dor_002dlist-_0028_0029"></a><span><code class="code">delete-char-or-list ()</code><a class="copiable-link" href="#index-delete_002dchar_002dor_002dlist-_0028_0029"> &para;</a></span></dt>
<dd><p>Deletes the character under the cursor if not at the beginning or
end of the line (like <code class="code">delete-char</code>).
At the end of the line, it behaves identically to <code class="code">possible-completions</code>.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-complete_002dfilename-_0028M_002d_002f_0029"></a><span><code class="code">complete-filename (M-/)</code><a class="copiable-link" href="#index-complete_002dfilename-_0028M_002d_002f_0029"> &para;</a></span></dt>
<dd><p>Attempt filename completion on the text before point.
</p>
</dd>
<dt><a id="index-possible_002dfilename_002dcompletions-_0028C_002dx-_002f_0029"></a><span><code class="code">possible-filename-completions (C-x /)</code><a class="copiable-link" href="#index-possible_002dfilename_002dcompletions-_0028C_002dx-_002f_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point,
treating it as a filename.
</p>
</dd>
<dt><a id="index-complete_002dusername-_0028M_002d_007e_0029"></a><span><code class="code">complete-username (M-~)</code><a class="copiable-link" href="#index-complete_002dusername-_0028M_002d_007e_0029"> &para;</a></span></dt>
<dd><p>Attempt completion on the text before point, treating
it as a username.
</p>
</dd>
<dt><a id="index-possible_002dusername_002dcompletions-_0028C_002dx-_007e_0029"></a><span><code class="code">possible-username-completions (C-x ~)</code><a class="copiable-link" href="#index-possible_002dusername_002dcompletions-_0028C_002dx-_007e_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point,
treating it as a username.
</p>
</dd>
<dt><a id="index-complete_002dvariable-_0028M_002d_0024_0029"></a><span><code class="code">complete-variable (M-$)</code><a class="copiable-link" href="#index-complete_002dvariable-_0028M_002d_0024_0029"> &para;</a></span></dt>
<dd><p>Attempt completion on the text before point, treating
it as a shell variable.
</p>
</dd>
<dt><a id="index-possible_002dvariable_002dcompletions-_0028C_002dx-_0024_0029"></a><span><code class="code">possible-variable-completions (C-x $)</code><a class="copiable-link" href="#index-possible_002dvariable_002dcompletions-_0028C_002dx-_0024_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point,
treating it as a shell variable.
</p>
</dd>
<dt><a id="index-complete_002dhostname-_0028M_002d_0040_0029"></a><span><code class="code">complete-hostname (M-@)</code><a class="copiable-link" href="#index-complete_002dhostname-_0028M_002d_0040_0029"> &para;</a></span></dt>
<dd><p>Attempt completion on the text before point, treating
it as a hostname.
</p>
</dd>
<dt><a id="index-possible_002dhostname_002dcompletions-_0028C_002dx-_0040_0029"></a><span><code class="code">possible-hostname-completions (C-x @)</code><a class="copiable-link" href="#index-possible_002dhostname_002dcompletions-_0028C_002dx-_0040_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point,
treating it as a hostname.
</p>
</dd>
<dt><a id="index-complete_002dcommand-_0028M_002d_0021_0029"></a><span><code class="code">complete-command (M-!)</code><a class="copiable-link" href="#index-complete_002dcommand-_0028M_002d_0021_0029"> &para;</a></span></dt>
<dd><p>Attempt completion on the text before point, treating
it as a command name.
Command completion attempts to
match the text against aliases, reserved words, shell
functions, shell builtins, and finally executable filenames,
in that order.
</p>
</dd>
<dt><a id="index-possible_002dcommand_002dcompletions-_0028C_002dx-_0021_0029"></a><span><code class="code">possible-command-completions (C-x !)</code><a class="copiable-link" href="#index-possible_002dcommand_002dcompletions-_0028C_002dx-_0021_0029"> &para;</a></span></dt>
<dd><p>List the possible completions of the text before point,
treating it as a command name.
</p>
</dd>
<dt><a id="index-dynamic_002dcomplete_002dhistory-_0028M_002dTAB_0029"></a><span><code class="code">dynamic-complete-history (M-<kbd class="key">TAB</kbd>)</code><a class="copiable-link" href="#index-dynamic_002dcomplete_002dhistory-_0028M_002dTAB_0029"> &para;</a></span></dt>
<dd><p>Attempt completion on the text before point, comparing
the text against history list entries for possible
completion matches.
</p>
</dd>
<dt><a id="index-dabbrev_002dexpand-_0028_0029"></a><span><code class="code">dabbrev-expand ()</code><a class="copiable-link" href="#index-dabbrev_002dexpand-_0028_0029"> &para;</a></span></dt>
<dd><p>Attempt menu completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
</p>
</dd>
<dt><a id="index-complete_002dinto_002dbraces-_0028M_002d_007b_0029"></a><span><code class="code">complete-into-braces (M-{)</code><a class="copiable-link" href="#index-complete_002dinto_002dbraces-_0028M_002d_007b_0029"> &para;</a></span></dt>
<dd><p>Perform filename completion and insert the list of possible completions
enclosed within braces so the list is available to the shell
(see <a class="pxref" href="#Brace-Expansion">Brace Expansion</a>).
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Keyboard-Macros">
<div class="nav-panel">
<p>
Next: <a href="#Miscellaneous-Commands" accesskey="n" rel="next">Some Miscellaneous Commands</a>, Previous: <a href="#Commands-For-Completion" accesskey="p" rel="prev">Letting Readline Type For You</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Keyboard-Macros-1"><span>8.4.7 Keyboard Macros<a class="copiable-link" href="#Keyboard-Macros-1"> &para;</a></span></h4>
<dl class="ftable">
<dt><a id="index-start_002dkbd_002dmacro-_0028C_002dx-_0028_0029"></a><span><code class="code">start-kbd-macro (C-x ()</code><a class="copiable-link" href="#index-start_002dkbd_002dmacro-_0028C_002dx-_0028_0029"> &para;</a></span></dt>
<dd><p>Begin saving the characters typed into the current keyboard macro.
</p>
</dd>
<dt><a id="index-end_002dkbd_002dmacro-_0028C_002dx-_0029_0029"></a><span><code class="code">end-kbd-macro (C-x ))</code><a class="copiable-link" href="#index-end_002dkbd_002dmacro-_0028C_002dx-_0029_0029"> &para;</a></span></dt>
<dd><p>Stop saving the characters typed into the current keyboard macro
and save the definition.
</p>
</dd>
<dt><a id="index-call_002dlast_002dkbd_002dmacro-_0028C_002dx-e_0029"></a><span><code class="code">call-last-kbd-macro (C-x e)</code><a class="copiable-link" href="#index-call_002dlast_002dkbd_002dmacro-_0028C_002dx-e_0029"> &para;</a></span></dt>
<dd><p>Re-execute the last keyboard macro defined, by making the characters
in the macro appear as if typed at the keyboard.
</p>
</dd>
<dt><a id="index-print_002dlast_002dkbd_002dmacro-_0028_0029"></a><span><code class="code">print-last-kbd-macro ()</code><a class="copiable-link" href="#index-print_002dlast_002dkbd_002dmacro-_0028_0029"> &para;</a></span></dt>
<dd><p>Print the last keyboard macro defined in a format suitable for the
<var class="var">inputrc</var> file.
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Miscellaneous-Commands">
<div class="nav-panel">
<p>
Previous: <a href="#Keyboard-Macros" accesskey="p" rel="prev">Keyboard Macros</a>, Up: <a href="#Bindable-Readline-Commands" accesskey="u" rel="up">Bindable Readline Commands</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Some-Miscellaneous-Commands"><span>8.4.8 Some Miscellaneous Commands<a class="copiable-link" href="#Some-Miscellaneous-Commands"> &para;</a></span></h4>
<dl class="ftable">
<dt><a id="index-re_002dread_002dinit_002dfile-_0028C_002dx-C_002dr_0029"></a><span><code class="code">re-read-init-file (C-x C-r)</code><a class="copiable-link" href="#index-re_002dread_002dinit_002dfile-_0028C_002dx-C_002dr_0029"> &para;</a></span></dt>
<dd><p>Read in the contents of the <var class="var">inputrc</var> file, and incorporate
any bindings or variable assignments found there.
</p>
</dd>
<dt><a id="index-abort-_0028C_002dg_0029"></a><span><code class="code">abort (C-g)</code><a class="copiable-link" href="#index-abort-_0028C_002dg_0029"> &para;</a></span></dt>
<dd><p>Abort the current editing command and
ring the terminal&rsquo;s bell (subject to the setting of
<code class="code">bell-style</code>).
</p>
</dd>
<dt><a id="index-do_002dlowercase_002dversion-_0028M_002dA_002c-M_002dB_002c-M_002dx_002c-_002e_002e_002e_0029"></a><span><code class="code">do-lowercase-version (M-A, M-B, M-<var class="var">x</var>, &hellip;)</code><a class="copiable-link" href="#index-do_002dlowercase_002dversion-_0028M_002dA_002c-M_002dB_002c-M_002dx_002c-_002e_002e_002e_0029"> &para;</a></span></dt>
<dd><p>If the metafied character <var class="var">x</var> is upper case, run the command
that is bound to the corresponding metafied lower case character.
The behavior is undefined if <var class="var">x</var> is already lower case.
</p>
</dd>
<dt><a id="index-prefix_002dmeta-_0028ESC_0029"></a><span><code class="code">prefix-meta (<kbd class="key">ESC</kbd>)</code><a class="copiable-link" href="#index-prefix_002dmeta-_0028ESC_0029"> &para;</a></span></dt>
<dd><p>Metafy the next character typed.
Typing &lsquo;<samp class="samp"><kbd class="key">ESC</kbd> f</samp>&rsquo; is equivalent to typing <kbd class="kbd">M-f</kbd>.
</p>
</dd>
<dt><a id="index-undo-_0028C_002d_005f-or-C_002dx-C_002du_0029"></a><span><code class="code">undo (C-_ or C-x C-u)</code><a class="copiable-link" href="#index-undo-_0028C_002d_005f-or-C_002dx-C_002du_0029"> &para;</a></span></dt>
<dd><p>Incremental undo, separately remembered for each line.
</p>
</dd>
<dt><a id="index-revert_002dline-_0028M_002dr_0029"></a><span><code class="code">revert-line (M-r)</code><a class="copiable-link" href="#index-revert_002dline-_0028M_002dr_0029"> &para;</a></span></dt>
<dd><p>Undo all changes made to this line.
This is like executing the <code class="code">undo</code>
command enough times to get back to the initial state.
</p>
</dd>
<dt><a id="index-tilde_002dexpand-_0028M_002d_0026_0029"></a><span><code class="code">tilde-expand (M-&amp;)</code><a class="copiable-link" href="#index-tilde_002dexpand-_0028M_002d_0026_0029"> &para;</a></span></dt>
<dd><p>Perform tilde expansion on the current word.
</p>
</dd>
<dt><a id="index-set_002dmark-_0028C_002d_0040_0029"></a><span><code class="code">set-mark (C-@)</code><a class="copiable-link" href="#index-set_002dmark-_0028C_002d_0040_0029"> &para;</a></span></dt>
<dd><p>Set the mark to the point.
If a numeric argument is supplied, set the mark to that position.
</p>
</dd>
<dt><a id="index-exchange_002dpoint_002dand_002dmark-_0028C_002dx-C_002dx_0029"></a><span><code class="code">exchange-point-and-mark (C-x C-x)</code><a class="copiable-link" href="#index-exchange_002dpoint_002dand_002dmark-_0028C_002dx-C_002dx_0029"> &para;</a></span></dt>
<dd><p>Swap the point with the mark.
Set the current cursor position to the saved position,
then set the mark to the old cursor position.
</p>
</dd>
<dt><a id="index-character_002dsearch-_0028C_002d_005d_0029"></a><span><code class="code">character-search (C-])</code><a class="copiable-link" href="#index-character_002dsearch-_0028C_002d_005d_0029"> &para;</a></span></dt>
<dd><p>Read a character and move point to the next occurrence of that character.
A negative argument searches for previous occurrences.
</p>
</dd>
<dt><a id="index-character_002dsearch_002dbackward-_0028M_002dC_002d_005d_0029"></a><span><code class="code">character-search-backward (M-C-])</code><a class="copiable-link" href="#index-character_002dsearch_002dbackward-_0028M_002dC_002d_005d_0029"> &para;</a></span></dt>
<dd><p>Read a character and move point to the previous occurrence of that character.
A negative argument searches for subsequent occurrences.
</p>
</dd>
<dt><a id="index-skip_002dcsi_002dsequence-_0028_0029"></a><span><code class="code">skip-csi-sequence ()</code><a class="copiable-link" href="#index-skip_002dcsi_002dsequence-_0028_0029"> &para;</a></span></dt>
<dd><p>Read enough characters to consume a multi-key sequence such as those
defined for keys like Home and End.
CSI sequences begin with a Control Sequence Indicator (CSI), usually
<kbd class="kbd">ESC [</kbd>.
If this sequence is bound to &quot;\e[&quot;,
keys producing CSI sequences have no effect
unless explicitly bound to a Readline command,
instead of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to
<kbd class="kbd">ESC [</kbd>.
</p>
</dd>
<dt><a id="index-insert_002dcomment-_0028M_002d_0023_0029"></a><span><code class="code">insert-comment (M-#)</code><a class="copiable-link" href="#index-insert_002dcomment-_0028M_002d_0023_0029"> &para;</a></span></dt>
<dd><p>Without a numeric argument, insert the value of the <code class="code">comment-begin</code>
variable at the beginning of the current line.
If a numeric argument is supplied, this command acts as a toggle:  if
the characters at the beginning of the line do not match the value
of <code class="code">comment-begin</code>, insert the value; otherwise delete
the characters in <code class="code">comment-begin</code> from the beginning of the line.
In either case, the line is accepted as if a newline had been typed.
The default value of <code class="code">comment-begin</code> causes this command
to make the current line a shell comment.
If a numeric argument causes the comment character to be removed, the line
will be executed by the shell.
</p>
</dd>
<dt><a id="index-dump_002dfunctions-_0028_0029"></a><span><code class="code">dump-functions ()</code><a class="copiable-link" href="#index-dump_002dfunctions-_0028_0029"> &para;</a></span></dt>
<dd><p>Print all of the functions and their key bindings
to the Readline output stream.
If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an <var class="var">inputrc</var> file.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-dump_002dvariables-_0028_0029"></a><span><code class="code">dump-variables ()</code><a class="copiable-link" href="#index-dump_002dvariables-_0028_0029"> &para;</a></span></dt>
<dd><p>Print all of the settable variables and their values
to the Readline output stream.
If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an <var class="var">inputrc</var> file.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-dump_002dmacros-_0028_0029"></a><span><code class="code">dump-macros ()</code><a class="copiable-link" href="#index-dump_002dmacros-_0028_0029"> &para;</a></span></dt>
<dd><p>Print all of the Readline key sequences bound to macros and the
strings they output
to the Readline output stream.
If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an <var class="var">inputrc</var> file.
This command is unbound by default.
</p>
</dd>
<dt><a id="index-execute_002dnamed_002dcommand-_0028M_002dx_0029"></a><span><code class="code">execute-named-command (M-x)</code><a class="copiable-link" href="#index-execute_002dnamed_002dcommand-_0028M_002dx_0029"> &para;</a></span></dt>
<dd><p>Read a bindable Readline command name from the input and execute the
function to which it&rsquo;s bound, as if the key sequence to which it was
bound appeared in the input.
If this function is supplied with a numeric argument, it passes that
argument to the function it executes.
</p>
</dd>
<dt><a id="index-spell_002dcorrect_002dword-_0028C_002dx-s_0029"></a><span><code class="code">spell-correct-word (C-x s)</code><a class="copiable-link" href="#index-spell_002dcorrect_002dword-_0028C_002dx-s_0029"> &para;</a></span></dt>
<dd><p>Perform spelling correction on the current word, treating it as a directory
or filename, in the same way as the <code class="code">cdspell</code> shell option.
Word boundaries are the same as those used by <code class="code">shell-forward-word</code>.
</p>
</dd>
<dt><a id="index-glob_002dcomplete_002dword-_0028M_002dg_0029"></a><span><code class="code">glob-complete-word (M-g)</code><a class="copiable-link" href="#index-glob_002dcomplete_002dword-_0028M_002dg_0029"> &para;</a></span></dt>
<dd><p>Treat the word before point as a pattern for pathname expansion,
with an asterisk implicitly appended, then use the pattern to
generate a list of matching file names for possible completions.
</p>
</dd>
<dt><a id="index-glob_002dexpand_002dword-_0028C_002dx-_002a_0029"></a><span><code class="code">glob-expand-word (C-x *)</code><a class="copiable-link" href="#index-glob_002dexpand_002dword-_0028C_002dx-_002a_0029"> &para;</a></span></dt>
<dd><p>Treat the word before point as a pattern for pathname expansion,
and insert the list of matching file names, replacing the word.
If a numeric argument is supplied, append a &lsquo;<samp class="samp">*</samp>&rsquo; before
pathname expansion.
</p>
</dd>
<dt><a id="index-glob_002dlist_002dexpansions-_0028C_002dx-g_0029"></a><span><code class="code">glob-list-expansions (C-x g)</code><a class="copiable-link" href="#index-glob_002dlist_002dexpansions-_0028C_002dx-g_0029"> &para;</a></span></dt>
<dd><p>Display the list of expansions that would have been generated by
<code class="code">glob-expand-word</code>, and redisplay the line.
If a numeric argument is supplied, append a &lsquo;<samp class="samp">*</samp>&rsquo; before
pathname expansion.
</p>
</dd>
<dt><a id="index-shell_002dexpand_002dline-_0028M_002dC_002de_0029"></a><span><code class="code">shell-expand-line (M-C-e)</code><a class="copiable-link" href="#index-shell_002dexpand_002dline-_0028M_002dC_002de_0029"> &para;</a></span></dt>
<dd><p>Expand the line by performing shell word expansions.  
This performs alias and history expansion,
$&rsquo;<var class="var">string</var>&rsquo; and $&quot;<var class="var">string</var>&quot; quoting,
tilde expansion, parameter and variable expansion, arithmetic expansion,
command and process substitution,
word splitting, and quote removal.  
An explicit argument suppresses command and process substitution.
</p>
</dd>
<dt><a id="index-history_002dexpand_002dline-_0028M_002d_005e_0029"></a><span><code class="code">history-expand-line (M-^)</code><a class="copiable-link" href="#index-history_002dexpand_002dline-_0028M_002d_005e_0029"> &para;</a></span></dt>
<dd><p>Perform history expansion on the current line.
</p>
</dd>
<dt><a id="index-magic_002dspace-_0028_0029"></a><span><code class="code">magic-space ()</code><a class="copiable-link" href="#index-magic_002dspace-_0028_0029"> &para;</a></span></dt>
<dd><p>Perform history expansion on the current line and insert a space
(see <a class="pxref" href="#History-Interaction">History Expansion</a>).
</p>
</dd>
<dt><a id="index-alias_002dexpand_002dline-_0028_0029"></a><span><code class="code">alias-expand-line ()</code><a class="copiable-link" href="#index-alias_002dexpand_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Perform alias expansion on the current line (see <a class="pxref" href="#Aliases">Aliases</a>).
</p>
</dd>
<dt><a id="index-history_002dand_002dalias_002dexpand_002dline-_0028_0029"></a><span><code class="code">history-and-alias-expand-line ()</code><a class="copiable-link" href="#index-history_002dand_002dalias_002dexpand_002dline-_0028_0029"> &para;</a></span></dt>
<dd><p>Perform history and alias expansion on the current line.
</p>
</dd>
<dt><a id="index-insert_002dlast_002dargument-_0028M_002d_002e-or-M_002d_005f_0029"></a><span><code class="code">insert-last-argument (M-. or M-_)</code><a class="copiable-link" href="#index-insert_002dlast_002dargument-_0028M_002d_002e-or-M_002d_005f_0029"> &para;</a></span></dt>
<dd><p>A synonym for <code class="code">yank-last-arg</code>.
</p>
</dd>
<dt><a id="index-edit_002dand_002dexecute_002dcommand-_0028C_002dx-C_002de_0029"></a><span><code class="code">edit-and-execute-command (C-x C-e)</code><a class="copiable-link" href="#index-edit_002dand_002dexecute_002dcommand-_0028C_002dx-C_002de_0029"> &para;</a></span></dt>
<dd><p>Invoke an editor on the current command line, and execute the result as shell
commands.
Bash attempts to invoke
<code class="code">$VISUAL</code>, <code class="code">$EDITOR</code>, and <code class="code">emacs</code>
as the editor, in that order.
</p>
</dd>
<dt><a id="index-display_002dshell_002dversion-_0028C_002dx-C_002dv_0029"></a><span><code class="code">display-shell-version (C-x C-v)</code><a class="copiable-link" href="#index-display_002dshell_002dversion-_0028C_002dx-C_002dv_0029"> &para;</a></span></dt>
<dd><p>Display version information about the current instance of Bash.
</p>


</dd>
</dl>

<hr>
</div>
</div>
<div class="section-level-extent" id="Readline-vi-Mode">
<div class="nav-panel">
<p>
Next: <a href="#Programmable-Completion" accesskey="n" rel="next">Programmable Completion</a>, Previous: <a href="#Bindable-Readline-Commands" accesskey="p" rel="prev">Bindable Readline Commands</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Readline-vi-Mode-1"><span>8.5 Readline vi Mode<a class="copiable-link" href="#Readline-vi-Mode-1"> &para;</a></span></h3>

<p>While the Readline library does not have a full set of <code class="code">vi</code>
editing functions, it does contain enough to allow simple editing
of the line.
The Readline <code class="code">vi</code> mode behaves as specified in the
<code class="code">sh</code> description in the <small class="sc">POSIX</small> standard.
</p>
<p>You can use the &lsquo;<samp class="samp">set -o emacs</samp>&rsquo; and &lsquo;<samp class="samp">set -o vi</samp>&rsquo;
commands (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>)
to switch interactively between <code class="code">emacs</code> and <code class="code">vi</code>
editing modes,
The Readline default is <code class="code">emacs</code> mode.
</p>
<p>When you enter a line in <code class="code">vi</code> mode, you are already placed in
&lsquo;insertion&rsquo; mode, as if you had typed an &lsquo;<samp class="samp">i</samp>&rsquo;.  Pressing <kbd class="key">ESC</kbd>
switches you into &lsquo;command&rsquo; mode, where you can edit the text of the
line with the standard <code class="code">vi</code> movement keys, move to previous
history lines with &lsquo;<samp class="samp">k</samp>&rsquo; and subsequent lines with &lsquo;<samp class="samp">j</samp>&rsquo;, and
so forth.
</p>
<hr>
</div>
<div class="section-level-extent" id="Programmable-Completion">
<div class="nav-panel">
<p>
Next: <a href="#Programmable-Completion-Builtins" accesskey="n" rel="next">Programmable Completion Builtins</a>, Previous: <a href="#Readline-vi-Mode" accesskey="p" rel="prev">Readline vi Mode</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Programmable-Completion-1"><span>8.6 Programmable Completion<a class="copiable-link" href="#Programmable-Completion-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-programmable-completion"></a>

<p>When the user attempts word completion
for a command or an argument to a command for which a
completion specification (a <em class="dfn">compspec</em>) has been defined
using the <code class="code">complete</code> builtin
(see <a class="pxref" href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a>),
Readline invokes the programmable completion facilities. 
</p>
<p>First, Bash identifies the command name.
If a compspec has been defined for that command, the
compspec is used to generate the list of possible completions for the word.
If the command word is the empty string (completion attempted at the
beginning of an empty line), Bash uses any compspec defined with
the <samp class="option">-E</samp> option to <code class="code">complete</code>.
The <samp class="option">-I</samp> option to <code class="code">complete</code>
indicates that the command word is the first non-assignment word
on the line, or after a command delimiter such as
&lsquo;<samp class="samp">;</samp>&rsquo; or &lsquo;<samp class="samp">|</samp>&rsquo;.
This usually indicates command name completion.
</p>
<p>If the command word is a full pathname, Bash
searches for a compspec for the full pathname first.
If there is no compspec for the full pathname, Bash attempts to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec,
or if there is no compspec for the command word,
Bash uses any compspec defined with
the <samp class="option">-D</samp> option to <code class="code">complete</code> as the default.
If there is no default compspec, Bash performs alias expansion
on the command word as a final resort,
and attempts to find a compspec for the command word
resulting from any successful expansion.
</p>
<p>If a compspec is not found, Bash performs its default completion
described above (see <a class="pxref" href="#Commands-For-Completion">Letting Readline Type For You</a>).
Otherwise, once a compspec has been found, Bash uses it to generate
the list of matching words.
</p>
<p>First, Bash performs the <var class="var">actions</var> specified by the compspec.
This only returns matches which are prefixes
of the word being completed.
When the <samp class="option">-f</samp> or <samp class="option">-d</samp> option is used for filename or
directory name completion, Bash uses shell the variable <code class="env">FIGNORE</code>
to filter the matches.
See <a class="xref" href="#Bash-Variables">Bash Variables</a>, for a description of <code class="env">FIGNORE</code>.
</p>
<p>Next, programmable completion generates matches
specified by a pathname expansion pattern
supplied as an argument to the
<samp class="option">-G</samp> option.
The words generated by the pattern need not match the word being completed.
Bash uses the <code class="env">FIGNORE</code> 
variable to filter the matches, but does not use the
<code class="env">GLOBIGNORE</code> shell variable.
</p>
<p>Next, completion considers
the string specified as the argument to the <samp class="option">-W</samp> option.
The string is first split using the characters in the <code class="env">IFS</code>
special variable as delimiters.
This honors shell quoting within the string, in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of <code class="env">IFS</code>.
Each word is then expanded using
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, and arithmetic expansion,
as described above (see <a class="pxref" href="#Shell-Expansions">Shell Expansions</a>).
The results are split using the rules described above
(see <a class="pxref" href="#Word-Splitting">Word Splitting</a>).
The results of the expansion are prefix-matched against the word being
completed, and the matching words become possible completions.
</p>
<p>After these matches have been generated,
Bash executes any shell function or command
specified with the <samp class="option">-F</samp> and <samp class="option">-C</samp> options.
When the command or function is invoked, Bash
assigns values to the
<code class="env">COMP_LINE</code>,
<code class="env">COMP_POINT</code>,
<code class="env">COMP_KEY</code>,
and
<code class="env">COMP_TYPE</code>
variables as described above
(see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
If a shell function is being invoked, Bash
also sets the
<code class="env">COMP_WORDS</code>
and
<code class="env">COMP_CWORD</code>
variables.
When the function or command is invoked,
the first argument ($1) is the name of the command whose arguments
are being completed,
the second argument ($2) is the word being completed,
and the third argument ($3) is the word preceding the word being
completed on the current command line.
There is no filtering of the generated completions against the 
word being completed;
the function or command has complete freedom in generating the matches
and they do not need to match a prefix of the word.
</p>
<p>Any function specified with <samp class="option">-F</samp> is invoked first.
The function may use any of the shell facilities, including the
<code class="code">compgen</code> and <code class="code">compopt</code> builtins described below
(see <a class="pxref" href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a>), to generate the matches.
It must put the possible completions in the <code class="env">COMPREPLY</code> array
variable, one per array element.
</p>
<p>Next, any command specified with the <samp class="option">-C</samp> option is invoked
in an environment equivalent to command substitution.
It should print a list of completions, one per line, to
the standard output.
Backslash will escape a newline, if necessary.
These are added to the set of possible completions.
</p>
<p>After generating all of the possible completions,
Bash applies any filter
specified with the <samp class="option">-X</samp> option to the completions in the list.
The filter is a pattern as used for pathname expansion; a &lsquo;<samp class="samp">&amp;</samp>&rsquo;
in the pattern is replaced with the text of the word being completed.
A literal &lsquo;<samp class="samp">&amp;</samp>&rsquo; may be escaped with a backslash; the backslash
is removed before attempting a match.
Any completion that matches the pattern is removed from the list.
A leading &lsquo;<samp class="samp">!</samp>&rsquo; negates the pattern;
in this case Bash removes
any completion that does not match the pattern.
If the <code class="code">nocasematch</code> shell option is enabled
(see the description of <code class="code">shopt</code> in <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a>),
Bash performs the match without regard to the case
of alphabetic characters.
</p>
<p>Finally, programmable completion adds
any prefix and suffix specified with the
<samp class="option">-P</samp> and <samp class="option">-S</samp>
options, respectively, to each completion,
and returns the result
to Readline as the list of possible completions.
</p>
<p>If the previously-applied actions do not generate any matches, and the
<samp class="option">-o dirnames</samp> option was supplied to <code class="code">complete</code> when the
compspec was defined, Bash attempts directory name completion. 
</p>
<p>If the <samp class="option">-o plusdirs</samp> option was supplied to <code class="code">complete</code> when
the compspec was defined, Bash attempts directory name completion
and adds any  matches to the set of possible completions.
</p>
<p>By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions.
The default Bash completions and the Readline default
of filename completion are disabled.
If the <samp class="option">-o bashdefault</samp> option was supplied to <code class="code">complete</code> when
the compspec was defined, and the compspec generates no matches,
Bash attempts its default completions.
If the compspec and, if attempted, the default Bash completions
generate no matches,
and the <samp class="option">-o default</samp> option was supplied to
<code class="code">complete</code> when the compspec was defined,
programmable completion performs Readline&rsquo;s default completion.
</p>
<p>The options supplied to <code class="code">complete</code> and <code class="code">compopt</code>
can control how Readline treats the completions.
For instance, the <samp class="option">-o fullquote</samp> option tells Readline
to quote the matches as if they were filenames.
See the description of <code class="code">complete</code>
(see <a class="pxref" href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a>)
for details.
</p>
<p>When a compspec indicates that it wants directory name completion,
the programmable completion functions force Readline
to append a slash to completed names which are symbolic links
to directories, subject to the value of the
<var class="var">mark-directories</var> Readline variable,
regardless of the setting of the
<var class="var">mark-symlinked-directories</var> Readline variable.
</p>
<p>There is some support for dynamically modifying completions.
This is most useful when used in combination with a default completion
specified with <samp class="option">-D</samp>.
It&rsquo;s possible for shell functions executed as completion functions
to indicate that completion should be retried by returning an
exit status of 124.
If a shell function returns 124, and changes
the compspec associated with the command on which completion is being
attempted (supplied as the first argument when the function is executed),
programmable completion restarts from the beginning, with an
attempt to find a new compspec for that command.
This can be used to build a set of completions dynamically
as completion is attempted, rather than loading them all at once.
</p>
<p>For instance, assuming that there is a library of compspecs, each kept in a
file corresponding to the name of the command, the following default
completion function would load completions dynamically:
</p>
<div class="example">
<pre class="example-preformatted">_completion_loader()
{
    . &quot;/etc/bash_completion.d/$1.sh&quot; &gt;/dev/null 2&gt;&amp;1 &amp;&amp; return 124
}
complete -D -F _completion_loader -o bashdefault -o default
</pre></div>

<hr>
</div>
<div class="section-level-extent" id="Programmable-Completion-Builtins">
<div class="nav-panel">
<p>
Next: <a href="#A-Programmable-Completion-Example" accesskey="n" rel="next">A Programmable Completion Example</a>, Previous: <a href="#Programmable-Completion" accesskey="p" rel="prev">Programmable Completion</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Programmable-Completion-Builtins-1"><span>8.7 Programmable Completion Builtins<a class="copiable-link" href="#Programmable-Completion-Builtins-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-completion-builtins"></a>

<p>Three builtin commands are available to manipulate the programmable completion
facilities: one to specify how the arguments to a particular command are to
be completed, and two to modify the completion as it is happening.
</p>
<dl class="table">
<dt><a id="index-compgen"></a><span><code class="code">compgen</code><a class="copiable-link" href="#index-compgen"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted"><code class="code">compgen [-V <var class="var">varname</var>] [<var class="var">option</var>] [<var class="var">word</var>]</code>
</pre></div>

<p>Generate possible completion matches for <var class="var">word</var> according to
the <var class="var">option</var>s, which may be any option accepted by the
<code class="code">complete</code>
builtin with the exceptions of
<samp class="option">-p</samp>,
<samp class="option">-r</samp>,
<samp class="option">-D</samp>,
<samp class="option">-E</samp>,
and
<samp class="option">-I</samp>,
and write the matches to the standard output.
</p>
<p>If the <samp class="option">-V</samp> option is supplied, <code class="code">compgen</code> stores the generated
completions into the indexed array variable <var class="var">varname</var> instead of writing
them to the standard output.
</p>
<p>When using the <samp class="option">-F</samp> or <samp class="option">-C</samp> options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.
</p>
<p>The matches will be generated in the same way as if the programmable
completion code had generated them directly from a completion specification
with the same flags.
If <var class="var">word</var> is specified, only those completions matching <var class="var">word</var>
will be displayed or stored.
</p>
<p>The return value is true unless an invalid option is supplied, or no
matches were generated.
</p>
</dd>
<dt><a id="index-complete"></a><span><code class="code">complete</code><a class="copiable-link" href="#index-complete"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted"><code class="code">complete [-abcdefgjksuv] [-o <var class="var">comp-option</var>] [-DEI] [-A <var class="var">action</var>]
[-G <var class="var">globpat</var>] [-W <var class="var">wordlist</var>] [-F <var class="var">function</var>] [-C <var class="var">command</var>]
[-X <var class="var">filterpat</var>] [-P <var class="var">prefix</var>] [-S <var class="var">suffix</var>] <var class="var">name</var> [<var class="var">name</var> ...]</code>
<code class="code">complete -pr [-DEI] [<var class="var">name</var> ...]</code>
</pre></div>

<p>Specify how arguments to each <var class="var">name</var> should be completed.
</p>
<p>If the <samp class="option">-p</samp> option is supplied, or if no options or <var class="var">name</var>s
are supplied, print existing completion specifications
in a way that allows them to be reused as input.
The <samp class="option">-r</samp> option removes a completion specification for
each <var class="var">name</var>, or, if no <var class="var">name</var>s are supplied, all
completion specifications.
</p>
<p>The <samp class="option">-D</samp> option indicates that other supplied options and actions should
apply to the &ldquo;default&rdquo; command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The <samp class="option">-E</samp> option indicates that other supplied options and actions should
apply to &ldquo;empty&rdquo; command completion; that is, completion attempted on a 
blank line.
The <samp class="option">-I</samp> option indicates that other supplied options and actions should
apply to completion on the initial non-assignment word on the line, or after a
command delimiter such as &lsquo;<samp class="samp">;</samp>&rsquo; or &lsquo;<samp class="samp">|</samp>&rsquo;, which is usually command
name completion.
If multiple options are supplied, the <samp class="option">-D</samp> option takes precedence
over <samp class="option">-E</samp>, and both take precedence over <samp class="option">-I</samp>.
If any of <samp class="option">-D</samp>, <samp class="option">-E</samp>, or <samp class="option">-I</samp> are supplied, any other
<var class="var">name</var> arguments are ignored; these completions only apply to the case
specified by the option.
</p>
<p>The process of applying these completion specifications when word completion
is attempted is described above (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
</p>
<p>Other options, if specified, have the following meanings.
The arguments to the <samp class="option">-G</samp>, <samp class="option">-W</samp>, and <samp class="option">-X</samp> options
(and, if necessary, the <samp class="option">-P</samp> and <samp class="option">-S</samp> options)
should be quoted to protect them from expansion before the
<code class="code">complete</code> builtin is invoked.
</p>
<dl class="table">
<dt><code class="code">-o <var class="var">comp-option</var></code></dt>
<dd><p>The <var class="var">comp-option</var> controls several aspects of the compspec&rsquo;s behavior
beyond the simple generation of completions.
<var class="var">comp-option</var> may be one of: 
</p>
<dl class="table">
<dt><code class="code">bashdefault</code></dt>
<dd><p>Perform the rest of the default Bash completions if the compspec
generates no matches.
</p>
</dd>
<dt><code class="code">default</code></dt>
<dd><p>Use Readline&rsquo;s default filename completion if the compspec generates
no matches.
</p>
</dd>
<dt><code class="code">dirnames</code></dt>
<dd><p>Perform directory name completion if the compspec generates no matches.
</p>
</dd>
<dt><code class="code">filenames</code></dt>
<dd><p>Tell Readline that the compspec generates filenames, so it can perform
any filename-specific processing (such as adding a slash to directory names,
quoting special characters, or suppressing trailing spaces).
This option is intended to be used with shell functions specified
with <samp class="option">-F</samp>.
</p>
</dd>
<dt><code class="code">fullquote</code></dt>
<dd><p>Tell Readline to quote all the completed words even if they are not
filenames.
</p>
</dd>
<dt><code class="code">noquote</code></dt>
<dd><p>Tell Readline not to quote the completed words if they are filenames
(quoting filenames is the default).
</p>
</dd>
<dt><code class="code">nosort</code></dt>
<dd><p>Tell Readline not to sort the list of possible completions alphabetically.
</p>
</dd>
<dt><code class="code">nospace</code></dt>
<dd><p>Tell Readline not to append a space (the default) to words completed at
the end of the line.
</p>
</dd>
<dt><code class="code">plusdirs</code></dt>
<dd><p>After generating any matches defined by the compspec, 
attempt directory name completion and add any
matches to the results of the other actions.
</p></dd>
</dl>

</dd>
<dt><code class="code">-A <var class="var">action</var></code></dt>
<dd><p>The <var class="var">action</var> may be one of the following to generate a list of possible
completions:
</p>
<dl class="table">
<dt><code class="code">alias</code></dt>
<dd><p>Alias names.
May also be specified as <samp class="option">-a</samp>.
</p>
</dd>
<dt><code class="code">arrayvar</code></dt>
<dd><p>Array variable names.
</p>
</dd>
<dt><code class="code">binding</code></dt>
<dd><p>Readline key binding names (see <a class="pxref" href="#Bindable-Readline-Commands">Bindable Readline Commands</a>).
</p>
</dd>
<dt><code class="code">builtin</code></dt>
<dd><p>Names of shell builtin commands.
May also be specified as <samp class="option">-b</samp>.
</p>
</dd>
<dt><code class="code">command</code></dt>
<dd><p>Command names.
May also be specified as <samp class="option">-c</samp>.
</p>
</dd>
<dt><code class="code">directory</code></dt>
<dd><p>Directory names.
May also be specified as <samp class="option">-d</samp>.
</p>
</dd>
<dt><code class="code">disabled</code></dt>
<dd><p>Names of disabled shell builtins.
</p>
</dd>
<dt><code class="code">enabled</code></dt>
<dd><p>Names of enabled shell builtins.
</p>
</dd>
<dt><code class="code">export</code></dt>
<dd><p>Names of exported shell variables.
May also be specified as <samp class="option">-e</samp>.
</p>
</dd>
<dt><code class="code">file</code></dt>
<dd><p>File and directory names, similar to Readline&rsquo;s filename completion.
May also be specified as <samp class="option">-f</samp>.
</p>
</dd>
<dt><code class="code">function</code></dt>
<dd><p>Names of shell functions.
</p>
</dd>
<dt><code class="code">group</code></dt>
<dd><p>Group names.
May also be specified as <samp class="option">-g</samp>.
</p>
</dd>
<dt><code class="code">helptopic</code></dt>
<dd><p>Help topics as accepted by the <code class="code">help</code> builtin (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p>
</dd>
<dt><code class="code">hostname</code></dt>
<dd><p>Hostnames, as taken from the file specified by the
<code class="env">HOSTFILE</code> shell variable (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
</p>
</dd>
<dt><code class="code">job</code></dt>
<dd><p>Job names, if job control is active.
May also be specified as <samp class="option">-j</samp>.
</p>
</dd>
<dt><code class="code">keyword</code></dt>
<dd><p>Shell reserved words.
May also be specified as <samp class="option">-k</samp>.
</p>
</dd>
<dt><code class="code">running</code></dt>
<dd><p>Names of running jobs, if job control is active.
</p>
</dd>
<dt><code class="code">service</code></dt>
<dd><p>Service names.
May also be specified as <samp class="option">-s</samp>.
</p>
</dd>
<dt><code class="code">setopt</code></dt>
<dd><p>Valid arguments for the <samp class="option">-o</samp> option to the <code class="code">set</code> builtin
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
</p>
</dd>
<dt><code class="code">shopt</code></dt>
<dd><p>Shell option names as accepted by the <code class="code">shopt</code> builtin
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p>
</dd>
<dt><code class="code">signal</code></dt>
<dd><p>Signal names.
</p>
</dd>
<dt><code class="code">stopped</code></dt>
<dd><p>Names of stopped jobs, if job control is active.
</p>
</dd>
<dt><code class="code">user</code></dt>
<dd><p>User names.
May also be specified as <samp class="option">-u</samp>.
</p>
</dd>
<dt><code class="code">variable</code></dt>
<dd><p>Names of all shell variables.
May also be specified as <samp class="option">-v</samp>.
</p></dd>
</dl>

</dd>
<dt><code class="code">-C <var class="var">command</var></code></dt>
<dd><p><var class="var">command</var> is executed in a subshell environment, and its output is
used as the possible completions.
Arguments are passed as with the <samp class="option">-F</samp> option.
</p>
</dd>
<dt><code class="code">-F <var class="var">function</var></code></dt>
<dd><p>The shell function <var class="var">function</var> is executed in the current shell
environment.
When it is executed,
the first argument ($1) is the name of the command whose arguments are
being completed,
the second argument ($2) is the word being completed, and
the third argument ($3) is the word preceding the word being completed,
as described above (see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
When <code class="code">function</code> finishes,
programmable completion retrieves
the possible completions from the value
of the <code class="env">COMPREPLY</code> array variable.
</p>
</dd>
<dt><code class="code">-G <var class="var">globpat</var></code></dt>
<dd><p>Expand the filename expansion pattern <var class="var">globpat</var> to generate
the possible completions.
</p>
</dd>
<dt><code class="code">-P <var class="var">prefix</var></code></dt>
<dd><p>Add <var class="var">prefix</var> to the beginning of each possible completion
after all other options have been applied.
</p>
</dd>
<dt><code class="code">-S <var class="var">suffix</var></code></dt>
<dd><p>Append <var class="var">suffix</var> to each possible completion
after all other options have been applied.
</p>
</dd>
<dt><code class="code">-W <var class="var">wordlist</var></code></dt>
<dd><p>Split the <var class="var">wordlist</var> using the characters in the
<code class="env">IFS</code> special variable as delimiters, and expand
each resulting word.
Shell quoting is honored within <var class="var">wordlist</var>
in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of <code class="env">IFS</code>.
The possible completions are the members of the resultant list which
match a prefix of the word being completed.
</p>
</dd>
<dt><code class="code">-X <var class="var">filterpat</var></code></dt>
<dd><p><var class="var">filterpat</var> is a pattern as used for filename expansion.
It is applied to the list of possible completions generated by the
preceding options and arguments, and each completion matching
<var class="var">filterpat</var> is removed from the list.
A leading &lsquo;<samp class="samp">!</samp>&rsquo; in <var class="var">filterpat</var> negates the pattern; in this
case, any completion not matching <var class="var">filterpat</var> is removed.
</p></dd>
</dl>

<p>The return value is true unless an invalid option is supplied, an option
other than
<samp class="option">-p</samp>,
<samp class="option">-r</samp>,
<samp class="option">-D</samp>,
<samp class="option">-E</samp>,
or
<samp class="option">-I</samp>
is supplied without a <var class="var">name</var>
argument, an attempt is made to remove a completion specification for
a <var class="var">name</var> for which no specification exists, or
an error occurs adding a completion specification.
</p>
</dd>
<dt><a id="index-compopt"></a><span><code class="code">compopt</code><a class="copiable-link" href="#index-compopt"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted"><code class="code">compopt</code> [-o <var class="var">option</var>] [-DEI] [+o <var class="var">option</var>] [<var class="var">name</var>]
</pre></div>
<p>Modify completion options for each <var class="var">name</var> according to the
<var class="var">option</var>s, or for the currently-executing completion if no <var class="var">name</var>s
are supplied.
If no <var class="var">option</var>s are given, display the completion options for each
<var class="var">name</var> or the current completion.
The possible values of <var class="var">option</var> are those valid for the <code class="code">complete</code>
builtin described above.
</p>
<p>The <samp class="option">-D</samp> option indicates that other supplied options should
apply to the &ldquo;default&rdquo; command completion;
the <samp class="option">-E</samp> option indicates that other supplied options should
apply to &ldquo;empty&rdquo; command completion; and
the <samp class="option">-I</samp> option indicates that other supplied options should
apply to completion on the initial word on the line.
These are determined in the same way as the <code class="code">complete</code> builtin.
</p>
<p>If multiple options are supplied, the <samp class="option">-D</samp> option takes precedence
over <samp class="option">-E</samp>, and both take precedence over <samp class="option">-I</samp>
</p>
<p>The return value is true unless an invalid option is supplied, an attempt
is made to modify the options for a <var class="var">name</var> for which no completion
specification exists, or an output error occurs.
</p></dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="A-Programmable-Completion-Example">
<div class="nav-panel">
<p>
Previous: <a href="#Programmable-Completion-Builtins" accesskey="p" rel="prev">Programmable Completion Builtins</a>, Up: <a href="#Command-Line-Editing" accesskey="u" rel="up">Command Line Editing</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="A-Programmable-Completion-Example-1"><span>8.8 A Programmable Completion Example<a class="copiable-link" href="#A-Programmable-Completion-Example-1"> &para;</a></span></h3>

<p>The most common way to obtain additional completion functionality beyond
the default actions <code class="code">complete</code> and <code class="code">compgen</code> provide is to use
a shell function and bind it to a particular command using <code class="code">complete -F</code>.
</p>
<p>The following function provides completions for the <code class="code">cd</code> builtin.
It is a reasonably good example of what shell functions must do when
used for completion.
This function uses the word passed as <code class="code">$2</code> to determine the
directory name to complete.
You can also use the
<code class="code">COMP_WORDS</code> array variable; the current word is indexed by the
<code class="code">COMP_CWORD</code> variable.
</p>
<p>The function relies on the <code class="code">complete</code> and <code class="code">compgen</code> builtins
to do much of the work, adding only the things that the Bash <code class="code">cd</code>
does beyond accepting basic directory names:
tilde expansion (see <a class="pxref" href="#Tilde-Expansion">Tilde Expansion</a>),
searching directories in <var class="var">$CDPATH</var>, which is described above
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>),
and basic support for the <code class="code">cdable_vars</code> shell option
(see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>).
<code class="code">_comp_cd</code> modifies the value of <var class="var">IFS</var> so that it contains only
a newline to accommodate file names containing spaces and tabs &ndash;
<code class="code">compgen</code> prints the possible completions it generates one per line.
</p>
<p>Possible completions go into the <var class="var">COMPREPLY</var> array variable, one
completion per array element.
The programmable completion system retrieves
the completions from there when the function returns.
</p>
<div class="example">
<pre class="example-preformatted"># A completion function for the cd builtin
# based on the cd completion function from the bash_completion package
_comp_cd()
{
    local IFS=$' \t\n'    # normalize IFS
    local cur _skipdot _cdpath
    local i j k

    # Tilde expansion, which also expands tilde to full pathname
    case &quot;$2&quot; in
    \~*)    eval cur=&quot;$2&quot; ;;
    *)      cur=$2 ;;
    esac

    # no cdpath or absolute pathname -- straight directory completion
    if [[ -z &quot;${CDPATH:-}&quot; ]] || [[ &quot;$cur&quot; == @(./*|../*|/*) ]]; then
        # compgen prints paths one per line; could also use while loop
        IFS=$'\n'
        COMPREPLY=( $(compgen -d -- &quot;$cur&quot;) )
        IFS=$' \t\n'
    # CDPATH+directories in the current directory if not in CDPATH
    else
        IFS=$'\n'
        _skipdot=false
        # preprocess CDPATH to convert null directory names to .
        _cdpath=${CDPATH/#:/.:}
        _cdpath=${_cdpath//::/:.:}
        _cdpath=${_cdpath/%:/:.}
        for i in ${_cdpath//:/$'\n'}; do
            if [[ $i -ef . ]]; then _skipdot=true; fi
            k=&quot;${#COMPREPLY[@]}&quot;
            for j in $( compgen -d -- &quot;$i/$cur&quot; ); do
                COMPREPLY[k++]=${j#$i/}        # cut off directory
            done
        done
        $_skipdot || COMPREPLY+=( $(compgen -d -- &quot;$cur&quot;) )
        IFS=$' \t\n'
    fi

    # variable names if appropriate shell option set and no completions
    if shopt -q cdable_vars &amp;&amp; [[ ${#COMPREPLY[@]} -eq 0 ]]; then
        COMPREPLY=( $(compgen -v -- &quot;$cur&quot;) )
    fi

    return 0
}
</pre></div>

<p>We install the completion function using the <samp class="option">-F</samp> option to
<code class="code">complete</code>:
</p>
<div class="example">
<pre class="example-preformatted"># Tell readline to quote appropriate and append slashes to directories;
# use the bash default completion for other arguments
complete -o filenames -o nospace -o bashdefault -F _comp_cd cd
</pre></div>

<p>Since we&rsquo;d like Bash and Readline to take care of some
of the other details for us, we use several other options to tell Bash
and Readline what to do.
The <samp class="option">-o filenames</samp> option tells Readline
that the possible completions should be treated as filenames, and quoted
appropriately.
That option will also cause Readline to append a slash to
filenames it can determine are directories (which is why we might want to
extend <code class="code">_comp_cd</code> to append a slash if we&rsquo;re using directories found
via <var class="var">CDPATH</var>: Readline can&rsquo;t tell those completions are directories).
The <samp class="option">-o nospace</samp> option tells Readline to not append a space
character to the directory name, in case we want to append to it.
The <samp class="option">-o bashdefault</samp> option brings in the rest of the &ldquo;Bash default&rdquo;
completions &ndash; possible completions that Bash adds to the default Readline
set.
These include things like command name completion, variable completion
for words beginning with &lsquo;<samp class="samp">$</samp>&rsquo; or &lsquo;<samp class="samp">${</samp>&rsquo;, completions containing
pathname expansion patterns (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>), and so on.
</p>
<p>Once installed using <code class="code">complete</code>, <code class="code">_comp_cd</code> will be called every
time we attempt word completion for a <code class="code">cd</code> command.
</p>
<p>Many more examples &ndash; an extensive collection of completions for most of
the common GNU, Unix, and Linux commands &ndash; are available as part of the
bash_completion project.  This is installed by default on many GNU/Linux
distributions.  Originally written by Ian Macdonald, the project now lives
at <a class="url" href="https://github.com/scop/bash-completion/">https://github.com/scop/bash-completion/</a>.
There are ports for other systems such as Solaris and Mac OS X.
</p>
<p>An older version of the bash_completion package is distributed with bash
in the <samp class="file">examples/complete</samp> subdirectory.
</p>
<a class="index-entry-id" id="index-History_002c-how-to-use"></a>

<hr>
</div>
</div>
<div class="chapter-level-extent" id="Using-History-Interactively">
<div class="nav-panel">
<p>
Next: <a href="#Installing-Bash" accesskey="n" rel="next">Installing Bash</a>, Previous: <a href="#Command-Line-Editing" accesskey="p" rel="prev">Command Line Editing</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Using-History-Interactively-1"><span>9 Using History Interactively<a class="copiable-link" href="#Using-History-Interactively-1"> &para;</a></span></h2>


<p>This chapter describes how to use the <small class="sc">GNU</small> History Library
interactively, from a user&rsquo;s standpoint.
It should be considered a user&rsquo;s guide.
For information on using the <small class="sc">GNU</small> History Library in other programs,
see the <small class="sc">GNU</small> Readline Library Manual.
</p>

<ul class="mini-toc">
<li><a href="#Bash-History-Facilities" accesskey="1">Bash History Facilities</a></li>
<li><a href="#Bash-History-Builtins" accesskey="2">Bash History Builtins</a></li>
<li><a href="#History-Interaction" accesskey="3">History Expansion</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Bash-History-Facilities">
<div class="nav-panel">
<p>
Next: <a href="#Bash-History-Builtins" accesskey="n" rel="next">Bash History Builtins</a>, Up: <a href="#Using-History-Interactively" accesskey="u" rel="up">Using History Interactively</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-History-Facilities-1"><span>9.1 Bash History Facilities<a class="copiable-link" href="#Bash-History-Facilities-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-command-history"></a>
<a class="index-entry-id" id="index-history-list"></a>

<p>When the <samp class="option">-o history</samp> option to the <code class="code">set</code> builtin
is enabled (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>),
the shell provides access to the <em class="dfn">command history</em>,
the list of commands previously typed.
The value of the <code class="env">HISTSIZE</code> shell variable is used as the
number of commands to save in a history list:
the shell saves the text of the last <code class="env">$HISTSIZE</code>
commands (default 500).
The shell stores each command in the history list prior to
parameter and variable expansion
but after history expansion is performed, subject to the
values of the shell variables
<code class="env">HISTIGNORE</code> and <code class="env">HISTCONTROL</code>.
</p>
<p>When the shell starts up, Bash initializes the history list
by reading history entries from the
file named by the <code class="env">HISTFILE</code> variable (default <samp class="file">~/.bash_history</samp>).
This is referred to as the <em class="dfn">history file</em>.
The history file is truncated, if necessary,
to contain no more than the number of history entries
specified by the value of the <code class="env">HISTFILESIZE</code> variable.
If <code class="env">HISTFILESIZE</code> is unset, or set to null, a non-numeric value,
or a numeric value less than zero, the history file is not truncated.
</p>
<p>When the history file is read,
lines beginning with the history comment character followed immediately
by a digit are interpreted as timestamps for the following history entry.
These timestamps are optionally displayed depending on the value of the
<code class="env">HISTTIMEFORMAT</code> variable (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
When present, history timestamps delimit history entries, making
multi-line entries possible.
</p>
<p>When a shell with history enabled exits, Bash copies the last
<code class="env">$HISTSIZE</code> entries from the history list to the file
named by <code class="env">$HISTFILE</code>.
If the <code class="code">histappend</code> shell option is set (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>),
Bash appends the entries to the history file,
otherwise it overwrites the history file.
If <code class="env">HISTFILE</code> is unset or null,
or if the history file is unwritable, the history is not saved.
After saving the history, Bash truncates the history file
to contain no more than <code class="env">$HISTFILESIZE</code>
lines as described above.
</p>
<p>If the <code class="env">HISTTIMEFORMAT</code>
variable is set, the shell writes the timestamp information
associated with each history entry to the history file,
marked with the history comment character,
so timestamps are preserved across shell sessions.
When the history file is read, lines beginning with
the history comment character followed immediately by a digit are
interpreted as timestamps for the following history entry.
As above, when using <code class="env">HISTTIMEFORMAT</code>,
the timestamps delimit multi-line history entries.
</p>
<p>The <code class="code">fc</code> builtin command will list or edit and re-execute a
portion of the history list.
The <code class="code">history</code> builtin can display or
modify the history list and manipulate the history file.
When using command-line editing, search commands
are available in each editing mode that provide access to the
history list (see <a class="pxref" href="#Commands-For-History">Commands For Manipulating The History</a>). 
</p>
<p>The shell allows control over which commands are saved on the history list. 
The <code class="env">HISTCONTROL</code> and <code class="env">HISTIGNORE</code>
variables are used to save only a subset of the commands entered.
If the <code class="code">cmdhist</code> shell option is
enabled, the shell attempts to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The <code class="code">lithist</code>
shell option modifies <code class="code">cmdhist</code> by saving
the command with embedded newlines instead of semicolons.
The <code class="code">shopt</code> builtin is used to set these options.
See <a class="xref" href="#The-Shopt-Builtin">The Shopt Builtin</a>, for a description of <code class="code">shopt</code>.
</p>
<hr>
</div>
<div class="section-level-extent" id="Bash-History-Builtins">
<div class="nav-panel">
<p>
Next: <a href="#History-Interaction" accesskey="n" rel="next">History Expansion</a>, Previous: <a href="#Bash-History-Facilities" accesskey="p" rel="prev">Bash History Facilities</a>, Up: <a href="#Using-History-Interactively" accesskey="u" rel="up">Using History Interactively</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Bash-History-Builtins-1"><span>9.2 Bash History Builtins<a class="copiable-link" href="#Bash-History-Builtins-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-history-builtins"></a>

<p>Bash provides two builtin commands which manipulate the
history list and history file.
</p>
<dl class="table">
<dt><a id="index-fc"></a><span><code class="code">fc</code><a class="copiable-link" href="#index-fc"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted"><code class="code">fc [-e <var class="var">ename</var>] [-lnr] [<var class="var">first</var>] [<var class="var">last</var>]</code>
<code class="code">fc -s [<var class="var">pat</var>=<var class="var">rep</var>] [<var class="var">command</var>]</code>
</pre></div>

<p>The first form selects a range of commands from
<var class="var">first</var> to <var class="var">last</var>
from the history list and displays or edits and re-executes them.
Both <var class="var">first</var> and
<var class="var">last</var> may be specified as a string (to locate the most recent
command beginning with that string) or as a number (an index into the
history list, where a negative number is used as an offset from the
current command number).
</p>
<p>When listing, a <var class="var">first</var> or <var class="var">last</var> of 0 is equivalent to -1
and -0 is equivalent to the current command (usually the <code class="code">fc</code>
command);
otherwise 0 is equivalent to -1 and -0 is invalid.
</p>
<p>If <var class="var">last</var> is not specified, it is set to
the current command for listing and to <var class="var">first</var> otherwise.
If <var class="var">first</var> is not specified, it is set to the previous
command for editing and &minus;16 for listing.
</p>
<p>If the <samp class="option">-l</samp> flag is supplied,
the commands are listed on standard output.
The <samp class="option">-n</samp> flag suppresses the command numbers when listing.
The <samp class="option">-r</samp> flag reverses the order of the listing.
</p>
<p>Otherwise, <code class="code">fc</code> invokes the editor named by
<var class="var">ename</var> on a file containing those commands.
If <var class="var">ename</var> is not supplied, <code class="code">fc</code> uses the value of the following
variable expansion: <code class="code">${FCEDIT:-${EDITOR:-vi}}</code>.
This says to use the
value of the <code class="env">FCEDIT</code> variable if set, or the value of the
<code class="env">EDITOR</code> variable if that is set, or <code class="code">vi</code> if neither is set.
When editing is complete, <code class="code">fc</code> reads the file of edited commands
and echoes and executes them.
</p>
<p>In the second form, <code class="code">fc</code> re-executes <var class="var">command</var> after
replacing each instance of <var class="var">pat</var> in the selected command with <var class="var">rep</var>.
<var class="var">command</var> is interpreted the same as <var class="var">first</var> above.
</p>
<p>A useful alias to use with the <code class="code">fc</code> command is <code class="code">r='fc -s'</code>, so
that typing &lsquo;<samp class="samp">r cc</samp>&rsquo; runs the last command beginning with <code class="code">cc</code>
and typing &lsquo;<samp class="samp">r</samp>&rsquo; re-executes the last command (see <a class="pxref" href="#Aliases">Aliases</a>).
</p>
<p>If the first form is used, the return value is zero unless an invalid
option is encountered or <var class="var">first</var> or <var class="var">last</var>
specify history lines out of range.
When editing and re-executing a file of commands,
the return value is the value of the last command executed
or failure if an error occurs with the temporary file.
If the second form is used, the return status
is that of the re-executed command, unless
<var class="var">command</var> does not specify a valid history entry, in which case
<code class="code">fc</code> returns a non-zero status.
</p>
</dd>
<dt><a id="index-history"></a><span><code class="code">history</code><a class="copiable-link" href="#index-history"> &para;</a></span></dt>
<dd><div class="example">
<pre class="example-preformatted">history [<var class="var">n</var>]
history -c
history -d <var class="var">offset</var>
history -d <var class="var">start</var>-<var class="var">end</var>
history [-anrw] [<var class="var">filename</var>]
history -ps <var class="var">arg</var>
</pre></div>

<p>With no options, display the history list with numbers.
Entries prefixed with a &lsquo;<samp class="samp">*</samp>&rsquo; have been modified.
An argument of <var class="var">n</var> lists only the last <var class="var">n</var> entries.
If the shell variable <code class="env">HISTTIMEFORMAT</code> is set and not null,
it is used as a format string for <code class="code">strftime</code>(3) to display
the time stamp associated with each displayed history entry.
If <code class="code">history</code> uses <code class="env">HISTTIMEFORMAT</code>, it does not print an
intervening space between the formatted time stamp and the history entry.
</p>
<p>Options, if supplied, have the following meanings:
</p>
<dl class="table">
<dt><code class="code">-c</code></dt>
<dd><p>Clear the history list.
This may be combined with the other options to replace the history list.
</p>
</dd>
<dt><code class="code">-d <var class="var">offset</var></code></dt>
<dd><p>Delete the history entry at position <var class="var">offset</var>.
If <var class="var">offset</var> is positive, it should be specified as it appears when
the history is displayed.
If <var class="var">offset</var> is negative, it is interpreted as relative to one greater
than the last history position, so negative indices count back from the
end of the history, and an index of &lsquo;<samp class="samp">-1</samp>&rsquo; refers to the current
<code class="code">history -d</code> command.
</p>
</dd>
<dt><code class="code">-d <var class="var">start</var>-<var class="var">end</var></code></dt>
<dd><p>Delete the range of history entries between positions <var class="var">start</var> and
<var class="var">end</var>, inclusive.
Positive and negative values for <var class="var">start</var> and <var class="var">end</var>
are interpreted as described above.
</p>
</dd>
<dt><code class="code">-a</code></dt>
<dd><p>Append the &quot;new&quot; history lines to the history file.
These are history lines entered since the beginning of the current
Bash session, but not already appended to the history file.
</p>
</dd>
<dt><code class="code">-n</code></dt>
<dd><p>Read the history lines not already read from the history file
and add them to the current history list.
These are lines appended to the history
file since the beginning of the current Bash session.
</p>
</dd>
<dt><code class="code">-r</code></dt>
<dd><p>Read the history file and append its contents to the history list.
</p>
</dd>
<dt><code class="code">-w</code></dt>
<dd><p>Write the current history list to the history file, overwriting
the history file.
</p>
</dd>
<dt><code class="code">-p</code></dt>
<dd><p>Perform history substitution on the <var class="var">arg</var>s and display the result
on the standard output, without storing the results in the history list.
</p>
</dd>
<dt><code class="code">-s</code></dt>
<dd><p>Add the <var class="var">arg</var>s to the end of the history list as a single entry.
The last command in the history list is removed before adding the <var class="var">arg</var>s.
</p>
</dd>
</dl>

<p>If a <var class="var">filename</var> argument is supplied
with any of the <samp class="option">-w</samp>, <samp class="option">-r</samp>, <samp class="option">-a</samp>, or <samp class="option">-n</samp>
options, Bash uses <var class="var">filename</var> as the history file.
If not, it uses the value of the <code class="env">HISTFILE</code> variable.
If <code class="env">HISTFILE</code> is unset or null, these options have no effect.
</p>
<p>If the <code class="env">HISTTIMEFORMAT</code>
variable is set, <code class="code">history</code> writes the time stamp information
associated with each history entry to the history file,  
marked with the history comment character as described above.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history entry.
</p>
<p>The return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
<var class="var">offset</var> or range is supplied as an argument to <samp class="option">-d</samp>, or the
history expansion supplied as an argument to <samp class="option">-p</samp> fails.
</p>
</dd>
</dl>

<hr>
</div>
<div class="section-level-extent" id="History-Interaction">
<div class="nav-panel">
<p>
Previous: <a href="#Bash-History-Builtins" accesskey="p" rel="prev">Bash History Builtins</a>, Up: <a href="#Using-History-Interactively" accesskey="u" rel="up">Using History Interactively</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="History-Expansion"><span>9.3 History Expansion<a class="copiable-link" href="#History-Expansion"> &para;</a></span></h3>
<a class="index-entry-id" id="index-history-expansion"></a>

<p>The shell
provides a history expansion feature that is similar
to the history expansion provided by <code class="code">csh</code>
(also referred to as history substitution where appropriate).
This section describes the syntax used to manipulate the
history information.
</p>
<p>History expansion is enabled by default for interactive shells,
and can be disabled using the <samp class="option">+H</samp> option to the <code class="code">set</code>
builtin command (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
Non-interactive shells do not perform history expansion by default,
but it can be enabled with <code class="code">set -H</code>.
</p>
<p>History expansions introduce words from the history list into
the input stream, making it easy to repeat commands, insert the
arguments to a previous command into the current input line, or
fix errors in previous commands quickly.
</p>
<p>History expansion is performed immediately after a complete line
is read, before the shell breaks it into words, and is performed
on each line individually.
Bash attempts to inform the history
expansion functions about quoting still in effect from previous lines.
</p>
<p>History expansion takes place in two parts.
The first is to determine
which entry from the history list should be used during substitution.
The second is to select portions of that entry to include into the
current one.
</p>
<p>The entry selected from the history is called the <em class="dfn">event</em>,
and the portions of that entry that are acted upon are <em class="dfn">words</em>.
Various <em class="dfn">modifiers</em> are available to manipulate the selected words.
The entry is split into words in the same fashion that Bash
does when reading input,
so that several words surrounded by quotes are considered one word.
The <em class="dfn">event designator</em> selects the event, the optional
<em class="dfn">word designator</em> selects words from the event, and
various optional <em class="dfn">modifiers</em> are available to manipulate the
selected words.
</p>
<p>History expansions are introduced by the appearance of the
history expansion character, which is &lsquo;<samp class="samp">!</samp>&rsquo; by default.
History expansions may appear anywhere in the input, but do not nest.
</p>
<p>History expansion implements shell-like quoting conventions:
a backslash can be used to remove the special handling for the next character;
single quotes enclose verbatim sequences of characters, and can be used to
inhibit history expansion;
and characters enclosed within double quotes may be subject to history
expansion, since backslash can escape the history expansion character,
but single quotes may not, since they are not treated specially within
double quotes.
</p>
<p>When using the shell, only &lsquo;<samp class="samp">\</samp>&rsquo; and &lsquo;<samp class="samp">'</samp>&rsquo; may be used to escape the
history expansion character, but the history expansion character is
also treated as quoted if it immediately precedes the closing double quote
in a double-quoted string. 
</p>
<p>Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, &lsquo;<samp class="samp">=</samp>&rsquo;,
and the other shell metacharacters.
</p>
<p>There is a special abbreviation for substitution, active when the
<var class="var">quick substitution</var> character
(described above under <code class="code">histchars</code>)
is the first character on the line.
It selects the previous history list entry, using an event designator
equivalent to <code class="code">!!</code>,
and substitutes one string for another in that entry.
It is described below (see <a class="pxref" href="#Event-Designators">Event Designators</a>).
This is the only history expansion that does not begin with the history
expansion character.
</p>
<p>Several shell options settable with the <code class="code">shopt</code>
builtin (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
modify history expansion behavior
If the <code class="code">histverify</code> shell option is enabled, and Readline
is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the Readline
editing buffer for further modification.
If Readline is being used, and the <code class="code">histreedit</code>
shell option is enabled, a failed history expansion is
reloaded into the Readline editing buffer for correction.
</p>
<p>The <samp class="option">-p</samp> option to the <code class="code">history</code> builtin command
shows what a history expansion will do before using it.
The <samp class="option">-s</samp> option to the <code class="code">history</code> builtin may be used to
add commands to the end of the history list without actually executing
them, so that they are available for subsequent recall.
This is most useful in conjunction with Readline.
</p>
<p>The shell allows control of the various characters used by the
history expansion mechanism with the <code class="code">histchars</code> variable,
as explained above (see <a class="pxref" href="#Bash-Variables">Bash Variables</a>).
The shell uses the history comment character to mark history
timestamps when writing the history file.
</p>

<ul class="mini-toc">
<li><a href="#Event-Designators" accesskey="1">Event Designators</a></li>
<li><a href="#Word-Designators" accesskey="2">Word Designators</a></li>
<li><a href="#Modifiers" accesskey="3">Modifiers</a></li>
</ul>
<hr>
<div class="subsection-level-extent" id="Event-Designators">
<div class="nav-panel">
<p>
Next: <a href="#Word-Designators" accesskey="n" rel="next">Word Designators</a>, Up: <a href="#History-Interaction" accesskey="u" rel="up">History Expansion</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Event-Designators-1"><span>9.3.1 Event Designators<a class="copiable-link" href="#Event-Designators-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-event-designators"></a>

<p>An event designator is a reference to an entry in the history list.
The event designator consists of the portion of the word beginning
with the history expansion character, and ending with the word designator
if one is present, or the end of the word.
Unless the reference is absolute, events are relative to the current
position in the history list.
<a class="index-entry-id" id="index-history-events"></a>
</p>
<dl class="table">
<dt><code class="code">!</code></dt>
<dd><p>Start a history substitution, except when followed by a space, tab,
the end of the line, &lsquo;<samp class="samp">=</samp>&rsquo;,
or the rest of the shell metacharacters defined above
(see <a class="pxref" href="#Definitions">Definitions</a>).
</p>
</dd>
<dt><code class="code">!<var class="var">n</var></code></dt>
<dd><p>Refer to history list entry <var class="var">n</var>.
</p>
</dd>
<dt><code class="code">!-<var class="var">n</var></code></dt>
<dd><p>Refer to the history entry minus <var class="var">n</var>.
</p>
</dd>
<dt><code class="code">!!</code></dt>
<dd><p>Refer to the previous entry.
This is a synonym for &lsquo;<samp class="samp">!-1</samp>&rsquo;.
</p>
</dd>
<dt><code class="code">!<var class="var">string</var></code></dt>
<dd><p>Refer to the most recent command
preceding the current position in the history list
starting with <var class="var">string</var>.
</p>
</dd>
<dt><code class="code">!?<var class="var">string</var>[?]</code></dt>
<dd><p>Refer to the most recent command
preceding the current position in the history list
containing <var class="var">string</var>.
The trailing
&lsquo;<samp class="samp">?</samp>&rsquo; may be omitted if the <var class="var">string</var> is followed immediately by
a newline.
If <var class="var">string</var> is missing, this uses
the string from the most recent search;
it is an error if there is no previous search string.
</p>
</dd>
<dt><code class="code">^<var class="var">string1</var>^<var class="var">string2</var>^</code></dt>
<dd><p>Quick Substitution.
Repeat the last command, replacing <var class="var">string1</var> with <var class="var">string2</var>.
Equivalent to <code class="code">!!:s^<var class="var">string1</var>^<var class="var">string2</var>^</code>.
</p>
</dd>
<dt><code class="code">!#</code></dt>
<dd><p>The entire command line typed so far.
</p>
</dd>
</dl>

<hr>
</div>
<div class="subsection-level-extent" id="Word-Designators">
<div class="nav-panel">
<p>
Next: <a href="#Modifiers" accesskey="n" rel="next">Modifiers</a>, Previous: <a href="#Event-Designators" accesskey="p" rel="prev">Event Designators</a>, Up: <a href="#History-Interaction" accesskey="u" rel="up">History Expansion</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Word-Designators-1"><span>9.3.2 Word Designators<a class="copiable-link" href="#Word-Designators-1"> &para;</a></span></h4>

<p>Word designators are used to select desired words from the event.
They are optional; if the word designator isn&rsquo;t supplied, the history
expansion uses the entire event.
A &lsquo;<samp class="samp">:</samp>&rsquo; separates the event specification from the word designator.
It may be omitted if the word designator begins with a &lsquo;<samp class="samp">^</samp>&rsquo;, &lsquo;<samp class="samp">$</samp>&rsquo;,
&lsquo;<samp class="samp">*</samp>&rsquo;, &lsquo;<samp class="samp">-</samp>&rsquo;, or &lsquo;<samp class="samp">%</samp>&rsquo;.
Words are numbered from the beginning of the line,
with the first word being denoted by 0 (zero).
That first word is usually the command word, and the arguments begin
with the second word.
Words are inserted into the current line separated by single spaces.
</p>
<p>For example,
</p>
<dl class="table">
<dt><code class="code">!!</code></dt>
<dd><p>designates the preceding command.
When you type this, the preceding command is repeated in toto.
</p>
</dd>
<dt><code class="code">!!:$</code></dt>
<dd><p>designates the last word of the preceding command.
This may be shortened to <code class="code">!$</code>.
</p>
</dd>
<dt><code class="code">!fi:2</code></dt>
<dd><p>designates the second argument of the most recent command starting with
the letters <code class="code">fi</code>.
</p></dd>
</dl>

<p>Here are the word designators:
</p>
<dl class="table">
<dt><code class="code">0 (zero)</code></dt>
<dd><p>The <code class="code">0</code>th word.
For the shell, and many other, applications, this is the command word.
</p>
</dd>
<dt><code class="code"><var class="var">n</var></code></dt>
<dd><p>The <var class="var">n</var>th word.
</p>
</dd>
<dt><code class="code">^</code></dt>
<dd><p>The first argument: word 1.
</p>
</dd>
<dt><code class="code">$</code></dt>
<dd><p>The last word.
This is usually the last argument, but expands to the
zeroth word if there is only one word in the line.
</p>
</dd>
<dt><code class="code">%</code></dt>
<dd><p>The first word matched by the most recent &lsquo;<samp class="samp">?<var class="var">string</var>?</samp>&rsquo; search,
if the search string begins with a character that is part of a word.
By default, searches begin at the end of each line and proceed to the
beginning, so the first word matched is the one closest to the end of
the line.
</p>
</dd>
<dt><code class="code"><var class="var">x</var>-<var class="var">y</var></code></dt>
<dd><p>A range of words; &lsquo;<samp class="samp">-<var class="var">y</var></samp>&rsquo; abbreviates &lsquo;<samp class="samp">0-<var class="var">y</var></samp>&rsquo;.
</p>
</dd>
<dt><code class="code">*</code></dt>
<dd><p>All of the words, except the <code class="code">0</code>th.
This is a synonym for &lsquo;<samp class="samp">1-$</samp>&rsquo;.
It is not an error to use &lsquo;<samp class="samp">*</samp>&rsquo; if there is just one word in the event;
it expands to the empty string in that case.
</p>
</dd>
<dt><code class="code"><var class="var">x</var>*</code></dt>
<dd><p>Abbreviates &lsquo;<samp class="samp"><var class="var">x</var>-$</samp>&rsquo;.
</p>
</dd>
<dt><code class="code"><var class="var">x</var>-</code></dt>
<dd><p>Abbreviates &lsquo;<samp class="samp"><var class="var">x</var>-$</samp>&rsquo; like &lsquo;<samp class="samp"><var class="var">x</var>*</samp>&rsquo;, but omits the last word.
If &lsquo;<samp class="samp">x</samp>&rsquo; is missing, it defaults to 0.
</p>
</dd>
</dl>

<p>If a word designator is supplied without an event specification, the
previous command is used as the event, equivalent to <code class="code">!!</code>. 
</p>
<hr>
</div>
<div class="subsection-level-extent" id="Modifiers">
<div class="nav-panel">
<p>
Previous: <a href="#Word-Designators" accesskey="p" rel="prev">Word Designators</a>, Up: <a href="#History-Interaction" accesskey="u" rel="up">History Expansion</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h4 class="subsection" id="Modifiers-1"><span>9.3.3 Modifiers<a class="copiable-link" href="#Modifiers-1"> &para;</a></span></h4>

<p>After the optional word designator, you can add a sequence of one or more
of the following modifiers, each preceded by a &lsquo;<samp class="samp">:</samp>&rsquo;.
These modify, or edit, the word or words selected from the history event.
</p>
<dl class="table">
<dt><code class="code">h</code></dt>
<dd><p>Remove a trailing filename component, leaving only the head.
</p>
</dd>
<dt><code class="code">t</code></dt>
<dd><p>Remove all leading filename components, leaving the tail.
</p>
</dd>
<dt><code class="code">r</code></dt>
<dd><p>Remove a trailing suffix of the form &lsquo;<samp class="samp">.<var class="var">suffix</var></samp>&rsquo;, leaving
the basename.
</p>
</dd>
<dt><code class="code">e</code></dt>
<dd><p>Remove all but the trailing suffix.
</p>
</dd>
<dt><code class="code">p</code></dt>
<dd><p>Print the new command but do not execute it.
</p>
</dd>
<dt><code class="code">q</code></dt>
<dd><p>Quote the substituted words, escaping further substitutions.
</p>
</dd>
<dt><code class="code">x</code></dt>
<dd><p>Quote the substituted words as with &lsquo;<samp class="samp">q</samp>&rsquo;,
but break into words at spaces, tabs, and newlines.
The &lsquo;<samp class="samp">q</samp>&rsquo; and &lsquo;<samp class="samp">x</samp>&rsquo; modifiers are mutually exclusive;
expansion uses the last one supplied.
</p>
</dd>
<dt><code class="code">s/<var class="var">old</var>/<var class="var">new</var>/</code></dt>
<dd><p>Substitute <var class="var">new</var> for the first occurrence of <var class="var">old</var> in the
event line.
Any character may be used as the delimiter in place of &lsquo;<samp class="samp">/</samp>&rsquo;.
The delimiter may be quoted in <var class="var">old</var> and <var class="var">new</var>
with a single backslash.
If &lsquo;<samp class="samp">&amp;</samp>&rsquo; appears in <var class="var">new</var>, it is replaced with <var class="var">old</var>.
A single backslash quotes the &lsquo;<samp class="samp">&amp;</samp>&rsquo; in <var class="var">old</var> and <var class="var">new</var>.
If <var class="var">old</var> is null, it is set to the last <var class="var">old</var>
substituted, or, if no previous history substitutions took place,
the last <var class="var">string</var>
in a !?<var class="var">string</var><code class="code">[?]</code>
search.
If <var class="var">new</var> is null, each matching <var class="var">old</var> is deleted.
The final delimiter is optional if it is the last
character on the input line.
</p>
</dd>
<dt><code class="code">&amp;</code></dt>
<dd><p>Repeat the previous substitution.
</p>
</dd>
<dt><code class="code">g</code></dt>
<dt><code class="code">a</code></dt>
<dd><p>Cause changes to be applied over the entire event line.
This is used in conjunction with
&lsquo;<samp class="samp">s</samp>&rsquo;, as in <code class="code">gs/<var class="var">old</var>/<var class="var">new</var>/</code>,
or with &lsquo;<samp class="samp">&amp;</samp>&rsquo;.
</p>
</dd>
<dt><code class="code">G</code></dt>
<dd><p>Apply the following &lsquo;<samp class="samp">s</samp>&rsquo; or &lsquo;<samp class="samp">&amp;</samp>&rsquo; modifier once to each word
in the event.
</p>
</dd>
</dl>

<hr>
</div>
</div>
</div>
<div class="chapter-level-extent" id="Installing-Bash">
<div class="nav-panel">
<p>
Next: <a href="#Reporting-Bugs" accesskey="n" rel="next">Reporting Bugs</a>, Previous: <a href="#Using-History-Interactively" accesskey="p" rel="prev">Using History Interactively</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="chapter" id="Installing-Bash-1"><span>10 Installing Bash<a class="copiable-link" href="#Installing-Bash-1"> &para;</a></span></h2>

<p>This chapter provides basic instructions for installing Bash on
the various supported platforms.
The distribution supports the
<small class="sc">GNU</small> operating systems, nearly every version of Unix, and several
non-Unix systems such as BeOS and Interix.
Other independent ports exist for Windows platforms.
</p>

<ul class="mini-toc">
<li><a href="#Basic-Installation" accesskey="1">Basic Installation</a></li>
<li><a href="#Compilers-and-Options" accesskey="2">Compilers and Options</a></li>
<li><a href="#Compiling-For-Multiple-Architectures" accesskey="3">Compiling For Multiple Architectures</a></li>
<li><a href="#Installation-Names" accesskey="4">Installation Names</a></li>
<li><a href="#Specifying-the-System-Type" accesskey="5">Specifying the System Type</a></li>
<li><a href="#Sharing-Defaults" accesskey="6">Sharing Defaults</a></li>
<li><a href="#Operation-Controls" accesskey="7">Operation Controls</a></li>
<li><a href="#Optional-Features" accesskey="8">Optional Features</a></li>
</ul>
<hr>
<div class="section-level-extent" id="Basic-Installation">
<div class="nav-panel">
<p>
Next: <a href="#Compilers-and-Options" accesskey="n" rel="next">Compilers and Options</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Basic-Installation-1"><span>10.1 Basic Installation<a class="copiable-link" href="#Basic-Installation-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-installation"></a>
<a class="index-entry-id" id="index-configuration"></a>
<a class="index-entry-id" id="index-Bash-installation"></a>
<a class="index-entry-id" id="index-Bash-configuration"></a>

<p>These are installation instructions for Bash.
</p>
<p>The simplest way to compile Bash is:
</p>
<ol class="enumerate">
<li> <code class="code">cd</code> to the directory containing the source code and type
&lsquo;<samp class="samp">./configure</samp>&rsquo; to configure Bash for your system.
If you&rsquo;re using <code class="code">csh</code> on an old version of System V, you might
need to type &lsquo;<samp class="samp">sh ./configure</samp>&rsquo; instead to prevent <code class="code">csh</code>
from trying to execute <code class="code">configure</code> itself.

<p>Running <code class="code">configure</code> takes some time.
While running, it prints messages telling which features it is
checking for.
</p>
</li><li> Type &lsquo;<samp class="samp">make</samp>&rsquo; to compile Bash and build the <code class="code">bashbug</code> bug
reporting script.

</li><li> Optionally, type &lsquo;<samp class="samp">make tests</samp>&rsquo; to run the Bash test suite.

</li><li> Type &lsquo;<samp class="samp">make install</samp>&rsquo; to install <code class="code">bash</code> and <code class="code">bashbug</code>.
This will also install the manual pages and Info file, message translation
files, some supplemental documentation, a number of example loadable
builtin commands, and a set of header files for developing loadable
builtins.
You may need additional privileges to install <code class="code">bash</code> to your
desired destination, which may require &lsquo;<samp class="samp">sudo make install</samp>&rsquo;.
More information about controlling the locations where <code class="code">bash</code> and
other files are installed is below (see <a class="pxref" href="#Installation-Names">Installation Names</a>).

</li></ol>

<p>The <code class="code">configure</code> shell script attempts to guess correct
values for various system-dependent variables used during compilation.
It uses those values to create a <samp class="file">Makefile</samp> in
each directory of the package (the top directory, the
<samp class="file">builtins</samp>, <samp class="file">doc</samp>, <samp class="file">po</samp>, and <samp class="file">support</samp> directories,
each directory under <samp class="file">lib</samp>, and several others).
It also creates a
<samp class="file">config.h</samp> file containing system-dependent definitions. 
Finally, it creates a shell script named <code class="code">config.status</code> that you
can run in the future to recreate the current configuration, a
file <samp class="file">config.cache</samp> that saves the results of its tests to
speed up reconfiguring, and a file <samp class="file">config.log</samp> containing
compiler output (useful mainly for debugging <code class="code">configure</code>). 
If at some point
<samp class="file">config.cache</samp> contains results you don&rsquo;t want to keep, you
may remove or edit it. 
</p>
<p>To find out more about the options and arguments that the
<code class="code">configure</code> script understands, type 
</p>
<div class="example">
<pre class="example-preformatted">bash-4.2$ ./configure --help
</pre></div>

<p>at the Bash prompt in your Bash source directory.
</p>
<p>If you want to build Bash in a directory separate from the source
directory &ndash; to build for multiple architectures, for example &ndash;
just use the full path to the configure script.
The following commands
will build Bash in a directory under <samp class="file">/usr/local/build</samp> from
the source code in <samp class="file">/usr/local/src/bash-4.4</samp>:
</p>
<div class="example">
<pre class="example-preformatted">mkdir /usr/local/build/bash-4.4
cd /usr/local/build/bash-4.4
bash /usr/local/src/bash-4.4/configure
make
</pre></div>

<p>See <a class="ref" href="#Compiling-For-Multiple-Architectures">Compiling For Multiple Architectures</a> for more information
about building in a directory separate from the source.
</p>
<p>If you need to do unusual things to compile Bash, please
try to figure out how <code class="code">configure</code> could check whether or not
to do them, and mail diffs or instructions to
<a class="email" href="mailto:bash-maintainers@gnu.org">bash-maintainers@gnu.org</a> so they can be
considered for the next release.
</p>
<p>The file <samp class="file">configure.ac</samp> is used to create <code class="code">configure</code>
by a program called Autoconf.
You only need <samp class="file">configure.ac</samp> if you want to change it or regenerate
<code class="code">configure</code> using a newer version of Autoconf.
If you do this, make sure you are using Autoconf version 2.69 or
newer.
</p>
<p>You can remove the program binaries and object files from the
source code directory by typing &lsquo;<samp class="samp">make clean</samp>&rsquo;.
To also remove the
files that <code class="code">configure</code> created (so you can compile Bash for
a different kind of computer), type &lsquo;<samp class="samp">make distclean</samp>&rsquo;.
</p>
<hr>
</div>
<div class="section-level-extent" id="Compilers-and-Options">
<div class="nav-panel">
<p>
Next: <a href="#Compiling-For-Multiple-Architectures" accesskey="n" rel="next">Compiling For Multiple Architectures</a>, Previous: <a href="#Basic-Installation" accesskey="p" rel="prev">Basic Installation</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Compilers-and-Options-1"><span>10.2 Compilers and Options<a class="copiable-link" href="#Compilers-and-Options-1"> &para;</a></span></h3>

<p>Some systems require unusual options for compilation or linking
that the <code class="code">configure</code> script does not know about.
You can give <code class="code">configure</code> initial values for variables by setting
them in the environment.
Using a Bourne-compatible shell, you can do that on the command line
like this:
</p>
<div class="example">
<pre class="example-preformatted">CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
</pre></div>

<p>On systems that have the <code class="code">env</code> program, you can do it like this:
</p>
<div class="example">
<pre class="example-preformatted">env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
</pre></div>

<p>The configuration process uses GCC to build Bash if it
is available.
</p>
<hr>
</div>
<div class="section-level-extent" id="Compiling-For-Multiple-Architectures">
<div class="nav-panel">
<p>
Next: <a href="#Installation-Names" accesskey="n" rel="next">Installation Names</a>, Previous: <a href="#Compilers-and-Options" accesskey="p" rel="prev">Compilers and Options</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Compiling-For-Multiple-Architectures-1"><span>10.3 Compiling For Multiple Architectures<a class="copiable-link" href="#Compiling-For-Multiple-Architectures-1"> &para;</a></span></h3>

<p>You can compile Bash for more than one kind of computer at the
same time, by placing the object files for each architecture in their
own directory.
To do this, you must use a version of <code class="code">make</code> that
supports the <code class="code">VPATH</code> variable, such as GNU <code class="code">make</code>.
<code class="code">cd</code> to the
directory where you want the object files and executables to go and run
the <code class="code">configure</code> script from the source directory
(see <a class="pxref" href="#Basic-Installation">Basic Installation</a>).
You may need to
supply the <samp class="option">--srcdir=PATH</samp> argument to tell <code class="code">configure</code>
where the source files are.
<code class="code">configure</code> automatically checks for the
source code in the directory that <code class="code">configure</code> is in and in <samp class="file">..</samp>.
</p>
<p>If you have to use a <code class="code">make</code> that does not support the <code class="code">VPATH</code>
variable, you can compile Bash for one architecture at a
time in the source code directory.
After you have installed
Bash for one architecture, use &lsquo;<samp class="samp">make distclean</samp>&rsquo; before
reconfiguring for another architecture.
</p>
<p>Alternatively, if your system supports symbolic links, you can use the
<samp class="file">support/mkclone</samp> script to create a build tree which has
symbolic links back to each file in the source directory.
Here&rsquo;s an
example that creates a build directory in the current directory from a
source directory <samp class="file">/usr/gnu/src/bash-2.0</samp>:
</p>
<div class="example">
<pre class="example-preformatted">bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
</pre></div>

<p>The <code class="code">mkclone</code> script requires Bash, so you must have already built
Bash for at least one architecture before you can create build
directories for other architectures.
</p>
<hr>
</div>
<div class="section-level-extent" id="Installation-Names">
<div class="nav-panel">
<p>
Next: <a href="#Specifying-the-System-Type" accesskey="n" rel="next">Specifying the System Type</a>, Previous: <a href="#Compiling-For-Multiple-Architectures" accesskey="p" rel="prev">Compiling For Multiple Architectures</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Installation-Names-1"><span>10.4 Installation Names<a class="copiable-link" href="#Installation-Names-1"> &para;</a></span></h3>

<p>By default, &lsquo;<samp class="samp">make install</samp>&rsquo; will install into
<samp class="file">/usr/local/bin</samp>, <samp class="file">/usr/local/man</samp>, etc.;
that is, the <em class="dfn">installation prefix</em> defaults to <samp class="file">/usr/local</samp>.
You can specify an installation prefix other than <samp class="file">/usr/local</samp> by
giving <code class="code">configure</code> the option <samp class="option">--prefix=<var class="var">PATH</var></samp>,
or by specifying a value for the <code class="env">prefix</code> &lsquo;<samp class="samp">make</samp>&rsquo;
variable when running &lsquo;<samp class="samp">make install</samp>&rsquo;
(e.g., &lsquo;<samp class="samp">make install prefix=<var class="var">PATH</var></samp>&rsquo;).
The <code class="env">prefix</code> variable provides a default for <code class="env">exec_prefix</code> and
other variables used when installing Bash.
</p>
<p>You can specify separate installation prefixes for
architecture-specific files and architecture-independent files. 
If you give <code class="code">configure</code> the option
<samp class="option">--exec-prefix=<var class="var">PATH</var></samp>, &lsquo;<samp class="samp">make install</samp>&rsquo; will use
<var class="var">PATH</var> as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.
</p>
<p>If you would like to change the installation locations for a single run,
you can specify these variables as arguments to <code class="code">make</code>:
&lsquo;<samp class="samp">make install exec_prefix=/</samp>&rsquo; will install <code class="code">bash</code> and
<code class="code">bashbug</code> into <samp class="file">/bin</samp> instead of the default <samp class="file">/usr/local/bin</samp>.
</p>
<p>If you want to see the files Bash will install and where it will install
them without changing anything on your system, specify the variable
<code class="env">DESTDIR</code> as an argument to <code class="code">make</code>.
Its value should be the absolute directory path you&rsquo;d like to use as the
root of your sample installation tree.
For example,
</p>
<div class="example">
<pre class="example-preformatted">mkdir /fs1/bash-install
make install DESTDIR=/fs1/bash-install
</pre></div>

<p>will install <code class="code">bash</code> into <samp class="file">/fs1/bash-install/usr/local/bin/bash</samp>,
the documentation into directories within
<samp class="file">/fs1/bash-install/usr/local/share</samp>, the example loadable builtins into
<samp class="file">/fs1/bash-install/usr/local/lib/bash</samp>, and so on.
You can use the usual <code class="env">exec_prefix</code> and <code class="env">prefix</code> variables to alter
the directory paths beneath the value of <code class="env">DESTDIR</code>.
</p>
<p>The GNU Makefile standards provide a more complete description of these
variables and their effects.
</p>
<hr>
</div>
<div class="section-level-extent" id="Specifying-the-System-Type">
<div class="nav-panel">
<p>
Next: <a href="#Sharing-Defaults" accesskey="n" rel="next">Sharing Defaults</a>, Previous: <a href="#Installation-Names" accesskey="p" rel="prev">Installation Names</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Specifying-the-System-Type-1"><span>10.5 Specifying the System Type<a class="copiable-link" href="#Specifying-the-System-Type-1"> &para;</a></span></h3>

<p>There may be some features <code class="code">configure</code> can not figure out
automatically, but needs to determine by the type of host Bash
will run on.
Usually <code class="code">configure</code> can figure that
out, but if it prints a message saying it can not guess the host
type, give it the <samp class="option">--host=TYPE</samp> option.
&lsquo;<samp class="samp">TYPE</samp>&rsquo; can
either be a short name for the system type, such as &lsquo;<samp class="samp">sun4</samp>&rsquo;,
or a canonical name with three fields: &lsquo;<samp class="samp">CPU-COMPANY-SYSTEM</samp>&rsquo;
(e.g., &lsquo;<samp class="samp">i386-unknown-freebsd4.2</samp>&rsquo;).
</p>
<p>See the file <samp class="file">support/config.sub</samp> for the possible
values of each field. 
</p>
<hr>
</div>
<div class="section-level-extent" id="Sharing-Defaults">
<div class="nav-panel">
<p>
Next: <a href="#Operation-Controls" accesskey="n" rel="next">Operation Controls</a>, Previous: <a href="#Specifying-the-System-Type" accesskey="p" rel="prev">Specifying the System Type</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Sharing-Defaults-1"><span>10.6 Sharing Defaults<a class="copiable-link" href="#Sharing-Defaults-1"> &para;</a></span></h3>

<p>If you want to set default values for <code class="code">configure</code> scripts to
share, you can create a site shell script called
<code class="code">config.site</code> that gives default values for variables like
<code class="code">CC</code>, <code class="code">cache_file</code>, and <code class="code">prefix</code>.
<code class="code">configure</code>
looks for <samp class="file">PREFIX/share/config.site</samp> if it exists, then
<samp class="file">PREFIX/etc/config.site</samp> if it exists.
Or, you can set the
<code class="code">CONFIG_SITE</code> environment variable to the location of the site
script.
A warning: the Bash <code class="code">configure</code> looks for a site script,
but not all <code class="code">configure</code> scripts do.
</p>
<hr>
</div>
<div class="section-level-extent" id="Operation-Controls">
<div class="nav-panel">
<p>
Next: <a href="#Optional-Features" accesskey="n" rel="next">Optional Features</a>, Previous: <a href="#Sharing-Defaults" accesskey="p" rel="prev">Sharing Defaults</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Operation-Controls-1"><span>10.7 Operation Controls<a class="copiable-link" href="#Operation-Controls-1"> &para;</a></span></h3>

<p><code class="code">configure</code> recognizes the following options to control how it
operates.
</p>
<dl class="table">
<dt><code class="code">--cache-file=<var class="var">file</var></code></dt>
<dd><p>Use and save the results of the tests in
<var class="var">file</var> instead of <samp class="file">./config.cache</samp>.
Set <var class="var">file</var> to <samp class="file">/dev/null</samp> to disable caching,
for debugging <code class="code">configure</code>. 
</p>
</dd>
<dt><code class="code">--help</code></dt>
<dd><p>Print a summary of the options to <code class="code">configure</code>, and exit.
</p>
</dd>
<dt><code class="code">--quiet</code></dt>
<dt><code class="code">--silent</code></dt>
<dt><code class="code">-q</code></dt>
<dd><p>Do not print messages saying which checks are being made.
</p>
</dd>
<dt><code class="code">--srcdir=<var class="var">dir</var></code></dt>
<dd><p>Look for the Bash source code in directory <var class="var">dir</var>.
Usually <code class="code">configure</code> can determine that directory automatically.
</p>
</dd>
<dt><code class="code">--version</code></dt>
<dd><p>Print the version of Autoconf used to generate the <code class="code">configure</code>
script, and exit.
</p></dd>
</dl>

<p><code class="code">configure</code> also accepts some other, not widely used, boilerplate
options.
&lsquo;<samp class="samp">configure --help</samp>&rsquo; prints the complete list.
</p>
<hr>
</div>
<div class="section-level-extent" id="Optional-Features">
<div class="nav-panel">
<p>
Previous: <a href="#Operation-Controls" accesskey="p" rel="prev">Operation Controls</a>, Up: <a href="#Installing-Bash" accesskey="u" rel="up">Installing Bash</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="section" id="Optional-Features-1"><span>10.8 Optional Features<a class="copiable-link" href="#Optional-Features-1"> &para;</a></span></h3>

<p>The Bash <code class="code">configure</code> has a number of <samp class="option">--enable-<var class="var">feature</var></samp>
options, where <var class="var">feature</var> indicates an optional part of Bash.
There are also several <samp class="option">--with-<var class="var">package</var></samp> options,
where <var class="var">package</var> is something like &lsquo;<samp class="samp">bash-malloc</samp>&rsquo; or &lsquo;<samp class="samp">afs</samp>&rsquo;.
To turn off the default use of a package, use
<samp class="option">--without-<var class="var">package</var></samp>.
To configure Bash without a feature
that is enabled by default, use <samp class="option">--disable-<var class="var">feature</var></samp>.
</p>
<p>Here is a complete list of the <samp class="option">--enable-</samp> and <samp class="option">--with-</samp>
options that the Bash <code class="code">configure</code> recognizes. 
</p>
<dl class="table">
<dt><code class="code">--with-afs</code></dt>
<dd><p>Define if you are using the Andrew File System from Transarc.
</p>
</dd>
<dt><code class="code">--with-bash-malloc</code></dt>
<dd><p>Use the Bash version of
<code class="code">malloc</code> in the directory <samp class="file">lib/malloc</samp>.
This is not the same
<code class="code">malloc</code> that appears in <small class="sc">GNU</small> libc, but a custom version
originally derived from the 4.2 <small class="sc">BSD</small> <code class="code">malloc</code>.
This <code class="code">malloc</code> is very fast, but wastes some space on each allocation,
though it uses several techniques to minimize the waste.
This option is enabled by default.
The <samp class="file">NOTES</samp> file contains a list of systems for
which this should be turned off, and <code class="code">configure</code> disables this
option automatically for a number of systems.
</p>
</dd>
<dt><code class="code">--with-curses</code></dt>
<dd><p>Use the curses library instead of the termcap library.
<code class="code">configure</code> usually chooses this automatically, since most systems
include the termcap functions in the curses library.
</p>
</dd>
<dt><code class="code">--with-gnu-malloc</code></dt>
<dd><p>A synonym for <code class="code">--with-bash-malloc</code>.
</p>
</dd>
<dt><code class="code">--with-installed-readline[=<var class="var">PREFIX</var>]</code></dt>
<dd><p>Define this to make Bash link with a locally-installed version of Readline
rather than the version in <samp class="file">lib/readline</samp>.
This works only with Readline 5.0 and later versions.
If <var class="var">PREFIX</var> is <code class="code">yes</code> or not
supplied, <code class="code">configure</code> uses the values of the make variables
<code class="code">includedir</code> and <code class="code">libdir</code>, which are subdirectories of <code class="code">prefix</code>
by default, to find the installed version of Readline if it is not in
the standard system include and library directories.
If <var class="var">PREFIX</var> is <code class="code">no</code>, Bash links with the version in
<samp class="file">lib/readline</samp>.
If <var class="var">PREFIX</var> is set to any other value, <code class="code">configure</code> treats it as
a directory pathname and looks for
the installed version of Readline in subdirectories of that directory
(include files in <var class="var">PREFIX</var>/<code class="code">include</code> and the library in
<var class="var">PREFIX</var>/<code class="code">lib</code>).
The Bash default is to link with a static library built in the
<samp class="file">lib/readline</samp> subdirectory of the build directory.
</p>
</dd>
<dt><code class="code">--with-libintl-prefix[=<var class="var">PREFIX</var>]</code></dt>
<dd><p>Define this to make Bash link with a locally-installed version of the
libintl library instead of the version in <samp class="file">lib/intl</samp>.
</p>
</dd>
<dt><code class="code">--with-libiconv-prefix[=<var class="var">PREFIX</var>]</code></dt>
<dd><p>Define this to make Bash look for libiconv in <var class="var">PREFIX</var> instead of the
standard system locations.
The Bash distribution does not include this library.
</p>
</dd>
<dt><code class="code">--enable-minimal-config</code></dt>
<dd><p>This produces a shell with minimal features, closer to the historical
Bourne shell.
</p></dd>
</dl>

<p>There are several <samp class="option">--enable-</samp> options that alter how Bash is
compiled, linked, and installed, rather than changing run-time features.
</p>
<dl class="table">
<dt><code class="code">--enable-largefile</code></dt>
<dd><p>Enable support for <a class="url" href="http://www.unix.org/version2/whatsnew/lfs20mar.html">large files</a> if the operating system requires special compiler options
to build programs which can access large files.
This is enabled by
default, if the operating system provides large file support.
</p>
</dd>
<dt><code class="code">--enable-profiling</code></dt>
<dd><p>This builds a Bash binary that produces profiling information to be
processed by <code class="code">gprof</code> each time it is executed.
</p>
</dd>
<dt><code class="code">--enable-separate-helpfiles</code></dt>
<dd><p>Use external files for the documentation displayed by the <code class="code">help</code> builtin
instead of storing the text internally.
</p>
</dd>
<dt><code class="code">--enable-static-link</code></dt>
<dd><p>This causes Bash to be linked statically, if <code class="code">gcc</code> is being used.
This could be used to build a version to use as root&rsquo;s shell.
</p>
</dd>
</dl>

<p>The &lsquo;<samp class="samp">minimal-config</samp>&rsquo; option can be used to disable all of
the following options, but it is processed first, so individual
options may be enabled using &lsquo;<samp class="samp">enable-<var class="var">feature</var></samp>&rsquo;. 
</p>
<p>All of the following options except for
&lsquo;<samp class="samp">alt-array-implementation</samp>&rsquo;,
&lsquo;<samp class="samp">disabled-builtins</samp>&rsquo;,
&lsquo;<samp class="samp">direxpand-default</samp>&rsquo;,
&lsquo;<samp class="samp">strict-posix-default</samp>&rsquo;,
and
&lsquo;<samp class="samp">xpg-echo-default</samp>&rsquo; are
enabled by default, unless the operating system does not provide the
necessary support.
</p>
<dl class="table">
<dt><code class="code">--enable-alias</code></dt>
<dd><p>Allow alias expansion and include the <code class="code">alias</code> and <code class="code">unalias</code>
builtins (see <a class="pxref" href="#Aliases">Aliases</a>).
</p>
</dd>
<dt><code class="code">--enable-alt-array-implementation</code></dt>
<dd><p>This builds Bash using an alternate implementation of arrays
(see <a class="pxref" href="#Arrays">Arrays</a>) that provides faster access at the expense of using
more memory (sometimes many times more, depending on how sparse an array is).
</p>
</dd>
<dt><code class="code">--enable-arith-for-command</code></dt>
<dd><p>Include support for the alternate form of the <code class="code">for</code> command
that behaves like the C language <code class="code">for</code> statement
(see <a class="pxref" href="#Looping-Constructs">Looping Constructs</a>).
</p>
</dd>
<dt><code class="code">--enable-array-variables</code></dt>
<dd><p>Include support for one-dimensional array shell variables
(see <a class="pxref" href="#Arrays">Arrays</a>).
</p>
</dd>
<dt><code class="code">--enable-bang-history</code></dt>
<dd><p>Include support for <code class="code">csh</code>-like history substitution
(see <a class="pxref" href="#History-Interaction">History Expansion</a>).
</p>
</dd>
<dt><code class="code">--enable-bash-source-fullpath-default</code></dt>
<dd><p>Set the default value of the <code class="code">bash_source_fullpath</code> shell option
described above under <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a> to be enabled.
This controls how filenames are assigned to the <code class="code">BASH_SOURCE</code>
array variable.
</p>
</dd>
<dt><code class="code">--enable-brace-expansion</code></dt>
<dd><p>Include <code class="code">csh</code>-like brace expansion
( <code class="code">b{a,b}c</code> &rarr; <code class="code">bac bbc</code> ).
See <a class="ref" href="#Brace-Expansion">Brace Expansion</a>, for a complete description.
</p>
</dd>
<dt><code class="code">--enable-casemod-attributes</code></dt>
<dd><p>Include support for case-modifying attributes in the <code class="code">declare</code> builtin
and assignment statements.
Variables with the <code class="code">uppercase</code> attribute,
for example, will have their values converted to uppercase upon assignment.
</p>
</dd>
<dt><code class="code">--enable-casemod-expansion</code></dt>
<dd><p>Include support for case-modifying word expansions.
</p>
</dd>
<dt><code class="code">--enable-command-timing</code></dt>
<dd><p>Include support for recognizing <code class="code">time</code> as a reserved word and for
displaying timing statistics for the pipeline following <code class="code">time</code>
(see <a class="pxref" href="#Pipelines">Pipelines</a>).
This allows timing pipelines, shell compound commands, shell builtins,
and shell functions, which an external command cannot do easily.
</p>
</dd>
<dt><code class="code">--enable-cond-command</code></dt>
<dd><p>Include support for the <code class="code">[[</code> conditional command.
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
</p>
</dd>
<dt><code class="code">--enable-cond-regexp</code></dt>
<dd><p>Include support for matching <small class="sc">POSIX</small> regular expressions using the
&lsquo;<samp class="samp">=~</samp>&rsquo; binary operator in the <code class="code">[[</code> conditional command.
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
</p>
</dd>
<dt><code class="code">--enable-coprocesses</code></dt>
<dd><p>Include support for coprocesses and the <code class="code">coproc</code> reserved word
(see <a class="pxref" href="#Pipelines">Pipelines</a>).
</p>
</dd>
<dt><code class="code">--enable-debugger</code></dt>
<dd><p>Include support for the Bash debugger (distributed separately).
</p>
</dd>
<dt><code class="code">--enable-dev-fd-stat-broken</code></dt>
<dd><p>If calling <code class="code">stat</code> on /dev/fd/<var class="var">N</var> returns different results than
calling <code class="code">fstat</code> on file descriptor <var class="var">N</var>, supply this option to
enable a workaround.
This has implications for conditional commands that test file attributes.
</p>
</dd>
<dt><code class="code">--enable-direxpand-default</code></dt>
<dd><p>Cause the <code class="code">direxpand</code> shell option (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>)
to be enabled by default when the shell starts.
It is normally disabled by default.
</p>
</dd>
<dt><code class="code">--enable-directory-stack</code></dt>
<dd><p>Include support for a <code class="code">csh</code>-like directory stack and the
<code class="code">pushd</code>, <code class="code">popd</code>, and <code class="code">dirs</code> builtins
(see <a class="pxref" href="#The-Directory-Stack">The Directory Stack</a>).
</p>
</dd>
<dt><code class="code">--enable-disabled-builtins</code></dt>
<dd><p>Allow builtin commands to be invoked via &lsquo;<samp class="samp">builtin xxx</samp>&rsquo;
even after <code class="code">xxx</code> has been disabled using &lsquo;<samp class="samp">enable -n xxx</samp>&rsquo;.
See <a class="ref" href="#Bash-Builtins">Bash Builtin Commands</a>, for details of the <code class="code">builtin</code> and
<code class="code">enable</code> builtin commands.
</p>
</dd>
<dt><code class="code">--enable-dparen-arithmetic</code></dt>
<dd><p>Include support for the <code class="code">((&hellip;))</code> command
(see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
</p>
</dd>
<dt><code class="code">--enable-extended-glob</code></dt>
<dd><p>Include support for the extended pattern matching features described
above under <a class="ref" href="#Pattern-Matching">Pattern Matching</a>.
</p>
</dd>
<dt><code class="code">--enable-extended-glob-default</code></dt>
<dd><p>Set the default value of the <code class="code">extglob</code> shell option described
above under <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a> to be enabled.
</p>
</dd>
<dt><code class="code">--enable-function-import</code></dt>
<dd><p>Include support for importing function definitions exported by another
instance of the shell from the environment.
This option is enabled by default.
</p>
</dd>
<dt><code class="code">--enable-glob-asciiranges-default</code></dt>
<dd><p>Set the default value of the <code class="code">globasciiranges</code> shell option described
above under <a class="ref" href="#The-Shopt-Builtin">The Shopt Builtin</a> to be enabled.
This controls the behavior of character ranges when used in pattern matching
bracket expressions.
</p>
</dd>
<dt><code class="code">--enable-help-builtin</code></dt>
<dd><p>Include the <code class="code">help</code> builtin, which displays help on shell builtins and
variables (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).
</p>
</dd>
<dt><code class="code">--enable-history</code></dt>
<dd><p>Include command history and the <code class="code">fc</code> and <code class="code">history</code>
builtin commands (see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>).
</p>
</dd>
<dt><code class="code">--enable-job-control</code></dt>
<dd><p>This enables the job control features (see <a class="pxref" href="#Job-Control">Job Control</a>),
if the operating system supports them.
</p>
</dd>
<dt><code class="code">--enable-multibyte</code></dt>
<dd><p>This enables support for multibyte characters if the operating
system provides the necessary support.
</p>
</dd>
<dt><code class="code">--enable-net-redirections</code></dt>
<dd><p>This enables the special handling of filenames of the form
<code class="code">/dev/tcp/<var class="var">host</var>/<var class="var">port</var></code> and
<code class="code">/dev/udp/<var class="var">host</var>/<var class="var">port</var></code>
when used in redirections (see <a class="pxref" href="#Redirections">Redirections</a>).
</p>
</dd>
<dt><code class="code">--enable-process-substitution</code></dt>
<dd><p>This enables process substitution (see <a class="pxref" href="#Process-Substitution">Process Substitution</a>) if
the operating system provides the necessary support.
</p>
</dd>
<dt><code class="code">--enable-progcomp</code></dt>
<dd><p>Enable the programmable completion facilities
(see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>).
If Readline is not enabled, this option has no effect.
</p>
</dd>
<dt><code class="code">--enable-prompt-string-decoding</code></dt>
<dd><p>Turn on the interpretation of a number of backslash-escaped characters
in the <code class="env">$PS0</code>, <code class="env">$PS1</code>, <code class="env">$PS2</code>, and <code class="env">$PS4</code> prompt
strings.
See <a class="ref" href="#Controlling-the-Prompt">Controlling the Prompt</a>, for a complete list of prompt
string escape sequences.
</p>
</dd>
<dt><code class="code">--enable-readline</code></dt>
<dd><p>Include support for command-line editing and history with the Bash
version of the Readline library (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>).
</p>
</dd>
<dt><code class="code">--enable-restricted</code></dt>
<dd><p>Include support for a <em class="dfn">restricted shell</em>.
If this is enabled,
Bash enters a restricted mode when called as <code class="code">rbash</code>.
See <a class="ref" href="#The-Restricted-Shell">The Restricted Shell</a>, for a description of restricted mode.
</p>
</dd>
<dt><code class="code">--enable-select</code></dt>
<dd><p>Include the <code class="code">select</code> compound command, which allows generation of
simple menus (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).
</p>
</dd>
<dt><code class="code">--enable-single-help-strings</code></dt>
<dd><p>Store the text displayed by the <code class="code">help</code> builtin as a single string for
each help topic.
This aids in translating the text to different languages.
You may need to disable this if your compiler cannot handle very long string
literals.
</p>
</dd>
<dt><code class="code">--enable-strict-posix-default</code></dt>
<dd><p>Make Bash <small class="sc">POSIX</small>-conformant by default (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>).
</p>
</dd>
<dt><code class="code">--enable-translatable-strings</code></dt>
<dd><p>Enable support for <code class="code">$&quot;<var class="var">string</var>&quot;</code> translatable strings
(see <a class="pxref" href="#Locale-Translation">Locale-Specific Translation</a>).
</p>
</dd>
<dt><code class="code">--enable-usg-echo-default</code></dt>
<dd><p>A synonym for <code class="code">--enable-xpg-echo-default</code>.
</p>
</dd>
<dt><code class="code">--enable-xpg-echo-default</code></dt>
<dd><p>Make the <code class="code">echo</code> builtin expand backslash-escaped characters by default,
without requiring the <samp class="option">-e</samp> option.
This sets the default value of the <code class="code">xpg_echo</code> shell option to <code class="code">on</code>,
which makes the Bash <code class="code">echo</code> behave more like the version specified in
the Single Unix Specification, version 3.
See <a class="xref" href="#Bash-Builtins">Bash Builtin Commands</a>, for a description of the escape sequences that
<code class="code">echo</code> recognizes.
</p></dd>
</dl>

<p>The file <samp class="file">config-top.h</samp> contains C Preprocessor
&lsquo;<samp class="samp">#define</samp>&rsquo; statements for options which are not settable from
<code class="code">configure</code>.
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.
</p>
<hr>
</div>
</div>
<div class="appendix-level-extent" id="Reporting-Bugs">
<div class="nav-panel">
<p>
Next: <a href="#Major-Differences-From-The-Bourne-Shell" accesskey="n" rel="next">Major Differences From The Bourne Shell</a>, Previous: <a href="#Installing-Bash" accesskey="p" rel="prev">Installing Bash</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="appendix" id="Reporting-Bugs-1"><span>Appendix A Reporting Bugs<a class="copiable-link" href="#Reporting-Bugs-1"> &para;</a></span></h2>

<p>Please report all bugs you find in Bash.
But first, you should
make sure that it really is a bug, and that it appears in the latest
version of Bash.
The latest released version of Bash is always available for FTP from
<a class="uref" href="ftp://ftp.gnu.org/pub/gnu/bash/">ftp://ftp.gnu.org/pub/gnu/bash/</a> and from
<a class="uref" href="http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz">http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz</a>.
</p>
<p>Once you have determined that a bug actually exists, use the
<code class="code">bashbug</code> command to submit a bug report or use the form at the
<a class="uref" href="https://savannah.gnu.org/projects/bash/">Bash project page</a>.
If you have a fix, you are encouraged to submit that as well!
Suggestions and &lsquo;philosophical&rsquo; bug reports may be mailed
to <a class="email" href="mailto:bug-bash@gnu.org">bug-bash@gnu.org</a> or <a class="email" href="mailto:help-bash@gnu.org">help-bash@gnu.org</a>.
</p>
<p>All bug reports should include:
</p><ul class="itemize mark-bullet">
<li>The version number of Bash.
</li><li>The hardware and operating system.
</li><li>The compiler used to compile Bash.
</li><li>A description of the bug behavior.
</li><li>A short script or &lsquo;recipe&rsquo; which exercises the bug and may be used
to reproduce it.
</li></ul>

<p><code class="code">bashbug</code> inserts the first three items automatically into
the template it provides for filing a bug report.
</p>
<p>Please send all reports concerning this manual to
<a class="email" href="mailto:bug-bash@gnu.org">bug-bash@gnu.org</a>.
</p>
<hr>
</div>
<div class="appendix-level-extent" id="Major-Differences-From-The-Bourne-Shell">
<div class="nav-panel">
<p>
Next: <a href="#GNU-Free-Documentation-License" accesskey="n" rel="next">GNU Free Documentation License</a>, Previous: <a href="#Reporting-Bugs" accesskey="p" rel="prev">Reporting Bugs</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="appendix" id="Major-Differences-From-The-Bourne-Shell-1"><span>Appendix B Major Differences From The Bourne Shell<a class="copiable-link" href="#Major-Differences-From-The-Bourne-Shell-1"> &para;</a></span></h2>

<p>Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell. 
Bash uses the <small class="sc">POSIX</small> standard as the specification of
how these features are to be implemented and how they should behave.
There are some
differences between the traditional Bourne shell and Bash; this
section quickly details the differences of significance.
A number of these differences are explained in greater depth in
previous sections.
This section uses the version of <code class="code">sh</code> included in SVR4.2 (the
last version of the historical Bourne shell) as the baseline reference.
</p>
<ul class="itemize mark-bullet">
<li>Bash is <small class="sc">POSIX</small>-conformant, even where the <small class="sc">POSIX</small> specification
differs from traditional <code class="code">sh</code> behavior (see <a class="pxref" href="#Bash-POSIX-Mode">Bash and POSIX</a>).

</li><li>Bash has multi-character invocation options (see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>).

</li><li>The Bash restricted mode is more useful (see <a class="pxref" href="#The-Restricted-Shell">The Restricted Shell</a>);
the SVR4.2 shell restricted mode is too limited.

</li><li>Bash has command-line editing (see <a class="pxref" href="#Command-Line-Editing">Command Line Editing</a>) and
the <code class="code">bind</code> builtin.

</li><li>Bash provides a programmable word completion mechanism
(see <a class="pxref" href="#Programmable-Completion">Programmable Completion</a>), and builtin commands
<code class="code">complete</code>, <code class="code">compgen</code>, and <code class="code">compopt</code>, to
manipulate it.

</li><li>Bash decodes a number of backslash-escape sequences in the prompt string
variables (<code class="code">PS0</code>, <code class="code">PS1</code>, <code class="code">PS2</code>, and <code class="code">PS4</code>)
(see <a class="pxref" href="#Controlling-the-Prompt">Controlling the Prompt</a>).

</li><li>Bash expands and displays the <code class="code">PS0</code> prompt string variable.

</li><li>Bash runs commands from the <code class="code">PROMPT_COMMAND</code> array variable before
issuing each primary prompt.

</li><li>Bash has command history (see <a class="pxref" href="#Bash-History-Facilities">Bash History Facilities</a>) and the
<code class="code">history</code> and <code class="code">fc</code> builtins to manipulate it.
The Bash history list maintains timestamp information and uses the
value of the <code class="code">HISTTIMEFORMAT</code> variable to display it.

</li><li>Bash implements <code class="code">csh</code>-like history expansion
(see <a class="pxref" href="#History-Interaction">History Expansion</a>).

</li><li>Bash supports the <code class="code">$'&hellip;'</code> quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes
(see <a class="pxref" href="#ANSI_002dC-Quoting">ANSI-C Quoting</a>).

</li><li>Bash supports the <code class="code">$&quot;&hellip;&quot;</code> quoting syntax and performs
locale-specific translation of the characters between the double
quotes.
The <samp class="option">-D</samp>, <samp class="option">--dump-strings</samp>, and <samp class="option">--dump-po-strings</samp>
invocation options list the translatable strings found in a script
(see <a class="pxref" href="#Locale-Translation">Locale-Specific Translation</a>).

</li><li>Bash includes brace expansion (see <a class="pxref" href="#Brace-Expansion">Brace Expansion</a>) and tilde
expansion (see <a class="pxref" href="#Tilde-Expansion">Tilde Expansion</a>).

</li><li>Bash implements command aliases and the <code class="code">alias</code> and <code class="code">unalias</code>
builtins (see <a class="pxref" href="#Aliases">Aliases</a>).

</li><li>Bash implements the <code class="code">!</code> reserved word to negate the return value of
a pipeline (see <a class="pxref" href="#Pipelines">Pipelines</a>).
This is very useful when an <code class="code">if</code> statement needs to act only if a
test fails.
The Bash &lsquo;<samp class="samp">-o pipefail</samp>&rsquo; option to <code class="code">set</code> will cause a pipeline to
return a failure status if any command fails
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li>Bash has the <code class="code">time</code> reserved word and command timing (see <a class="pxref" href="#Pipelines">Pipelines</a>).
The display of the timing statistics may be controlled with the
<code class="env">TIMEFORMAT</code> variable.

</li><li>Bash provides coprocesses and the <code class="code">coproc</code> reserved word
(see <a class="pxref" href="#Coprocesses">Coprocesses</a>).

</li><li>Bash implements the <code class="code">for (( <var class="var">expr1</var> ; <var class="var">expr2</var> ; <var class="var">expr3</var> ))</code>
arithmetic for command, similar to the C language (see <a class="pxref" href="#Looping-Constructs">Looping Constructs</a>).

</li><li>Bash includes the <code class="code">select</code> compound command, which allows the
generation of simple menus (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).

</li><li>Bash includes the <code class="code">[[</code> compound command, which makes conditional
testing part of the shell grammar (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>), including
optional regular expression matching.

</li><li>Bash provides optional case-insensitive matching for the <code class="code">case</code> and
<code class="code">[[</code> constructs  (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).

</li><li>Bash provides additional <code class="code">case</code> statement action list terminators:
&lsquo;<samp class="samp">;&amp;</samp>&rsquo; and &lsquo;<samp class="samp">;;&amp;</samp>&rsquo; (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>).

</li><li>Bash provides shell arithmetic,
the <code class="code">((</code> compound command (see <a class="pxref" href="#Conditional-Constructs">Conditional Constructs</a>),
the <code class="code">let</code> builtin,
and arithmetic expansion (see <a class="pxref" href="#Shell-Arithmetic">Shell Arithmetic</a>).

</li><li>Bash has one-dimensional array variables (see <a class="pxref" href="#Arrays">Arrays</a>), and the
appropriate variable expansions and assignment syntax to use them.
Several of the Bash builtins take options to act on arrays.
Bash provides a number of built-in array variables.

</li><li>Variables present in the shell&rsquo;s initial environment are automatically
exported to child processes (see <a class="pxref" href="#Command-Execution-Environment">Command Execution Environment</a>).
The Bourne shell does not normally do
this unless the variables are explicitly marked using the <code class="code">export</code>
command.

</li><li>Bash can expand positional parameters beyond <code class="code">$9</code> using
<code class="code">${<var class="var">num</var>}</code> (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>Bash supports the &lsquo;<samp class="samp">+=</samp>&rsquo; assignment operator, which appends to the value
of the variable named on the left hand side
(see <a class="pxref" href="#Shell-Parameters">Shell Parameters</a>).

</li><li>Bash includes the <small class="sc">POSIX</small> pattern removal &lsquo;<samp class="samp">%</samp>&rsquo;, &lsquo;<samp class="samp">#</samp>&rsquo;, &lsquo;<samp class="samp">%%</samp>&rsquo;
and &lsquo;<samp class="samp">##</samp>&rsquo; expansions to remove leading or trailing substrings from
variable values (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>The expansion <code class="code">${#xx}</code>, which returns the length of <code class="code">${xx}</code>,
is supported (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>The expansion <code class="code">${var:</code><var class="var">offset</var><code class="code">[:</code><var class="var">length</var><code class="code">]}</code>,
which expands to the substring of <code class="code">var</code>&rsquo;s value of length
<var class="var">length</var>, beginning at <var class="var">offset</var>, is present
(see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>The expansion
<code class="code">${<var class="var">var</var>/[/]</code><var class="var">pattern</var><code class="code">[/</code><var class="var">replacement</var><code class="code">]}</code>,
which matches <var class="var">pattern</var> and replaces it with <var class="var">replacement</var> in
the value of <var class="var">var</var>, is available (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>),
with a mechanism to use the matched text in <var class="var">replacement</var>.

</li><li>The expansion <code class="code">${!<var class="var">prefix</var>*}</code> expansion, which expands to
the names of all shell variables whose names begin with <var class="var">prefix</var>,
is available (see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>Bash has indirect variable expansion using <code class="code">${!word}</code>
(see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>) and implements the <code class="code">nameref</code>
variable attribute for automatic indirect variable expansion.

</li><li>Bash includes a set of parameter transformation word expansions of the
form <code class="code">${var@X}</code>, where &lsquo;<samp class="samp">X</samp>&rsquo; specifies the transformation
(see <a class="pxref" href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).

</li><li>The <small class="sc">POSIX</small> <code class="code">$()</code> form of command substitution
is implemented (see <a class="pxref" href="#Command-Substitution">Command Substitution</a>),
and preferred to the Bourne shell&rsquo;s <code class="code">``</code> (which
is also implemented for backwards compatibility).

</li><li>Bash implements a variant of command substitution that runs the enclosed
command in the current shell execution environment:
<code class="code">${ <var class="var">command</var>;}</code> or <code class="code">${|<var class="var">command</var>;}</code>
(see <a class="pxref" href="#Command-Substitution">Command Substitution</a>).

</li><li>Bash has process substitution (see <a class="pxref" href="#Process-Substitution">Process Substitution</a>).

</li><li>Bash automatically assigns variables that provide information about the
current user (<code class="env">UID</code>, <code class="env">EUID</code>, and <code class="env">GROUPS</code>), the current host
(<code class="env">HOSTTYPE</code>, <code class="env">OSTYPE</code>, <code class="env">MACHTYPE</code>, and <code class="env">HOSTNAME</code>),
and the instance of Bash that is running (<code class="env">BASH</code>,
<code class="env">BASH_VERSION</code>, and <code class="env">BASH_VERSINFO</code>).
See <a class="xref" href="#Bash-Variables">Bash Variables</a>, for details.

</li><li>Bash uses many variables to provide functionality and customize shell
behavior that the Bourne shell does not.
Examples include <code class="env">RANDOM</code>, <code class="env">SRANDOM</code>, <code class="env">EPOCHSECONDS</code>,
<code class="env">EPOCHREALTIME</code>,
<code class="env">TIMEFORMAT</code>, <code class="env">BASHPID</code>, <code class="env">BASH_XTRACEFD</code>,
<code class="env">GLOBIGNORE</code>, <code class="env">HISTIGNORE</code>, and <code class="env">BASH_VERSION</code>.
See <a class="xref" href="#Bash-Variables">Bash Variables</a>, for a complete list.

</li><li>Bash uses the <code class="code">GLOBSORT</code> shell variable to control how to sort
the results of filename expansion (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>).

</li><li>Bash uses the <code class="env">IFS</code> variable to split only the results of expansion,
not all words (see <a class="pxref" href="#Word-Splitting">Word Splitting</a>).
This closes a longstanding shell security hole.

</li><li>The filename expansion bracket expression code uses &lsquo;<samp class="samp">!</samp>&rsquo; and &lsquo;<samp class="samp">^</samp>&rsquo;
to negate the set of characters between the brackets
(see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>).
The Bourne shell uses only &lsquo;<samp class="samp">!</samp>&rsquo;.

</li><li>Bash implements the full set of <small class="sc">POSIX</small> filename expansion operators,
including character classes, equivalence classes, and
collating symbols (see <a class="pxref" href="#Filename-Expansion">Filename Expansion</a>).

</li><li>Bash implements extended pattern matching features when the <code class="code">extglob</code>
shell option is enabled (see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).

</li><li>The <code class="code">globstar</code> shell option extends filename expansion to recursively
scan directories and subdirectories for matching filenames
(see <a class="pxref" href="#Pattern-Matching">Pattern Matching</a>).

</li><li>It is possible to have a variable and a function with the same name;
<code class="code">sh</code> does not separate the two name spaces.

</li><li>Bash functions are permitted to have local variables using the
<code class="code">local</code> builtin, and thus users can write useful recursive functions
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>Bash performs filename expansion on filenames specified as operands
to input and output redirection operators (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li>Bash contains the &lsquo;<samp class="samp">&lt;&gt;</samp>&rsquo; redirection operator, allowing a file to be
opened for both reading and writing, and the &lsquo;<samp class="samp">&amp;&gt;</samp>&rsquo; redirection
operator, for directing standard output and standard error to the same
file (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li>Bash includes the &lsquo;<samp class="samp">&lt;&lt;&lt;</samp>&rsquo; redirection operator, allowing a string to
be used as the standard input to a command (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li>Bash implements the &lsquo;<samp class="samp">[n]&lt;&amp;<var class="var">word</var></samp>&rsquo; and &lsquo;<samp class="samp">[n]&gt;&amp;<var class="var">word</var></samp>&rsquo;
redirection operators, which move one file descriptor to another.

</li><li>Bash treats a number of filenames specially when they are
used in redirection operators (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li>Bash provides the {<var class="var">var</var>}&lt;<var class="var">word</var> capability to have the
shell allocate file descriptors for redirections and assign them
to <var class="var">var</var> (see <a class="pxref" href="#Redirections">Redirections</a>).
This works with multiple redirection operators.

</li><li>Bash can open network connections to arbitrary machines and services
with the redirection operators (see <a class="pxref" href="#Redirections">Redirections</a>).

</li><li>The <code class="code">noclobber</code> option is available to avoid overwriting existing
files with output redirection (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).
The &lsquo;<samp class="samp">&gt;|</samp>&rsquo; redirection operator may be used to override <code class="code">noclobber</code>.

</li><li>Variable assignments preceding commands affect only that command, even
builtins and functions (see <a class="pxref" href="#Environment">Environment</a>).
In <code class="code">sh</code>, all variable assignments 
preceding commands are global unless the command is executed from the
file system.

</li><li>Bash includes a number of features to support a separate debugger for
shell scripts:
variables (<code class="code">BASH_ARGC</code>, <code class="code">BASH_ARGV</code>, <code class="code">BASH_LINENO</code>,
<code class="code">BASH_SOURCE</code>),
the <code class="code">DEBUG</code>, <code class="code">RETURN</code>, and <code class="code">ERR</code> traps,
&lsquo;<samp class="samp">declare -F</samp>&rsquo;,
and
the <code class="code">caller</code> builtin.

</li><li>Bash implements a <code class="code">csh</code>-like directory stack, and provides the
<code class="code">pushd</code>, <code class="code">popd</code>, and <code class="code">dirs</code> builtins to manipulate it
(see <a class="pxref" href="#The-Directory-Stack">The Directory Stack</a>).
Bash also makes the directory stack visible as the value of the
<code class="env">DIRSTACK</code> shell variable.

</li><li>Bash allows a function to override a builtin with the same name, and provides
access to that builtin&rsquo;s functionality within the function via the
<code class="code">builtin</code> and <code class="code">command</code> builtins (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>Bash includes the <code class="code">caller</code> builtin
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>), which displays the context of
any active subroutine call (a shell function or a script executed with
the <code class="code">.</code> or <code class="code">source</code> builtins).
This supports the Bash debugger.

</li><li>The Bash <code class="code">cd</code> and <code class="code">pwd</code> builtins (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
each take <samp class="option">-L</samp> and <samp class="option">-P</samp> options to switch between logical and
physical modes.

</li><li>The <code class="code">command</code> builtin allows selectively skipping shell functions
when performing command lookup (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>Bash uses the <code class="code">declare</code> builtin to modify the full set of variable
and function attributes, and to assign values to variables.

</li><li>The <code class="code">disown</code> builtin can remove a job from the internal shell
job table (see <a class="pxref" href="#Job-Control-Builtins">Job Control Builtins</a>) or suppress sending
<code class="code">SIGHUP</code> to a job when the shell exits as the result of a
<code class="code">SIGHUP</code>.

</li><li>The <code class="code">enable</code> builtin (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>) can enable or disable
individual builtins
and implements support for dynamically loading
builtin commands from shared objects.

</li><li>The Bash <code class="code">exec</code> builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).

</li><li>Shell functions may be exported to children via the environment
using <code class="code">export -f</code> (see <a class="pxref" href="#Shell-Functions">Shell Functions</a>).

</li><li>The Bash <code class="code">export</code> and <code class="code">readonly</code> builtins
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a> can
take a <samp class="option">-f</samp> option to act on shell functions, a <samp class="option">-p</samp> option to
display variables with various attributes set in a format that can be
used as shell input, a <samp class="option">-n</samp> option to remove various variable
attributes, and &lsquo;<samp class="samp">name=value</samp>&rsquo; arguments to set variable attributes
and values simultaneously.

</li><li>The Bash <code class="code">hash</code> builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the <code class="env">$PATH</code>, using &lsquo;<samp class="samp">hash -p</samp>&rsquo;
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).

</li><li>Bash includes a <code class="code">help</code> builtin for quick reference to shell
facilities (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>Bash includes the <code class="code">mapfile</code> builtin to quickly read the contents
of a file into an indexed array variable (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>The <code class="code">printf</code> builtin is available to display formatted output
(see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>),
and has additional custom format specifiers and an option to assign
the formatted output directly to a shell variable.

</li><li>The Bash <code class="code">read</code> builtin (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>)
will read a line ending in &lsquo;<samp class="samp">\</samp>&rsquo; with
the <samp class="option">-r</samp> option, and will use the <code class="env">REPLY</code> variable as a
default if no non-option arguments are supplied.

</li><li>The <code class="code">read</code> builtin (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>)
accepts a prompt string with the <samp class="option">-p</samp> option and will use
Readline to obtain the line when given the <samp class="option">-e</samp> or <samp class="option">-E</samp>
options,
with the ability to insert text into the line using the <samp class="option">-i</samp>
option.
The <code class="code">read</code> builtin also has additional options to control input:
the <samp class="option">-s</samp> option will turn off echoing of input characters as
they are read, the <samp class="option">-t</samp> option will allow <code class="code">read</code> to time out
if input does not arrive within a specified number of seconds, the
<samp class="option">-n</samp> option will allow reading only a specified number of
characters rather than a full line, and the <samp class="option">-d</samp> option will read
until a particular character rather than newline.

</li><li>The <code class="code">return</code> builtin may be used to abort execution of scripts
executed with the <code class="code">.</code> or <code class="code">source</code> builtins
(see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).

</li><li>Bash has much more optional behavior controllable with the <code class="code">set</code>
builtin (see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li>The <samp class="option">-x</samp> (<samp class="option">xtrace</samp>) option displays commands other than
simple commands when performing an execution trace
(see <a class="pxref" href="#The-Set-Builtin">The Set Builtin</a>).

</li><li>Bash includes the <code class="code">shopt</code> builtin, for finer control of shell
optional capabilities (see <a class="pxref" href="#The-Shopt-Builtin">The Shopt Builtin</a>), and allows these options
to be set and unset at shell invocation (see <a class="pxref" href="#Invoking-Bash">Invoking Bash</a>).

</li><li>The <code class="code">test</code> builtin (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>)
is slightly different, as it implements the <small class="sc">POSIX</small> algorithm,
which specifies the behavior based on the number of arguments.

</li><li>The <code class="code">trap</code> builtin (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>) allows a
<code class="code">DEBUG</code> pseudo-signal specification, similar to <code class="code">EXIT</code>.
Commands specified with a <code class="code">DEBUG</code> trap are executed before every
simple command, <code class="code">for</code> command, <code class="code">case</code> command,
<code class="code">select</code> command, every arithmetic <code class="code">for</code> command, and before
the first command executes in a shell function.
The <code class="code">DEBUG</code> trap is not inherited by shell functions unless the
function has been given the <code class="code">trace</code> attribute or the
<code class="code">functrace</code> option has been enabled using the <code class="code">shopt</code> builtin.
The <code class="code">extdebug</code> shell option has additional effects on the
<code class="code">DEBUG</code> trap.

<p>The <code class="code">trap</code> builtin (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>) allows an
<code class="code">ERR</code> pseudo-signal specification, similar to <code class="code">EXIT</code> and <code class="code">DEBUG</code>.
Commands specified with an <code class="code">ERR</code> trap are executed after a simple
command fails, with a few exceptions.
The <code class="code">ERR</code> trap is not inherited by shell functions unless the
<code class="code">-o errtrace</code> option to the <code class="code">set</code> builtin is enabled.
</p>
<p>The <code class="code">trap</code> builtin (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>) allows a
<code class="code">RETURN</code> pseudo-signal specification, similar to
<code class="code">EXIT</code> and <code class="code">DEBUG</code>.
Commands specified with a <code class="code">RETURN</code> trap are executed before
execution resumes after a shell function or a shell script executed with
<code class="code">.</code> or <code class="code">source</code> returns.
The <code class="code">RETURN</code> trap is not inherited by shell functions unless the
function has been given the <code class="code">trace</code> attribute or the
<code class="code">functrace</code> option has been enabled using the <code class="code">shopt</code> builtin.
</p>
</li><li>The Bash <code class="code">type</code> builtin is more extensive and gives more information
about the names it finds (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>The <code class="code">ulimit</code> builtin provides control over many more per-process
resources (see <a class="pxref" href="#Bash-Builtins">Bash Builtin Commands</a>).

</li><li>The Bash <code class="code">umask</code> builtin uses the <samp class="option">-p</samp> option to display
the output in the form of a <code class="code">umask</code> command
that may be reused as input (see <a class="pxref" href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a>).

</li><li>The Bash <code class="code">wait</code> builtin has a <samp class="option">-n</samp> option to wait for the
next child to exit, possibly selecting from a list of supplied jobs,
and the <samp class="option">-p</samp> option to store information about a terminated
child process in a shell variable.

</li><li>The SVR4.2 shell behaves differently when invoked as <code class="code">jsh</code>
(it turns on job control).

</li><li>The SVR4.2 shell has two privilege-related builtins
(<code class="code">mldmode</code> and <code class="code">priv</code>) not present in Bash.

</li><li>Bash does not have the <code class="code">stop</code> or <code class="code">newgrp</code> builtins.

</li><li>Bash does not use the <code class="env">SHACCT</code> variable or perform shell accounting.

</li><li>The SVR4.2 <code class="code">sh</code> uses a <code class="env">TIMEOUT</code> variable like Bash uses
<code class="env">TMOUT</code>.

</li></ul>

<p>More features unique to Bash may be found in <a class="ref" href="#Bash-Features">Bash Features</a>.
</p>
<ul class="mini-toc">
<li><a href="#Implementation-Differences-From-The-SVR4_002e2-Shell" accesskey="1">Implementation Differences From The SVR4.2 Shell</a></li>
</ul>
<div class="appendixsec-level-extent" id="Implementation-Differences-From-The-SVR4_002e2-Shell">
<h3 class="appendixsec"><span>B.1 Implementation Differences From The SVR4.2 Shell<a class="copiable-link" href="#Implementation-Differences-From-The-SVR4_002e2-Shell"> &para;</a></span></h3>

<p>Since Bash is a completely new implementation, it does not suffer from
many of the limitations of the SVR4.2 shell.
For instance:
</p>
<ul class="itemize mark-bullet">
<li>Bash does not fork a subshell when redirecting into or out of
a shell control structure such as  an <code class="code">if</code> or <code class="code">while</code>
statement.

</li><li>Bash does not allow unbalanced quotes.
The SVR4.2 shell will silently
insert a needed closing quote at <code class="code">EOF</code> under certain circumstances.
This can be the cause of some hard-to-find errors.

</li><li>The SVR4.2 shell uses a baroque memory management scheme based on
trapping <code class="code">SIGSEGV</code>.
If the shell is started from a process with
<code class="code">SIGSEGV</code> blocked (e.g., by using the <code class="code">system()</code> C library
function call), it misbehaves badly.

</li><li>In a questionable attempt at security, the SVR4.2 shell,
when invoked without the <samp class="option">-p</samp> option, will alter its real
and effective <small class="sc">UID</small> and <small class="sc">GID</small> if they are less than some
magic threshold value, commonly 100.
This can lead to unexpected results.

</li><li>The SVR4.2 shell does not allow users to trap <code class="code">SIGSEGV</code>,
<code class="code">SIGALRM</code>, or <code class="code">SIGCHLD</code>.

</li><li>The SVR4.2 shell does not allow the <code class="env">IFS</code>, <code class="env">MAILCHECK</code>,
<code class="env">PATH</code>, <code class="env">PS1</code>, or <code class="env">PS2</code> variables to be unset.

</li><li>The SVR4.2 shell treats &lsquo;<samp class="samp">^</samp>&rsquo; as the undocumented equivalent of
&lsquo;<samp class="samp">|</samp>&rsquo;.

</li><li>Bash allows multiple option arguments when it is invoked (<code class="code">-x -v</code>);
the SVR4.2 shell allows only one option argument (<code class="code">-xv</code>).
In fact, some versions of the shell dump core if the second argument begins
with a &lsquo;<samp class="samp">-</samp>&rsquo;.

</li><li>The SVR4.2 shell exits a script if any builtin fails; Bash exits
a script only if one of the <small class="sc">POSIX</small> special builtins fails, and
only for certain failures, as enumerated in the <small class="sc">POSIX</small> standard.

</li><li>If the <code class="code">lastpipe</code> option is enabled, and job control is not active,
Bash runs the last element of a pipeline in the current shell execution
environment.

</li></ul>

<hr>
</div>
</div>
<div class="appendix-level-extent" id="GNU-Free-Documentation-License">
<div class="nav-panel">
<p>
Next: <a href="#Indexes" accesskey="n" rel="next">Indexes</a>, Previous: <a href="#Major-Differences-From-The-Bourne-Shell" accesskey="p" rel="prev">Major Differences From The Bourne Shell</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="appendix" id="GNU-Free-Documentation-License-1"><span>Appendix C GNU Free Documentation License<a class="copiable-link" href="#GNU-Free-Documentation-License-1"> &para;</a></span></h2>

<div class="center">Version 1.3, 3 November 2008
</div>

<div class="display">
<pre class="display-preformatted">Copyright &copy; 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
<a class="uref" href="http://fsf.org/">http://fsf.org/</a>

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
</pre></div>

<ol class="enumerate" start="0">
<li> PREAMBLE

<p>The purpose of this License is to make a manual, textbook, or other
functional and useful document <em class="dfn">free</em> in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.
</p>
<p>This License is a kind of &ldquo;copyleft&rdquo;, which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.
</p>
<p>We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book.  We recommend this License
principally for works whose purpose is instruction or reference.
</p>
</li><li> APPLICABILITY AND DEFINITIONS

<p>This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License.  Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein.  The &ldquo;Document&rdquo;, below,
refers to any such manual or work.  Any member of the public is a
licensee, and is addressed as &ldquo;you&rdquo;.  You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.
</p>
<p>A &ldquo;Modified Version&rdquo; of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.
</p>
<p>A &ldquo;Secondary Section&rdquo; is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document&rsquo;s overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject.  (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.)  The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.
</p>
<p>The &ldquo;Invariant Sections&rdquo; are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License.  If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant.  The Document may contain zero
Invariant Sections.  If the Document does not identify any Invariant
Sections then there are none.
</p>
<p>The &ldquo;Cover Texts&rdquo; are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License.  A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.
</p>
<p>A &ldquo;Transparent&rdquo; copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters.  A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text.  A copy that is not &ldquo;Transparent&rdquo; is called &ldquo;Opaque&rdquo;.
</p>
<p>Examples of suitable formats for Transparent copies include plain
<small class="sc">ASCII</small> without markup, Texinfo input format, LaTeX input
format, <abbr class="acronym">SGML</abbr> or <abbr class="acronym">XML</abbr> using a publicly available
<abbr class="acronym">DTD</abbr>, and standard-conforming simple <abbr class="acronym">HTML</abbr>,
PostScript or <abbr class="acronym">PDF</abbr> designed for human modification.  Examples
of transparent image formats include <abbr class="acronym">PNG</abbr>, <abbr class="acronym">XCF</abbr> and
<abbr class="acronym">JPG</abbr>.  Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, <abbr class="acronym">SGML</abbr> or
<abbr class="acronym">XML</abbr> for which the <abbr class="acronym">DTD</abbr> and/or processing tools are
not generally available, and the machine-generated <abbr class="acronym">HTML</abbr>,
PostScript or <abbr class="acronym">PDF</abbr> produced by some word processors for
output purposes only.
</p>
<p>The &ldquo;Title Page&rdquo; means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page.  For works in
formats which do not have any title page as such, &ldquo;Title Page&rdquo; means
the text near the most prominent appearance of the work&rsquo;s title,
preceding the beginning of the body of the text.
</p>
<p>The &ldquo;publisher&rdquo; means any person or entity that distributes copies
of the Document to the public.
</p>
<p>A section &ldquo;Entitled XYZ&rdquo; means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language.  (Here XYZ stands for a
specific section name mentioned below, such as &ldquo;Acknowledgements&rdquo;,
&ldquo;Dedications&rdquo;, &ldquo;Endorsements&rdquo;, or &ldquo;History&rdquo;.)  To &ldquo;Preserve the Title&rdquo;
of such a section when you modify the Document means that it remains a
section &ldquo;Entitled XYZ&rdquo; according to this definition.
</p>
<p>The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document.  These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.
</p>
</li><li> VERBATIM COPYING

<p>You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License.  You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute.  However, you may accept
compensation in exchange for copies.  If you distribute a large enough
number of copies you must also follow the conditions in section 3.
</p>
<p>You may also lend copies, under the same conditions stated above, and
you may publicly display copies.
</p>
</li><li> COPYING IN QUANTITY

<p>If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document&rsquo;s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover.  Both covers must also clearly and legibly identify
you as the publisher of these copies.  The front cover must present
the full title with all words of the title equally prominent and
visible.  You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.
</p>
<p>If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.
</p>
<p>If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.
</p>
<p>It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.
</p>
</li><li> MODIFICATIONS

<p>You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:
</p>
<ol class="enumerate" type="A" start="1">
<li> Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

</li><li> List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

</li><li> State on the Title page the name of the publisher of the
Modified Version, as the publisher.

</li><li> Preserve all the copyright notices of the Document.

</li><li> Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

</li><li> Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

</li><li> Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document&rsquo;s license notice.

</li><li> Include an unaltered copy of this License.

</li><li> Preserve the section Entitled &ldquo;History&rdquo;, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
there is no section Entitled &ldquo;History&rdquo; in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

</li><li> Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the &ldquo;History&rdquo; section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

</li><li> For any section Entitled &ldquo;Acknowledgements&rdquo; or &ldquo;Dedications&rdquo;, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

</li><li> Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

</li><li> Delete any section Entitled &ldquo;Endorsements&rdquo;.  Such a section
may not be included in the Modified Version.

</li><li> Do not retitle any existing section to be Entitled &ldquo;Endorsements&rdquo; or
to conflict in title with any Invariant Section.

</li><li> Preserve any Warranty Disclaimers.
</li></ol>

<p>If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version&rsquo;s license notice.
These titles must be distinct from any other section titles.
</p>
<p>You may add a section Entitled &ldquo;Endorsements&rdquo;, provided it contains
nothing but endorsements of your Modified Version by various
parties&mdash;for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.
</p>
<p>You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.
</p>
<p>The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.
</p>
</li><li> COMBINING DOCUMENTS

<p>You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.
</p>
<p>The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.
</p>
<p>In the combination, you must combine any sections Entitled &ldquo;History&rdquo;
in the various original documents, forming one section Entitled
&ldquo;History&rdquo;; likewise combine any sections Entitled &ldquo;Acknowledgements&rdquo;,
and any sections Entitled &ldquo;Dedications&rdquo;.  You must delete all
sections Entitled &ldquo;Endorsements.&rdquo;
</p>
</li><li> COLLECTIONS OF DOCUMENTS

<p>You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.
</p>
<p>You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.
</p>
</li><li> AGGREGATION WITH INDEPENDENT WORKS

<p>A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an &ldquo;aggregate&rdquo; if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation&rsquo;s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.
</p>
<p>If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document&rsquo;s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.
</p>
</li><li> TRANSLATION

<p>Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.
</p>
<p>If a section in the Document is Entitled &ldquo;Acknowledgements&rdquo;,
&ldquo;Dedications&rdquo;, or &ldquo;History&rdquo;, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.
</p>
</li><li> TERMINATION

<p>You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.
</p>
<p>However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.
</p>
<p>Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.
</p>
<p>Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.
</p>
</li><li> FUTURE REVISIONS OF THIS LICENSE

<p>The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
<a class="uref" href="http://www.gnu.org/copyleft/">http://www.gnu.org/copyleft/</a>.
</p>
<p>Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License &ldquo;or any later version&rdquo; applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.  If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy&rsquo;s public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.
</p>
</li><li> RELICENSING

<p>&ldquo;Massive Multiauthor Collaboration Site&rdquo; (or &ldquo;MMC Site&rdquo;) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works.  A
public wiki that anybody can edit is an example of such a server.  A
&ldquo;Massive Multiauthor Collaboration&rdquo; (or &ldquo;MMC&rdquo;) contained in the
site means any set of copyrightable works thus published on the MMC
site.
</p>
<p>&ldquo;CC-BY-SA&rdquo; means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
</p>
<p>&ldquo;Incorporate&rdquo; means to publish or republish a Document, in whole or
in part, as part of another Document.
</p>
<p>An MMC is &ldquo;eligible for relicensing&rdquo; if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.
</p>
<p>The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.
</p>
</li></ol>

<h3 class="heading" id="ADDENDUM_003a-How-to-use-this-License-for-your-documents"><span>ADDENDUM: How to use this License for your documents<a class="copiable-link" href="#ADDENDUM_003a-How-to-use-this-License-for-your-documents"> &para;</a></span></h3>

<p>To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">  Copyright (C)  <var class="var">year</var>  <var class="var">your name</var>.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
</pre></div></div>

<p>If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the &ldquo;with&hellip;Texts.&rdquo; line with this:
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">    with the Invariant Sections being <var class="var">list their titles</var>, with
    the Front-Cover Texts being <var class="var">list</var>, and with the Back-Cover Texts
    being <var class="var">list</var>.
</pre></div></div>

<p>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
</p>
<p>If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.
</p>


<hr>
</div>
<div class="appendix-level-extent" id="Indexes">
<div class="nav-panel">
<p>
Previous: <a href="#GNU-Free-Documentation-License" accesskey="p" rel="prev">GNU Free Documentation License</a>, Up: <a href="#Top" accesskey="u" rel="up">Bash Features</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h2 class="appendix" id="Indexes-1"><span>Appendix D Indexes<a class="copiable-link" href="#Indexes-1"> &para;</a></span></h2>


<ul class="mini-toc">
<li><a href="#Builtin-Index" accesskey="1">Index of Shell Builtin Commands</a></li>
<li><a href="#Reserved-Word-Index" accesskey="2">Index of Shell Reserved Words</a></li>
<li><a href="#Variable-Index" accesskey="3">Parameter and Variable Index</a></li>
<li><a href="#Function-Index" accesskey="4">Function Index</a></li>
<li><a href="#Concept-Index" accesskey="5">Concept Index</a></li>
</ul>
<hr>
<div class="appendixsec-level-extent" id="Builtin-Index">
<div class="nav-panel">
<p>
Next: <a href="#Reserved-Word-Index" accesskey="n" rel="next">Index of Shell Reserved Words</a>, Up: <a href="#Indexes" accesskey="u" rel="up">Indexes</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="appendixsec" id="Index-of-Shell-Builtin-Commands"><span>D.1 Index of Shell Builtin Commands<a class="copiable-link" href="#Index-of-Shell-Builtin-Commands"> &para;</a></span></h3>
<div class="printindex bt-printindex">
<table class="bt-letters-header-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-1"><b>:</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-2"><b>.</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-3"><b>[</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-J"><b>J</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-W"><b>W</b></a>
 &nbsp; 
</td></tr></table>
<table class="bt-entries-printindex">
<tr><td></td><th class="entries-header-printindex">Index Entry</th><th class="sections-header-printindex">Section</th></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_symbol-1">:</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_003a"><code>:</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_symbol-2">.</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_002e"><code>.</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_symbol-3">[</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_005b"><code>[</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-A">A</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-alias"><code>alias</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-B">B</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bg"><code>bg</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bind"><code>bind</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-break"><code>break</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-builtin"><code>builtin</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-C">C</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-caller"><code>caller</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-cd"><code>cd</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command"><code>command</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-compgen"><code>compgen</code></a></td><td class="printindex-index-section"><a href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete"><code>complete</code></a></td><td class="printindex-index-section"><a href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-compopt"><code>compopt</code></a></td><td class="printindex-index-section"><a href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-continue"><code>continue</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-D">D</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-declare"><code>declare</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dirs"><code>dirs</code></a></td><td class="printindex-index-section"><a href="#Directory-Stack-Builtins">Directory Stack Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-disown"><code>disown</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-E">E</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-echo"><code>echo</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-enable"><code>enable</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-eval"><code>eval</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-exec"><code>exec</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-exit"><code>exit</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-export"><code>export</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-F">F</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-false"><code>false</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-fc"><code>fc</code></a></td><td class="printindex-index-section"><a href="#Bash-History-Builtins">Bash History Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-fg"><code>fg</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-G">G</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-getopts"><code>getopts</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-H">H</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-hash"><code>hash</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-help"><code>help</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history"><code>history</code></a></td><td class="printindex-index-section"><a href="#Bash-History-Builtins">Bash History Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-J">J</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-jobs"><code>jobs</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-K">K</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill"><code>kill</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-L">L</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-let"><code>let</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-local"><code>local</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-logout"><code>logout</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-M">M</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-mapfile"><code>mapfile</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-P">P</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-popd"><code>popd</code></a></td><td class="printindex-index-section"><a href="#Directory-Stack-Builtins">Directory Stack Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-printf"><code>printf</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-pushd"><code>pushd</code></a></td><td class="printindex-index-section"><a href="#Directory-Stack-Builtins">Directory Stack Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-pwd"><code>pwd</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-R">R</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-read"><code>read</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-readarray"><code>readarray</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-readonly"><code>readonly</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-return"><code>return</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-S">S</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-set"><code>set</code></a></td><td class="printindex-index-section"><a href="#The-Set-Builtin">The Set Builtin</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shift"><code>shift</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shopt"><code>shopt</code></a></td><td class="printindex-index-section"><a href="#The-Shopt-Builtin">The Shopt Builtin</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-source"><code>source</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-suspend"><code>suspend</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-T">T</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-test"><code>test</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-times"><code>times</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-trap"><code>trap</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-true"><code>true</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-type"><code>type</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-typeset"><code>typeset</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-U">U</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-ulimit"><code>ulimit</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-umask"><code>umask</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unalias"><code>unalias</code></a></td><td class="printindex-index-section"><a href="#Bash-Builtins">Bash Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unset"><code>unset</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Builtins">Bourne Shell Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Builtin-Index_bt_letter-W">W</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-wait"><code>wait</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Builtins">Job Control Builtins</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
</table>
<table class="bt-letters-footer-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-1"><b>:</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-2"><b>.</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_symbol-3"><b>[</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-J"><b>J</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Builtin-Index_bt_letter-W"><b>W</b></a>
 &nbsp; 
</td></tr></table>
</div>

<hr>
</div>
<div class="appendixsec-level-extent" id="Reserved-Word-Index">
<div class="nav-panel">
<p>
Next: <a href="#Variable-Index" accesskey="n" rel="next">Parameter and Variable Index</a>, Previous: <a href="#Builtin-Index" accesskey="p" rel="prev">Index of Shell Builtin Commands</a>, Up: <a href="#Indexes" accesskey="u" rel="up">Indexes</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="appendixsec" id="Index-of-Shell-Reserved-Words"><span>D.2 Index of Shell Reserved Words<a class="copiable-link" href="#Index-of-Shell-Reserved-Words"> &para;</a></span></h3>
<div class="printindex rw-printindex">
<table class="rw-letters-header-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-1"><b>!</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-2"><b>[</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-3"><b>]</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-4"><b>{</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-5"><b>}</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-W"><b>W</b></a>
 &nbsp; 
</td></tr></table>
<table class="rw-entries-printindex">
<tr><td></td><th class="entries-header-printindex">Index Entry</th><th class="sections-header-printindex">Section</th></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_symbol-1">!</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0021"><code>!</code></a></td><td class="printindex-index-section"><a href="#Pipelines">Pipelines</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_symbol-2">[</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_005b_005b"><code>[[</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_symbol-3">]</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_005d_005d"><code>]]</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_symbol-4">{</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_007b"><code>{</code></a></td><td class="printindex-index-section"><a href="#Command-Grouping">Command Grouping</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_symbol-5">}</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_007d"><code>}</code></a></td><td class="printindex-index-section"><a href="#Command-Grouping">Command Grouping</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-C">C</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-case"><code>case</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-D">D</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-do"><code>do</code></a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-done"><code>done</code></a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-E">E</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-elif"><code>elif</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-else"><code>else</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-esac"><code>esac</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-F">F</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-fi"><code>fi</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-for"><code>for</code></a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-function"><code>function</code></a></td><td class="printindex-index-section"><a href="#Shell-Functions">Shell Functions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-I">I</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-if"><code>if</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-in"><code>in</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-S">S</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-select"><code>select</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-T">T</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-then"><code>then</code></a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-time"><code>time</code></a></td><td class="printindex-index-section"><a href="#Pipelines">Pipelines</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-U">U</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-until"><code>until</code></a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Reserved-Word-Index_rw_letter-W">W</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-while"><code>while</code></a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
</table>
<table class="rw-letters-footer-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-1"><b>!</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-2"><b>[</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-3"><b>]</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-4"><b>{</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_symbol-5"><b>}</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Reserved-Word-Index_rw_letter-W"><b>W</b></a>
 &nbsp; 
</td></tr></table>
</div>

<hr>
</div>
<div class="appendixsec-level-extent" id="Variable-Index">
<div class="nav-panel">
<p>
Next: <a href="#Function-Index" accesskey="n" rel="next">Function Index</a>, Previous: <a href="#Reserved-Word-Index" accesskey="p" rel="prev">Index of Shell Reserved Words</a>, Up: <a href="#Indexes" accesskey="u" rel="up">Indexes</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="appendixsec" id="Parameter-and-Variable-Index"><span>D.3 Parameter and Variable Index<a class="copiable-link" href="#Parameter-and-Variable-Index"> &para;</a></span></h3>
<div class="printindex vr-printindex">
<table class="vr-letters-header-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-1"><b>_</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-2"><b>-</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-3"><b>!</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-4"><b>?</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-5"><b>@</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-6"><b>*</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-7"><b>#</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-8"><b>$</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-9"><b>0</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-V"><b>V</b></a>
 &nbsp; 
</td></tr></table>
<table class="vr-entries-printindex">
<tr><td></td><th class="entries-header-printindex">Index Entry</th><th class="sections-header-printindex">Section</th></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-1">_</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_005f"><code>_</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-2">-</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_002d"><code>-</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-3">!</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0021-1"><code>!</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-4">?</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_003f"><code>?</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-5">@</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0040"><code>@</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-6">*</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_002a"><code>*</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-7">#</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0023"><code>#</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-8">$</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024"><code>$</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_005f"><code>$_</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_002d"><code>$-</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_0021"><code>$!</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_003f"><code>$?</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_0040"><code>$@</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_002a"><code>$*</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_0023"><code>$#</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_0024_0024"><code>$$</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-_00240"><code>$0</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_symbol-9">0</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-0"><code>0</code></a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-A">A</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-active_002dregion_002dend_002dcolor"><code>active-region-end-color</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-active_002dregion_002dstart_002dcolor"><code>active-region-start-color</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-auto_005fresume"><code>auto_resume</code></a></td><td class="printindex-index-section"><a href="#Job-Control-Variables">Job Control Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-B">B</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH"><code>BASH</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fALIASES"><code>BASH_ALIASES</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fARGC"><code>BASH_ARGC</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fARGV"><code>BASH_ARGV</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fARGV0"><code>BASH_ARGV0</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fCMDS"><code>BASH_CMDS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fCOMMAND"><code>BASH_COMMAND</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fCOMPAT"><code>BASH_COMPAT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fENV"><code>BASH_ENV</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fEXECUTION_005fSTRING"><code>BASH_EXECUTION_STRING</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fLINENO"><code>BASH_LINENO</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fLOADABLES_005fPATH"><code>BASH_LOADABLES_PATH</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fMONOSECONDS"><code>BASH_MONOSECONDS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fREMATCH"><code>BASH_REMATCH</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fSOURCE"><code>BASH_SOURCE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fSUBSHELL"><code>BASH_SUBSHELL</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fTRAPSIG"><code>BASH_TRAPSIG</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fVERSINFO"><code>BASH_VERSINFO</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fVERSION"><code>BASH_VERSION</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASH_005fXTRACEFD"><code>BASH_XTRACEFD</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASHOPTS"><code>BASHOPTS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-BASHPID"><code>BASHPID</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bell_002dstyle"><code>bell-style</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bind_002dtty_002dspecial_002dchars"><code>bind-tty-special-chars</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-blink_002dmatching_002dparen"><code>blink-matching-paren</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-C">C</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-CDPATH"><code>CDPATH</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-CHILD_005fMAX"><code>CHILD_MAX</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-colored_002dcompletion_002dprefix"><code>colored-completion-prefix</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-colored_002dstats"><code>colored-stats</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COLUMNS"><code>COLUMNS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-comment_002dbegin"><code>comment-begin</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fCWORD"><code>COMP_CWORD</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fKEY"><code>COMP_KEY</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fLINE"><code>COMP_LINE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fPOINT"><code>COMP_POINT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fTYPE"><code>COMP_TYPE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fWORDBREAKS"><code>COMP_WORDBREAKS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMP_005fWORDS"><code>COMP_WORDS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion_002ddisplay_002dwidth"><code>completion-display-width</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion_002dignore_002dcase"><code>completion-ignore-case</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion_002dmap_002dcase"><code>completion-map-case</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion_002dprefix_002ddisplay_002dlength"><code>completion-prefix-display-length</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion_002dquery_002ditems"><code>completion-query-items</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COMPREPLY"><code>COMPREPLY</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-convert_002dmeta"><code>convert-meta</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-COPROC"><code>COPROC</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-D">D</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-DIRSTACK"><code>DIRSTACK</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-disable_002dcompletion"><code>disable-completion</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-E">E</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-echo_002dcontrol_002dcharacters"><code>echo-control-characters</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-editing_002dmode"><code>editing-mode</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-EMACS"><code>EMACS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-emacs_002dmode_002dstring"><code>emacs-mode-string</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-enable_002dactive_002dregion-The"><code>enable-active-region The</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-enable_002dbracketed_002dpaste"><code>enable-bracketed-paste</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-enable_002dkeypad"><code>enable-keypad</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-enable_002dmeta_002dkey"><code>enable-meta-key</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-ENV"><code>ENV</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-EPOCHREALTIME"><code>EPOCHREALTIME</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-EPOCHSECONDS"><code>EPOCHSECONDS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-EUID"><code>EUID</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-EXECIGNORE"><code>EXECIGNORE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expand_002dtilde"><code>expand-tilde</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-F">F</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-FCEDIT"><code>FCEDIT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-FIGNORE"><code>FIGNORE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-force_002dmeta_002dprefix"><code>force-meta-prefix</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-FUNCNAME"><code>FUNCNAME</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-FUNCNEST"><code>FUNCNEST</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-G">G</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-GLOBIGNORE"><code>GLOBIGNORE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-GLOBSORT"><code>GLOBSORT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-GROUPS"><code>GROUPS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-H">H</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-histchars"><code>histchars</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTCMD"><code>HISTCMD</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTCONTROL"><code>HISTCONTROL</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTFILE"><code>HISTFILE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTFILESIZE"><code>HISTFILESIZE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTIGNORE"><code>HISTIGNORE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dpreserve_002dpoint"><code>history-preserve-point</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dsize"><code>history-size</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTSIZE"><code>HISTSIZE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HISTTIMEFORMAT"><code>HISTTIMEFORMAT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HOME"><code>HOME</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-horizontal_002dscroll_002dmode"><code>horizontal-scroll-mode</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HOSTFILE"><code>HOSTFILE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HOSTNAME"><code>HOSTNAME</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-HOSTTYPE"><code>HOSTTYPE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-I">I</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-IFS"><code>IFS</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-IGNOREEOF"><code>IGNOREEOF</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-input_002dmeta"><code>input-meta</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-INPUTRC"><code>INPUTRC</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-INSIDE_005fEMACS"><code>INSIDE_EMACS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-isearch_002dterminators"><code>isearch-terminators</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-K">K</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-keymap"><code>keymap</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-L">L</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LANG"><code>LANG</code></a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LANG-1"><code>LANG</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fALL"><code>LC_ALL</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fCOLLATE"><code>LC_COLLATE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fCTYPE"><code>LC_CTYPE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fMESSAGES"><code>LC_MESSAGES</code></a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fMESSAGES-1"><code>LC_MESSAGES</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fNUMERIC"><code>LC_NUMERIC</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LC_005fTIME"><code>LC_TIME</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LINENO"><code>LINENO</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-LINES"><code>LINES</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-M">M</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-MACHTYPE"><code>MACHTYPE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-MAIL"><code>MAIL</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-MAILCHECK"><code>MAILCHECK</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-MAILPATH"><code>MAILPATH</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-MAPFILE"><code>MAPFILE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-mark_002dmodified_002dlines"><code>mark-modified-lines</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-mark_002dsymlinked_002ddirectories"><code>mark-symlinked-directories</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-match_002dhidden_002dfiles"><code>match-hidden-files</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-menu_002dcomplete_002ddisplay_002dprefix"><code>menu-complete-display-prefix</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-meta_002dflag"><code>meta-flag</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-O">O</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-OLDPWD"><code>OLDPWD</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-OPTARG"><code>OPTARG</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-OPTERR"><code>OPTERR</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-OPTIND"><code>OPTIND</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-OSTYPE"><code>OSTYPE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-output_002dmeta"><code>output-meta</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-P">P</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-page_002dcompletions"><code>page-completions</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PATH"><code>PATH</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PIPESTATUS"><code>PIPESTATUS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-POSIXLY_005fCORRECT"><code>POSIXLY_CORRECT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PPID"><code>PPID</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PROMPT_005fCOMMAND"><code>PROMPT_COMMAND</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PROMPT_005fDIRTRIM"><code>PROMPT_DIRTRIM</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PS0"><code>PS0</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PS1"><code>PS1</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PS2"><code>PS2</code></a></td><td class="printindex-index-section"><a href="#Bourne-Shell-Variables">Bourne Shell Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PS3"><code>PS3</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PS4"><code>PS4</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-PWD"><code>PWD</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-R">R</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-RANDOM"><code>RANDOM</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-READLINE_005fARGUMENT"><code>READLINE_ARGUMENT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-READLINE_005fLINE"><code>READLINE_LINE</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-READLINE_005fMARK"><code>READLINE_MARK</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-READLINE_005fPOINT"><code>READLINE_POINT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-REPLY"><code>REPLY</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-revert_002dall_002dat_002dnewline"><code>revert-all-at-newline</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-S">S</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-search_002dignore_002dcase"><code>search-ignore-case</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-SECONDS"><code>SECONDS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-SHELL"><code>SHELL</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-SHELLOPTS"><code>SHELLOPTS</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-SHLVL"><code>SHLVL</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-show_002dall_002dif_002dambiguous"><code>show-all-if-ambiguous</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-show_002dall_002dif_002dunmodified"><code>show-all-if-unmodified</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-show_002dmode_002din_002dprompt"><code>show-mode-in-prompt</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-skip_002dcompleted_002dtext"><code>skip-completed-text</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-SRANDOM"><code>SRANDOM</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-T">T</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-TEXTDOMAIN"><code>TEXTDOMAIN</code></a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-TEXTDOMAINDIR"><code>TEXTDOMAINDIR</code></a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-TIMEFORMAT"><code>TIMEFORMAT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-TMOUT"><code>TMOUT</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-TMPDIR"><code>TMPDIR</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-U">U</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-UID"><code>UID</code></a></td><td class="printindex-index-section"><a href="#Bash-Variables">Bash Variables</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Variable-Index_vr_letter-V">V</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-vi_002dcmd_002dmode_002dstring"><code>vi-cmd-mode-string</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-vi_002dins_002dmode_002dstring"><code>vi-ins-mode-string</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-visible_002dstats"><code>visible-stats</code></a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
</table>
<table class="vr-letters-footer-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-1"><b>_</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-2"><b>-</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-3"><b>!</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-4"><b>?</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-5"><b>@</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-6"><b>*</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-7"><b>#</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-8"><b>$</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_symbol-9"><b>0</b></a>
 &nbsp; 
<br>
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Variable-Index_vr_letter-V"><b>V</b></a>
 &nbsp; 
</td></tr></table>
</div>

<hr>
</div>
<div class="appendixsec-level-extent" id="Function-Index">
<div class="nav-panel">
<p>
Next: <a href="#Concept-Index" accesskey="n" rel="next">Concept Index</a>, Previous: <a href="#Variable-Index" accesskey="p" rel="prev">Parameter and Variable Index</a>, Up: <a href="#Indexes" accesskey="u" rel="up">Indexes</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="appendixsec" id="Function-Index-1"><span>D.4 Function Index<a class="copiable-link" href="#Function-Index-1"> &para;</a></span></h3>
<div class="printindex fn-printindex">
<table class="fn-letters-header-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Function-Index_fn_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-N"><b>N</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-Q"><b>Q</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-Y"><b>Y</b></a>
 &nbsp; 
</td></tr></table>
<table class="fn-entries-printindex">
<tr><td></td><th class="entries-header-printindex">Index Entry</th><th class="sections-header-printindex">Section</th></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-A">A</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-abort-_0028C_002dg_0029"><code>abort (C-g)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-accept_002dline-_0028Newline-or-Return_0029"><code>accept-line (Newline or Return)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-alias_002dexpand_002dline-_0028_0029"><code>alias-expand-line ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-B">B</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-backward_002dchar-_0028C_002db_0029"><code>backward-char (C-b)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-backward_002ddelete_002dchar-_0028Rubout_0029"><code>backward-delete-char (Rubout)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-backward_002dkill_002dline-_0028C_002dx-Rubout_0029"><code>backward-kill-line (C-x Rubout)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-backward_002dkill_002dword-_0028M_002dDEL_0029"><code>backward-kill-word (M-<kbd class="key">DEL</kbd>)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-backward_002dword-_0028M_002db_0029"><code>backward-word (M-b)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-beginning_002dof_002dhistory-_0028M_002d_003c_0029"><code>beginning-of-history (M-&lt;)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-beginning_002dof_002dline-_0028C_002da_0029"><code>beginning-of-line (C-a)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bracketed_002dpaste_002dbegin-_0028_0029"><code>bracketed-paste-begin ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-C">C</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-call_002dlast_002dkbd_002dmacro-_0028C_002dx-e_0029"><code>call-last-kbd-macro (C-x e)</code></a></td><td class="printindex-index-section"><a href="#Keyboard-Macros">Keyboard Macros</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-capitalize_002dword-_0028M_002dc_0029"><code>capitalize-word (M-c)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-character_002dsearch-_0028C_002d_005d_0029"><code>character-search (C-])</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-character_002dsearch_002dbackward-_0028M_002dC_002d_005d_0029"><code>character-search-backward (M-C-])</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-clear_002ddisplay-_0028M_002dC_002dl_0029"><code>clear-display (M-C-l)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-clear_002dscreen-_0028C_002dl_0029"><code>clear-screen (C-l)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete-_0028TAB_0029"><code>complete (<kbd class="key">TAB</kbd>)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dcommand-_0028M_002d_0021_0029"><code>complete-command (M-!)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dfilename-_0028M_002d_002f_0029"><code>complete-filename (M-/)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dhostname-_0028M_002d_0040_0029"><code>complete-hostname (M-@)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dinto_002dbraces-_0028M_002d_007b_0029"><code>complete-into-braces (M-{)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dusername-_0028M_002d_007e_0029"><code>complete-username (M-~)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-complete_002dvariable-_0028M_002d_0024_0029"><code>complete-variable (M-$)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-copy_002dbackward_002dword-_0028_0029"><code>copy-backward-word ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-copy_002dforward_002dword-_0028_0029"><code>copy-forward-word ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-copy_002dregion_002das_002dkill-_0028_0029"><code>copy-region-as-kill ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-D">D</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dabbrev_002dexpand-_0028_0029"><code>dabbrev-expand ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-delete_002dchar-_0028C_002dd_0029"><code>delete-char (C-d)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-delete_002dchar_002dor_002dlist-_0028_0029"><code>delete-char-or-list ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-delete_002dhorizontal_002dspace-_0028_0029"><code>delete-horizontal-space ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-digit_002dargument-_0028M_002d0_002c-M_002d1_002c-_002e_002e_002e-M_002d_002d_0029"><code>digit-argument (<kbd class="kbd">M-0</kbd>, <kbd class="kbd">M-1</kbd>, &hellip; <kbd class="kbd">M--</kbd>)</code></a></td><td class="printindex-index-section"><a href="#Numeric-Arguments">Numeric Arguments</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-display_002dshell_002dversion-_0028C_002dx-C_002dv_0029"><code>display-shell-version (C-x C-v)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-do_002dlowercase_002dversion-_0028M_002dA_002c-M_002dB_002c-M_002dx_002c-_002e_002e_002e_0029"><code>do-lowercase-version (M-A, M-B, M-<var class="var">x</var>, &hellip;)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-downcase_002dword-_0028M_002dl_0029"><code>downcase-word (M-l)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dump_002dfunctions-_0028_0029"><code>dump-functions ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dump_002dmacros-_0028_0029"><code>dump-macros ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dump_002dvariables-_0028_0029"><code>dump-variables ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dynamic_002dcomplete_002dhistory-_0028M_002dTAB_0029"><code>dynamic-complete-history (M-<kbd class="key">TAB</kbd>)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-E">E</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-edit_002dand_002dexecute_002dcommand-_0028C_002dx-C_002de_0029"><code>edit-and-execute-command (C-x C-e)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-end_002dkbd_002dmacro-_0028C_002dx-_0029_0029"><code>end-kbd-macro (C-x ))</code></a></td><td class="printindex-index-section"><a href="#Keyboard-Macros">Keyboard Macros</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-end_002dof_002dfile-_0028usually-C_002dd_0029"><code><i class="i">end-of-file</i> (usually C-d)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-end_002dof_002dhistory-_0028M_002d_003e_0029"><code>end-of-history (M-&gt;)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-end_002dof_002dline-_0028C_002de_0029"><code>end-of-line (C-e)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-exchange_002dpoint_002dand_002dmark-_0028C_002dx-C_002dx_0029"><code>exchange-point-and-mark (C-x C-x)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-execute_002dnamed_002dcommand-_0028M_002dx_0029"><code>execute-named-command (M-x)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-export_002dcompletions-_0028_0029"><code>export-completions ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-F">F</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-fetch_002dhistory-_0028_0029"><code>fetch-history ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-forward_002dbackward_002ddelete_002dchar-_0028_0029"><code>forward-backward-delete-char ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-forward_002dchar-_0028C_002df_0029"><code>forward-char (C-f)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-forward_002dsearch_002dhistory-_0028C_002ds_0029"><code>forward-search-history (C-s)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-forward_002dword-_0028M_002df_0029"><code>forward-word (M-f)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-G">G</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-glob_002dcomplete_002dword-_0028M_002dg_0029"><code>glob-complete-word (M-g)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-glob_002dexpand_002dword-_0028C_002dx-_002a_0029"><code>glob-expand-word (C-x *)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-glob_002dlist_002dexpansions-_0028C_002dx-g_0029"><code>glob-list-expansions (C-x g)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-H">H</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dand_002dalias_002dexpand_002dline-_0028_0029"><code>history-and-alias-expand-line ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dexpand_002dline-_0028M_002d_005e_0029"><code>history-expand-line (M-^)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dsearch_002dbackward-_0028_0029"><code>history-search-backward ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dsearch_002dforward-_0028_0029"><code>history-search-forward ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dsubstring_002dsearch_002dbackward-_0028_0029"><code>history-substring-search-backward ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history_002dsubstring_002dsearch_002dforward-_0028_0029"><code>history-substring-search-forward ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-I">I</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-insert_002dcomment-_0028M_002d_0023_0029"><code>insert-comment (M-#)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-insert_002dcompletions-_0028M_002d_002a_0029"><code>insert-completions (M-*)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-insert_002dlast_002dargument-_0028M_002d_002e-or-M_002d_005f_0029"><code>insert-last-argument (M-. or M-_)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-K">K</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill_002dline-_0028C_002dk_0029"><code>kill-line (C-k)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill_002dregion-_0028_0029"><code>kill-region ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill_002dwhole_002dline-_0028_0029"><code>kill-whole-line ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill_002dword-_0028M_002dd_0029"><code>kill-word (M-d)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-M">M</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-magic_002dspace-_0028_0029"><code>magic-space ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-menu_002dcomplete-_0028_0029"><code>menu-complete ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-menu_002dcomplete_002dbackward-_0028_0029"><code>menu-complete-backward ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-N">N</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-next_002dhistory-_0028C_002dn_0029"><code>next-history (C-n)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-next_002dscreen_002dline-_0028_0029"><code>next-screen-line ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-non_002dincremental_002dforward_002dsearch_002dhistory-_0028M_002dn_0029"><code>non-incremental-forward-search-history (M-n)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-non_002dincremental_002dreverse_002dsearch_002dhistory-_0028M_002dp_0029"><code>non-incremental-reverse-search-history (M-p)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-O">O</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-operate_002dand_002dget_002dnext-_0028C_002do_0029"><code>operate-and-get-next (C-o)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-overwrite_002dmode-_0028_0029"><code>overwrite-mode ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-P">P</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dcommand_002dcompletions-_0028C_002dx-_0021_0029"><code>possible-command-completions (C-x !)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dcompletions-_0028M_002d_003f_0029"><code>possible-completions (M-?)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dfilename_002dcompletions-_0028C_002dx-_002f_0029"><code>possible-filename-completions (C-x /)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dhostname_002dcompletions-_0028C_002dx-_0040_0029"><code>possible-hostname-completions (C-x @)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dusername_002dcompletions-_0028C_002dx-_007e_0029"><code>possible-username-completions (C-x ~)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-possible_002dvariable_002dcompletions-_0028C_002dx-_0024_0029"><code>possible-variable-completions (C-x $)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Completion">Commands For Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-prefix_002dmeta-_0028ESC_0029"><code>prefix-meta (<kbd class="key">ESC</kbd>)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-previous_002dhistory-_0028C_002dp_0029"><code>previous-history (C-p)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-previous_002dscreen_002dline-_0028_0029"><code>previous-screen-line ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-print_002dlast_002dkbd_002dmacro-_0028_0029"><code>print-last-kbd-macro ()</code></a></td><td class="printindex-index-section"><a href="#Keyboard-Macros">Keyboard Macros</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-Q">Q</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-quoted_002dinsert-_0028C_002dq-or-C_002dv_0029"><code>quoted-insert (C-q or C-v)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-R">R</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-re_002dread_002dinit_002dfile-_0028C_002dx-C_002dr_0029"><code>re-read-init-file (C-x C-r)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-redraw_002dcurrent_002dline-_0028_0029"><code>redraw-current-line ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-reverse_002dsearch_002dhistory-_0028C_002dr_0029"><code>reverse-search-history (C-r)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-revert_002dline-_0028M_002dr_0029"><code>revert-line (M-r)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-S">S</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-self_002dinsert-_0028a_002c-b_002c-A_002c-1_002c-_0021_002c-_002e_002e_002e_0029"><code>self-insert (a, b, A, 1, !, &hellip;)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-set_002dmark-_0028C_002d_0040_0029"><code>set-mark (C-@)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dbackward_002dkill_002dword-_0028_0029"><code>shell-backward-kill-word ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dbackward_002dword-_0028M_002dC_002db_0029"><code>shell-backward-word (M-C-b)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dexpand_002dline-_0028M_002dC_002de_0029"><code>shell-expand-line (M-C-e)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dforward_002dword-_0028M_002dC_002df_0029"><code>shell-forward-word (M-C-f)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Moving">Commands For Moving</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dkill_002dword-_0028M_002dC_002dd_0029"><code>shell-kill-word (M-C-d)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002dtranspose_002dwords-_0028M_002dC_002dt_0029"><code>shell-transpose-words (M-C-t)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-skip_002dcsi_002dsequence-_0028_0029"><code>skip-csi-sequence ()</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-spell_002dcorrect_002dword-_0028C_002dx-s_0029"><code>spell-correct-word (C-x s)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-start_002dkbd_002dmacro-_0028C_002dx-_0028_0029"><code>start-kbd-macro (C-x ()</code></a></td><td class="printindex-index-section"><a href="#Keyboard-Macros">Keyboard Macros</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-T">T</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-tilde_002dexpand-_0028M_002d_0026_0029"><code>tilde-expand (M-&amp;)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-transpose_002dchars-_0028C_002dt_0029"><code>transpose-chars (C-t)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-transpose_002dwords-_0028M_002dt_0029"><code>transpose-words (M-t)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-U">U</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-undo-_0028C_002d_005f-or-C_002dx-C_002du_0029"><code>undo (C-_ or C-x C-u)</code></a></td><td class="printindex-index-section"><a href="#Miscellaneous-Commands">Miscellaneous Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-universal_002dargument-_0028_0029"><code>universal-argument ()</code></a></td><td class="printindex-index-section"><a href="#Numeric-Arguments">Numeric Arguments</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unix_002dfilename_002drubout-_0028_0029"><code>unix-filename-rubout ()</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unix_002dline_002ddiscard-_0028C_002du_0029"><code>unix-line-discard (C-u)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unix_002dword_002drubout-_0028C_002dw_0029"><code>unix-word-rubout (C-w)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-upcase_002dword-_0028M_002du_0029"><code>upcase-word (M-u)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Text">Commands For Text</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Function-Index_fn_letter-Y">Y</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-yank-_0028C_002dy_0029"><code>yank (C-y)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-yank_002dlast_002darg-_0028M_002d_002e-or-M_002d_005f_0029"><code>yank-last-arg (M-. or M-_)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-yank_002dnth_002darg-_0028M_002dC_002dy_0029"><code>yank-nth-arg (M-C-y)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-History">Commands For History</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-yank_002dpop-_0028M_002dy_0029"><code>yank-pop (M-y)</code></a></td><td class="printindex-index-section"><a href="#Commands-For-Killing">Commands For Killing</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
</table>
<table class="fn-letters-footer-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Function-Index_fn_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-G"><b>G</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-N"><b>N</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-Q"><b>Q</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Function-Index_fn_letter-Y"><b>Y</b></a>
 &nbsp; 
</td></tr></table>
</div>

<hr>
</div>
<div class="appendixsec-level-extent" id="Concept-Index">
<div class="nav-panel">
<p>
Previous: <a href="#Function-Index" accesskey="p" rel="prev">Function Index</a>, Up: <a href="#Indexes" accesskey="u" rel="up">Indexes</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<h3 class="appendixsec" id="Concept-Index-1"><span>D.5 Concept Index<a class="copiable-link" href="#Concept-Index-1"> &para;</a></span></h3>
<div class="printindex cp-printindex">
<table class="cp-letters-header-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Concept-Index_cp_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-J"><b>J</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-N"><b>N</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-Q"><b>Q</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-V"><b>V</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-W"><b>W</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-Y"><b>Y</b></a>
 &nbsp; 
</td></tr></table>
<table class="cp-entries-printindex">
<tr><td></td><th class="entries-header-printindex">Index Entry</th><th class="sections-header-printindex">Section</th></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-A">A</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-alias-expansion">alias expansion</a></td><td class="printindex-index-section"><a href="#Aliases">Aliases</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-arithmetic-evaluation">arithmetic evaluation</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-arithmetic-expansion">arithmetic expansion</a></td><td class="printindex-index-section"><a href="#Arithmetic-Expansion">Arithmetic Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-arithmetic-operators">arithmetic operators</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-arithmetic_002c-shell">arithmetic, shell</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-arrays">arrays</a></td><td class="printindex-index-section"><a href="#Arrays">Arrays</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-B">B</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-background">background</a></td><td class="printindex-index-section"><a href="#Job-Control-Basics">Job Control Basics</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Bash-configuration">Bash configuration</a></td><td class="printindex-index-section"><a href="#Basic-Installation">Basic Installation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Bash-installation">Bash installation</a></td><td class="printindex-index-section"><a href="#Basic-Installation">Basic Installation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-binary-arithmetic-operators">binary arithmetic operators</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-bitwise-arithmetic-operators">bitwise arithmetic operators</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Bourne-shell">Bourne shell</a></td><td class="printindex-index-section"><a href="#Basic-Shell-Features">Basic Shell Features</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-brace-expansion">brace expansion</a></td><td class="printindex-index-section"><a href="#Brace-Expansion">Brace Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-builtin-1">builtin</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-C">C</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-editing">command editing</a></td><td class="printindex-index-section"><a href="#Readline-Bare-Essentials">Readline Bare Essentials</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-execution">command execution</a></td><td class="printindex-index-section"><a href="#Command-Search-and-Execution">Command Search and Execution</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-expansion">command expansion</a></td><td class="printindex-index-section"><a href="#Simple-Command-Expansion">Simple Command Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-history">command history</a></td><td class="printindex-index-section"><a href="#Bash-History-Facilities">Bash History Facilities</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-search">command search</a></td><td class="printindex-index-section"><a href="#Command-Search-and-Execution">Command Search and Execution</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-substitution">command substitution</a></td><td class="printindex-index-section"><a href="#Command-Substitution">Command Substitution</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-command-timing">command timing</a></td><td class="printindex-index-section"><a href="#Pipelines">Pipelines</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-compound">commands, compound</a></td><td class="printindex-index-section"><a href="#Compound-Commands">Compound Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-conditional">commands, conditional</a></td><td class="printindex-index-section"><a href="#Conditional-Constructs">Conditional Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-grouping">commands, grouping</a></td><td class="printindex-index-section"><a href="#Command-Grouping">Command Grouping</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-lists">commands, lists</a></td><td class="printindex-index-section"><a href="#Lists">Lists</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-looping">commands, looping</a></td><td class="printindex-index-section"><a href="#Looping-Constructs">Looping Constructs</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-pipelines">commands, pipelines</a></td><td class="printindex-index-section"><a href="#Pipelines">Pipelines</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-shell">commands, shell</a></td><td class="printindex-index-section"><a href="#Shell-Commands">Shell Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-commands_002c-simple">commands, simple</a></td><td class="printindex-index-section"><a href="#Simple-Commands">Simple Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-comments_002c-shell">comments, shell</a></td><td class="printindex-index-section"><a href="#Comments">Comments</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Compatibility-Level">Compatibility Level</a></td><td class="printindex-index-section"><a href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Compatibility-Mode">Compatibility Mode</a></td><td class="printindex-index-section"><a href="#Shell-Compatibility-Mode">Shell Compatibility Mode</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-completion-builtins">completion builtins</a></td><td class="printindex-index-section"><a href="#Programmable-Completion-Builtins">Programmable Completion Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-conditional-arithmetic-operator">conditional arithmetic operator</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-configuration">configuration</a></td><td class="printindex-index-section"><a href="#Basic-Installation">Basic Installation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-control-operator">control operator</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-coprocess">coprocess</a></td><td class="printindex-index-section"><a href="#Coprocesses">Coprocesses</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-D">D</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-directory-stack">directory stack</a></td><td class="printindex-index-section"><a href="#The-Directory-Stack">The Directory Stack</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-dollar_002dsingle-quote-quoting">dollar-single quote quoting</a></td><td class="printindex-index-section"><a href="#ANSI_002dC-Quoting">ANSI-C Quoting</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-E">E</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-editing-command-lines">editing command lines</a></td><td class="printindex-index-section"><a href="#Readline-Bare-Essentials">Readline Bare Essentials</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-environment">environment</a></td><td class="printindex-index-section"><a href="#Environment">Environment</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-evaluation_002c-arithmetic">evaluation, arithmetic</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-event-designators">event designators</a></td><td class="printindex-index-section"><a href="#Event-Designators">Event Designators</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-execution-environment">execution environment</a></td><td class="printindex-index-section"><a href="#Command-Execution-Environment">Command Execution Environment</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-exit-status">exit status</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-exit-status-1">exit status</a></td><td class="printindex-index-section"><a href="#Exit-Status">Exit Status</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion">expansion</a></td><td class="printindex-index-section"><a href="#Shell-Expansions">Shell Expansions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-arithmetic">expansion, arithmetic</a></td><td class="printindex-index-section"><a href="#Arithmetic-Expansion">Arithmetic Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-brace">expansion, brace</a></td><td class="printindex-index-section"><a href="#Brace-Expansion">Brace Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-filename">expansion, filename</a></td><td class="printindex-index-section"><a href="#Filename-Expansion">Filename Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-parameter">expansion, parameter</a></td><td class="printindex-index-section"><a href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-pathname">expansion, pathname</a></td><td class="printindex-index-section"><a href="#Filename-Expansion">Filename Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expansion_002c-tilde">expansion, tilde</a></td><td class="printindex-index-section"><a href="#Tilde-Expansion">Tilde Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expressions_002c-arithmetic">expressions, arithmetic</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-expressions_002c-conditional">expressions, conditional</a></td><td class="printindex-index-section"><a href="#Bash-Conditional-Expressions">Bash Conditional Expressions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-F">F</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-field">field</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-filename">filename</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-filename-expansion">filename expansion</a></td><td class="printindex-index-section"><a href="#Filename-Expansion">Filename Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-foreground">foreground</a></td><td class="printindex-index-section"><a href="#Job-Control-Basics">Job Control Basics</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-functions_002c-shell">functions, shell</a></td><td class="printindex-index-section"><a href="#Shell-Functions">Shell Functions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-H">H</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history-builtins">history builtins</a></td><td class="printindex-index-section"><a href="#Bash-History-Builtins">Bash History Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history-events">history events</a></td><td class="printindex-index-section"><a href="#Event-Designators">Event Designators</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history-expansion">history expansion</a></td><td class="printindex-index-section"><a href="#History-Interaction">History Interaction</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-history-list">history list</a></td><td class="printindex-index-section"><a href="#Bash-History-Facilities">Bash History Facilities</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-History_002c-how-to-use">History, how to use</a></td><td class="printindex-index-section"><a href="#A-Programmable-Completion-Example">A Programmable Completion Example</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-I">I</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-identifier">identifier</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-initialization-file_002c-readline">initialization file, readline</a></td><td class="printindex-index-section"><a href="#Readline-Init-File">Readline Init File</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-installation">installation</a></td><td class="printindex-index-section"><a href="#Basic-Installation">Basic Installation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-interaction_002c-readline">interaction, readline</a></td><td class="printindex-index-section"><a href="#Readline-Interaction">Readline Interaction</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-interactive-shell">interactive shell</a></td><td class="printindex-index-section"><a href="#Invoking-Bash">Invoking Bash</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-interactive-shell-1">interactive shell</a></td><td class="printindex-index-section"><a href="#Interactive-Shells">Interactive Shells</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-internationalization">internationalization</a></td><td class="printindex-index-section"><a href="#Locale-Translation">Locale Translation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-internationalized-scripts">internationalized scripts</a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-J">J</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-job">job</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-job-control">job control</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-job-control-1">job control</a></td><td class="printindex-index-section"><a href="#Job-Control-Basics">Job Control Basics</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-K">K</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-kill-ring">kill ring</a></td><td class="printindex-index-section"><a href="#Readline-Killing-Commands">Readline Killing Commands</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-killing-text">killing text</a></td><td class="printindex-index-section"><a href="#Readline-Killing-Commands">Readline Killing Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-L">L</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-localization">localization</a></td><td class="printindex-index-section"><a href="#Locale-Translation">Locale Translation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-login-shell">login shell</a></td><td class="printindex-index-section"><a href="#Invoking-Bash">Invoking Bash</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-M">M</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-matching_002c-pattern">matching, pattern</a></td><td class="printindex-index-section"><a href="#Pattern-Matching">Pattern Matching</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-metacharacter">metacharacter</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-N">N</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-name">name</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-native-languages">native languages</a></td><td class="printindex-index-section"><a href="#Locale-Translation">Locale Translation</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-notation_002c-readline">notation, readline</a></td><td class="printindex-index-section"><a href="#Readline-Bare-Essentials">Readline Bare Essentials</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-O">O</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-operator_002c-shell">operator, shell</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-P">P</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-parameter-expansion">parameter expansion</a></td><td class="printindex-index-section"><a href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-parameters">parameters</a></td><td class="printindex-index-section"><a href="#Shell-Parameters">Shell Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-parameters_002c-positional">parameters, positional</a></td><td class="printindex-index-section"><a href="#Positional-Parameters">Positional Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-parameters_002c-special">parameters, special</a></td><td class="printindex-index-section"><a href="#Special-Parameters">Special Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-pathname-expansion">pathname expansion</a></td><td class="printindex-index-section"><a href="#Filename-Expansion">Filename Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-pattern-matching">pattern matching</a></td><td class="printindex-index-section"><a href="#Pattern-Matching">Pattern Matching</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-pipeline">pipeline</a></td><td class="printindex-index-section"><a href="#Pipelines">Pipelines</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-POSIX">POSIX</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-POSIX-description">POSIX description</a></td><td class="printindex-index-section"><a href="#Bash-POSIX-Mode">Bash POSIX Mode</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-POSIX-Mode">POSIX Mode</a></td><td class="printindex-index-section"><a href="#Bash-POSIX-Mode">Bash POSIX Mode</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-process-group">process group</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-process-group-ID">process group ID</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-process-substitution">process substitution</a></td><td class="printindex-index-section"><a href="#Process-Substitution">Process Substitution</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-programmable-completion">programmable completion</a></td><td class="printindex-index-section"><a href="#Programmable-Completion">Programmable Completion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-prompting">prompting</a></td><td class="printindex-index-section"><a href="#Controlling-the-Prompt">Controlling the Prompt</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-Q">Q</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-quoting">quoting</a></td><td class="printindex-index-section"><a href="#Quoting">Quoting</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-quoting_002c-ANSI">quoting, ANSI</a></td><td class="printindex-index-section"><a href="#ANSI_002dC-Quoting">ANSI-C Quoting</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-R">R</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-Readline_002c-how-to-use">Readline, how to use</a></td><td class="printindex-index-section"><a href="#Job-Control-Variables">Job Control Variables</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-redirection">redirection</a></td><td class="printindex-index-section"><a href="#Redirections">Redirections</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-reserved-word">reserved word</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-reserved-words">reserved words</a></td><td class="printindex-index-section"><a href="#Reserved-Words">Reserved Words</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-restricted-shell">restricted shell</a></td><td class="printindex-index-section"><a href="#The-Restricted-Shell">The Restricted Shell</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-return-status">return status</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-S">S</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell-arithmetic">shell arithmetic</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell-function">shell function</a></td><td class="printindex-index-section"><a href="#Shell-Functions">Shell Functions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell-script">shell script</a></td><td class="printindex-index-section"><a href="#Shell-Scripts">Shell Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell-variable">shell variable</a></td><td class="printindex-index-section"><a href="#Shell-Parameters">Shell Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-shell_002c-interactive">shell, interactive</a></td><td class="printindex-index-section"><a href="#Interactive-Shells">Interactive Shells</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-signal">signal</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-signal-handling">signal handling</a></td><td class="printindex-index-section"><a href="#Signals">Signals</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-special-builtin">special builtin</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-special-builtin-1">special builtin</a></td><td class="printindex-index-section"><a href="#Special-Builtins">Special Builtins</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-startup-files">startup files</a></td><td class="printindex-index-section"><a href="#Bash-Startup-Files">Bash Startup Files</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-string-translations">string translations</a></td><td class="printindex-index-section"><a href="#Creating-Internationalized-Scripts">Creating Internationalized Scripts</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-suspending-jobs">suspending jobs</a></td><td class="printindex-index-section"><a href="#Job-Control-Basics">Job Control Basics</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-T">T</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-tilde-expansion">tilde expansion</a></td><td class="printindex-index-section"><a href="#Tilde-Expansion">Tilde Expansion</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-token">token</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-translation_002c-native-languages">translation, native languages</a></td><td class="printindex-index-section"><a href="#Locale-Translation">Locale Translation</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-U">U</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-unary-arithmetic-operators">unary arithmetic operators</a></td><td class="printindex-index-section"><a href="#Shell-Arithmetic">Shell Arithmetic</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-V">V</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-variable_002c-shell">variable, shell</a></td><td class="printindex-index-section"><a href="#Shell-Parameters">Shell Parameters</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-variables_002c-readline">variables, readline</a></td><td class="printindex-index-section"><a href="#Readline-Init-File-Syntax">Readline Init File Syntax</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-W">W</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-word">word</a></td><td class="printindex-index-section"><a href="#Definitions">Definitions</a></td></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-word-splitting">word splitting</a></td><td class="printindex-index-section"><a href="#Word-Splitting">Word Splitting</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><th id="Concept-Index_cp_letter-Y">Y</th></tr>
<tr><td></td><td class="printindex-index-entry"><a href="#index-yanking-text">yanking text</a></td><td class="printindex-index-section"><a href="#Readline-Killing-Commands">Readline Killing Commands</a></td></tr>
<tr><td colspan="3"><hr></td></tr>
</table>
<table class="cp-letters-footer-printindex"><tr><th>Jump to: &nbsp; </th><td><a class="summary-letter-printindex" href="#Concept-Index_cp_letter-A"><b>A</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-B"><b>B</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-C"><b>C</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-D"><b>D</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-H"><b>H</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-I"><b>I</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-J"><b>J</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-K"><b>K</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-L"><b>L</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-M"><b>M</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-N"><b>N</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-O"><b>O</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-P"><b>P</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-Q"><b>Q</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-R"><b>R</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-S"><b>S</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-T"><b>T</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-U"><b>U</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-V"><b>V</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-W"><b>W</b></a>
 &nbsp; 
<a class="summary-letter-printindex" href="#Concept-Index_cp_letter-Y"><b>Y</b></a>
 &nbsp; 
</td></tr></table>
</div>

</div>
</div>
</div>



</body>
</html>