     [file version.texi not included]

This text is a brief description of the features that are present in the Bash shell (version VERSION, UPDATED).

This is Edition EDITION, last updated UPDATED, of The GNU Bash Reference Manual, for Bash, Version VERSION.

Copyright © 1988--2020 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''.

Bash Features
*************

This text is a brief description of the features that are present in the Bash shell (version VERSION, UPDATED). The Bash home page is http://www.gnu.org/software/bash/.

This is Edition EDITION, last updated UPDATED, of The GNU Bash Reference Manual, for Bash, Version VERSION.

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 (sh), the Korn Shell (ksh), and the C-shell (csh and its successor, tcsh).  The following menu breaks the features up into categories, noting which features were inspired by other shells and which are specific to Bash.

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.

Introduction
************

What is Bash?
=============

Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the Bourne-Again SHell, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition Bell Labs Research version of Unix.

Bash is largely compatible with sh and incorporates useful features from the Korn shell ksh and the C shell csh. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools portion of the IEEE POSIX specification (IEEE Standard 1003.1). It offers functional improvements over sh for both interactive and programming use.

While the GNU operating system provides other shells, including a version of csh, Bash is the default shell. Like other GNU software, Bash is quite portable.  It currently runs on nearly every version of Unix and a few other operating systems - independently-supported ports exist for MS-DOS, OS/2, and Windows platforms.

What is a shell?
================

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.

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 GNU utilities.  The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves.  These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.

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.

A shell allows execution of GNU 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 redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments.

Shells also provide a small set of built-in commands (builtins) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, cd, break, continue, and exec cannot be implemented outside of the shell because they directly manipulate the shell itself. The history, getopts, kill, or pwd 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.

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.

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.  Each of these features is described in this manual.

Definitions
***********

These definitions are used throughout the remainder of this manual.

POSIX
     A family of open system standards based on Unix.  Bash is primarily concerned with the Shell and Utilities portion of the POSIX 1003.1 standard.

blank
     A space or tab character.

builtin
     A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system.

control operator
     A token that performs a control function.  It is a newline
     or one of the following:
     ||, &&, &, ;, ;;, ;&, ;;&,
     |, |&, (, or ).

exit status
     The value returned by a command to its caller.  The value is restricted to eight bits, so the maximum value is 255.

field
     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.

filename
     A string of characters used to identify a file.

job
     A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group.

job control
     A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes.

metacharacter
     A character that, when unquoted, separates words.  A metacharacter is a space, tab, newline, or one of the following characters: |, &, ;, (, ), <, or >.

name
     A word consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore.  Names are used as shell variable and function names. Also referred to as an identifier.

operator
     A control operator or a redirection operator. See Redirections, for a list of redirection operators. Operators contain at least one unquoted metacharacter.

process group
     A collection of related processes each having the same process group ID.

process group ID
     A unique identifier that represents a process group during its lifetime.

reserved word
     A word that has a special meaning to the shell.  Most reserved words introduce shell flow control constructs, such as for and while.

return status
     A synonym for exit status.

signal
     A mechanism by which a process may be notified by the kernel of an event occurring in the system.

special builtin
     A shell builtin command that has been classified as special by the POSIX standard.

token
     A sequence of characters considered a single unit by the shell. It is either a word or an operator.

word
     A sequence of characters treated as a unit by the shell. Words may not include unquoted metacharacters.

Basic Shell Features
********************

Bash is an acronym for Bourne-Again SHell. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the POSIX specification for the `standard' Unix shell.

This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.

Shell Syntax
============

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 (#), and the rest of that line.

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.

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's exit status, and makes that exit status available for further inspection or processing.

Shell Operation
---------------

The following is a brief description of the shell's operation when it reads and executes a command.  Basically, the shell does the following:

 1. Reads its input from a file (see Shell Scripts), from a string supplied as an argument to the -c invocation option (see Invoking Bash), or from the user's terminal.

 2. Breaks the input into words and operators, obeying the quoting rules described in Quoting.  These tokens are separated by metacharacters.  Alias expansion is performed by this step (see Aliases).

 3. Parses the tokens into simple and compound commands (see Shell Commands).

 4. Performs the various shell expansions (see Shell Expansions), breaking the expanded tokens into lists of filenames (see Filename Expansion) and commands and arguments.

 5. Performs any necessary redirections (see Redirections) and removes the redirection operators and their operands from the argument list.

 6. Executes the command (see Executing Commands).

 7. Optionally waits for the command to complete and collects its exit status (see Exit Status).

Quoting
-------

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.

Each of the shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see History Interaction), the HISTORY EXPANSION character, usually !, must be quoted to prevent history expansion.  See Bash History Facilities, for more details concerning history expansion.

There are three quoting mechanisms: the ESCAPE CHARACTER, single quotes, and double quotes.

Escape Character
................

A non-quoted backslash \ is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of newline.  If a \newline pair appears, and the backslash itself is not quoted, the \newline is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

Single Quotes
.............

Enclosing characters in single quotes (') 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.

Double Quotes
.............

Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. When the shell is in POSIX mode (see Bash POSIX Mode), the ! has no special meaning within double quotes, even when history expansion is enabled. The characters $ and ` retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline. Within double quotes, backslashes that are followed by one of these characters are removed.  Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see Shell Parameter Expansion).

ANSI-C Quoting
..............

Words of the form $'STRING' are treated specially.  The word expands to STRING, with backslash-escaped characters replaced as specified by the ANSI C standard.  Backslash escape sequences, if present, are decoded as follows:

\a
     alert (bell)

\b
     backspace

\e
\E
     an escape character (not ANSI C)

\f
     form feed

\n
     newline

\r
     carriage return

\t
     horizontal tab

\v
     vertical tab

\\
     backslash

\'
     single quote

\"
     double quote

\?
     question mark

\NNN
     the eight-bit character whose value is the octal value NNN (one to three octal digits)

\xHH
     the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)

\uHHHH
     the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)

\UHHHHHHHH
     the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)

\cX
     a control-X character

The expanded result is single-quoted, as if the dollar sign had not been present.

Locale-Specific Translation
...........................

A double-quoted string preceded by a dollar sign ($) will cause the string to be translated according to the current locale. The GETTEXT infrastructure performs the message catalog lookup and translation, using the LC_MESSAGES and TEXTDOMAIN shell variables, as explained below. See the gettext documentation for additional details. If the current locale is C or POSIX, or if there are no translations available, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.

Some systems use the message catalog selected by the LC_MESSAGES shell variable.  Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding a suffix of .mo.  If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files.  Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.

Comments
--------

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see The Shopt Builtin), a word beginning with # causes that word and all remaining characters on that line to be ignored.  An interactive shell without the interactive_comments option enabled does not allow comments.  The interactive_comments option is on by default in interactive shells. See Interactive Shells, for a description of what makes a shell interactive.

Shell Commands
==============

A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces.

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.

Reserved Words
--------------

Reserved words are words that have special meaning to the shell. They are used to begin and end the shell's compound commands.

The following words are recognized as reserved when unquoted and the first word of a command (see below for exceptions):

if   then   elif
else   fi   time
for   in   until
while   do   done
case   esac   coproc
select   function
{   }   [[   ]]   !

in is recognized as a reserved word if it is the third word of a case or select command. in and do are recognized as reserved words if they are the third word in a for command.

Simple Commands
---------------

A simple command is the kind of command encountered most often. It's just a sequence of words separated by blanks, terminated by one of the shell's control operators (see Definitions).  The first word generally specifies a command to be executed, with the rest of the words being that command's arguments.

The return status (see Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+N if the command was terminated by signal N.

Pipelines
---------

A pipeline is a sequence of one or more commands separated by one of the control operators | or |&.

The format for a pipeline is

     [time [-p]] [!] COMMAND1 [ | or |& COMMAND2 ] ...

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's output.  This connection is performed before any redirections specified by the command.

If |& is used, COMMAND1's standard error, in addition to its standard output, is connected to COMMAND2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is performed after any redirections specified by the command.

The reserved word time causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The -p option changes the output format to that specified by POSIX. When the shell is in POSIX mode (see Bash POSIX Mode), it does not recognize time as a reserved word if the next token begins with a -. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed. See Bash Variables, for a description of the available formats. The use of time as a reserved word permits the timing of shell builtins, shell functions, and pipelines.  An external time command cannot time these easily.

When the shell is in POSIX mode (see Bash POSIX Mode), time may be followed by a newline.  In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable may be used to specify the format of the time information.

If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell, which is a separate process (see Command Execution Environment). If the lastpipe option is enabled using the shopt builtin (see The Shopt Builtin), the last element of a pipeline may be run by the shell process.

The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (see The Set Builtin). If pipefail is enabled, the pipeline'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 ! precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.

Lists of Commands
-----------------

A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or a newline.

Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.

A sequence of one or more newlines may appear in a list to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator &, the shell executes the command asynchronously in a subshell. This is known as executing the command in the BACKGROUND, and these are referred to as ASYNCHRONOUS 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 Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Commands separated by a ; 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.

AND and OR lists are sequences of one or more pipelines separated by the control operators && and ||, respectively.  AND and OR lists are executed with left associativity.

An AND list has the form

     COMMAND1 && COMMAND2

COMMAND2 is executed if, and only if, COMMAND1 returns an exit status of zero (success).

An OR list has the form

     COMMAND1 || COMMAND2

COMMAND2 is executed if, and only if, COMMAND1 returns a non-zero exit status.

The return status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands
-----------------

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 Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.

In most cases a list of commands in a compound command'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.

Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.

Looping Constructs
..................

Bash supports the following looping constructs.

Note that wherever a ; appears in the description of a command's syntax, it may be replaced with one or more newlines.

until
     The syntax of the until command is:

          until TEST-COMMANDS; do CONSEQUENT-COMMANDS; done

     Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit status which is not zero. The return status is the exit status of the last command executed in CONSEQUENT-COMMANDS, or zero if none was executed.

while
     The syntax of the while command is:

          while TEST-COMMANDS; do CONSEQUENT-COMMANDS; done

     Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit status of zero. The return status is the exit status of the last command executed in CONSEQUENT-COMMANDS, or zero if none was executed.

for
     The syntax of the for command is:

          for NAME [ [in [WORDS ...] ] ; ] do COMMANDS; done

     Expand WORDS (see Shell Expansions), and execute COMMANDS once for each member in the resultant list, with NAME bound to the current member. If in WORDS is not present, the for command executes the COMMANDS once for each positional parameter that is set, as if in "$@" had been specified (see Special Parameters).

     The return status is the exit status of the last command that executes. If there are no items in the expansion of WORDS, no commands are executed, and the return status is zero.

     An alternate form of the for command is also supported:

          for (( EXPR1 ; EXPR2 ; EXPR3 )) ; do COMMANDS ; done

     First, the arithmetic expression EXPR1 is evaluated according to the rules described below (see Shell Arithmetic). The arithmetic expression EXPR2 is then evaluated repeatedly until it evaluates to zero. Each time EXPR2 evaluates to a non-zero value, COMMANDS are executed and the arithmetic expression EXPR3 is evaluated. 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 COMMANDS that is executed, or false if any of the expressions is invalid.

The break and continue builtins (see Bourne Shell Builtins) may be used to control loop execution.

Conditional Constructs
......................

if
     The syntax of the if command is:

          if TEST-COMMANDS; then
            CONSEQUENT-COMMANDS;
          [elif MORE-TEST-COMMANDS; then
            MORE-CONSEQUENTS;]
          [else ALTERNATE-CONSEQUENTS;]
          fi

     The TEST-COMMANDS list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. If TEST-COMMANDS returns a non-zero status, each elif list is executed in turn, and if its exit status is zero, the corresponding MORE-CONSEQUENTS is executed and the command completes. If else ALTERNATE-CONSEQUENTS is present, and the final command in the final if or elif clause has a non-zero exit status, then ALTERNATE-CONSEQUENTS is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

case
     The syntax of the case command is:

          case WORD in
              [ [(] PATTERN [| PATTERN]...) COMMAND-LIST ;;]...
          esac

     case will selectively execute the COMMAND-LIST corresponding to the first PATTERN that matches WORD. The match is performed according to the rules described below in Pattern Matching. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The | is used to separate multiple patterns, and the ) operator terminates a pattern list. A list of patterns and an associated command-list is known as a CLAUSE.

     Each clause must be terminated with ;;, ;&, or ;;&. The WORD undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal (see Shell Parameter Expansion) before matching is attempted.  Each PATTERN undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

     There may be an arbitrary number of case clauses, each terminated by a ;;, ;&, or ;;&. The first pattern that matches determines the command-list that is executed. It's a common idiom to use * as the final pattern to define the default case, since that pattern will always match.

     Here is an example using case in a script that could be used to describe one interesting feature of an animal:

          echo -n "Enter the name of an animal: "
          read ANIMAL
          echo -n "The $ANIMAL has "
          case $ANIMAL in
            horse | dog | cat) echo -n "four";;
            man | kangaroo ) echo -n "two";;
            *) echo -n "an unknown number of";;
          esac
          echo " legs."

     If the ;; operator is used, no subsequent matches are attempted after the first pattern match. Using ;&  in place of ;; causes execution to continue with the COMMAND-LIST associated with the next clause, if any. Using ;;& in place of ;; causes the shell to test the patterns in the next clause, if any, and execute any associated COMMAND-LIST on a successful match, continuing the case statement execution as if the pattern list had not matched.

     The return status is zero if no PATTERN is matched.  Otherwise, the return status is the exit status of the COMMAND-LIST executed.

select

     The select construct allows the easy generation of menus. It has almost the same syntax as the for command:

          select NAME [in WORDS ...]; do COMMANDS; done

     The list of words following in is expanded, generating a list of items.  The set of expanded words is printed on the standard error output stream, each preceded by a number.  If the in WORDS is omitted, the positional parameters are printed, as if in "$@" had been specified. The PS3 prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of NAME is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the select command completes. Any other value read causes NAME to be set to null. The line read is saved in the variable REPLY.

     The COMMANDS are executed after each selection until a break command is executed, at which point the select command completes.

     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.

          select fname in *;
          do
          	echo you picked $fname \($REPLY\)
          	break;
          done

((...))

          (( EXPRESSION ))

     The arithmetic EXPRESSION is evaluated according to the rules described below (see Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1.  This is exactly equivalent to

          let "EXPRESSION"

     See Bash Builtins, for a full description of the let builtin.

[[...]]

          [[ EXPRESSION ]]

     Return a status of 0 or 1 depending on the evaluation of the conditional expression EXPRESSION. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries.

     When used with [[, the < and > operators sort lexicographically using the current locale.

     When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching, as if the extglob shell option were enabled. The = operator is identical to ==. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force the quoted portion to be matched as a string.

     An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (using the POSIX regcomp and regexec interfaces usually described in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force the quoted portion to be matched as a string. Bracket expressions in regular expressions must be treated carefully, since normal quoting characters lose their meanings between brackets. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched as a string.

     The pattern will match if it matches any part of the string. Anchor the pattern using the ^ and $ regular expression operators to force it to match the entire string. The array variable BASH_REMATCH records which parts of the string matched the pattern. The element of BASH_REMATCH 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 BASH_REMATCH indices. The element of BASH_REMATCH with index N is the portion of the string matching the Nth parenthesized subexpression.

     For example, the following will match a line (stored in the shell variable LINE) if there is a sequence of characters anywhere in the value consisting of any number, including zero, of characters in the space character class, zero or one instances of a, then a b:

          [[ $line =~ [[:space:]]*(a)?b ]]

     That means values like aab and   aaaaaab will match, as will a line containing a b anywhere in its value.

     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. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell's quote removal. Using a shell variable to store the pattern decreases these problems. For example, the following is equivalent to the above:

          pattern='[[:space:]]*(a)?b'
          [[ $line =~ $pattern ]]

     If you want to match a character that's special to the regular expression grammar, it has to be quoted to remove its special meaning. This means that in the pattern xxx.txt, the . matches any character in the string (its usual regular expression meaning), but in the pattern "xxx.txt" it can only match a literal .. Shell programmers should take special care with backslashes, since backslashes are used both by the shell and regular expressions to remove the special meaning from the following character. The following two sets of commands are not equivalent:

          pattern='\.'

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

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

     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 backslash removes the special meaning from ., so the literal . matches. If the string in the first examples were anything other than ., say a, the pattern would not match, because the quoted . in the pattern loses its special meaning of matching any single character.

     Expressions may be combined using the following operators, listed in decreasing order of precedence:

     ( EXPRESSION )
          Returns the value of EXPRESSION. This may be used to override the normal precedence of operators.

     ! EXPRESSION
          True if EXPRESSION is false.

     EXPRESSION1 && EXPRESSION2
          True if both EXPRESSION1 and EXPRESSION2 are true.

     EXPRESSION1 || EXPRESSION2
          True if either EXPRESSION1 or EXPRESSION2 is true.

     The && and || operators do not evaluate EXPRESSION2 if the value of EXPRESSION1 is sufficient to determine the return value of the entire conditional expression.

Grouping Commands
.................

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.

()

          ( LIST )

     Placing a list of commands between parentheses causes a subshell environment to be created (see Command Execution Environment), and each of the commands in LIST to be executed in that subshell.  Since the LIST is executed in a subshell, variable assignments do not remain in effect after the subshell completes.

{}

          { LIST; }

     Placing a list of commands between curly braces causes the list to be executed in the current shell context.  No subshell is created. The semicolon (or newline) following LIST is required.

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 LIST by blanks 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 LIST by whitespace.

The exit status of both of these constructs is the exit status of LIST.

Coprocesses
-----------

A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator, with a two-way pipe established between the executing shell and the coprocess.

The format for a coprocess is:

     coproc [NAME] COMMAND [REDIRECTIONS]

This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if COMMAND is a simple command (see Simple Commands); otherwise, it is interpreted as the first word of the simple command.

When the coprocess is executed, the shell creates an array variable (see Arrays) named NAME in the context of the executing shell. The standard output of COMMAND is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[0]. The standard input of COMMAND is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified by the command (see Redirections). 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.

The process ID of the shell spawned to execute the coprocess is available as the value of the variable NAME_PID. The wait builtin command may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command, the coproc command always returns success. The return status of a coprocess is the exit status of COMMAND.

GNU Parallel
------------

There are ways to run commands in parallel that are not built into Bash. GNU Parallel is a tool to do just that.

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 xargs or feed commands from its input sources to several different instances of Bash.

For a complete description, refer to the GNU Parallel documentation.  A few examples should provide a brief introduction to its use.

For example, it is easy to replace xargs to gzip all html files in the current directory and its subdirectories:

     find . -type f -name '*.html' -print | parallel gzip

If you need to protect special characters such as newlines in file names, use find's -print0 option and parallel's -0 option.

You can use Parallel to move files from the current directory when the number of files is too large to process with one mv invocation:

     printf '%s\n' * | parallel mv {} destdir

As you can see, the {} is replaced with each line read from standard input. While using ls will work in most instances, it is not sufficient to deal with all filenames. printf is a shell builtin, and therefore is not subject to the kernel's limit on the number of arguments to a program, so you can use * (but see below about the dotglob shell option). If you need to accommodate special characters in filenames, you can use

     printf '%s\0' * | parallel -0 mv {} destdir

as alluded to above.

This will run as many mv commands as there are files in the current directory. You can emulate a parallel xargs by adding the -X option:

     printf '%s\0' * | parallel -0 -X mv {} destdir

(You may have to modify the pattern if you have the dotglob option enabled.)

GNU Parallel can replace certain common idioms that operate on lines read from a file (in this case, filenames listed one per line):

     	while IFS= read -r x; do
     		do-something1 "$x" "config-$x"
     		do-something2 < "$x"
     	done < file | process-output

with a more compact syntax reminiscent of lambdas:

     cat list | parallel "do-something1 {} config-{} ; do-something2 < {}" |
                process-output

Parallel provides a built-in mechanism to remove filename extensions, which lends itself to batch file transformations or renaming:

     ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}"

This will recompress all files in the current directory with names ending in .gz using bzip2, running one job per CPU (-j+0) in parallel. (We use ls for brevity here; using find as above is more robust in the face of filenames containing unexpected characters.) Parallel can take arguments from the command line; the above can also be written as

     parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

If a command generates output, you may want to preserve the input order in the output.  For instance, the following command

     {
         echo foss.org.my ;
         echo debian.org ;
         echo freenetproject.org ;
     } | parallel traceroute

will display as output the traceroute invocation that finishes first. Adding the -k option

     {
         echo foss.org.my ;
         echo debian.org ;
         echo freenetproject.org ;
     } | parallel -k traceroute

will ensure that the output of traceroute foss.org.my is displayed first.

Finally, Parallel can be used to run a sequence of shell commands in parallel, similar to cat file | bash. It is not uncommon to take a list of filenames, create a series of shell commands to operate on them, and feed that list of commands to a shell. Parallel can speed this up.  Assuming that file contains a list of shell commands, one per line,

     parallel -j 10 < file

will evaluate the commands using the shell (since no explicit command is supplied as an argument), in blocks of ten shell jobs at a time.

Shell Functions
===============

Shell functions are a way to group commands for later execution using a single name for the group.  They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.

Functions are declared using this syntax:

     FNAME () COMPOUND-COMMAND [ REDIRECTIONS ]

or

     function FNAME [()] COMPOUND-COMMAND [ REDIRECTIONS ]

This defines a shell function named FNAME.  The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The BODY of the function is the compound command COMPOUND-COMMAND (see Compound Commands). That command is usually a LIST enclosed between { and }, but may be any compound command listed above, with one exception: If the function reserved word is used, but the parentheses are not supplied, the braces are required. COMPOUND-COMMAND is executed whenever FNAME is specified as the name of a command. When the shell is in POSIX mode (see Bash POSIX Mode), FNAME must be a valid shell NAME and may not be the same as one of the special builtins (see Special Builtins). In default mode, a function name can be any unquoted shell word that does not contain $. Any redirections (see Redirections) associated with the shell function are performed when the function is executed. A function definition may be deleted using the -f option to the unset builtin (see Bourne Shell Builtins).

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.

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 blanks 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. Also, when using the braces, the LIST must be terminated by a semicolon, a &, or a newline.

When a function is executed, the arguments to the function become the positional parameters during its execution (see Positional Parameters). The special parameter # that expands to the number of positional parameters is updated to reflect the change. Special parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is executing.

All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the DEBUG and RETURN traps are not inherited unless the function has been given the trace attribute using the declare builtin or the -o functrace option has been enabled with the set builtin, (in which case all functions inherit the DEBUG and RETURN traps), and the ERR trap is not inherited unless the -o errtrace shell option has been enabled. See Bourne Shell Builtins, for the description of the trap builtin.

The FUNCNEST 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.

If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the RETURN trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter # are restored to the values they had prior to the function's execution.  If a numeric argument is given to return, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the return.

Variables local to the function may be declared with the local builtin.  These variables are visible only to the function and the commands it invokes.  This is particularly important when a shell function calls other functions.

Local variables "shadow" variables with the same name declared at previous scopes.  For instance, a local variable declared in a function hides a global variable of the same name: references and assignments refer to the local variable, leaving the global variable unmodified. When the function returns, the global variable is once again visible.

The shell uses DYNAMIC SCOPING to control a variable'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.

For example, if a variable VAR is declared as local in function FUNC1, and FUNC1 calls another function FUNC2, references to VAR made from within FUNC2 will resolve to the local variable VAR from FUNC1, shadowing any global variable named VAR.

The following script demonstrates this behavior. When executed, the script displays

     In func2, var = func1 local

     func1()
     {
         local var='func1 local'
         func2
     }

     func2()
     {
         echo "In func2, var = $var"
     }

     var=global
     func1

The unset builtin also acts using the same dynamic scope: if a variable is local to the current scope, unset will unset 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 will remain so 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 will become visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed will become visible.

Function names and definitions may be listed with the -f option to the declare (typeset) builtin command (see Bash Builtins). The -F option to declare or typeset will list the function names only (and optionally the source file and line number, if the extdebug shell option is enabled). Functions may be exported so that subshells automatically have them defined with the -f option to the export builtin (see Bourne Shell Builtins).

Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, no limit is placed on the number of recursive  calls.

Shell Parameters
================

A PARAMETER is an entity that stores values. It can be a name, a number, or one of the special characters listed below. A VARIABLE is a parameter denoted by a name. A variable has a VALUE and zero or more ATTRIBUTES. Attributes are assigned using the declare builtin command (see the description of the declare builtin in Bash Builtins).

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 unset builtin command.

A variable may be assigned to by a statement of the form

     NAME=[VALUE]

If VALUE is not given, the variable is assigned the null string.  All VALUEs undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below).  If the variable has its integer attribute set, then VALUE is evaluated as an arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion). Word splitting is not performed, with the exception of "$@" as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (DECLARATION commands). When in POSIX mode (see Bash POSIX Mode), these builtins may appear in a command after one or more instances of the command builtin and retain these assignment statement properties.

In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the += operator can be used to append to or add to the variable's previous value. This includes arguments to builtin commands such as declare that accept assignment statements (DECLARATION commands). When += is applied to a variable for which the INTEGER attribute has been set, VALUE is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When += is applied to an array variable using compound assignment (see Arrays), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays),  or added as additional key-value pairs in an associative array. When applied to a string-valued variable, VALUE is expanded and appended to the variable's value.

A variable can be assigned the NAMEREF attribute using the -n option to the declare or local builtin commands (see Bash Builtins) to create a NAMEREF, 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'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

     declare -n ref=$1

inside the function creates a nameref variable REF whose value is the variable name passed as the first argument. References and assignments to REF, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1.

If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be 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 -n option to the unset builtin (see Bourne Shell Builtins). Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable will be unset.

Positional Parameters
---------------------

A POSITIONAL PARAMETER is a parameter denoted by one or more digits, other than the single digit 0.  Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameter N may be referenced as ${N}, or as $N when N consists of a single digit. Positional parameters may not be assigned to with assignment statements. The set and shift builtins are used to set and unset them (see Shell Builtin Commands). The positional parameters are temporarily replaced when a shell function is executed (see Shell Functions).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.

Special Parameters
------------------

The shell treats several parameters specially.  These parameters may only be referenced; assignment to them is not allowed.

*
     ($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where it is performed, those words are subject to further word splitting and filename expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable.  That is, "$*" is equivalent to "$1C$2C...", where C is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.

@
     ($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word.  That is, "$@" is equivalent to "$1" "$2" .... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).

#
     ($#) Expands to the number of positional parameters in decimal.

?
     ($?) Expands to the exit status of the most recently executed foreground pipeline.

-
     ($-, a hyphen.)  Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option).

$
     ($$) Expands to the process ID of the shell.  In a () subshell, it expands to the process ID of the invoking shell, not the subshell.

!
     ($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin (see Job Control Builtins).

0
     ($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 Shell Scripts), $0 is set to the name of that file. If Bash is started with the -c option (see Invoking Bash), then $0 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.

Shell Expansions
================

Expansion is performed on the command line after it has been split into tokens.  There are seven kinds of expansion performed:

  * brace expansion

  * tilde expansion

  * parameter and variable expansion

  * command substitution

  * arithmetic expansion

  * word splitting

  * filename expansion

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; and filename expansion.

On systems that can support it, there is an additional expansion available: PROCESS SUBSTITUTION. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.

After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (QUOTE REMOVAL).

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 "$@" and $* (see Special Parameters), and "${NAME[@]}" and ${NAME[*]} (see Arrays).

After all expansions, quote removal (see Quote Removal) is performed.

Brace Expansion
---------------

Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to FILENAME EXPANSION (see Filename Expansion), but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional PREAMBLE, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional POSTSCRIPT. 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.

Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,

     bash$ echo a{d,c,b}e
     ade ace abe

A sequence expression takes the form {X..Y[..INCR]}, where X and Y are either integers or single characters, and INCR, an optional increment, is an integer. When integers are supplied, the expression expands to each number between X and Y, inclusive. Supplied integers may be prefixed with 0 to force each term to have the same width. When either X or Y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands to each character lexicographically between X and Y, inclusive, using the default C locale. Note that both X and Y must be of the same type. When the increment is supplied, it is used as the difference between each term.  The default increment is 1 or -1 as appropriate.

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.

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.

A { or , may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion, and inhibits brace expansion until the closing }.

This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:

     mkdir /usr/local/src/bash/{old,new,dist,bugs}

or

     chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Tilde Expansion
---------------

If a word begins with an unquoted tilde character (~), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a TILDE-PREFIX. 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 LOGIN NAME. If this login name is the null string, the tilde is replaced with the value of the HOME shell variable. If HOME is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.

If the tilde-prefix is ~+, the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is ~-, the value of the shell variable OLDPWD, if it is set, is substituted.

If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a + or a -, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the dirs builtin invoked with the characters following tilde in the tilde-prefix as an argument (see The Directory Stack). If the tilde-prefix, sans the tilde, consists of a number without a leading + or -, + is assumed.

If the login name is invalid, or the tilde expansion fails, the word is left unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately following a : or the first =. In these cases, tilde expansion is also performed. Consequently, one may use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

~
     The value of $HOME

~/foo
     $HOME/foo

~fred/foo
     The subdirectory foo of the home directory of the user fred

~+/foo
     $PWD/foo

~-/foo
     ${OLDPWD-'~-'}/foo

~N
     The string that would be displayed by dirs +N

~+N
     The string that would be displayed by dirs +N

~-N
     The string that would be displayed by dirs -N

Bash also performs tilde expansion on words satisfying the conditions of variable assignments (see Shell Parameters) when they appear as arguments to simple commands. Bash does not do this, except for the DECLARATION commands listed above, when in POSIX mode.

Shell Parameter Expansion
-------------------------

The $ 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.

When braces are used, the matching ending brace is the first } not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

The basic form of parameter expansion is ${PARAMETER}. The value of PARAMETER is substituted. The PARAMETER is a shell parameter as described above (see Shell Parameters) or an array reference (see Arrays). The braces are required when PARAMETER is a positional parameter with more than one digit, or when PARAMETER is followed by a character that is not to be interpreted as part of its name.

If the first character of PARAMETER is an exclamation point (!), and PARAMETER is not a NAMEREF, it introduces a level of indirection. Bash uses the value formed by expanding the rest of PARAMETER as the new PARAMETER; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original PARAMETER. This is known as indirect expansion. The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If PARAMETER is a nameref, this expands to the name of the variable referenced by PARAMETER instead of performing the complete indirect expansion. The exceptions to this are the expansions of ${!PREFIX*} and ${!NAME[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

In each of the cases below, WORD is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the form described below (e.g., :-), 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 PARAMETER's existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

${PARAMETER:-WORD}
     If PARAMETER is unset or null, the expansion of WORD is substituted.  Otherwise, the value of PARAMETER is substituted.

${PARAMETER:=WORD}
     If PARAMETER is unset or null, the expansion of WORD is assigned to PARAMETER. The value of PARAMETER is then substituted. Positional parameters and special parameters may not be assigned to in this way.

${PARAMETER:?WORD}
     If PARAMETER is null or unset, the expansion of WORD (or a message to that effect if WORD is not present) is written to the standard error and the shell, if it is not interactive, exits.  Otherwise, the value of PARAMETER is substituted.

${PARAMETER:+WORD}
     If PARAMETER is null or unset, nothing is substituted, otherwise the expansion of WORD is substituted.

${PARAMETER:OFFSET}
${PARAMETER:OFFSET:LENGTH}
     This is referred to as Substring Expansion. It expands to up to LENGTH characters of the value of PARAMETER starting at the character specified by OFFSET. If PARAMETER is @, an indexed array subscripted by @ or *, or an associative array name, the results differ as described below. If LENGTH is omitted, it expands to the substring of the value of PARAMETER starting at the character specified by OFFSET and extending to the end of the value. LENGTH and OFFSET are arithmetic expressions (see Shell Arithmetic).

     If OFFSET evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of PARAMETER. If LENGTH evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of PARAMETER rather than a number of characters, and the expansion is the characters between OFFSET and that result. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :- expansion.

     Here are some examples illustrating substring expansion on parameters and subscripted arrays:

     $ 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

     If PARAMETER is @, the result is LENGTH positional parameters beginning at OFFSET. A negative OFFSET is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter. It is an expansion error if LENGTH evaluates to a number less than zero.

     The following examples illustrate substring expansion using positional parameters:

     $ 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 < 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}

     If PARAMETER is an indexed array name subscripted by @ or *, the result is the LENGTH members of the array beginning with ${PARAMETER[OFFSET]}. A negative OFFSET is taken relative to one greater than the maximum index of the specified array. It is an expansion error if LENGTH evaluates to a number less than zero.

     These examples show how you can use substring expansion with indexed arrays:

     $ 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 < 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}

     Substring expansion applied to an associative array produces undefined results.

     Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If OFFSET is 0, and the positional parameters are used, $0 is prefixed to the list.

${!PREFIX*}
${!PREFIX@}
     Expands to the names of variables whose names begin with PREFIX, separated by the first character of the IFS special variable. When @ is used and the expansion appears within double quotes, each variable name expands to a separate word.

${!NAME[@]}
${!NAME[*]}
     If NAME is an array variable, expands to the list of array indices (keys) assigned in NAME. If NAME is not an array, expands to 0 if NAME is set and null otherwise. When @ is used and the expansion appears within double quotes, each key expands to a separate word.

${#PARAMETER}
     The length in characters of the expanded value of PARAMETER is substituted. If PARAMETER is * or @, the value substituted is the number of positional parameters. If PARAMETER is an array name subscripted by * or @, the value substituted is the number of elements in the array. If PARAMETER is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of PARAMETER, so negative indices count back from the end of the array, and an index of -1 references the last element.

${PARAMETER#WORD}
${PARAMETER##WORD}
     The WORD is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching).  If the pattern matches the beginning of the expanded value of PARAMETER, then the result of the expansion is the expanded value of PARAMETER with the shortest matching pattern (the # case) or the longest matching pattern (the ## case) deleted. If PARAMETER is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

${PARAMETER%WORD}
${PARAMETER%%WORD}
     The WORD is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching). If the pattern matches a trailing portion of the expanded value of PARAMETER, then the result of the expansion is the value of PARAMETER with the shortest matching pattern (the % case) or the longest matching pattern (the %% case) deleted. If PARAMETER is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

${PARAMETER/PATTERN/STRING}

     The PATTERN is expanded to produce a pattern just as in filename expansion. PARAMETER is expanded and the longest match of PATTERN against its value is replaced with STRING. The match is performed according to the rules described below (see Pattern Matching). If PATTERN begins with /, all matches of PATTERN are replaced with STRING.  Normally only the first match is replaced. If PATTERN begins with #, it must match at the beginning of the expanded value of PARAMETER. If PATTERN begins with %, it must match at the end of the expanded value of PARAMETER. If STRING is null, matches of PATTERN are deleted and the / following PATTERN may be omitted. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. If PARAMETER is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

${PARAMETER^PATTERN}
${PARAMETER^^PATTERN}
${PARAMETER,PATTERN}
${PARAMETER,,PATTERN}
     This expansion modifies the case of alphabetic characters in PARAMETER. The PATTERN is expanded to produce a pattern just as in filename expansion. Each character in the expanded value of PARAMETER is tested against PATTERN, and, if it matches the pattern, its case is converted. The pattern should not attempt to match more than one character. The ^ operator converts lowercase letters matching PATTERN to uppercase; the , operator converts matching uppercase letters to lowercase. The ^^ and ,, expansions convert each matched character in the expanded value; the ^ and , expansions match and convert only the first character in the expanded value. If PATTERN is omitted, it is treated like a ?, which matches every character. If PARAMETER is @ or *, the case modification operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with @ or *, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.

${PARAMETER@OPERATOR}
     The expansion is either a transformation of the value of PARAMETER or information about PARAMETER itself, depending on the value of OPERATOR.  Each OPERATOR is a single letter:

     U
          The expansion is a string that is the value of PARAMETER with lowercase alphabetic characters converted to uppercase.

     u
          The expansion is a string that is the value of PARAMETER with the first character converted to uppercase, if it is alphabetic.

     L
          The expansion is a string that is the value of PARAMETER with uppercase alphabetic characters converted to lowercase.

     Q
          The expansion is a string that is the value of PARAMETER quoted in a format that can be reused as input.

     E
          The expansion is a string that is the value of PARAMETER with backslash escape sequences expanded as with the $'...' quoting mechanism.

     P
          The expansion is a string that is the result of expanding the value of PARAMETER as if it were a prompt string (see Controlling the Prompt).

     A
          The expansion is a string in the form of an assignment statement or declare command that, if evaluated, will recreate PARAMETER with its attributes and value.

     K
          Produces a possibly-quoted version of the value of PARAMETER, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays).

     a
          The expansion is a string consisting of flag values representing PARAMETER's attributes.

     If PARAMETER is @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If PARAMETER is an array variable subscripted with @ or *, the operation is applied to each member of the array in turn, and the expansion is the resultant list.

     The result of the expansion is subject to word splitting and filename expansion as described below.

Command Substitution
--------------------

Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:

     $(COMMAND)

or

     `COMMAND`

Bash performs the expansion by executing COMMAND 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 $(cat FILE) can be replaced by the equivalent but faster $(< FILE).

When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the command substitution. When using the $(COMMAND) form, all characters between the parentheses make up the command; none are treated specially.

Command substitutions may be nested.  To nest when using the backquoted form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.

Arithmetic Expansion
--------------------

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result.  The format for arithmetic expansion is:

     $(( EXPRESSION ))

The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. 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. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.

Process Substitution
--------------------

Process substitution allows a process's input or output to be referred to using a filename. It takes the form of

     <(LIST)

or

     >(LIST)

The process LIST 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. If the >(LIST) form is used, writing to the file will provide input for LIST.  If the <(LIST) form is used, the file passed as an argument should be read to obtain the output of LIST. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection. Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

Word Splitting
--------------

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators. If IFS is unset, or its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and <newline> at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space, tab, and newline are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field.  A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs.

Explicit null arguments ("" or '') 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. If a parameter with no value is expanded within double quotes, a null argument results and is retained and passed to a command as an empty string. When a quoted null argument appears as part of a word whose expansion is non-null, the null argument is removed. That is, the word -d'' becomes -d after word splitting and null argument removal.

Note that if no expansion occurs, no splitting is performed.

Filename Expansion
------------------

After word splitting, unless the -f option has been set (see The Set Builtin), Bash scans each word for the characters *, ?, and [. If one of these characters appears, and is not quoted, then the word is regarded as a PATTERN, and replaced with an alphabetically sorted list of filenames matching the pattern (see Pattern Matching). If no matching filenames are found, and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option is set, and no matches are found, the word is removed. If the failglob shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.

When a pattern is used for filename expansion, the character . at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. The filenames . and .. must always be matched explicitly, even if dotglob is set. In other cases, the . character is not treated specially.

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 Pattern Matching).

See the description of shopt in The Shopt Builtin, for a description of the nocaseglob, nullglob, failglob, and dotglob options.

The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern.  If GLOBIGNORE is set, each matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. If the nocaseglob option is set, the matching against the patterns in GLOBIGNORE is performed without regard to case. The filenames . and .. are always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option, so all other filenames beginning with a . will match. To get the old behavior of ignoring filenames beginning with a ., make .* one of the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is unset.

Pattern Matching
................

Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL 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.

The special pattern characters have the following meanings:

*
     Matches any string, including the null string. When the globstar shell option is enabled, and * is used in a filename expansion context, two adjacent *s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a /, two adjacent *s will match only directories and subdirectories.

?
     Matches any single character.

[...]
     Matches any one of the enclosed characters.  A pair of characters separated by a hyphen denotes a RANGE EXPRESSION; any character that falls between those two characters, inclusive, using the current locale's collating sequence and character set, is matched.  If the first character following the [ is a !  or a ^ then any character not enclosed is matched.  A - may be matched by including it as the first or last character in the set.  A ] may be matched by including it as the first character in the set. The sorting order of characters in range expressions is determined by the current locale and the values of the LC_COLLATE and LC_ALL shell variables, if set.

     For example, in the default C locale, [a-dx-z] is equivalent to [abcdxyz].  Many locales sort characters in dictionary order, and in these locales [a-dx-z] is typically not equivalent to [abcdxyz]; it might be equivalent to [aBbCcDdxXyYz], for example.  To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the LC_COLLATE or LC_ALL environment variable to the value C, or enable the globasciiranges shell option.

     Within [ and ], CHARACTER CLASSES can be specified using the syntax [:CLASS:], where CLASS is one of the following classes defined in the POSIX standard:

          alnum   alpha   ascii   blank   cntrl   digit   graph   lower
          print   punct   space   upper   word    xdigit

     A character class matches any character belonging to that class. The word character class matches letters, digits, and the character _.

     Within [ and ], an EQUIVALENCE CLASS can be specified using the syntax [=C=], which matches all characters with the same collation weight (as defined by the current locale) as the character C.

     Within [ and ], the syntax [.SYMBOL.] matches the collating symbol SYMBOL.

If the extglob shell option is enabled using the shopt builtin, several extended pattern matching operators are recognized. In the following description, a PATTERN-LIST is a list of one or more patterns separated by a |. Composite patterns may be formed using one or more of the following sub-patterns:

?(PATTERN-LIST)
     Matches zero or one occurrence of the given patterns.

*(PATTERN-LIST)
     Matches zero or more occurrences of the given patterns.

+(PATTERN-LIST)
     Matches one or more occurrences of the given patterns.

@(PATTERN-LIST)
     Matches one of the given patterns.

!(PATTERN-LIST)
     Matches anything except one of the given patterns.

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.

Quote Removal
-------------

After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are removed.

Redirections
============

Before a command is executed, its input and output may be REDIRECTED using a special notation interpreted by the shell. Redirection allows commands' 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. Redirection may also be used to 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.

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {VARNAME}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {VARNAME}.  If >&- or <&- is preceded by {VARNAME}, the value of VARNAME defines the file descriptor to close. If {VARNAME} is supplied, the redirection persists beyond the scope of the command, allowing the shell programmer to manage the file descriptor's lifetime manually.

In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0).  If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).

The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant.  For example, the command

     ls > DIRLIST 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file DIRLIST, while the command

     ls 2>&1 > DIRLIST

directs only the standard output to file DIRLIST, because the standard error was made a copy of the standard output before the standard output was redirected to DIRLIST.

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 will use them; otherwise it will emulate them internally with the behavior described below.

/dev/fd/FD
     If FD is a valid integer, file descriptor FD is duplicated.

/dev/stdin
     File descriptor 0 is duplicated.

/dev/stdout
     File descriptor 1 is duplicated.

/dev/stderr
     File descriptor 2 is duplicated.

/dev/tcp/HOST/PORT
     If HOST is a valid hostname or Internet address, and PORT is an integer port number or service name, Bash attempts to open the corresponding TCP socket.

/dev/udp/HOST/PORT
     If HOST is a valid hostname or Internet address, and PORT is an integer port number or service name, Bash attempts to open the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.

Note that the exec builtin command can make redirections take effect in the current shell.

Redirecting Input
-----------------

Redirection of input causes the file whose name results from the expansion of WORD to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.

The general format for redirecting input is:

     [N]<WORD

Redirecting Output
------------------

Redirection of output causes the file whose name results from the expansion of WORD to be opened for writing on file descriptor N, or the standard output (file descriptor 1) if N is not specified.  If the file does not exist it is created; if it does exist it is truncated to zero size.

The general format for redirecting output is:

     [N]>[|]WORD

If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name results from the expansion of WORD exists and is a regular file. If the redirection operator is >|, or the redirection operator is > and the noclobber option is not enabled, the redirection is attempted even if the file named by WORD exists.

Appending Redirected Output
---------------------------

Redirection of output in this fashion causes the file whose name results from the expansion of WORD to be opened for appending on file descriptor N, or the standard output (file descriptor 1) if N is not specified.  If the file does not exist it is created.

The general format for appending output is:

     [N]>>WORD

Redirecting Standard Output and Standard Error
----------------------------------------------

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of WORD.

There are two formats for redirecting standard output and standard error:

     &>WORD

and

     >&WORD

Of the two forms, the first is preferred. This is semantically equivalent to

     >WORD 2>&1

When using the second form, WORD may not expand to a number or -.  If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.

Appending Standard Output and Standard Error
--------------------------------------------

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of WORD.

The format for appending standard output and standard error is:

     &>>WORD

This is semantically equivalent to

     >>WORD 2>&1

(see Duplicating File Descriptors below).

Here Documents
--------------

This type of redirection instructs the shell to read input from the current source until a line containing only WORD (with no trailing blanks) is seen.  All of the lines read up to that point are then used as the standard input (or file descriptor N if N is specified) for a command.

The format of here-documents is:

     [N]<<[-]WORD
             HERE-DOCUMENT
     DELIMITER

No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on WORD.  If any part of WORD is quoted, the DELIMITER is the result of quote removal on WORD, and the lines in the here-document are not expanded. If WORD is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline is ignored, and \ must be used to quote the characters \, $, and `.

If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing DELIMITER. This allows here-documents within shell scripts to be indented in a natural fashion.

Here Strings
------------

A variant of here documents, the format is:

     [N]<<< WORD

The WORD 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 N if N is specified).

Duplicating File Descriptors
----------------------------

The redirection operator

     [N]<&WORD

is used to duplicate input file descriptors. If WORD expands to one or more digits, the file descriptor denoted by N is made to be a copy of that file descriptor. If the digits in WORD do not specify a file descriptor open for input, a redirection error occurs. If WORD evaluates to -, file descriptor N is closed. If N is not specified, the standard input (file descriptor 0) is used.

The operator

     [N]>&WORD

is used similarly to duplicate output file descriptors.  If N is not specified, the standard output (file descriptor 1) is used. If the digits in WORD do not specify a file descriptor open for output, a redirection error occurs. If WORD evaluates to -, file descriptor N is closed. As a special case, if N is omitted, and WORD does not expand to one or more digits or -, the standard output and standard error are redirected as described previously.

Moving File Descriptors
-----------------------

The redirection operator

     [N]<&DIGIT-

moves the file descriptor DIGIT to file descriptor N, or the standard input (file descriptor 0) if N is not specified. DIGIT is closed after being duplicated to N.

Similarly, the redirection operator

     [N]>&DIGIT-

moves the file descriptor DIGIT to file descriptor N, or the standard output (file descriptor 1) if N is not specified.

Opening File Descriptors for Reading and Writing
------------------------------------------------

The redirection operator

     [N]<>WORD

causes the file whose name is the expansion of WORD to be opened for both reading and writing on file descriptor N, or on file descriptor 0 if N is not specified.  If the file does not exist, it is created.

Executing Commands
==================

Simple Command Expansion
------------------------

When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right, in the following order.

 1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.

 2. The words that are not variable assignments or redirections are expanded (see Shell Expansions). 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.

 3. Redirections are performed as described above (see Redirections).

 4. The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment.  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.

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.

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 status of zero.

Command Search and Execution
----------------------------

After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.

 1. 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 Shell Functions.

 2. 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.

 3. If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of $PATH 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 PATH searches (see the description of hash in Bourne Shell Builtins). A full search of the directories in $PATH is performed 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 command_not_found_handle. If that function exists, it is invoked in a separate execution environment with the original command and the original command's arguments as its arguments, and the function'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.

 4. 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.

 5. 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 SHELL SCRIPT and the shell executes it as described in Shell Scripts.

 6. If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status.

Command Execution Environment
-----------------------------

The shell has an EXECUTION ENVIRONMENT, which consists of the following:

  * open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin

  * the current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation

  * the file creation mode mask as set by umask or inherited from the shell's parent

  * current traps set by trap

  * shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the environment

  * shell functions defined during execution or inherited from the shell's parent in the environment

  * options enabled at invocation (either by default or with command-line arguments) or by set

  * options enabled by shopt (see The Shopt Builtin)

  * shell aliases defined with alias (see Aliases)

  * various process IDs, including those of background jobs (see Lists), the value of $$, and the value of $PPID

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.

  * the shell's open files, plus any modifications and additions specified by redirections to the command

  * the current working directory

  * the file creation mode mask

  * shell variables and functions marked for export, along with variables exported for the command, passed in the environment (see Environment)

  * traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the shell's execution environment.

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 are also executed in a subshell environment.  Changes made to the subshell environment cannot affect the shell's execution environment.

Subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell.  When not in POSIX mode, Bash clears the -e option in such subshells.

If a command is followed by a & and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.

Environment
-----------

When a program is invoked it is given an array of strings called the ENVIRONMENT. This is a list of name-value pairs, of the form name=value.

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 EXPORT to child processes.  Executed commands inherit the environment. The export and declare -x commands allow parameters and functions to be added to and deleted from the environment.  If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old.  The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the unset and export -n commands, plus any additions via the export and declare -x commands.

The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in Shell Parameters. These assignment statements affect only the environment seen by that command.

If the -k option is set (see The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.

When Bash invokes an external command, the variable $_ is set to the full pathname of the command and passed to that command in its environment.

Exit Status
-----------

The exit status of an executed command is the value returned by the WAITPID 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.

For the shell's purposes, a command which exits with a zero exit status has succeeded. 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. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.

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.

If a command fails because of an error during expansion or redirection, the exit status is greater than zero.

The exit status is used by the Bash conditional commands (see Conditional Constructs) and some of the list constructs (see Lists).

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.

Signals
-------

When Bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill 0 does not kill an interactive shell), and SIGINT is caught and handled (so that the wait builtin is interruptible). When Bash receives a SIGINT, it breaks out of any executing loops. In all cases, Bash ignores SIGQUIT. If job control is in effect (see Job Control), Bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from sending the SIGHUP signal to a particular job, it should be removed from the jobs table with the disown builtin (see Job Control Builtins) or marked to not receive SIGHUP using disown -h.

If the  huponexit shell option has been set with shopt (see The Shopt Builtin), Bash sends a SIGHUP to all jobs when an interactive login shell exits.

If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait builtin, the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.

Shell Scripts
=============

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 -c nor -s option is supplied (see Invoking Bash), Bash reads and executes commands from the file, then exits.  This mode of operation creates a non-interactive shell.  The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.

When Bash runs a shell script, it sets the special parameter 0 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.

A shell script may be made executable by using the chmod command to turn on the execute bit.  When Bash finds such a file while searching the $PATH for a command, it spawns a subshell to execute it.  In other words, executing

     filename ARGUMENTS

is equivalent to executing

     bash filename ARGUMENTS

if filename 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 hash in Bourne Shell Builtins) are retained by the child.

Most versions of Unix make this a part of the operating system's command execution mechanism.  If the first line of a script begins with the two characters #!, 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, awk, Perl, or some other interpreter and write the rest of the script file in that language.

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's not portable to assume that using more than one argument will work.

Bash scripts often begin with #! /bin/bash (assuming that Bash has been installed in /bin), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell. It's a common idiom to use env to find bash even if it's been installed in another directory: #!/usr/bin/env bash will find the first occurrence of bash in $PATH.

Shell Builtin Commands
**********************

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 Simple Commands), 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.

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.

Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Job Control Builtins), the directory stack (see Directory Stack Builtins), the command history (see Bash History Builtins), and the programmable completion facilities (see Programmable Completion Builtins).

Many of the builtins have been extended by POSIX or Bash.

Unless otherwise noted, each builtin command documented as accepting options preceded by - accepts -- to signify the end of the options. The :, true, false, and test/[ builtins do not accept options and do not treat -- specially. The exit, logout, return, break, continue, let, and shift builtins accept and process arguments beginning with - without requiring --. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with - as invalid options and require -- to prevent this interpretation.

Bourne Shell Builtins
=====================

The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard.

:    (a colon)

          : [ARGUMENTS]

     Do nothing beyond expanding ARGUMENTS and performing redirections. The return status is zero.

.    (a period)

          . FILENAME [ARGUMENTS]

     Read and execute commands from the FILENAME argument in the current shell context.  If FILENAME does not contain a slash, the PATH variable is used to find FILENAME. When Bash is not in POSIX mode, the current directory is searched if FILENAME is not found in $PATH. If any ARGUMENTS are supplied, they become the positional parameters when FILENAME is executed.  Otherwise the positional parameters are unchanged. If the -T option is enabled, source inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and restored around the call to source, and source unsets the DEBUG trap while it executes. If -T is not set, and the sourced file changes the DEBUG trap, the new value is retained when source completes. The return status is the exit status of the last command executed, or zero if no commands are executed.  If FILENAME is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source.

break

          break [N]

     Exit from a for, while, until, or select loop. If N is supplied, the Nth enclosing loop is exited. N must be greater than or equal to 1. The return status is zero unless N is not greater than or equal to 1.

cd

          cd [-L|[-P [-e]] [-@] [DIRECTORY]

     Change the current working directory to DIRECTORY. If DIRECTORY is not supplied, the value of the HOME shell variable is used. Any additional arguments following DIRECTORY are ignored. If the shell variable CDPATH exists, it is used as a search path: each directory name in CDPATH is searched for DIRECTORY, with alternative directory names in CDPATH separated by a colon (:). If DIRECTORY begins with a slash, CDPATH is not used.

     The -P option means to not follow symbolic links: symbolic links are resolved while cd is traversing DIRECTORY and before processing an instance of .. in DIRECTORY.

     By default, or when the -L option is supplied, symbolic links in DIRECTORY are resolved after cd processes an instance of .. in DIRECTORY.

     If .. appears in DIRECTORY, it is processed by removing the immediately preceding pathname component, back to a slash or the beginning of DIRECTORY.

     If the -e option is supplied with -P and the current working directory cannot be successfully determined after a successful directory change, cd will return an unsuccessful status.

     On systems that support it, the -@ option presents the extended attributes associated with a file as a directory.

     If DIRECTORY is -, it is converted to $OLDPWD before the directory change is attempted.

     If a non-empty directory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output.

     The return status is zero if the directory is successfully changed, non-zero otherwise.

continue

          continue [N]

     Resume the next iteration of an enclosing for, while, until, or select loop. If N is supplied, the execution of the Nth enclosing loop is resumed. N must be greater than or equal to 1. The return status is zero unless N is not greater than or equal to 1.

eval

          eval [ARGUMENTS]

     The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of eval. If there are no arguments or only empty arguments, the return status is zero.

exec

          exec [-cl] [-a NAME] [COMMAND [ARGUMENTS]]

     If COMMAND is supplied, it replaces the shell without creating a new process. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to COMMAND. This is what the login program does. The -c option causes COMMAND to be executed with an empty environment. If -a is supplied, the shell passes NAME as the zeroth argument to COMMAND. If COMMAND cannot be executed for some reason, a non-interactive shell exits, unless the execfail shell option is enabled.  In that case, it returns failure. An interactive shell returns failure if the file cannot be executed. A subshell exits unconditionally if exec fails. If no COMMAND is 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.

exit

          exit [N]

     Exit the shell, returning a status of N to the shell's parent. If N is omitted, the exit status is that of the last command executed. Any trap on EXIT is executed before the shell terminates.

export

          export [-fn] [-p] [NAME[=VALUE]]

     Mark each NAME to be passed to child processes in the environment.  If the -f option is supplied, the NAMEs refer to shell functions; otherwise the names refer to shell variables. The -n option means to no longer mark each NAME for export. If no NAMES are supplied, or if the -p option is given, a list of names of all exported variables is displayed. The -p option displays output in a form that may be reused as input. If a variable name is followed by =VALUE, the value of the variable is set to VALUE.

     The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a shell function.

getopts

          getopts OPTSTRING NAME [ARG ...]

     getopts is used by shell scripts to parse positional parameters. OPTSTRING 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 (:) and question mark (?) may not be used as option characters. Each time it is invoked, getopts places the next option in the shell variable NAME, initializing NAME if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used.

     When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and NAME is set to ?.

     getopts normally parses the positional parameters, but if more arguments are supplied as ARG values, getopts parses those instead.

     getopts can report errors in two ways.  If the first character of OPTSTRING is a colon, SILENT error reporting is used.  In normal operation, diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable OPTERR is set to 0, no error messages will be displayed, even if the first character of optstring is not a colon.

     If an invalid option is seen, getopts places ? into NAME and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed.

     If a required argument is not found, and getopts is not silent, a question mark (?) is placed in NAME, OPTARG is unset, and a diagnostic message is printed. If getopts is silent, then a colon (:) is placed in NAME and OPTARG is set to the option character found.

hash

          hash [-r] [-p FILENAME] [-dt] [NAME]

     Each time hash is invoked, it remembers the full pathnames of the commands specified as NAME arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in $PATH. Any previously-remembered pathname is discarded. The -p option inhibits the path search, and FILENAME is used as the location of NAME. The -r option causes the shell to forget all remembered locations. The -d option causes the shell to forget the remembered location of each NAME. If the -t option is supplied, the full pathname to which each NAME corresponds is printed.  If multiple NAME arguments are supplied with -t, the NAME is printed before the hashed full pathname. The -l option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only -l is supplied, information about remembered commands is printed. The return status is zero unless a NAME is not found or an invalid option is supplied.

pwd

          pwd [-LP]

     Print the absolute pathname of the current working directory. If the -P option is supplied, the pathname printed will not contain symbolic links. If the -L 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.

readonly

          readonly [-aAf] [-p] [NAME[=VALUE]] ...

     Mark each NAME as readonly. The values of these names may not be changed by subsequent assignment. If the -f option is supplied, each NAME refers to a shell function. The -a option means each NAME refers to an indexed array variable; the -A option means each NAME refers to an associative array variable. If both options are supplied, -A takes precedence. If no NAME arguments are given, or if the -p option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =VALUE, the value of the variable is set to VALUE. The return status is zero unless an invalid option is supplied, one of the NAME arguments is not a valid shell variable or function name, or the -f option is supplied with a name that is not a shell function.

return

          return [N]

     Cause a shell function to stop executing and return the value N to its caller. If N is not supplied, the return value is the exit status of the last command executed in the function. If return is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If return is executed during a DEBUG trap, the last command used to determine the status is the last command executed by the trap handler before return was invoked. return may also be used to terminate execution of a script being executed with the . (source) builtin, returning either N or the exit status of the last command executed within the script as the exit status of the script. If N is supplied, the return value is its least significant 8 bits. Any command associated with the RETURN trap is executed before execution resumes after the function or script. The return status is non-zero if return is supplied a non-numeric argument or is used outside a function and not during the execution of a script by . or source.

shift

          shift [N]

     Shift the positional parameters to the left by N. The positional parameters from N+1 ... $# are renamed to $1 ... $#-N. Parameters represented by the numbers $# down to $#-N+1 are unset. N must be a non-negative number less than or equal to $#. If N is zero or greater than $#, the positional parameters are not changed. If N is not supplied, it is assumed to be 1. The return status is zero unless N is greater than $# or less than zero, non-zero otherwise.

test
[

          test EXPR

     Evaluate a conditional expression EXPR 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 Bash Conditional Expressions. test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options.

     When the [ form is used, the last argument to the command must be a ].

     Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. Operator precedence is used when there are five or more arguments.

     ! EXPR
          True if EXPR is false.

     ( EXPR )
          Returns the value of EXPR. This may be used to override the normal precedence of operators.

     EXPR1 -a EXPR2
          True if both EXPR1 and EXPR2 are true.

     EXPR1 -o EXPR2
          True if either EXPR1 or EXPR2 is true.

     The test and [ builtins evaluate conditional expressions using a set of rules based on the number of arguments.

     0 arguments
          The expression is false.

     1 argument
          The expression is true if, and only if, the argument is not null.

     2 arguments
          If the first argument is !, 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 Bash Conditional Expressions), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false.

     3 arguments
          The following conditions are applied in the order listed.

           1. If the second argument is one of the binary conditional operators (see Bash Conditional Expressions), the result of the expression is the result of the binary test using the first and third arguments as operands. The -a and -o operators are considered binary operators when there are three arguments.

           2. If the first argument is !, the value is the negation of the two-argument test using the second and third arguments.

           3. If the first argument is exactly ( and the third argument is exactly ), the result is the one-argument test of the second argument.

           4. Otherwise, the expression is false.

     4 arguments
          If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.

     5 or more arguments
          The expression is parsed and evaluated according to precedence using the rules listed above.

     When used with test or [, the < and > operators sort lexicographically using ASCII ordering.

times

          times

     Print out the user and system times used by the shell and its children. The return status is zero.

trap

          trap [-lp] [ARG] [SIGSPEC ...]

     The commands in ARG are to be read and executed when the shell receives signal SIGSPEC.  If ARG is absent (and there is a single SIGSPEC) or equal to -, each specified signal's disposition is reset to the value it had when the shell was started. If ARG is the null string, then the signal specified by each SIGSPEC is ignored by the shell and commands it invokes. If ARG is not present and -p has been supplied, the shell displays the trap commands associated with each SIGSPEC. If no arguments are supplied, or only -p is given, trap prints the list of commands associated with each signal number in a form that may be reused as shell input. The -l option causes the shell to print a list of signal names and their corresponding numbers. Each SIGSPEC is either a signal name or a signal number. Signal names are case insensitive and the SIG prefix is optional.

     If a SIGSPEC is 0 or EXIT, ARG is executed when the shell exits. If a SIGSPEC is DEBUG, the command ARG is executed before every simple command, for command, case command, select command, every arithmetic for command, and before the first command executes in a shell function. Refer to the description of the extdebug option to the shopt builtin (see The Shopt Builtin) for details of its effect on the DEBUG trap. If a SIGSPEC is RETURN, the command ARG is executed each time a shell function or a script executed with the . or source builtins finishes executing.

     If a SIGSPEC is ERR, the command ARG 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 ERR trap is not executed if the failed command is part of the command list immediately following an until or while keyword, part of the test following the if or elif reserved words, part of a command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return status is being inverted using !. These are the same conditions obeyed by the errexit (-e) option.

     Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created.

     The return status is zero unless a SIGSPEC does not specify a valid signal.

umask

          umask [-p] [-S] [MODE]

     Set the shell process's file creation mask to MODE.  If MODE 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 chmod command.  If MODE is omitted, the current value of the mask is printed.  If the -S option is supplied without a MODE argument, the mask is printed in a symbolic format. If the  -p option is supplied, and MODE 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 MODE argument is supplied, and non-zero otherwise.

     Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from 7.  Thus, a umask of 022 results in permissions of 755.

unset

          unset [-fnv] [NAME]

     Remove each variable or function NAME. If the -v option is given, each NAME refers to a shell variable and that variable is removed. If the -f option is given, the NAMEs refer to shell functions, and the function definition is removed. If the -n option is supplied, and NAME is a variable with the NAMEREF attribute, NAME will be unset rather than the variable it references. -n has no effect if the -f option is supplied. If no options are supplied, each NAME 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. 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 NAME is readonly.

Bash Builtin Commands
=====================

This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX standard.

alias

          alias [-p] [NAME[=VALUE] ...]

     Without arguments or with the -p option, alias prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, an alias is defined for each NAME whose VALUE is given.  If no VALUE is given, the name and value of the alias is printed. Aliases are described in Aliases.

bind

          bind [-m KEYMAP] [-lpsvPSVX]
          bind [-m KEYMAP] [-q FUNCTION] [-u FUNCTION] [-r KEYSEQ]
          bind [-m KEYMAP] -f FILENAME
          bind [-m KEYMAP] -x KEYSEQ:SHELL-COMMAND
          bind [-m KEYMAP] KEYSEQ:FUNCTION-NAME
          bind [-m KEYMAP] KEYSEQ:READLINE-COMMAND

     Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument;  e.g., "\C-x\C-r":re-read-init-file.

     Options, if supplied, have the following meanings:

     -m KEYMAP
          Use KEYMAP as the keymap to be affected by the subsequent bindings.  Acceptable KEYMAP names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command (vi-move is also a synonym); emacs is equivalent to emacs-standard.

     -l
          List the names of all Readline functions.

     -p
          Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file.

     -P
          List current Readline function names and bindings.

     -v
          Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file.

     -V
          List current Readline variable names and values.

     -s
          Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file.

     -S
          Display Readline key sequences bound to macros and the strings they output.

     -f FILENAME
          Read key bindings from FILENAME.

     -q FUNCTION
          Query about which keys invoke the named FUNCTION.

     -u FUNCTION
          Unbind all keys bound to the named FUNCTION.

     -r KEYSEQ
          Remove any current binding for KEYSEQ.

     -x KEYSEQ:SHELL-COMMAND
          Cause SHELL-COMMAND to be executed whenever KEYSEQ is entered. When SHELL-COMMAND is executed, the shell sets the READLINE_LINE variable to the contents of the Readline line buffer and the READLINE_POINT and READLINE_MARK variables to the current location of the insertion point and the saved insertion point (the MARK), respectively. If the executed command changes the value of any of READLINE_LINE, READLINE_POINT, or READLINE_MARK, those new values will be reflected in the editing state.

     -X
          List all key sequences bound to shell commands and the associated commands in a format that can be reused as input.

     The return status is zero unless an invalid option is supplied or an error occurs.

builtin

          builtin [SHELL-BUILTIN [ARGS]]

     Run a shell builtin, passing it ARGS, 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 SHELL-BUILTIN is not a shell builtin command.

caller

          caller [EXPR]

     Returns the context of any active subroutine call (a shell function or a script executed with the . or source builtins).

     Without EXPR, caller displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as EXPR, caller 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.

     The return value is 0 unless the shell is not executing a subroutine call or EXPR does not correspond to a valid position in the call stack.

command

          command [-pVv] COMMAND [ARGUMENTS ...]

     Runs COMMAND with ARGUMENTS ignoring any shell function named COMMAND. Only shell builtin commands or commands found by searching the PATH are executed. If there is a shell function named ls, running command ls within the function will execute the external command ls instead of calling the function recursively. The -p option means to use a default value for PATH that is guaranteed to find all of the standard utilities. The return status in this case is 127 if COMMAND cannot be found or an error occurred, and the exit status of COMMAND otherwise.

     If either the -V or -v option is supplied, a description of COMMAND is printed.  The -v option causes a single word indicating the command or file name used to invoke COMMAND to be displayed; the -V option produces a more verbose description.  In this case, the return status is zero if COMMAND is found, and non-zero if not.

declare

          declare [-aAfFgiIlnrtux] [-p] [NAME[=VALUE] ...]

     Declare variables and give them attributes.  If no NAMEs are given, then display the values of variables instead.

     The -p option will display the attributes and values of each NAME. When -p is used with NAME arguments, additional options, other than -f and -F, are ignored.

     When -p is supplied without NAME arguments, declare will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with -p, declare will display the attributes and values of all shell variables.  The -f option will restrict the display to shell functions.

     The -F option inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug shell option is enabled using shopt (see The Shopt Builtin), the source file name and line number where each NAME is defined are displayed as well. -F implies -f.

     The -g option forces variables to be created or modified at the global scope, even when declare is executed in a shell function. It is ignored in all other cases.

     The -I option causes local variables to inherit the attributes (except the NAMEREF attribute) and value of any existing variable with the same NAME at a surrounding scope. If there is no existing variable, the local variable is initially unset.

     The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:

     -a
          Each NAME is an indexed array variable (see Arrays).

     -A
          Each NAME is an associative array variable (see Arrays).

     -f
          Use function names only.

     -i
          The variable is to be treated as an integer; arithmetic evaluation (see Shell Arithmetic) is performed when the variable is assigned a value.

     -l
          When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.

     -n
          Give each NAME the NAMEREF attribute, making it a name reference to another variable. That other variable is defined by the value of NAME. All references, assignments, and attribute modifications to NAME, except for those using or changing the -n attribute itself, are performed on the variable referenced by NAME's value. The nameref attribute cannot be applied to array variables.

     -r
          Make NAMEs readonly.  These names cannot then be assigned values by subsequent assignment statements or unset.

     -t
          Give each NAME the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling shell. The trace attribute has no special meaning for variables.

     -u
          When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.

     -x
          Mark each NAME for export to subsequent commands via the environment.

     Using + instead of - turns off the attribute instead, with the exceptions that +a and +A may not be used to destroy array variables and +r will not remove the readonly attribute. When used in a function, declare makes each NAME local, as with the local command, unless the -g option is used. If a variable name is followed by =VALUE, the value of the variable is set to VALUE.

     When using -a or -A and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments.

     The return status is zero unless an invalid option is encountered, an attempt is made to define a function using -f foo=bar, 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 Arrays), one of the NAMES 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 -f.

echo

          echo [-neE] [ARG ...]

     Output the ARGs, separated by spaces, terminated with a newline. The return status is 0 unless a write error occurs. If -n is specified, the trailing newline is suppressed. If the -e option is given, interpretation of the following backslash-escaped characters is enabled. The -E option disables the interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo shell option may be used to dynamically determine whether or not echo expands these escape characters by default. echo does not interpret -- to mean the end of options.

     echo interprets the following escape sequences:

     \a
          alert (bell)

     \b
          backspace

     \c
          suppress further output

     \e
     \E
          escape

     \f
          form feed

     \n
          new line

     \r
          carriage return

     \t
          horizontal tab

     \v
          vertical tab

     \\
          backslash

     \0NNN
          the eight-bit character whose value is the octal value NNN (zero to three octal digits)

     \xHH
          the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)

     \uHHHH
          the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)

     \UHHHHHHHH
          the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)

enable

          enable [-a] [-dnps] [-f FILENAME] [NAME ...]

     Enable and disable builtin shell commands. Disabling a builtin allows a disk command 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 disk commands. If -n is used, the NAMEs become disabled.  Otherwise NAMEs are enabled.  For example, to use the test binary found via $PATH instead of the shell builtin version, type enable -n test.

     If the -p option is supplied, or no NAME arguments appear, a list of shell builtins is printed.  With no other arguments, the list consists of all enabled shell builtins. The -a option means to list each builtin with an indication of whether or not it is enabled.

     The -f option means to load the new builtin command NAME from shared object FILENAME, on systems that support dynamic loading. The -d option will delete a builtin loaded with -f.

     If there are no options, a list of the shell builtins is displayed. The -s option restricts enable to the POSIX special builtins.  If -s is used with -f, the new builtin becomes a special builtin (see Special Builtins).

     The return status is zero unless a NAME is not a shell builtin or there is an error loading a new builtin from a shared object.

help

          help [-dms] [PATTERN]

     Display helpful information about builtin commands. If PATTERN is specified, help gives detailed help on all commands matching PATTERN, otherwise a list of the builtins is printed.

     Options, if supplied, have the following meanings:

     -d
          Display a short description of each PATTERN

     -m
          Display the description of each PATTERN in a manpage-like format

     -s
          Display only a short usage synopsis for each PATTERN

     The return status is zero unless no command matches PATTERN.

let

          let EXPRESSION [EXPRESSION ...]

     The let builtin allows arithmetic to be performed on shell variables.  Each EXPRESSION is evaluated according to the rules given below in Shell Arithmetic.  If the last EXPRESSION evaluates to 0, let returns 1; otherwise 0 is returned.

local

          local [OPTION] NAME[=VALUE] ...

     For each argument, a local variable named NAME is created, and assigned VALUE. The OPTION can be any of the options accepted by declare. local can only be used within a function; it makes the variable NAME have a visible scope restricted to that function and its children. If NAME is -, the set of shell options is made local to the function in which local is invoked: shell options changed using the set builtin inside the function are restored to their original values when the function returns. The restore is effected as if a series of set commands were executed to restore the values that were in place before the function. The return status is zero unless local is used outside a function, an invalid NAME is supplied, or NAME is a readonly variable.

logout

          logout [N]

     Exit a login shell, returning a status of N to the shell's parent.

mapfile

          mapfile [-d DELIM] [-n COUNT] [-O ORIGIN] [-s COUNT]
              [-t] [-u FD] [-C CALLBACK] [-c QUANTUM] [ARRAY]

     Read lines from the standard input into the indexed array variable ARRAY, or from file descriptor FD if the -u option is supplied. The variable MAPFILE is the default ARRAY. Options, if supplied, have the following meanings:

     -d
          The first character of DELIM is used to terminate each input line, rather than newline. If DELIM is the empty string, mapfile will terminate a line when it reads a NUL character.

     -n
          Copy at most COUNT lines.  If COUNT is 0, all lines are copied.

     -O
          Begin assigning to ARRAY at index ORIGIN. The default index is 0.

     -s
          Discard the first COUNT lines read.

     -t
          Remove a trailing DELIM (default newline) from each line read.

     -u
          Read lines from file descriptor FD instead of the standard input.

     -C
          Evaluate CALLBACK each time QUANTUM lines are read. The -c option specifies QUANTUM.

     -c
          Specify the number of lines read between each call to CALLBACK.

     If -C is specified without -c, the default quantum is 5000. When CALLBACK  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. CALLBACK is evaluated after the line is read but before the array element is assigned.

     If not supplied with an explicit origin, mapfile will clear ARRAY before assigning to it.

     mapfile returns successfully unless an invalid option or option argument is supplied, ARRAY is invalid or unassignable, or ARRAY is not an indexed array.

printf

          printf [-v VAR] FORMAT [ARGUMENTS]

     Write the formatted ARGUMENTS to the standard output under the control of the FORMAT. The -v option causes the output to be assigned to the variable VAR rather than being printed to the standard output.

     The FORMAT 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 ARGUMENT. In addition to the standard printf(1) formats, printf interprets the following extensions:

     %b
          Causes printf to expand backslash escape sequences in the corresponding ARGUMENT in the same way as echo -e (see Bash Builtins).

     %q
          Causes printf to output the corresponding ARGUMENT in a format that can be reused as shell input.

     %(DATEFMT)T
          Causes printf to output the date-time string resulting from using DATEFMT as a format string for strftime(3). The corresponding ARGUMENT is an integer representing the number of seconds since the epoch. Two special argument values may be used: -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 given. This is an exception to the usual printf behavior.

     The %b, %q, and %T directives 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.

     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 ASCII value of the following character.

     The FORMAT is reused as necessary to consume all of the ARGUMENTS. If the FORMAT requires more ARGUMENTS 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 on failure.

read

          read [-ers] [-a ANAME] [-d DELIM] [-i TEXT] [-n NCHARS]
              [-N NCHARS] [-p PROMPT] [-t TIMEOUT] [-u FD] [NAME ...]

     One line is read from the standard input, or from the file descriptor FD supplied as an argument to the -u option, split into words as described above in Word Splitting, and the first word is assigned to the first NAME, the second word to the second NAME, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last NAME. 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 IFS variable are used to split the line into words using the same rules the shell uses for expansion (described above in Word Splitting). The backslash character \ may be used to remove any special meaning for the next character read and for line continuation.

     Options, if supplied, have the following meanings:

     -a ANAME
          The words are assigned to sequential indices of the array variable ANAME, starting at 0. All elements are removed from ANAME before the assignment. Other NAME arguments are ignored.

     -d DELIM
          The first character of DELIM is used to terminate the input line, rather than newline. If DELIM is the empty string, read will terminate a line when it reads a NUL character.

     -e
          Readline (see Command Line Editing) is used to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline's default filename completion.

     -i TEXT
          If Readline is being used to read the line, TEXT is placed into the editing buffer before editing begins.

     -n NCHARS
          read returns after reading NCHARS characters rather than waiting for a complete line of input, but honors a delimiter if fewer than NCHARS characters are read before the delimiter.

     -N NCHARS
          read returns after reading exactly NCHARS characters rather than waiting for a complete line of input, unless EOF is encountered or read times out. Delimiter characters encountered in the input are not treated specially and do not cause read to return until NCHARS characters are read. The result is not split on the characters in IFS; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the -r option below).

     -p PROMPT
          Display PROMPT, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.

     -r
          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.

     -s
          Silent mode.  If input is coming from a terminal, characters are not echoed.

     -t TIMEOUT
          Cause read to time out and return failure if a complete line of input (or a specified number of characters) is not read within TIMEOUT seconds. TIMEOUT  may be a decimal number with a fractional portion following the decimal point. This option is only effective if read is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If read times out, read saves any partial input read into the specified variable NAME. If TIMEOUT is 0, read returns immediately, without trying to read any data.  The exit status is 0 if input is available on the specified file descriptor, non-zero otherwise. The exit status is greater than 128 if the timeout is exceeded.

     -u FD
          Read input from file descriptor FD.

     If no NAMEs are supplied, the line read, without the ending delimiter but otherwise unmodified, is assigned to the variable REPLY. The exit status is zero, unless end-of-file is encountered, read 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 -u.

readarray

          readarray [-d DELIM] [-n COUNT] [-O ORIGIN] [-s COUNT]
              [-t] [-u FD] [-C CALLBACK] [-c QUANTUM] [ARRAY]

     Read lines from the standard input into the indexed array variable ARRAY, or from file descriptor FD if the -u option is supplied.

     A synonym for mapfile.

source

          source FILENAME

     A synonym for . (see Bourne Shell Builtins).

type

          type [-afptP] [NAME ...]

     For each NAME, indicate how it would be interpreted if used as a command name.

     If the -t option is used, type prints a single word which is one of alias, function, builtin, file or keyword, if NAME is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the NAME is not found, then nothing is printed, and type returns a failure status.

     If the -p option is used, type either returns the name of the disk file that would be executed, or nothing if -t would not return file.

     The -P option forces a path search for each NAME, even if -t would not return file.

     If a command is hashed, -p and -P print the hashed value, which is not necessarily the file that appears first in $PATH.

     If the -a option is used, type returns all of the places that contain an executable named FILE. This includes aliases and functions, if and only if the -p option is not also used.

     If the -f option is used, type does not attempt to find shell functions, as with the command builtin.

     The return status is zero if all of the NAMES are found, non-zero if any are not found.

typeset

          typeset [-afFgrxilnrtux] [-p] [NAME[=VALUE] ...]

     The typeset command is supplied for compatibility with the Korn shell. It is a synonym for the declare builtin command.

ulimit

          ulimit [-HS] -a
          ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [LIMIT]

     ulimit provides control over the resources available to processes started by the shell, on systems that allow such control.  If an option is given, it is interpreted as follows:

     -S
          Change and report the soft limit associated with a resource.

     -H
          Change and report the hard limit associated with a resource.

     -a
          All current limits are reported; no limits are set.

     -b
          The maximum socket buffer size.

     -c
          The maximum size of core files created.

     -d
          The maximum size of a process's data segment.

     -e
          The maximum scheduling priority ("nice").

     -f
          The maximum size of files written by the shell and its children.

     -i
          The maximum number of pending signals.

     -k
          The maximum number of kqueues that may be allocated.

     -l
          The maximum size that may be locked into memory.

     -m
          The maximum resident set size (many systems do not honor this limit).

     -n
          The maximum number of open file descriptors (most systems do not allow this value to be set).

     -p
          The pipe buffer size.

     -q
          The maximum number of bytes in POSIX message queues.

     -r
          The maximum real-time scheduling priority.

     -s
          The maximum stack size.

     -t
          The maximum amount of cpu time in seconds.

     -u
          The maximum number of processes available to a single user.

     -v
          The maximum amount of virtual memory available to the shell, and, on some systems, to its children.

     -x
          The maximum number of file locks.

     -P
          The maximum number of pseudoterminals.

     -R
          The maximum time a real-time process can run before blocking, in microseconds.

     -T
          The maximum number of threads.

     If LIMIT is given, and the -a option is not used, LIMIT is the new value of the specified resource. The special LIMIT values hard, soft, and unlimited 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, the current value of the soft limit for the specified resource is printed, unless the -H 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 -H nor -S is supplied, both the hard and soft limits are set. If no option is given, then -f is assumed.  Values are in 1024-byte increments, except for -t, which is in seconds; -R, which is in microseconds; -p, which is in units of 512-byte blocks; -P, -T, -b, -k, -n and -u, which are unscaled values; and, when in POSIX Mode (see Bash POSIX Mode), -c and -f, which are in 512-byte increments.

     The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit.

unalias

          unalias [-a] [NAME ... ]

     Remove each NAME from the list of aliases.  If -a is supplied, all aliases are removed. Aliases are described in Aliases.

Modifying Shell Behavior
========================

The Set Builtin
---------------

This builtin is so complicated that it deserves its own section.  set allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables.

set

          set [--abefhkmnptuvxBCEHPT] [-o OPTION-NAME] [ARGUMENT ...]
          set [+abefhkmnptuvxBCEHPT] [+o OPTION-NAME] [ARGUMENT ...]

     If no options or arguments are supplied, set 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 POSIX mode, only shell variables are listed.

     When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:

     -a
          Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.

     -b
          Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.

     -e
          Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a list (see Lists), or a compound command (see Compound Commands) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return status is being inverted with !. If a compound command other than a subshell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit. A trap on ERR, if set, is executed before the shell exits.

          This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.

          If a compound command or shell function executes in a context where -e is being ignored, none of the commands executed within the compound command or function body will be affected by the -e setting, even if -e is set and a command returns a failure status. If a compound command or shell function sets -e while executing in a context where -e is ignored, that setting will not have any effect until the compound command or the command containing the function call completes.

     -f
          Disable filename expansion (globbing).

     -h
          Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.

     -k
          All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.

     -m
          Job control is enabled (see Job Control). All processes run in a separate process group. When a background job completes, the shell prints a line containing its exit status.

     -n
          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.

     -o OPTION-NAME

          Set the option corresponding to OPTION-NAME:

          allexport
               Same as -a.

          braceexpand
               Same as -B.

          emacs
               Use an emacs-style line editing interface (see Command Line Editing). This also affects the editing interface used for read -e.

          errexit
               Same as -e.

          errtrace
               Same as -E.

          functrace
               Same as -T.

          hashall
               Same as -h.

          histexpand
               Same as -H.

          history
               Enable command history, as described in Bash History Facilities. This option is on by default in interactive shells.

          ignoreeof
               An interactive shell will not exit upon reading EOF.

          keyword
               Same as -k.

          monitor
               Same as -m.

          noclobber
               Same as -C.

          noexec
               Same as -n.

          noglob
               Same as -f.

          nolog
               Currently ignored.

          notify
               Same as -b.

          nounset
               Same as -u.

          onecmd
               Same as -t.

          physical
               Same as -P.

          pipefail
               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.

          posix
               Change the behavior of Bash where the default operation differs from the POSIX standard to match the standard (see Bash POSIX Mode). This is intended to make Bash behave as a strict superset of that standard.

          privileged
               Same as -p.

          verbose
               Same as -v.

          vi
               Use a vi-style line editing interface. This also affects the editing interface used for read -e.

          xtrace
               Same as -x.

     -p
          Turn on privileged mode. In this mode, the $BASH_ENV and $ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH and GLOBIGNORE 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 -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p 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.

     -t
          Exit after reading and executing one command.

     -u
          Treat unset variables and parameters other than the special parameters @ or * as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.

     -v
          Print shell input lines as they are read.

     -x
          Print a trace of simple commands, for commands, case commands, select commands, and arithmetic for commands and their arguments or associated word lists after they are expanded and before they are executed.  The value of the PS4 variable is expanded and the resultant value is printed before the command and its expanded arguments.

     -B
          The shell will perform brace expansion (see Brace Expansion). This option is on by default.

     -C
          Prevent output redirection using >, >&, and <> from overwriting existing files.

     -E
          If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.

     -H
          Enable ! style history substitution (see History Interaction). This option is on by default for interactive shells.

     -P
          If set, do not resolve symbolic links when performing commands such as cd which change the current directory.  The physical directory is used instead.  By default, Bash follows the logical chain of directories when performing commands which change the current directory.

          For example, if /usr/sys is a symbolic link to /usr/local/sys then:

               $ cd /usr/sys; echo $PWD
               /usr/sys
               $ cd ..; pwd
               /usr

          If set -P is on, then:

               $ cd /usr/sys; echo $PWD
               /usr/local/sys
               $ cd ..; pwd
               /usr/local

     -T
          If set, any trap on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The DEBUG and RETURN traps are normally not inherited in such cases.

     --
          If no arguments follow this option, then the positional parameters are unset.  Otherwise, the positional parameters are set to the ARGUMENTS, even if some of them begin with a -.

     -
          Signal the end of options, cause all remaining ARGUMENTS to be assigned to the positional parameters.  The -x and -v  options are turned off. If there are no arguments, the positional parameters remain unchanged.

     Using + rather than - 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 $-.

     The remaining N ARGUMENTS are positional parameters and are assigned, in order, to $1, $2, ...  $N. The special parameter # is set to N.

     The return status is always zero unless an invalid option is supplied.

The Shopt Builtin
-----------------

This builtin allows you to change additional shell optional behavior.

shopt

          shopt [-pqsu] [-o] [OPTNAME ...]

     Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the -o option is used, those available with the -o option to the set builtin command (see The Set Builtin). With no options, or with the -p option, a list of all settable options is displayed, with an indication of whether or not each is set; if OPTNAMES are supplied, the output is restricted to those options. The -p option causes output to be displayed in a form that may be reused as input. Other options have the following meanings:

     -s
          Enable (set) each OPTNAME.

     -u
          Disable (unset) each OPTNAME.

     -q
          Suppresses normal output; the return status indicates whether the OPTNAME is set or unset. If multiple OPTNAME arguments are given with -q, the return status is zero if all OPTNAMES are enabled; non-zero otherwise.

     -o
          Restricts the values of OPTNAME to be those defined for the -o option to the set builtin (see The Set Builtin).

     If either -s or -u is used with no OPTNAME arguments, shopt shows only those options which are set or unset, respectively.

     Unless otherwise noted, the shopt options are disabled (off) by default.

     The return status when listing options is zero if all OPTNAMES are enabled, non-zero otherwise.  When setting or unsetting options, the return status is zero unless an OPTNAME is not a valid shell option.

     The list of shopt options is:

     assoc_expand_once
          If set, the shell suppresses multiple evaluation of associative array subscripts during arithmetic expression evaluation, while executing builtins that can perform variable assignments, and while executing builtins that perform array dereferencing.

     autocd
          If set, a command name that is the name of a directory is executed as if it were the argument to the cd command. This option is only used by interactive shells.

     cdable_vars
          If this is set, an argument to the cd builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.

     cdspell
          If set, minor errors in the spelling of a directory component in a cd command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells.

     checkhash
          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, a normal path search is performed.

     checkjobs
          If set, Bash lists the status of any stopped and running jobs before exiting an interactive shell.  If any jobs are running, this causes the exit to be deferred until a second exit is attempted without an intervening command (see Job Control). The shell always postpones exiting if any jobs are stopped.

     checkwinsize
          If set, Bash checks the window size after each external (non-builtin) command and, if necessary, updates the values of LINES and COLUMNS. This option is enabled by default.

     cmdhist
          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 Bash History Facilities).

     compat31
     compat32
     compat40
     compat41
     compat42
     compat43
     compat44
          These control aspects of the shell's compatibility mode (see Shell Compatibility Mode).

     complete_fullquote
          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.

     direxpand
          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.

     dirspell
          If set, Bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist.

     dotglob
          If set, Bash includes filenames beginning with a `.' in the results of filename expansion. The filenames . and .. must always be matched explicitly, even if dotglob is set.

     execfail
          If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command.  An interactive shell does not exit if exec fails.

     expand_aliases
          If set, aliases are expanded as described below under Aliases, Aliases. This option is enabled by default for interactive shells.

     extdebug
          If set at shell invocation, or in a shell startup file, arrange to execute the debugger profile before the shell starts, identical to the --debugger option. If set after invocation, behavior intended for use by debuggers is enabled:

           1. The -F option to the declare builtin (see Bash Builtins) displays the source file name and line number corresponding to each function name supplied as an argument.

           2. If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.

           3. If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source builtins), the shell simulates a call to return.

           4. BASH_ARGC and BASH_ARGV are updated as described in their descriptions (see Bash Variables).

           5. Function tracing is enabled: command substitution, shell functions, and subshells invoked with ( COMMAND ) inherit the DEBUG and RETURN traps.

           6. Error tracing is enabled: command substitution, shell functions, and subshells invoked with ( COMMAND ) inherit the ERR trap.

     extglob
          If set, the extended pattern matching features described above (see Pattern Matching) are enabled.

     extquote
          If set, $'STRING' and $"STRING" quoting is performed within ${PARAMETER} expansions enclosed in double quotes.  This option is enabled by default.

     failglob
          If set, patterns which fail to match filenames during filename expansion result in an expansion error.

     force_fignore
          If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See Bash Variables, for a description of FIGNORE. This option is enabled by default.

     globasciiranges
          If set, range expressions used in pattern matching bracket expressions (see Pattern Matching) behave as if in the traditional C locale when performing comparisons.  That is, the current locale's collating sequence is not taken into account, so b will not collate between A and B, and upper-case and lower-case ASCII characters will collate together.

     globstar
          If set, the pattern ** used in a filename expansion context will match all files and zero or more directories and subdirectories. If the pattern is followed by a /, only directories and subdirectories match.

     gnu_errfmt
          If set, shell error messages are written in the standard GNU error message format.

     histappend
          If set, the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file.

     histreedit
          If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution.

     histverify
          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.

     hostcomplete
          If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a @ is being completed (see Commands For Completion).  This option is enabled by default.

     huponexit
          If set, Bash will send SIGHUP to all jobs when an interactive login shell exits (see Signals).

     inherit_errexit
          If set, command substitution inherits the value of the errexit option, instead of unsetting it in the subshell environment. This option is enabled when POSIX mode is enabled.

     interactive_comments
          Allow a word beginning with # to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default.

     lastpipe
          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.

     lithist
          If enabled, and the cmdhist option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.

     localvar_inherit
          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 NAMEREF attribute is not inherited.

     localvar_unset
          If set, calling unset 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.

     login_shell
          The shell sets this option if it is started as a login shell (see Invoking Bash). The value may not be changed.

     mailwarn
          If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message "The mail in MAILFILE has been read" is displayed.

     no_empty_cmd_completion
          If set, and Readline is being used, Bash will not attempt to search the PATH for possible completions when completion is attempted on an empty line.

     nocaseglob
          If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion.

     nocasematch
          If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional commands, when performing pattern substitution word expansions, or when filtering possible completions as part of programmable completion.

     nullglob
          If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves.

     progcomp
          If set, the programmable completion facilities (see Programmable Completion) are enabled. This option is enabled by default.

     progcomp_alias
          If set, and programmable completion is enabled, Bash treats a command name that doesn'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.

     promptvars
          If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (see Controlling the Prompt). This option is enabled by default.

     restricted_shell
          The shell sets this option if it is started in restricted mode (see The Restricted Shell). 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.

     shift_verbose
          If this is set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters.

     sourcepath
          If set, the source builtin uses the value of PATH to find the directory containing the file supplied as an argument. This option is enabled by default.

     xpg_echo
          If set, the echo builtin expands backslash-escape sequences by default.

Special Builtins
================

For historical reasons, the POSIX standard has classified several builtin commands as special. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects:

 1. Special builtins are found before shell functions during command lookup.

 2. If a special builtin returns an error status, a non-interactive shell exits.

 3. Assignment statements preceding the command stay in effect in the shell environment after the command completes.

When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in Bash POSIX Mode.

These are the POSIX special builtins:

     break : . continue eval exec exit export readonly return set
     shift trap unset

Shell Variables
***************

This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables.

Bourne Shell Variables
======================

Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.

CDPATH
     A colon-separated list of directories used as a search path for the cd builtin command.

HOME
     The current user's home directory; the default for the cd builtin command. The value of this variable is also used by tilde expansion (see Tilde Expansion).

IFS
     A list of characters that separate fields; used when the shell splits words as part of expansion.

MAIL
     If this parameter is set to a filename or directory name and the MAILPATH variable is not set, Bash informs the user of the arrival of mail in the specified file or Maildir-format directory.

MAILPATH
     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 ?. When used in the text of the message, $_ expands to the name of the current mail file.

OPTARG
     The value of the last option argument processed by the getopts builtin.

OPTIND
     The index of the last option argument processed by the getopts builtin.

PATH
     A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon.

PS1
     The primary prompt string.  The default value is \s-\v\$ . See Controlling the Prompt, for the complete list of escape sequences that are expanded before PS1 is displayed.

PS2
     The secondary prompt string.  The default value is > . PS2 is expanded in the same way as PS1 before being displayed.

Bash Variables
==============

These variables are set or used by Bash, but other shells do not normally treat them specially.

A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Job Control Variables).

_
     ($_, an underscore.) 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, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.

BASH
     The full pathname used to execute the current instance of Bash.

BASHOPTS
     A colon-separated list of enabled shell options.  Each word in the list is a valid argument for the -s option to the shopt builtin command (see The Shopt Builtin). The options appearing in BASHOPTS are those reported as on by shopt. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files.  This variable is readonly.

BASHPID
     Expands to the process ID of the current Bash process. This differs from $$ under certain circumstances, such as subshells that do not require Bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset.

BASH_ALIASES
     An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. (see Bourne Shell Builtins). 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 BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset.

BASH_ARGC
     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 . or source) is at the top of the stack.  When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values.

BASH_ARGV
     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 parameters supplied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values.

BASH_ARGV0
     When referenced, this variable expands to the name of the shell or shell script (identical to $0; see Special Parameters, for the description of special parameter 0). Assignment to BASH_ARGV0 causes the value assigned to also be assigned to $0. If BASH_ARGV0 is unset, it loses its special properties, even if it is subsequently reset.

BASH_CMDS
     An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin (see Bourne Shell Builtins). Elements added to this array appear in the hash table; however, unsetting array elements currently does not cause command names to be removed from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMMAND
     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 BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMPAT
     The value is used to set the shell's compatibility level. See Shell Compatibility Mode, 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 BASH_COMPAT is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT 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. The valid values correspond to the compatibility levels described below (see Shell Compatibility Mode). For example, 4.2 and 42 are valid values that correspond to the compat42 shopt option and set the compatibility level to 42. The current version is also a valid value.

BASH_ENV
     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.  See Bash Startup Files.

BASH_EXECUTION_STRING
     The command argument to the -c invocation option.

BASH_LINENO
     An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number.

BASH_LOADABLES_PATH
     A colon-separated list of directories in which the shell looks for dynamically loadable builtins specified by the enable command.

BASH_REMATCH
     An array variable whose members are assigned by the =~ binary operator to the [[ conditional command (see Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index N is the portion of the string matching the Nth parenthesized subexpression.

BASH_SOURCE
     An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}

BASH_SUBSHELL
     Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.

BASH_VERSINFO
     A readonly array variable (see Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:

     BASH_VERSINFO[0]
          The major version number (the RELEASE).

     BASH_VERSINFO[1]
          The minor version number (the VERSION).

     BASH_VERSINFO[2]
          The patch level.

     BASH_VERSINFO[3]
          The build version.

     BASH_VERSINFO[4]
          The release status (e.g., BETA1).

     BASH_VERSINFO[5]
          The value of MACHTYPE.

BASH_VERSION
     The version number of the current instance of Bash.

BASH_XTRACEFD
     If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when set -x is enabled to that file descriptor. This allows tracing output to be separated from diagnostic and error messages. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.

CHILD_MAX
     Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent.

COLUMNS
     Used by the select command to determine the terminal width when printing selection lists. Automatically set if the checkwinsize option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a SIGWINCH.

COMP_CWORD
     An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).

COMP_LINE
     The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

COMP_POINT
     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 ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

COMP_TYPE
     Set to an integer value corresponding to the type of completion attempted that caused a completion function to be called: TAB, for normal completion, ?, for listing completions after successive tabs, !, for listing alternatives on partial word completion, @, to list completions if the word is not unmodified, or %, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

COMP_KEY
     The key (or final key of a key sequence) used to invoke the current completion function.

COMP_WORDBREAKS
     The set of characters that the Readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.

COMP_WORDS
     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 COMP_WORDBREAKS as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).

COMPREPLY
     An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion). Each array element contains one possible completion.

COPROC
     An array variable created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses).

DIRSTACK
     An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assignment to this variable will not change the current directory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.

EMACS
     If Bash finds this variable in the environment when the shell starts with value t, it assumes that the shell is running in an Emacs shell buffer and disables line editing.

ENV
     Expanded and executed similarlty to BASH_ENV (see Bash Startup Files) when an interactive shell is invoked in POSIX Mode (see Bash POSIX Mode).

EPOCHREALTIME
     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 TIME for the definition of Epoch). Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.

EPOCHSECONDS
     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 TIME for the definition of Epoch). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently reset.

EUID
     The numeric effective user id of the current user.  This variable is readonly.

EXECIGNORE
     A colon-separated list of shell patterns (see Pattern Matching) defining the list of filenames to be ignored by command search using PATH. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH lookup. This does not affect the behavior of the [, test, and [[ commands. Full pathnames in the command hash table are not subject to EXECIGNORE. 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 extglob shell option.

FCEDIT
     The editor used as a default by the -e option to the fc builtin command.

FIGNORE
     A colon-separated list of suffixes to ignore when performing filename completion. A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames.  A sample value is .o:~

FUNCNAME
     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 "main". This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

     This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using this information.

FUNCNEST
     If set to a numeric value greater than 0, defines a maximum function nesting level.  Function invocations that exceed this nesting level will cause the current command to abort.

GLOBIGNORE
     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 GLOBIGNORE, it is removed from the list of matches. The pattern matching honors the setting of the extglob shell option.

GROUPS
     An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses its special properties, even if it is subsequently reset.

histchars
     Up to three characters which control history expansion, quick substitution, and tokenization (see History Interaction). The first character is the HISTORY EXPANSION character, that is, the character which signifies the start of a history expansion, normally !.  The second character is the character which signifies `quick substitution' when seen as the first character on a line, normally ^.  The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, usually #.  The history comment character causes history substitution to be skipped 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.

HISTCMD
     The history number, or index in the history list, of the current command. Assignments to HISTCMD are ignored. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.

HISTCONTROL
     A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a space character are not saved in the history list. A value of ignoredups causes lines which match the previous history entry to not be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups 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 HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.

HISTFILE
     The name of the file to which the command history is saved.  The default value is ~/.bash_history.

HISTFILESIZE
     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 that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits. 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 HISTSIZE after reading any startup files.

HISTIGNORE
     A colon-separated list of patterns used to decide which command lines should be saved on the history list.  Each pattern is anchored at the beginning of the line and must match the complete line (no implicit * is appended).  Each pattern is tested against the line after the checks specified by HISTCONTROL are applied.  In addition to the normal shell pattern matching characters, & matches the previous history line.  & may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTIGNORE. The pattern matching honors the setting of the extglob shell option.

     HISTIGNORE subsumes the function of HISTCONTROL.  A pattern of & is identical to ignoredups, and a pattern of [ ]* is identical to ignorespace. Combining these two patterns, separating them with a colon, provides the functionality of ignoreboth.

HISTSIZE
     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.

HISTTIMEFORMAT
     If this variable is set and not null, its value is used as a format string for STRFTIME to print the time stamp associated with each history entry displayed by the history builtin. If this variable is set, time stamps are written 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.

HOSTFILE
     Contains the name of a file in the same format as /etc/hosts 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 HOSTFILE is set, but has no value, or does not name a readable file, Bash attempts to read /etc/hosts to obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared.

HOSTNAME
     The name of the current host.

HOSTTYPE
     A string describing the machine Bash is running on.

IGNOREEOF
     Controls the action of the shell on receipt of an EOF character as the sole input.  If set, the value denotes the number of consecutive EOF characters that can be read as the first character on an input line before the shell will exit.  If the variable exists but does not have a numeric value, or has no value, then the default is 10. If the variable does not exist, then EOF signifies the end of input to the shell.  This is only in effect for interactive shells.

INPUTRC
     The name of the Readline initialization file, overriding the default of ~/.inputrc.

INSIDE_EMACS
     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 TERM.

LANG
     Used to determine the locale category for any category not specifically selected with a variable starting with LC_.

LC_ALL
     This variable overrides the value of LANG and any other LC_ variable specifying a locale category.

LC_COLLATE
     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 Filename Expansion).

LC_CTYPE
     This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see Filename Expansion).

LC_MESSAGES
     This variable determines the locale used to translate double-quoted strings preceded by a $ (see Locale Translation).

LC_NUMERIC
     This variable determines the locale category used for number formatting.

LC_TIME
     This variable determines the locale category used for data and time formatting.

LINENO
     The line number in the script or shell function currently executing. If LINENO is unset, it loses its special properties, even if it is subsequently reset.

LINES
     Used by the select command to determine the column length for printing selection lists. Automatically set if the checkwinsize option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a SIGWINCH.

MACHTYPE
     A string that fully describes the system type on which Bash is executing, in the standard GNU CPU-COMPANY-SYSTEM format.

MAILCHECK
     How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL 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.

MAPFILE
     An array variable created to hold the text read by the mapfile builtin when no variable name is supplied.

OLDPWD
     The previous working directory as set by the cd builtin.

OPTERR
     If set to the value 1, Bash displays error messages generated by the getopts builtin command.

OSTYPE
     A string describing the operating system Bash is running on.

PIPESTATUS
     An array variable (see Arrays) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command).

POSIXLY_CORRECT
     If this variable is in the environment when Bash starts, the shell enters POSIX mode (see Bash POSIX Mode) before reading the startup files, as if the --posix invocation option had been supplied. If it is set while the shell is running, Bash enables POSIX mode, as if the command

          set -o posix

     had been executed. When the shell enters POSIX mode, it sets this variable if it was not already set.

PPID
     The process ID of the shell's parent process.  This variable is readonly.

PROMPT_COMMAND
     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 ($PS1). If this is set but not an array variable, its value is used as a command to execute instead.

PROMPT_DIRTRIM
     If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see Controlling the Prompt). Characters removed are replaced with an ellipsis.

PS0
     The value of this parameter is expanded like PS1 and displayed by interactive shells after reading a command and before the command is executed.

PS3
     The value of this variable is used as the prompt for the select command.  If this variable is not set, the select command prompts with #?

PS4
     The value of this parameter is expanded like PS1 and the expanded value is the prompt printed before the command line is echoed when the -x option is set (see The Set Builtin). The first character of the expanded value is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is + .

PWD
     The current working directory as set by the cd builtin.

RANDOM
     Each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to this variable seeds the random number generator. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.

READLINE_LINE
     The contents of the Readline line buffer, for use with bind -x (see Bash Builtins).

READLINE_MARK
     The position of the MARK (saved insertion point) in the Readline line buffer, for use with bind -x (see Bash Builtins). The characters between the insertion point and the mark are often called the REGION.

READLINE_POINT
     The position of the insertion point in the Readline line buffer, for use with bind -x (see Bash Builtins).

REPLY
     The default variable for the read builtin.

SECONDS
     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 is always determined by querying the system clock. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.

SHELL
     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's login shell.

SHELLOPTS
     A colon-separated list of enabled shell options.  Each word in the list is a valid argument for the -o option to the set builtin command (see The Set Builtin). The options appearing in SHELLOPTS are those reported as on by set -o. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files.  This variable is readonly.

SHLVL
     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.

SRANDOM
     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 /dev/urandom or arc4random, 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 SRANDOM is unset, it loses its special properties, even if it is subsequently reset.

TIMEFORMAT
     The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The % 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 braces denote optional portions.

     %%
          A literal %.

     %[P][l]R
          The elapsed time in seconds.

     %[P][l]U
          The number of CPU seconds spent in user mode.

     %[P][l]S
          The number of CPU seconds spent in system mode.

     %P
          The CPU percentage, computed as (%U + %S) / %R.

     The optional P 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. At most three places after the decimal point may be specified; values of P greater than 3 are changed to 3. If P is not specified, the value 3 is used.

     The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of P determines whether or not the fraction is included.

     If this variable is not set, Bash acts as if it had the value

          $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'

     If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.

TMOUT
     If set to a value greater than zero, TMOUT is treated as the default timeout for the read builtin (see Bash Builtins). The select command (see Conditional Constructs) terminates if input does not arrive after TMOUT seconds when input is coming from a terminal.

     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.

TMPDIR
     If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell's use.

UID
     The numeric real user id of the current user.  This variable is readonly.

Bash Features
*************

This chapter describes features unique to Bash.

Invoking Bash
=============

     bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o OPTION]
         [-O SHOPT_OPTION] [ARGUMENT ...]
     bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o OPTION]
         [-O SHOPT_OPTION] -c STRING [ARGUMENT ...]
     bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o OPTION]
         [-O SHOPT_OPTION] [ARGUMENT ...]

All of the single-character options used with the set builtin (see The Set Builtin) 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.

--debugger
     Arrange for the debugger profile to be executed before the shell starts.  Turns on extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin).

--dump-po-strings
     A list of all double-quoted strings preceded by $ is printed on the standard output in the GNU gettext PO (portable object) file format. Equivalent to -D except for the output format.

--dump-strings
     Equivalent to -D.

--help
     Display a usage message on standard output and exit successfully.

--init-file FILENAME
--rcfile FILENAME
     Execute commands from FILENAME (instead of ~/.bashrc) in an interactive shell.

--login
     Equivalent to -l.

--noediting
     Do not use the GNU Readline library (see Command Line Editing) to read  command lines when the shell is interactive.

--noprofile
     Don't load the system-wide startup file /etc/profile or any of the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile when Bash is invoked as a login shell.

--norc
     Don't read the ~/.bashrc initialization file in an interactive shell.  This is on by default if the shell is invoked as sh.

--posix
     Change the behavior of Bash where the default operation differs from the POSIX standard to match the standard.  This is intended to make Bash behave as a strict superset of that standard.  See Bash POSIX Mode, for a description of the Bash POSIX mode.

--restricted
     Make the shell a restricted shell (see The Restricted Shell).

--verbose
     Equivalent to -v.  Print shell input lines as they're read.

--version
     Show version information for this instance of Bash on the standard output and exit successfully.

There are several single-character options that may be supplied at invocation which are not available with the set builtin.

-c
     Read and execute commands from the first non-option argument COMMAND_STRING, then exit. If there are arguments after the COMMAND_STRING, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages.

-i
     Force the shell to run interactively.  Interactive shells are described in Interactive Shells.

-l
     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 exec -l bash. When the shell is not interactive, the login shell startup files will be executed. exec bash -l or exec bash --login will replace the current shell with a Bash login shell. See Bash Startup Files, for a description of the special behavior of a login shell.

-r
     Make the shell a restricted shell (see The Restricted Shell).

-s
     If this option is present, or if no arguments remain after option processing, then commands are read 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.

-D
     A list of all double-quoted strings preceded by $ is printed on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX (see Locale Translation). This implies the -n option; no commands will be executed.

[-+]O [SHOPT_OPTION]
     SHOPT_OPTION is one of the shell options accepted by the shopt builtin (see The Shopt Builtin). If SHOPT_OPTION is present, -O sets the value of that option; +O unsets it. If SHOPT_OPTION is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input.

--
     A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments.

A login shell is one whose first character of argument zero is -, or one invoked with the --login option.

An interactive shell is one started without non-option arguments, unless -s is specified, without specifying the -c option, and whose input and output are both connected to terminals (as determined by isatty(3)), or one started with the -i option.  See Interactive Shells, for more information.

If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is assumed to be the name of a file containing shell commands (see Shell Scripts). When Bash is invoked in this fashion, $0 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'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 Startup Files
==================

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 Tilde Expansion).

Interactive shells are described in Interactive Shells.

Invoked as an interactive login shell, or with --login
......................................................

When Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, Bash reads and executes commands from the file ~/.bash_logout, if it exists.

Invoked as an interactive non-login shell
.........................................

When an interactive shell that is not a login shell is started, Bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile FILE option will force Bash to read and execute commands from FILE instead of ~/.bashrc.

So, typically, your ~/.bash_profile contains the line

     if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

after (or before) any login-specific initializations.

Invoked non-interactively
.........................

When Bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV 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:

     if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the filename.

As noted above, if a non-interactive shell is invoked with the --login option, Bash attempts to read and execute commands from the login shell startup files.

Invoked with name sh
....................

If Bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.

When invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh, Bash looks for the variable ENV, 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 sh does not attempt to read and execute commands from any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files.

When invoked as sh, Bash enters POSIX mode after the startup files are read.

Invoked in POSIX mode
.....................

When Bash is started in POSIX mode, as with the --posix command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.

Invoked by remote shell daemon
..............................

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the remote shell daemon, usually rshd, or the secure shell daemon sshd. If Bash determines it is being run in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force another file to be read, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified.

Invoked with unequal effective and real UID/GIDs
................................................

If Bash is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

Interactive Shells
==================

What is an Interactive Shell?
-----------------------------

An interactive shell is one started without non-option arguments, unless -s is specified, without specifying the -c option, and whose input and error output are both connected to terminals (as determined by isatty(3)), or one started with the -i option.

An interactive shell generally reads from and writes to a user's terminal.

The -s invocation option may be used to set the positional parameters when an interactive shell is started.

Is this Shell Interactive?
--------------------------

To determine within a startup script whether or not Bash is running interactively, test the value of the - special parameter. It contains i when the shell is interactive.  For example:

     case "$-" in
     *i*)	echo This shell is interactive ;;
     *)	echo This shell is not interactive ;;
     esac

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells.  Thus:

     if [ -z "$PS1" ]; then
             echo This shell is not interactive
     else
             echo This shell is interactive
     fi

Interactive Shell Behavior
--------------------------

When the shell is running interactively, it changes its behavior in several ways.

 1. Startup files are read and executed as described in Bash Startup Files.

 2. Job Control (see Job Control) is enabled by default.  When job control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

 3. Bash expands and displays PS1 before reading the first line of a command, and expands and displays PS2 before reading the second and subsequent lines of a multi-line command. Bash expands and displays PS0 after it reads a command but before executing it. See Controlling the Prompt, for a complete list of prompt string escape sequences.

 4. Bash executes the values of the set elements of the PROMPT_COMMANDS array variable as commands before printing the primary prompt, $PS1 (see Bash Variables).

 5. Readline (see Command Line Editing) is used to read commands from the user's terminal.

 6. Bash inspects the value of the ignoreeof option to set -o instead of exiting immediately when it receives an EOF on its standard input when reading a command (see The Set Builtin).

 7. Command history (see Bash History Facilities) and history expansion (see History Interaction) are enabled by default. Bash will save the command history to the file named by $HISTFILE when a shell with history enabled exits.

 8. Alias expansion (see Aliases) is performed by default.

 9. In the absence of any traps, Bash ignores SIGTERM (see Signals).

 10. In the absence of any traps, SIGINT is caught and handled (see Signals). SIGINT will interrupt some shell builtins.

 11. An interactive login shell sends a SIGHUP to all jobs on exit if the huponexit shell option has been enabled (see Signals).

 12. The -n invocation option is ignored, and set -n has no effect (see The Set Builtin).

 13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH, and MAILCHECK shell variables (see Bash Variables).

 14. Expansion errors due to references to unbound shell variables after set -u has been enabled will not cause the shell to exit (see The Set Builtin).

 15. The shell will not exit on expansion errors caused by VAR being unset or null in ${VAR:?WORD} expansions (see Shell Parameter Expansion).

 16. Redirection errors encountered by shell builtins will not cause the shell to exit.

 17. When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see Bash POSIX Mode).

 18. A failed exec will not cause the shell to exit (see Bourne Shell Builtins).

 19. Parser syntax errors will not cause the shell to exit.

 20. Simple spelling correction for directory arguments to the cd builtin is enabled by default (see the description of the cdspell option to the shopt builtin in The Shopt Builtin).

 21. The shell will check the value of the TMOUT variable and exit if a command is not read within the specified number of seconds after printing $PS1 (see Bash Variables).

Bash Conditional Expressions
============================

Conditional expressions are used by the [[ compound command and the test and [ builtin commands. The test and [ commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.

Expressions may be unary or binary, and are formed from the following primaries. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. 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 will use them; otherwise it will emulate them internally with this behavior: If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the FILE argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

When used with [[, the < and > operators sort lexicographically using the current locale. The test command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.

See the description of the test builtin command (section see Bash Builtins below) for the handling of parameters (i.e. missing parameters).

-a FILE
     True if FILE exists.

-b FILE
     True if FILE exists and is a block special file.

-c FILE
     True if FILE exists and is a character special file.

-d FILE
     True if FILE exists and is a directory.

-e FILE
     True if FILE exists.

-f FILE
     True if FILE exists and is a regular file.

-g FILE
     True if FILE exists and its set-group-id bit is set.

-h FILE
     True if FILE exists and is a symbolic link.

-k FILE
     True if FILE exists and its "sticky" bit is set.

-p FILE
     True if FILE exists and is a named pipe (FIFO).

-r FILE
     True if FILE exists and is readable.

-s FILE
     True if FILE exists and has a size greater than zero.

-t FD
     True if file descriptor FD is open and refers to a terminal.

-u FILE
     True if FILE exists and its set-user-id bit is set.

-w FILE
     True if FILE exists and is writable.

-x FILE
     True if FILE exists and is executable.

-G FILE
     True if FILE exists and is owned by the effective group id.

-L FILE
     True if FILE exists and is a symbolic link.

-N FILE
     True if FILE exists and has been modified since it was last read.

-O FILE
     True if FILE exists and is owned by the effective user id.

-S FILE
     True if FILE exists and is a socket.

FILE1 -ef FILE2
     True if FILE1 and FILE2 refer to the same device and inode numbers.

FILE1 -nt FILE2
     True if FILE1 is newer (according to modification date) than FILE2, or if FILE1 exists and FILE2 does not.

FILE1 -ot FILE2
     True if FILE1 is older than FILE2, or if FILE2 exists and FILE1 does not.

-o OPTNAME
     True if the shell option OPTNAME is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).

-v VARNAME
     True if the shell variable VARNAME is set (has been assigned a value).

-R VARNAME
     True if the shell variable VARNAME is set and is a name reference.

-z STRING
     True if the length of STRING is zero.

-n STRING
STRING
     True if the length of STRING is non-zero.

STRING1 == STRING2
STRING1 = STRING2
     True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs).

     = should be used with the test command for POSIX conformance.

STRING1 != STRING2
     True if the strings are not equal.

STRING1 < STRING2
     True if STRING1 sorts before STRING2 lexicographically.

STRING1 > STRING2
     True if STRING1 sorts after STRING2 lexicographically.

ARG1 OP ARG2
     OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true if ARG1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to ARG2, respectively.  ARG1 and ARG2 may be positive or negative integers. When used with the [[ command, ARG1 and ARG2 are evaluated as arithmetic expressions (see Shell Arithmetic).

Shell Arithmetic
================

The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by using the (( compound command, the let builtin, or the -i option to the declare builtin.

Evaluation is done in fixed-width integers 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.

ID++ ID--
     variable post-increment and post-decrement

++ID --ID
     variable pre-increment and pre-decrement

- +
     unary minus and plus

! ~
     logical and bitwise negation

**
     exponentiation

* / %
     multiplication, division, remainder

+ -
     addition, subtraction

<< >>
     left and right bitwise shifts

<= >= < >
     comparison

== !=
     equality and inequality

&
     bitwise AND

^
     bitwise exclusive OR

|
     bitwise OR

&&
     logical AND

||
     logical OR

expr ? expr : expr
     conditional operator

= *= /= %= += -= <<= >>= &= ^= |=
     assignment

expr1 , expr2
     comma

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. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the INTEGER attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its INTEGER attribute turned on to be used in an expression.

Integer constants follow the C language definition, without suffixes or character constants. Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal.  Otherwise, numbers take the form [BASE#]N, where the optional BASE is a decimal number between 2 and 64 representing the arithmetic base, and N is a number in that base. If BASE# is omitted, then base 10 is used. When specifying N, if a non-digit is required, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If BASE is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in order of precedence.  Sub-expressions in parentheses are evaluated first and may override the precedence rules above.

Aliases
=======

ALIASES allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands.

The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The characters /, $, `, = 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 ls to "ls -F", for instance, and Bash does not try to recursively expand the replacement text. If the last character of the alias value is a BLANK, then the next command word following the alias is also checked for alias expansion.

Aliases are created and listed with the alias command, and removed with the unalias command.

There is no mechanism for using arguments in the replacement text, as in csh. If arguments are needed, a shell function should be used (see Shell Functions).

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see The Shopt Builtin).

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 next line of input is read. The commands following the alias definition on that line 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 alias in compound commands.

For almost every purpose, shell functions are preferred over aliases.

Arrays
======

Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the declare builtin will explicitly declare 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 integers (including arithmetic expressions (see Shell Arithmetic)) and are zero-based; associative arrays use arbitrary strings. Unless otherwise noted, indexed array indices must be non-negative integers.

An indexed array is created automatically if any variable is assigned to using the syntax

     NAME[SUBSCRIPT]=VALUE

The SUBSCRIPT is treated as an arithmetic expression that must evaluate to a number. To explicitly declare an array, use

     declare -a NAME

The syntax

     declare -a NAME[SUBSCRIPT]

is also accepted; the SUBSCRIPT is ignored.

Associative arrays are created using

     declare -A NAME

Attributes may be specified for an array variable using the declare and readonly builtins.  Each attribute applies to all members of an array.

Arrays are assigned to using compound assignments of the form

     NAME=(VALUE1 VALUE2 ... )

where each VALUE may be of the form [SUBSCRIPT]=STRING. Indexed array assignments do not require anything but STRING. 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.

Each VALUE in the list undergoes all the shell expansions described above (see Shell Expansions).

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: NAME=(KEY1 VALUE1 KEY2 VALUE2 ... ). These are treated identically to NAME=( [KEY1]=VALUE1 [KEY2]=VALUE2 ... ). 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.

This syntax is also accepted by the declare builtin.  Individual array elements may be assigned to using the NAME[SUBSCRIPT]=VALUE syntax introduced above.

When assigning to an indexed array, if NAME is subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of NAME, so negative indices count back from the end of the array, and an index of -1 references the last element.

Any element of an array may be referenced using ${NAME[SUBSCRIPT]}. The braces are required to avoid conflicts with the shell's filename expansion operators.  If the SUBSCRIPT is @ or *, the word expands to all members of the array NAME.  These subscripts differ only when the word appears within double quotes. If the word is double-quoted, ${NAME[*]} expands to a single word with the value of each array member separated by the first character of the IFS variable, and ${NAME[@]} expands each element of NAME to a separate word.  When there are no array members, ${NAME[@]} 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 original word, and the expansion of the last parameter is joined with the last part of the original word. This is analogous to the expansion of the special parameters @ and *. ${#NAME[SUBSCRIPT]} expands to the length of ${NAME[SUBSCRIPT]}. If SUBSCRIPT is @ or *, the expansion is the number of elements in the array. If the SUBSCRIPT 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.

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 legal, and bash will create an array if necessary.

An array variable is considered set if a subscript has been assigned a value.  The null string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values. ${!NAME[@]} and ${!NAME[*]} expand to the indices assigned in array variable NAME. The treatment when in double quotes is similar to the expansion of the special parameters @ and * within double quotes.

The unset builtin is used to destroy arrays. unset NAME[SUBSCRIPT] destroys the array element at index SUBSCRIPT. Negative subscripts to indexed arrays are interpreted as described above. Unsetting the last element of an array variable does not unset the variable. unset NAME, where NAME is an array, removes the entire array.  A subscript of * or @ also removes the entire array.

When using a variable name with a subscript as an argument to a command, such as with unset, without using the word expansion syntax described above, the argument is subject to the shell's filename expansion. If filename expansion is not desired, the argument should be quoted.

The declare, local, and readonly builtins each accept a -a option to specify an indexed array and a -A option to specify an associative array. If both options are supplied, -A takes precedence. The read builtin accepts a -a 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 set and declare builtins display array values in a way that allows them to be reused as input.

The Directory Stack
===================

The directory stack is a list of recently-visited directories.  The pushd builtin adds directories to the stack as it changes the current directory, and the popd builtin removes specified directories from the stack and changes the current directory to the directory removed.  The dirs builtin displays the contents of the directory stack.  The current directory is always the "top" of the directory stack.

The contents of the directory stack are also visible as the value of the DIRSTACK shell variable.

Directory Stack Builtins
------------------------

dirs

          dirs [-clpv] [+N | -N]

     Display the list of currently remembered directories.  Directories are added to the list with the pushd command; the popd command removes directories from the list. The current directory is always the first directory in the stack.

     -c
          Clears the directory stack by deleting all of the elements.

     -l
          Produces a listing using full pathnames; the default listing format uses a tilde to denote the home directory.

     -p
          Causes dirs to print the directory stack with one entry per line.

     -v
          Causes dirs to print the directory stack with one entry per line, prefixing each entry with its index in the stack.

     +N
          Displays the Nth directory (counting from the left of the list printed by dirs when invoked without options), starting with zero.

     -N
          Displays the Nth directory (counting from the right of the list printed by dirs when invoked without options), starting with zero.

popd

          popd [-n] [+N | -N]

     When no arguments are given, popd removes the top directory from the stack and performs a cd to the new top directory. The elements are numbered from 0 starting at the first directory listed with dirs; that is, popd is equivalent to popd +0.

     -n
          Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated.

     +N
          Removes the Nth directory (counting from the left of the list printed by dirs), starting with zero.

     -N
          Removes the Nth directory (counting from the right of the list printed by dirs), starting with zero.

pushd

          pushd [-n] [+N | -N | DIR]

     Save the current directory on the top of the directory stack and then cd to DIR. With no arguments, pushd exchanges the top two directories and makes the new top the current directory.

     -n
          Suppresses the normal change of directory when rotating or adding directories to the stack, so that only the stack is manipulated.

     +N
          Brings the Nth directory (counting from the left of the list printed by dirs, starting with zero) to the top of the list by rotating the stack.

     -N
          Brings the Nth directory (counting from the right of the list printed by dirs, starting with zero) to the top of the list by rotating the stack.

     DIR
          Makes DIR be the top of the stack, making it the new current directory as if it had been supplied as an argument to the cd builtin.

Controlling the Prompt
======================

Bash examines the value of the array variable PROMPT_COMMANDS just before printing each primary prompt. If any elements in  PROMPT_COMMANDS are set and non-null, Bash executes each value, in numeric order, just as if it had been typed on the command line.

In addition, the following table describes the special characters which can appear in the prompt variables PS0, PS1, PS2, and PS4:

\a
     A bell character.

\d
     The date, in "Weekday Month Date" format (e.g., "Tue May 26").

\D{FORMAT}
     The FORMAT is passed to strftime(3) and the result is inserted into the prompt string; an empty FORMAT results in a locale-specific time representation.  The braces are required.

\e
     An escape character.

\h
     The hostname, up to the first `.'.

\H
     The hostname.

\j
     The number of jobs currently managed by the shell.

\l
     The basename of the shell's terminal device name.

\n
     A newline.

\r
     A carriage return.

\s
     The name of the shell, the basename of $0 (the portion following the final slash).

\t
     The time, in 24-hour HH:MM:SS format.

\T
     The time, in 12-hour HH:MM:SS format.

\@
     The time, in 12-hour am/pm format.

\A
     The time, in 24-hour HH:MM format.

\u
     The username of the current user.

\v
     The version of Bash (e.g., 2.00)

\V
     The release of Bash, version + patchlevel (e.g., 2.00.0)

\w
     The current working directory, with $HOME abbreviated with a tilde (uses the $PROMPT_DIRTRIM variable).

\W
     The basename of $PWD, with $HOME abbreviated with a tilde.

\!
     The history number of this command.

\#
     The command number of this command.

\$
     If the effective uid is 0, #, otherwise $.

\NNN
     The character whose ASCII code is the octal value NNN.

\\
     A backslash.

\[
     Begin a sequence of non-printing characters.  This could be used to embed a terminal control sequence into the prompt.

\]
     End a sequence of non-printing characters.

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 Bash History Facilities), while the command number is the position in the sequence of commands executed during the current shell session.

After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see The Shopt Builtin). This can have unwanted side effects if escaped portions of the string appear within command substitution or contain characters special to word expansion.

The Restricted Shell
====================

If Bash is started with the name rbash, or the --restricted or -r option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to bash with the exception that the following are disallowed or not performed:

  * Changing directories with the cd builtin.

  * Setting or unsetting the values of the SHELL, PATH, HISTFILE, ENV, or BASH_ENV variables.

  * Specifying command names containing slashes.

  * Specifying a filename containing a slash as an argument to the . builtin command.

  * Specifying a filename containing a slash as an argument to the history builtin command.

  * Specifying a filename containing a slash as an argument to the -p option to the hash builtin command.

  * Importing function definitions from the shell environment at startup.

  * Parsing the value of SHELLOPTS from the shell environment at startup.

  * Redirecting output using the >, >|, <>, >&, &>, and >> redirection operators.

  * Using the exec builtin to replace the shell with another command.

  * Adding or deleting builtin commands with the -f and -d options to the enable builtin.

  * Using the enable builtin command to enable disabled shell builtins.

  * Specifying the -p option to the command builtin.

  * Turning off restricted mode with set +r or set +o restricted.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see Shell Scripts), rbash turns off any restrictions in the shell spawned to execute the script.

The restricted shell mode is only one component of a useful restricted environment. It should be accompanied by setting PATH to a value that allows execution of only a few verified commands (commands that allow shell escapes are particularly vulnerable), leaving the user in a non-writable directory other than his home directory 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., VISUAL or PAGER).

Modern systems provide more secure ways to implement a restricted environment, such as jails, zones, or containers.

Bash POSIX Mode
===============

Starting Bash with the --posix command-line option or executing set -o posix while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.

When invoked as sh, Bash enters POSIX mode after reading the startup files.

The following list is what's changed when `POSIX mode' is in effect:

 1. Bash ensures that the POSIXLY_CORRECT variable is set.

 2. When a command in the hash table no longer exists, Bash will re-search $PATH to find the new location.  This is also available with shopt -s checkhash.

 3. 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 $PATH search.

 4. The message printed by the job control code and builtins when a job exits with a non-zero status is `Done(status)'.

 5. The message printed by the job control code and builtins when a job is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example, SIGTSTP.

 6. Alias expansion is always enabled, even in non-interactive shells.

 7. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.

 8. The POSIX PS1 and PS2 expansions of ! to the history number and !! to ! are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.

 9. The POSIX startup files are executed ($ENV) rather than the normal Bash files.

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

 11. The default history file is ~/.sh_history (this is the default value of $HISTFILE).

 12. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.

 13. Redirection operators do not perform word splitting on the word in the redirection.

 14. Function names must be valid shell names.  That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit.  Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.

 15. Function names may not be the same as one of the POSIX special builtins.

 16. POSIX special builtins are found before shell functions during command lookup.

 17. When printing shell function definitions (e.g., by type), Bash does not print the function keyword.

 18. Literal tildes that appear as the first character in elements of the PATH variable are not expanded as described above under Tilde Expansion.

 19. The time reserved word may be used by itself as a command.  When used in this way, it displays timing statistics for the shell and its completed children.  The TIMEFORMAT variable controls the format of the timing information.

 20. When parsing and expanding a ${...} 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.

 21. The parser does not recognize time as a reserved word if the next token begins with a -.

 22. The ! character does not introduce history expansion within a double-quoted string, even if the histexpand option is enabled.

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

 24. 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.

 25. 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.

 26. A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable.

 27. Non-interactive shells exit if FILENAME in . FILENAME is not found.

 28. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.

 29. Non-interactive shells exit if a parameter expansion error occurs.

 30. Non-interactive shells exit if there is a syntax error in a script read with the . or source builtins, or in a string processed by the eval builtin.

 31. While variable indirection is available, it may not be applied to the # and ? special parameters.

 32. When expanding the * special parameter in a pattern context where the expansion is double-quoted does not treat the $* as if it were double-quoted.

 33. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.

 34. The command builtin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, assignment builtins lose their assignment statement expansion properties when preceded by command.

 35. The bg 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.

 36. The output of kill -l prints all the signal names on a single line, separated by spaces, without the SIG prefix.

 37. The kill builtin does not accept signal names with a SIG prefix.

 38. The export and readonly builtin commands display their output in the format required by POSIX.

 39. The trap builtin displays signal names without the leading SIG.

 40. The trap builtin doesn'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 - as the first argument.

 41. trap -p displays signals whose dispositions are set to SIG_DFL and those that were ignored when the shell started.

 42. The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.

 43. Enabling POSIX mode has the effect of setting the inherit_errexit option, so subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When the inherit_errexit option is not enabled, Bash clears the -e option in such subshells.

 44. Enabling POSIX mode has the effect of setting the shift_verbose option, so numeric arguments to shift that exceed the number of positional parameters will result in an error message.

 45. When the alias builtin displays alias definitions, it does not display them with a leading alias  unless the -p option is supplied.

 46. When the set builtin is invoked without options, it does not display shell function names and definitions.

 47. When the set builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.

 48. When the cd builtin is invoked in LOGICAL mode, and the pathname constructed from $PWD and the directory name supplied as an argument does not refer to an existing directory, cd will fail instead of falling back to PHYSICAL mode.

 49. When the cd builtin cannot change a directory because the length of the pathname constructed from $PWD and the directory name supplied as an argument exceeds PATH_MAX when all symbolic links are expanded, cd will fail instead of attempting to use only the supplied directory name.

 50. The pwd 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 -P option.

 51. When listing the history, the fc builtin does not include an indication of whether or not a history entry has been modified.

 52. The default editor used by fc is ed.

 53. The type and command 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 $PATH.

 54. The vi editing mode will invoke the vi editor directly when the v command is run, instead of checking $VISUAL and $EDITOR.

 55. When the xpg_echo option is enabled, Bash does not attempt to interpret any arguments to echo as options.  Each argument is displayed, after escape characters are converted.

 56. The ulimit builtin uses a block size of 512 bytes for the -c and -f options.

 57. The arrival of SIGCHLD  when a trap is set on SIGCHLD does not interrupt the wait builtin and cause it to return immediately. The trap command is run once for each child that exits.

 58. The read builtin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executing read, the trap handler executes and read returns an exit status greater than 128.

 59. Bash removes an exited background process's status from the list of such statuses after the wait builtin is used to obtain it.

There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:

 1. The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset, rather than defaulting directly to ed.  fc uses ed if EDITOR is unset.

 2. As noted above, Bash requires the xpg_echo option to be enabled for the echo builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by specifying the --enable-strict-posix-default to configure when building (see Optional Features).

Shell Compatibility Mode
========================

Bash-4.0 introduced the concept of a `shell compatibility level', specified as a set of options to the shopt builtin (compat31, compat32, compat40, compat41, and so on). There is only one current compatibility level -- 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's intended to be a temporary solution.

This section does not mention behavior that is standard for a particular version (e.g., setting compat32 means that quoting the rhs of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2 and above).

If a user enables, say, compat32, 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 [[ command came in bash-4.1, and earlier versions used ASCII-based comparisons, so enabling compat32 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.

Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value assigned to this variable (a decimal version number like 4.2, or an integer corresponding to the compatNN option, like 42) determines the compatibility level.

Starting with bash-4.4, Bash has begun deprecating older compatibility levels. Eventually, the options will be removed in favor of BASH_COMPAT.

Bash-5.0 is the final version for which there will be an individual shopt option for the previous version. Users should use BASH_COMPAT on bash-5.0 and later versions.

The following table describes the behavior changes controlled by each compatibility level setting. The compatNN tag is used as shorthand for setting the compatibility level to NN using one of the following mechanisms. For versions prior to bash-5.0, the compatibility level may be set using the corresponding compatNN shopt option. For bash-4.3 and later versions, the BASH_COMPAT variable is preferred, and it is required for bash-5.1 and later versions.

compat31

       * quoting the rhs of the [[ command's regexp matching operator (=~) has no special effect

compat32

       * interrupting a command list such as "a ; b ; c" causes the execution of the next command in the list (in bash-4.0 and later versions, the shell acts as if it received the interrupt, so interrupting one command in a list aborts the execution of the entire list)

compat40

       * the < and > operators to the [[ 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's collation sequence and strcoll(3).

compat41

       * in posix mode, time may be followed by options and still be recognized as a reserved word (this is POSIX interpretation 267)

       * in posix mode, the parser requires that an even number of single quotes occur in the WORD portion of a double-quoted ${...} parameter expansion and treats them specially, so that characters within the single quotes are considered quoted (this is POSIX interpretation 221)

compat42

       * the replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2

       * in posix mode, single quotes are considered special when expanding the WORD portion of a double-quoted ${...} parameter expansion and can be used to quote a closing brace or other special character (this is part of POSIX interpretation 221); in later versions, single quotes are not special within double-quoted word expansions

compat43

       * the shell does not print a warning message if an attempt is made to use a quoted compound assignment as an argument to declare (declare -a foo='(1 2)'). Later versions warn that this usage is deprecated

       * word expansion errors are considered non-fatal errors that cause the current command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit)

       * when executing a shell function, the loop state (while/until/etc.) is not reset, so break or continue in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this

compat44

       * the shell sets up the values used by BASH_ARGV and BASH_ARGC so they can expand to the shell's positional parameters even if extended debugging mode is not enabled

       * a subshell inherits loops from its parent context, so break or continue will cause the subshell to exit. Bash-5.0 and later reset the loop state to prevent the exit

       * variable assignments preceding builtins like export and readonly that set attributes continue to affect variables with the same name in the calling environment even if the shell is not in posix mode

compat50 (set using BASH_COMPAT)

       * Bash-5.1 changed the way $RANDOM 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 RANDOM will produce the same sequence as in bash-5.0

       * 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 -l option is supplied.

Job Control
***********

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

Job Control Basics
==================

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's terminal driver and Bash.

The shell associates a JOB with each pipeline.  It keeps a table of currently executing jobs, which may be listed with the jobs command.  When Bash starts a job asynchronously, it prints a line that looks like:

     [1] 25647

indicating that this job is job number 1 and that the process ID 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 JOB abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the operating system maintains the notion of a current terminal process group ID.  Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the foreground.  Background processes are those whose process group ID differs from the terminal's; such processes are immune to keyboard-generated signals.  Only foreground processes are allowed to read from or, if the user so specifies with stty tostop, write to the terminal. Background processes which attempt to read from (write to when stty tostop is in effect) the terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel's terminal driver, which, unless caught, suspends the process.

If the operating system on which Bash is running supports job control, Bash contains facilities to use it.  Typing the SUSPEND character (typically ^Z, Control-Z) while a process is running causes that process to be stopped and returns control to Bash.  Typing the DELAYED SUSPEND character (typically ^Y, Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash.  The user then manipulates the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it.  A ^Z takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell.  The character % introduces a job specification (JOBSPEC).

Job number n may be referred to as %n. The symbols %% and  %+ refer to the shell's notion of the current job, which is the last job stopped while it was in the foreground or started in the background. A single % (with no accompanying job specification) also refers to the current job. The previous job may be referenced using %-. If there is only a single job, %+ and %- can both be used to refer to that job. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a +, and the previous job with a -.

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, %ce refers to a stopped job whose command name begins with ce. Using %?ce, on the other hand, refers to any job containing the string ce in its command line.  If the prefix or substring matches more than one job, Bash reports an error.

Simply naming a job can be used to bring it into the foreground: %1 is a synonym for fg %1, bringing job 1 from the background into the foreground.  Similarly, %1 & resumes job 1 in the background, equivalent to bg %1

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job's status so as to not interrupt any other output. If the -b option to the set builtin is enabled, Bash reports such changes immediately (see The Set Builtin). Any trap on SIGCHLD is executed for each child process that exits.

If an attempt to exit Bash is made while jobs are stopped, (or running, if the checkjobs option is enabled -- see The Shopt Builtin), the shell prints a warning message, and if the checkjobs option is enabled, lists the jobs and their statuses. The jobs command may then be used to inspect their status. If a second attempt to exit is made without an intervening command, Bash does not print another warning, and any stopped jobs are terminated.

When the shell is waiting for a job or process using the wait builtin, and job control is enabled, wait will return when the job changes state. The -f option causes wait to wait until the job or process terminates before returning.

Job Control Builtins
====================

bg

          bg [JOBSPEC ...]

     Resume each suspended job JOBSPEC in the background, as if it had been started with &. If JOBSPEC is not supplied, the current job is used. The return status is zero unless it is run when job control is not enabled, or, when run with job control enabled, any JOBSPEC was not found or specifies a job that was started without job control.

fg

          fg [JOBSPEC]

     Resume the job JOBSPEC in the foreground and make it the current job. If JOBSPEC is not supplied, the current job is used. 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, JOBSPEC does not specify a valid job or JOBSPEC specifies a job that was started without job control.

jobs

          jobs [-lnprs] [JOBSPEC]
          jobs -x COMMAND [ARGUMENTS]

     The first form lists the active jobs.  The options have the following meanings:

     -l
          List process IDs in addition to the normal information.

     -n
          Display information only about jobs that have changed status since the user was last notified of their status.

     -p
          List only the process ID of the job's process group leader.

     -r
          Display only running jobs.

     -s
          Display only stopped jobs.

     If JOBSPEC is given, output is restricted to information about that job. If JOBSPEC is not supplied, the status of all jobs is listed.

     If the -x option is supplied, jobs replaces any JOBSPEC found in COMMAND or ARGUMENTS with the corresponding process group ID, and executes COMMAND, passing it ARGUMENTs, returning its exit status.

kill

          kill [-s SIGSPEC] [-n SIGNUM] [-SIGSPEC] JOBSPEC or PID
          kill -l|-L [EXIT_STATUS]

     Send a signal specified by SIGSPEC or SIGNUM to the process named by job specification JOBSPEC or process ID PID. SIGSPEC is either a case-insensitive signal name such as SIGINT (with or without the SIG prefix) or a signal number; SIGNUM is a signal number. If SIGSPEC and SIGNUM are not present, SIGTERM is used. The -l option lists the signal names. If any arguments are supplied when -l is given, the names of the signals corresponding to the arguments are listed, and the return status is zero. EXIT_STATUS is a number specifying a signal number or the exit status of a process terminated by a signal. The -L option is equivalent to -l. 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.

wait

          wait [-fn] [-p VARNAME] [JOBSPEC or PID ...]

     Wait until the child process specified by each process ID PID or job specification JOBSPEC exits and return the exit status of the last command waited for. If a job spec is given, all processes in the job are waited for. If no arguments are given, wait waits for all running background jobs and the last-executed process substitution, if its process id is the same as $!, and the return status is zero. If the -n option is supplied, wait waits for a single job from the list of PIDS or JOBSPECS or, if no arguments are supplied, any job, to complete and returns its exit status. If none of the supplied arguments 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. If the -p option is supplied, the process or job identifier of the job for which the exit status is returned is assigned to the variable VARNAME named by the option argument. The variable will be unset initially, before any assignment. This is useful only when the -n option is supplied. Supplying the -f option, when job control is enabled, forces wait to wait for each PID or JOBSPEC to terminate before returning its status, intead of returning when it changes status. If neither JOBSPEC nor PID specifies an active child process of the shell, the return status is 127.

disown

          disown [-ar] [-h] [JOBSPEC ... | PID ... ]

     Without options, remove each JOBSPEC from the table of active jobs. If the -h option is given, the job is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If JOBSPEC is not present, and neither the -a nor the -r option is supplied, the current job is used. If no JOBSPEC is supplied, the -a option means to remove or mark all jobs; the -r option without a JOBSPEC argument restricts operation to running jobs.

suspend

          suspend [-f]

     Suspend the execution of this shell until it receives a SIGCONT signal. A login shell cannot be suspended; the -f option can be used to override this and force the suspension.

When job control is not active, the kill and wait builtins do not accept JOBSPEC arguments.  They must be supplied process IDs.

Job Control Variables
=====================

auto_resume
     This variable controls how the shell interacts with the user and job control.  If this variable exists then single word simple commands 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 the string typed, then the most recently accessed job will be selected. The name of a stopped job, in this context, is the command line used to start it.  If this variable is set to the value exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied needs to match a substring of the name of a stopped job.  The substring value provides functionality analogous to the %? job ID (see Job Control Basics). If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the % job ID.

     [file rluser.texi not included]
     [file hsuser.texi not included]

Installing Bash
***************

This chapter provides basic instructions for installing Bash on the various supported platforms.  The distribution supports the GNU operating systems, nearly every version of Unix, and several non-Unix systems such as BeOS and Interix. Other independent ports exist for MS-DOS, OS/2, and Windows platforms.

Basic Installation
==================

These are installation instructions for Bash.

The simplest way to compile Bash is:

 1. cd to the directory containing the source code and type ./configure to configure Bash for your system.  If you're using csh on an old version of System V, you might need to type sh ./configure instead to prevent csh from trying to execute configure itself.

     Running configure takes some time. While running, it prints messages telling which features it is checking for.

 2. Type make to compile Bash and build the bashbug bug reporting script.

 3. Optionally, type make tests to run the Bash test suite.

 4. Type make install to install bash and bashbug. This will also install the manual pages and Info file.

The configure shell script attempts to guess correct values for various system-dependent variables used during compilation.  It uses those values to create a Makefile in each directory of the package (the top directory, the builtins, doc, and support directories, each directory under lib, and several others).  It also creates a config.h file containing system-dependent definitions. Finally, it creates a shell script named config.status that you can run in the future to recreate the current configuration, a file config.cache that saves the results of its tests to speed up reconfiguring, and a file config.log containing compiler output (useful mainly for debugging configure). If at some point config.cache contains results you don't want to keep, you may remove or edit it.

To find out more about the options and arguments that the configure script understands, type

     bash-4.2$ ./configure --help

at the Bash prompt in your Bash source directory.

If you want to build Bash in a directory separate from the source directory -- to build for multiple architectures, for example -- just use the full path to the configure script. The following commands will build bash in a directory under /usr/local/build from the source code in /usr/local/src/bash-4.4:

     mkdir /usr/local/build/bash-4.4
     cd /usr/local/build/bash-4.4
     bash /usr/local/src/bash-4.4/configure
     make

See Compiling For Multiple Architectures for more information about building in a directory separate from the source.

If you need to do unusual things to compile Bash, please try to figure out how configure could check whether or not to do them, and mail diffs or instructions to bash-maintainers@gnu.org so they can be considered for the next release.

The file configure.ac is used to create configure by a program called Autoconf.  You only need configure.ac if you want to change it or regenerate configure using a newer version of Autoconf.  If you do this, make sure you are using Autoconf version 2.50 or newer.

You can remove the program binaries and object files from the source code directory by typing make clean.  To also remove the files that configure created (so you can compile Bash for a different kind of computer), type make distclean.

Compilers and Options
=====================

Some systems require unusual options for compilation or linking that the configure script does not know about.  You can give configure 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:

     CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure

On systems that have the env program, you can do it like this:

     env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure

The configuration process uses GCC to build Bash if it is available.

Compiling For Multiple Architectures
====================================

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 make that supports the VPATH variable, such as GNU make. cd to the directory where you want the object files and executables to go and run the configure script from the source directory (see Basic Installation). You may need to supply the --srcdir=PATH argument to tell configure where the source files are.  configure automatically checks for the source code in the directory that configure is in and in `..'.

If you have to use a make that does not supports the VPATH 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 make distclean before reconfiguring for another architecture.

Alternatively, if your system supports symbolic links, you can use the support/mkclone script to create a build tree which has symbolic links back to each file in the source directory.  Here's an example that creates a build directory in the current directory from a source directory /usr/gnu/src/bash-2.0:

     bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .

The mkclone script requires Bash, so you must have already built Bash for at least one architecture before you can create build directories for other architectures.

Installation Names
==================

By default, make install will install into /usr/local/bin, /usr/local/man, etc.  You can specify an installation prefix other than /usr/local by giving configure the option --prefix=PATH, or by specifying a value for the DESTDIR make variable when running make install.

You can specify separate installation prefixes for architecture-specific files and architecture-independent files. If you give configure the option --exec-prefix=PATH, make install will use PATH as the prefix for installing programs and libraries. Documentation and other data files will still use the regular prefix.

Specifying the System Type
==========================

There may be some features configure can not figure out automatically, but need to determine by the type of host Bash will run on.  Usually configure can figure that out, but if it prints a message saying it can not guess the host type, give it the --host=TYPE option.  TYPE can either be a short name for the system type, such as sun4, or a canonical name with three fields: CPU-COMPANY-SYSTEM (e.g., i386-unknown-freebsd4.2).

See the file support/config.sub for the possible values of each field.

Sharing Defaults
================

If you want to set default values for configure scripts to share, you can create a site shell script called config.site that gives default values for variables like CC, cache_file, and prefix.  configure looks for PREFIX/share/config.site if it exists, then PREFIX/etc/config.site if it exists.  Or, you can set the CONFIG_SITE environment variable to the location of the site script.  A warning: the Bash configure looks for a site script, but not all configure scripts do.

Operation Controls
==================

configure recognizes the following options to control how it operates.

--cache-file=FILE
     Use and save the results of the tests in FILE instead of ./config.cache.  Set FILE to /dev/null to disable caching, for debugging configure.

--help
     Print a summary of the options to configure, and exit.

--quiet
--silent
-q
     Do not print messages saying which checks are being made.

--srcdir=DIR
     Look for the Bash source code in directory DIR.  Usually configure can determine that directory automatically.

--version
     Print the version of Autoconf used to generate the configure script, and exit.

configure also accepts some other, not widely used, boilerplate options.  configure --help prints the complete list.

Optional Features
=================

The Bash configure has a number of --enable-FEATURE options, where FEATURE indicates an optional part of Bash. There are also several --with-PACKAGE options, where PACKAGE is something like bash-malloc or purify. To turn off the default use of a package, use --without-PACKAGE.  To configure Bash without a feature that is enabled by default, use --disable-FEATURE.

Here is a complete list of the --enable- and --with- options that the Bash configure recognizes.

--with-afs
     Define if you are using the Andrew File System from Transarc.

--with-bash-malloc
     Use the Bash version of malloc in the directory lib/malloc.  This is not the same malloc that appears in GNU libc, but an older version originally derived from the 4.2 BSD malloc.  This malloc is very fast, but wastes some space on each allocation. This option is enabled by default. The NOTES file contains a list of systems for which this should be turned off, and configure disables this option automatically for a number of systems.

--with-curses
     Use the curses library instead of the termcap library.  This should be supplied if your system has an inadequate or incomplete termcap database.

--with-gnu-malloc
     A synonym for --with-bash-malloc.

--with-installed-readline[=PREFIX]
     Define this to make Bash link with a locally-installed version of Readline rather than the version in lib/readline.  This works only with Readline 5.0 and later versions.  If PREFIX is yes or not supplied, configure uses the values of the make variables includedir and libdir, which are subdirectories of prefix by default, to find the installed version of Readline if it is not in the standard system include and library directories. If PREFIX is no, Bash links with the version in lib/readline. If PREFIX is set to any other value, configure treats it as a directory pathname and looks for the installed version of Readline in subdirectories of that directory (include files in PREFIX/include and the library in PREFIX/lib).

--with-purify
     Define this to use the Purify memory allocation checker from Rational Software.

--enable-minimal-config
     This produces a shell with minimal features, close to the historical Bourne shell.

There are several --enable- options that alter how Bash is compiled and linked, rather than changing run-time features.

--enable-largefile
     Enable support for large files (http://www.unix.org/version2/whatsnew/lfs20mar.html) 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.

--enable-profiling
     This builds a Bash binary that produces profiling information to be processed by gprof each time it is executed.

--enable-static-link
     This causes Bash to be linked statically, if gcc is being used. This could be used to build a version to use as root's shell.

The minimal-config option can be used to disable all of the following options, but it is processed first, so individual options may be enabled using enable-FEATURE.

All of the following options except for disabled-builtins, direxpand-default, and xpg-echo-default are enabled by default, unless the operating system does not provide the necessary support.

--enable-alias
     Allow alias expansion and include the alias and unalias builtins (see Aliases).

--enable-arith-for-command
     Include support for the alternate form of the for command that behaves like the C language for statement (see Looping Constructs).

--enable-array-variables
     Include support for one-dimensional array shell variables (see Arrays).

--enable-bang-history
     Include support for csh-like history substitution (see History Interaction).

--enable-brace-expansion
     Include csh-like brace expansion ( b{a,b}c ==> bac bbc ). See Brace Expansion, for a complete description.

--enable-casemod-attributes
     Include support for case-modifying attributes in the declare builtin and assignment statements.  Variables with the UPPERCASE attribute, for example, will have their values converted to uppercase upon assignment.

--enable-casemod-expansion
     Include support for case-modifying word expansions.

--enable-command-timing
     Include support for recognizing time as a reserved word and for displaying timing statistics for the pipeline following time (see Pipelines). This allows pipelines as well as shell builtins and functions to be timed.

--enable-cond-command
     Include support for the [[ conditional command. (see Conditional Constructs).

--enable-cond-regexp
     Include support for matching POSIX regular expressions using the =~ binary operator in the [[ conditional command. (see Conditional Constructs).

--enable-coprocesses
     Include support for coprocesses and the coproc reserved word (see Pipelines).

--enable-debugger
     Include support for the bash debugger (distributed separately).

--enable-dev-fd-stat-broken
     If calling stat on /dev/fd/N returns different results than calling fstat on file descriptor N, supply this option to enable a workaround. This has implications for conditional commands that test file attributes.

--enable-direxpand-default
     Cause the direxpand shell option (see The Shopt Builtin) to be enabled by default when the shell starts. It is normally disabled by default.

--enable-directory-stack
     Include support for a csh-like directory stack and the pushd, popd, and dirs builtins (see The Directory Stack).

--enable-disabled-builtins
     Allow builtin commands to be invoked via builtin xxx even after xxx has been disabled using enable -n xxx. See Bash Builtins, for details of the builtin and enable builtin commands.

--enable-dparen-arithmetic
     Include support for the ((...)) command (see Conditional Constructs).

--enable-extended-glob
     Include support for the extended pattern matching features described above under Pattern Matching.

--enable-extended-glob-default
     Set the default value of the EXTGLOB shell option described above under The Shopt Builtin to be enabled.

--enable-function-import
     Include support for importing function definitions exported by another instance of the shell from the environment.  This option is enabled by default.

--enable-glob-asciirange-default
     Set the default value of the GLOBASCIIRANGES shell option described above under The Shopt Builtin to be enabled. This controls the behavior of character ranges when used in pattern matching bracket expressions.

--enable-help-builtin
     Include the help builtin, which displays help on shell builtins and variables (see Bash Builtins).

--enable-history
     Include command history and the fc and history builtin commands (see Bash History Facilities).

--enable-job-control
     This enables the job control features (see Job Control), if the operating system supports them.

--enable-multibyte
     This enables support for multibyte characters if the operating system provides the necessary support.

--enable-net-redirections
     This enables the special handling of filenames of the form /dev/tcp/HOST/PORT and /dev/udp/HOST/PORT when used in redirections (see Redirections).

--enable-process-substitution
     This enables process substitution (see Process Substitution) if the operating system provides the necessary support.

--enable-progcomp
     Enable the programmable completion facilities (see Programmable Completion). If Readline is not enabled, this option has no effect.

--enable-prompt-string-decoding
     Turn on the interpretation of a number of backslash-escaped characters in the $PS0, $PS1, $PS2, and $PS4 prompt strings.  See Controlling the Prompt, for a complete list of prompt string escape sequences.

--enable-readline
     Include support for command-line editing and history with the Bash version of the Readline library (see Command Line Editing).

--enable-restricted
     Include support for a restricted shell.  If this is enabled, Bash, when called as rbash, enters a restricted mode.  See The Restricted Shell, for a description of restricted mode.

--enable-select
     Include the select compound command, which allows the generation of simple menus (see Conditional Constructs).

--enable-separate-helpfiles
     Use external files for the documentation displayed by the help builtin instead of storing the text internally.

--enable-single-help-strings
     Store the text displayed by the help 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.

--enable-strict-posix-default
     Make Bash POSIX-conformant by default (see Bash POSIX Mode).

--enable-usg-echo-default
     A synonym for --enable-xpg-echo-default.

--enable-xpg-echo-default
     Make the echo builtin expand backslash-escaped characters by default, without requiring the -e option. This sets the default value of the xpg_echo shell option to on, which makes the Bash echo behave more like the version specified in the Single Unix Specification, version 3. See Bash Builtins, for a description of the escape sequences that echo recognizes.

The file config-top.h contains C Preprocessor #define statements for options which are not settable from configure. 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.

Reporting Bugs
**************

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 version of Bash is always available for FTP from ftp://ftp.gnu.org/pub/gnu/bash/.

Once you have determined that a bug actually exists, use the bashbug command to submit a bug report. If you have a fix, you are encouraged to mail that as well! Suggestions and `philosophical' bug reports may be mailed to bug-bash@gnu.org or posted to the Usenet newsgroup gnu.bash.bug.

All bug reports should include:

  * The version number of Bash.

  * The hardware and operating system.

  * The compiler used to compile Bash.

  * A description of the bug behaviour.

  * A short script or `recipe' which exercises the bug and may be used to reproduce it.

bashbug inserts the first three items automatically into the template it provides for filing a bug report.

Please send all reports concerning this manual to bug-bash@gnu.org.

Major Differences From The Bourne Shell
***************************************

Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX standard as the specification of how these features are to be implemented.  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 sh included in SVR4.2 (the last version of the historical Bourne shell) as the baseline reference.

  * Bash is POSIX-conformant, even where the POSIX specification differs from traditional sh behavior (see Bash POSIX Mode).

  * Bash has multi-character invocation options (see Invoking Bash).

  * Bash has command-line editing (see Command Line Editing) and the bind builtin.

  * Bash provides a programmable word completion mechanism (see Programmable Completion), and builtin commands complete, compgen, and compopt, to manipulate it.

  * Bash has command history (see Bash History Facilities) and the history and fc builtins to manipulate it. The Bash history list maintains timestamp information and uses the value of the HISTTIMEFORMAT variable to display it.

  * Bash implements csh-like history expansion (see History Interaction).

  * Bash has one-dimensional array variables (see Arrays), 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.

  * The $'...' quoting syntax, which expands ANSI-C backslash-escaped characters in the text between the single quotes, is supported (see ANSI-C Quoting).

  * Bash supports the $"..." quoting syntax to do locale-specific translation of the characters between the double quotes.  The -D, --dump-strings, and --dump-po-strings invocation options list the translatable strings found in a script (see Locale Translation).

  * Bash implements the ! keyword to negate the return value of a pipeline (see Pipelines). Very useful when an if statement needs to act only if a test fails. The Bash -o pipefail option to set will cause a pipeline to return a failure status if any command fails.

  * Bash has the time reserved word and command timing (see Pipelines). The display of the timing statistics may be controlled with the TIMEFORMAT variable.

  * Bash implements the for (( EXPR1 ; EXPR2 ; EXPR3 )) arithmetic for command, similar to the C language (see Looping Constructs).

  * Bash includes the select compound command, which allows the generation of simple menus (see Conditional Constructs).

  * Bash includes the [[ compound command, which makes conditional testing part of the shell grammar (see Conditional Constructs), including optional regular expression matching.

  * Bash provides optional case-insensitive matching for the case and [[ constructs.

  * Bash includes brace expansion (see Brace Expansion) and tilde expansion (see Tilde Expansion).

  * Bash implements command aliases and the alias and unalias builtins (see Aliases).

  * Bash provides shell arithmetic, the (( compound command (see Conditional Constructs), and arithmetic expansion (see Shell Arithmetic).

  * Variables present in the shell's initial environment are automatically exported to child processes.  The Bourne shell does not normally do this unless the variables are explicitly marked using the export command.

  * Bash supports the += assignment operator, which appends to the value of the variable named on the left hand side.

  * Bash includes the POSIX pattern removal %, #, %% and ## expansions to remove leading or trailing substrings from variable values (see Shell Parameter Expansion).

  * The expansion ${#xx}, which returns the length of ${xx}, is supported (see Shell Parameter Expansion).

  * The expansion ${var:OFFSET[:LENGTH]}, which expands to the substring of var's value of length LENGTH, beginning at OFFSET, is present (see Shell Parameter Expansion).

  * The expansion ${var/[/]PATTERN[/REPLACEMENT]}, which matches PATTERN and replaces it with REPLACEMENT in the value of var, is available (see Shell Parameter Expansion).

  * The expansion ${!PREFIX*} expansion, which expands to the names of all shell variables whose names begin with PREFIX, is available (see Shell Parameter Expansion).

  * Bash has INDIRECT variable expansion using ${!word} (see Shell Parameter Expansion).

  * Bash can expand positional parameters beyond $9 using ${NUM}.

  * The POSIX $() form of command substitution is implemented (see Command Substitution), and preferred to the Bourne shell's `` (which is also implemented for backwards compatibility).

  * Bash has process substitution (see Process Substitution).

  * Bash automatically assigns variables that provide information about the current user (UID, EUID, and GROUPS), the current host (HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME), and the instance of Bash that is running (BASH, BASH_VERSION, and BASH_VERSINFO).  See Bash Variables, for details.

  * The IFS variable is used to split only the results of expansion, not all words (see Word Splitting). This closes a longstanding shell security hole.

  * The filename expansion bracket expression code uses ! and ^ to negate the set of characters between the brackets. The Bourne shell uses only !.

  * Bash implements the full set of POSIX filename expansion operators, including CHARACTER CLASSES, EQUIVALENCE CLASSES, and COLLATING SYMBOLS (see Filename Expansion).

  * Bash implements extended pattern matching features when the extglob shell option is enabled (see Pattern Matching).

  * It is possible to have a variable and a function with the same name; sh does not separate the two name spaces.

  * Bash functions are permitted to have local variables using the local builtin, and thus useful recursive functions may be written (see Bash Builtins).

  * Variable assignments preceding commands affect only that command, even builtins and functions (see Environment). In sh, all variable assignments preceding commands are global unless the command is executed from the file system.

  * Bash performs filename expansion on filenames specified as operands to input and output redirection operators (see Redirections).

  * Bash contains the <> redirection operator, allowing a file to be opened for both reading and writing, and the &> redirection operator, for directing standard output and standard error to the same file (see Redirections).

  * Bash includes the <<< redirection operator, allowing a string to be used as the standard input to a command.

  * Bash implements the [n]<&WORD and [n]>&WORD redirection operators, which move one file descriptor to another.

  * Bash treats a number of filenames specially when they are used in redirection operators (see Redirections).

  * Bash can open network connections to arbitrary machines and services with the redirection operators (see Redirections).

  * The noclobber option is available to avoid overwriting existing files with output redirection (see The Set Builtin). The >| redirection operator may be used to override noclobber.

  * The Bash cd and pwd builtins (see Bourne Shell Builtins) each take -L and -P options to switch between logical and physical modes.

  * Bash allows a function to override a builtin with the same name, and provides access to that builtin's functionality within the function via the builtin and command builtins (see Bash Builtins).

  * The command builtin allows selective disabling of functions when command lookup is performed (see Bash Builtins).

  * Individual builtins may be enabled or disabled using the enable builtin (see Bash Builtins).

  * The Bash exec 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 Bourne Shell Builtins).

  * Shell functions may be exported to children via the environment using export -f (see Shell Functions).

  * The Bash export, readonly, and declare builtins can take a -f option to act on shell functions, a -p option to display variables with various attributes set in a format that can be used as shell input, a -n option to remove various variable attributes, and name=value arguments to set variable attributes and values simultaneously.

  * The Bash hash builtin allows a name to be associated with an arbitrary filename, even when that filename cannot be found by searching the $PATH, using hash -p (see Bourne Shell Builtins).

  * Bash includes a help builtin for quick reference to shell facilities (see Bash Builtins).

  * The printf builtin is available to display formatted output (see Bash Builtins).

  * The Bash read builtin (see Bash Builtins) will read a line ending in \ with the -r option, and will use the REPLY variable as a default if no non-option arguments are supplied. The Bash read builtin also accepts a prompt string with the -p option and will use Readline to obtain the line when given the -e option. The read builtin also has additional options to control input: the -s option will turn off echoing of input characters as they are read, the -t option will allow read to time out if input does not arrive within a specified number of seconds, the -n option will allow reading only a specified number of characters rather than a full line, and the -d option will read until a particular character rather than newline.

  * The return builtin may be used to abort execution of scripts executed with the . or source builtins (see Bourne Shell Builtins).

  * Bash includes the shopt builtin, for finer control of shell optional capabilities (see The Shopt Builtin), and allows these options to be set and unset at shell invocation (see Invoking Bash).

  * Bash has much more optional behavior controllable with the set builtin (see The Set Builtin).

  * The -x (xtrace) option displays commands other than simple commands when performing an execution trace (see The Set Builtin).

  * The test builtin (see Bourne Shell Builtins) is slightly different, as it implements the POSIX algorithm, which specifies the behavior based on the number of arguments.

  * Bash includes the caller builtin, which displays the context of any active subroutine call (a shell function or a script executed with the . or source builtins).  This supports the bash debugger.

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

     The trap builtin (see Bourne Shell Builtins) allows an ERR pseudo-signal specification, similar to EXIT and DEBUG. Commands specified with an ERR trap are executed after a simple command fails, with a few exceptions. The ERR trap is not inherited by shell functions unless the -o errtrace option to the set builtin is enabled.

     The trap builtin (see Bourne Shell Builtins) allows a RETURN pseudo-signal specification, similar to EXIT and DEBUG. Commands specified with an RETURN trap are executed before execution resumes after a shell function or a shell script executed with . or source returns. The RETURN trap is not inherited by shell functions unless the function has been given the trace attribute or the functrace option has been enabled using the shopt builtin.

  * The Bash type builtin is more extensive and gives more information about the names it finds (see Bash Builtins).

  * The Bash umask builtin permits a -p option to cause the output to be displayed in the form of a umask command that may be reused as input (see Bourne Shell Builtins).

  * Bash implements a csh-like directory stack, and provides the pushd, popd, and dirs builtins to manipulate it (see The Directory Stack). Bash also makes the directory stack visible as the value of the DIRSTACK shell variable.

  * Bash interprets special backslash-escaped characters in the prompt strings when interactive (see Controlling the Prompt).

  * The Bash restricted mode is more useful (see The Restricted Shell); the SVR4.2 shell restricted mode is too limited.

  * The disown builtin can remove a job from the internal shell job table (see Job Control Builtins) or suppress the sending of SIGHUP to a job when the shell exits as the result of a SIGHUP.

  * Bash includes a number of features to support a separate debugger for shell scripts.

  * The SVR4.2 shell has two privilege-related builtins (mldmode and priv) not present in Bash.

  * Bash does not have the stop or newgrp builtins.

  * Bash does not use the SHACCT variable or perform shell accounting.

  * The SVR4.2 sh uses a TIMEOUT variable like Bash uses TMOUT.

More features unique to Bash may be found in Bash Features.

Implementation Differences From The SVR4.2 Shell
================================================

Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell.  For instance:

  * Bash does not fork a subshell when redirecting into or out of a shell control structure such as  an if or while statement.

  * Bash does not allow unbalanced quotes.  The SVR4.2 shell will silently insert a needed closing quote at EOF under certain circumstances. This can be the cause of some hard-to-find errors.

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

  * In a questionable attempt at security, the SVR4.2 shell, when invoked without the -p option, will alter its real and effective UID and GID if they are less than some magic threshold value, commonly 100. This can lead to unexpected results.

  * The SVR4.2 shell does not allow users to trap SIGSEGV, SIGALRM, or SIGCHLD.

  * The SVR4.2 shell does not allow the IFS, MAILCHECK, PATH, PS1, or PS2 variables to be unset.

  * The SVR4.2 shell treats ^ as the undocumented equivalent of |.

  * Bash allows multiple option arguments when it is invoked (-x -v); the SVR4.2 shell allows only one option argument (-xv).  In fact, some versions of the shell dump core if the second argument begins with a -.

  * The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the POSIX special builtins fails, and only for certain failures, as enumerated in the POSIX standard.

  * The SVR4.2 shell behaves differently when invoked as jsh (it turns on job control).

GNU Free Documentation License
******************************

     [file fdl.texi not included]

Indexes
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