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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
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><H1
><A
NAME="SUBSHELLS"
></A
>Chapter 21. Subshells</H1
><P
><A
NAME="SUBSHELLSREF"
></A
></P
><P
>Running a shell script launches a new process, a
        <I
CLASS="FIRSTTERM"
>subshell</I
>.</P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN18083"
></A
><P
><TT
CLASS="USERINPUT"
><B
>Definition:</B
></TT
>
          A <I
CLASS="FIRSTTERM"
>subshell</I
> is a
          <A
HREF="othertypesv.html#CHILDREF2"
>child process</A
> launched by a
          shell (or <I
CLASS="FIRSTTERM"
>shell script</I
>).</P
></DIV
></TD
></TR
></TABLE
><P
>A subshell is a separate instance of the command processor
        -- the <I
CLASS="FIRSTTERM"
>shell</I
> that gives you the prompt at
        the console or in an <I
CLASS="FIRSTTERM"
>xterm</I
> window. Just
        as your commands are interpreted at the command-line prompt,
        similarly does a script <A
HREF="timedate.html#BATCHPROCREF"
>batch-process</A
> a list of
        commands. Each shell script running is, in effect, a subprocess
        (<I
CLASS="FIRSTTERM"
>child process</I
>) of the <A
HREF="internal.html#FORKREF"
>parent</A
> shell.</P
><P
>A shell script can itself launch subprocesses. These
	<I
CLASS="FIRSTTERM"
>subshells</I
> let the script do
	parallel processing, in effect executing multiple subtasks
	simultaneously.</P
><P
>      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# subshell-test.sh
   3&nbsp;
   4&nbsp;(
   5&nbsp;# Inside parentheses, and therefore a subshell . . .
   6&nbsp;while [ 1 ]   # Endless loop.
   7&nbsp;do
   8&nbsp;  echo "Subshell running . . ."
   9&nbsp;done
  10&nbsp;)
  11&nbsp;
  12&nbsp;#  Script will run forever,
  13&nbsp;#+ or at least until terminated by a Ctl-C.
  14&nbsp;
  15&nbsp;exit $?  # End of script (but will never get here).
  16&nbsp;
  17&nbsp;
  18&nbsp;
  19&nbsp;Now, run the script:
  20&nbsp;sh subshell-test.sh
  21&nbsp;
  22&nbsp;And, while the script is running, from a different xterm:
  23&nbsp;ps -ef | grep subshell-test.sh
  24&nbsp;
  25&nbsp;UID       PID   PPID  C STIME TTY      TIME     CMD
  26&nbsp;500       2698  2502  0 14:26 pts/4    00:00:00 sh subshell-test.sh
  27&nbsp;500       2699  2698 21 14:26 pts/4    00:00:24 sh subshell-test.sh
  28&nbsp;
  29&nbsp;          ^^^^
  30&nbsp;
  31&nbsp;Analysis:
  32&nbsp;PID 2698, the script, launched PID 2699, the subshell.
  33&nbsp;
  34&nbsp;Note: The "UID ..." line would be filtered out by the "grep" command,
  35&nbsp;but is shown here for illustrative purposes.</PRE
></TD
></TR
></TABLE
>
      </P
><P
>In general, an <A
HREF="external.html#EXTERNALREF"
>external
	command</A
> in a script <A
HREF="internal.html#FORKREF"
>forks
	off</A
> a subprocess,
           <A
NAME="AEN18102"
HREF="#FTN.AEN18102"
>[1]</A
>
        whereas a Bash <A
HREF="internal.html#BUILTINREF"
>builtin</A
>
        does not. For this reason, builtins execute more quickly
        and use fewer system resources than their external command
        equivalents.</P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="SUBSHELLPARENS1"
></A
>Command List within
	Parentheses</B
></P
><DL
><DT
>( command1; command2; command3; ... )</DT
><DD
><P
>A command list embedded between
	  <TT
CLASS="REPLACEABLE"
><I
>parentheses</I
></TT
> runs as a
	  subshell.</P
></DD
></DL
></DIV
><P
><A
NAME="PARVIS"
></A
>Variables in a subshell are
        <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> visible outside the block of code
        in the subshell. They are not accessible to the <A
HREF="internal.html#FORKREF"
>parent process</A
>, to the shell
        that launched the subshell. These are, in effect,
        variables <A
HREF="localvar.html#LOCALREF"
>local</A
> to the
	<I
CLASS="FIRSTTERM"
>child process</I
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SUBSHELL"
></A
><P
><B
>Example 21-1. Variable scope in a subshell</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# subshell.sh
   3&nbsp;
   4&nbsp;echo
   5&nbsp;
   6&nbsp;echo "We are outside the subshell."
   7&nbsp;echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
   8&nbsp;# Bash, version 3, adds the new         $BASH_SUBSHELL variable.
   9&nbsp;echo; echo
  10&nbsp;
  11&nbsp;outer_variable=Outer
  12&nbsp;global_variable=
  13&nbsp;#  Define global variable for "storage" of
  14&nbsp;#+ value of subshell variable.
  15&nbsp;
  16&nbsp;(
  17&nbsp;echo "We are inside the subshell."
  18&nbsp;echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
  19&nbsp;inner_variable=Inner
  20&nbsp;
  21&nbsp;echo "From inside subshell, \"inner_variable\" = $inner_variable"
  22&nbsp;echo "From inside subshell, \"outer\" = $outer_variable"
  23&nbsp;
  24&nbsp;global_variable="$inner_variable"   #  Will this allow "exporting"
  25&nbsp;                                    #+ a subshell variable?
  26&nbsp;)
  27&nbsp;
  28&nbsp;echo; echo
  29&nbsp;echo "We are outside the subshell."
  30&nbsp;echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
  31&nbsp;echo
  32&nbsp;
  33&nbsp;if [ -z "$inner_variable" ]
  34&nbsp;then
  35&nbsp;  echo "inner_variable undefined in main body of shell"
  36&nbsp;else
  37&nbsp;  echo "inner_variable defined in main body of shell"
  38&nbsp;fi
  39&nbsp;
  40&nbsp;echo "From main body of shell, \"inner_variable\" = $inner_variable"
  41&nbsp;#  $inner_variable will show as blank (uninitialized)
  42&nbsp;#+ because variables defined in a subshell are "local variables".
  43&nbsp;#  Is there a remedy for this?
  44&nbsp;echo "global_variable = "$global_variable""  # Why doesn't this work?
  45&nbsp;
  46&nbsp;echo
  47&nbsp;
  48&nbsp;# =======================================================================
  49&nbsp;
  50&nbsp;# Additionally ...
  51&nbsp;
  52&nbsp;echo "-----------------"; echo
  53&nbsp;
  54&nbsp;var=41                                                 # Global variable.
  55&nbsp;
  56&nbsp;( let "var+=1"; echo "\$var INSIDE subshell = $var" )  # 42
  57&nbsp;
  58&nbsp;echo "\$var OUTSIDE subshell = $var"                   # 41
  59&nbsp;#  Variable operations inside a subshell, even to a GLOBAL variable
  60&nbsp;#+ do not affect the value of the variable outside the subshell!
  61&nbsp;
  62&nbsp;
  63&nbsp;exit 0
  64&nbsp;
  65&nbsp;#  Question:
  66&nbsp;#  --------
  67&nbsp;#  Once having exited a subshell,
  68&nbsp;#+ is there any way to reenter that very same subshell
  69&nbsp;#+ to modify or access the subshell variables?</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="variables2.html#BASHPIDREF"
>$BASHPID</A
> and
        <A
HREF="gotchas.html#SUBPIT"
>Example 34-2</A
>.</P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN18127"
></A
><P
><A
NAME="SCOPEREF"
></A
></P
><P
><TT
CLASS="USERINPUT"
><B
>Definition:</B
></TT
> The
	    <I
CLASS="FIRSTTERM"
>scope</I
> of a variable is the
	    context in which it has meaning, in which it has a
	    <I
CLASS="FIRSTTERM"
>value</I
> that can be referenced. For
	    example, the scope of a <A
HREF="localvar.html#LOCALREF1"
>local
	    variable</A
> lies only within the function,
	    block of code, or subshell within which it is defined,
	    while the scope of a <I
CLASS="FIRSTTERM"
>global</I
> variable
	    is the entire script in which it appears.</P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="SUBSHNLEVREF"
></A
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>While the <A
HREF="variables2.html#BASHSUBSHELLREF"
>$BASH_SUBSHELL</A
>
	      internal variable indicates the nesting level of a
	      subshell, the <A
HREF="variables2.html#SHLVLREF"
>$SHLVL</A
>
	      variable <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>shows no change</I
></SPAN
> within
	      a subshell.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;echo " \$BASH_SUBSHELL outside subshell       = $BASH_SUBSHELL"           # 0
   2&nbsp;  ( echo " \$BASH_SUBSHELL inside subshell        = $BASH_SUBSHELL" )     # 1
   3&nbsp;  ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
   4&nbsp;# ^ ^                           *** nested ***                        ^ ^
   5&nbsp;
   6&nbsp;echo
   7&nbsp;
   8&nbsp;echo " \$SHLVL outside subshell = $SHLVL"       # 3
   9&nbsp;( echo " \$SHLVL inside subshell  = $SHLVL" )   # 3 (No change!)</PRE
></TD
></TR
></TABLE
>
            </P
></TD
></TR
></TABLE
></DIV
><P
>Directory changes made in a subshell do not carry over to the
        parent shell.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ALLPROFS"
></A
><P
><B
>Example 21-2. List User Profiles</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# allprofs.sh: Print all user profiles.
   3&nbsp;
   4&nbsp;# This script written by Heiner Steven, and modified by the document author.
   5&nbsp;
   6&nbsp;FILE=.bashrc  #  File containing user profile,
   7&nbsp;              #+ was ".profile" in original script.
   8&nbsp;
   9&nbsp;for home in `awk -F: '{print $6}' /etc/passwd`
  10&nbsp;do
  11&nbsp;  [ -d "$home" ] || continue    # If no home directory, go to next.
  12&nbsp;  [ -r "$home" ] || continue    # If not readable, go to next.
  13&nbsp;  (cd $home; [ -e $FILE ] &#38;&#38; less $FILE)
  14&nbsp;done
  15&nbsp;
  16&nbsp;#  When script terminates, there is no need to 'cd' back to original directory,
  17&nbsp;#+ because 'cd $home' takes place in a subshell.
  18&nbsp;
  19&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>A subshell may be used to set up a <SPAN
CLASS="QUOTE"
>"dedicated
	      environment"</SPAN
> for a command group.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;COMMAND1
   2&nbsp;COMMAND2
   3&nbsp;COMMAND3
   4&nbsp;(
   5&nbsp;  IFS=:
   6&nbsp;  PATH=/bin
   7&nbsp;  unset TERMINFO
   8&nbsp;  set -C
   9&nbsp;  shift 5
  10&nbsp;  COMMAND4
  11&nbsp;  COMMAND5
  12&nbsp;  exit 3 # Only exits the subshell!
  13&nbsp;)
  14&nbsp;# The parent shell has not been affected, and the environment is preserved.
  15&nbsp;COMMAND6
  16&nbsp;COMMAND7</PRE
></TD
></TR
></TABLE
>

    As seen here, the <A
HREF="internal.html#EXITREF"
>exit</A
>
    command only terminates the subshell in which it is running,
    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> the parent shell or script.</P
><P
>One application of such a <SPAN
CLASS="QUOTE"
>"dedicated environment"</SPAN
>
        is testing whether a variable is defined.
              <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if (set -u; : $variable) 2&#62; /dev/null
   2&nbsp;then
   3&nbsp;  echo "Variable is set."
   4&nbsp;fi     #  Variable has been set in current script,
   5&nbsp;       #+ or is an an internal Bash variable,
   6&nbsp;       #+ or is present in environment (has been exported).
   7&nbsp;
   8&nbsp;# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
   9&nbsp;# or                    [[ ${variable-x} != x$variable ]]
  10&nbsp;# or                    [[ ${variable+x} = x ]]
  11&nbsp;# or                    [[ ${variable-x} != x ]]</PRE
></TD
></TR
></TABLE
></P
><P
>Another application is checking for a lock file:
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if (set -C; : &#62; lock_file) 2&#62; /dev/null
   2&nbsp;then
   3&nbsp;  :   # lock_file didn't exist: no user running the script
   4&nbsp;else
   5&nbsp;  echo "Another user is already running that script."
   6&nbsp;exit 65
   7&nbsp;fi
   8&nbsp;
   9&nbsp;#  Code snippet by Stphane Chazelas,
  10&nbsp;#+ with modifications by Paulo Marcel Coelho Aragao.</PRE
></TD
></TR
></TABLE
>
      </P
><P
>+</P
><P
>Processes may execute in parallel within different
        subshells. This permits breaking a complex task into subcomponents
        processed concurrently.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="PARALLEL-PROCESSES"
></A
><P
><B
>Example 21-3. Running parallel processes in subshells</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;	(cat list1 list2 list3 | sort | uniq &#62; list123) &#38;
   2&nbsp;	(cat list4 list5 list6 | sort | uniq &#62; list456) &#38;
   3&nbsp;	# Merges and sorts both sets of lists simultaneously.
   4&nbsp;	# Running in background ensures parallel execution.
   5&nbsp;	#
   6&nbsp;	# Same effect as
   7&nbsp;	#   cat list1 list2 list3 | sort | uniq &#62; list123 &#38;
   8&nbsp;	#   cat list4 list5 list6 | sort | uniq &#62; list456 &#38;
   9&nbsp;	
  10&nbsp;	wait   # Don't execute the next command until subshells finish.
  11&nbsp;	
  12&nbsp;	diff list123 list456</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Redirecting I/O to a subshell uses the <SPAN
CLASS="QUOTE"
>"|"</SPAN
> pipe
	  operator, as in <TT
CLASS="USERINPUT"
><B
>ls -al | (command)</B
></TT
>.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>A code block between <A
HREF="special-chars.html#CODEBLOCKREF"
>curly brackets</A
> does
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> launch a subshell.</P
><P
>{ command1; command2; command3; . . . commandN; }</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;var1=23
   2&nbsp;echo "$var1"   # 23
   3&nbsp;
   4&nbsp;{ var1=76; }
   5&nbsp;echo "$var1"   # 76</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18102"
HREF="subshells.html#AEN18102"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>An external command invoked with an <A
HREF="internal.html#EXECREF"
>exec</A
> does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
             (usually) fork off a subprocess / subshell.</P
></TD
></TR
></TABLE
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