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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
></P
><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
><P
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# subshell-test.sh
(
# Inside parentheses, and therefore a subshell . . .
while [ 1 ] # Endless loop.
do
echo "Subshell running . . ."
done
)
# Script will run forever,
#+ or at least until terminated by a Ctl-C.
exit $? # End of script (but will never get here).
Now, run the script:
sh subshell-test.sh
And, while the script is running, from a different xterm:
ps -ef | grep subshell-test.sh
UID PID PPID C STIME TTY TIME CMD
500 2698 2502 0 14:26 pts/4 00:00:00 sh subshell-test.sh
500 2699 2698 21 14:26 pts/4 00:00:24 sh subshell-test.sh
^^^^
Analysis:
PID 2698, the script, launched PID 2699, the subshell.
Note: The "UID ..." line would be filtered out by the "grep" command,
but is shown here for illustrative purposes.</PRE
></FONT
></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"
><SPAN
CLASS="footnote"
>[1]</SPAN
></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
><P
></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
<EM
>not</EM
> 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"
><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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# subshell.sh
echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new $BASH_SUBSHELL variable.
echo; echo
outer_variable=Outer
global_variable=
# Define global variable for "storage" of
#+ value of subshell variable.
(
echo "We are inside the subshell."
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner
echo "From inside subshell, \"inner_variable\" = $inner_variable"
echo "From inside subshell, \"outer\" = $outer_variable"
global_variable="$inner_variable" # Will this allow "exporting"
#+ a subshell variable?
)
echo; echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo
if [ -z "$inner_variable" ]
then
echo "inner_variable undefined in main body of shell"
else
echo "inner_variable defined in main body of shell"
fi
echo "From main body of shell, \"inner_variable\" = $inner_variable"
# $inner_variable will show as blank (uninitialized)
#+ because variables defined in a subshell are "local variables".
# Is there a remedy for this?
echo "global_variable = "$global_variable"" # Why doesn't this work?
echo
# =======================================================================
# Additionally ...
echo "-----------------"; echo
var=41 # Global variable.
( let "var+=1"; echo "\$var INSIDE subshell = $var" ) # 42
echo "\$var OUTSIDE subshell = $var" # 41
# Variable operations inside a subshell, even to a GLOBAL variable
#+ do not affect the value of the variable outside the subshell!
exit 0
# Question:
# --------
# Once having exited a subshell,
#+ is there any way to reenter that very same subshell
#+ to modify or access the subshell variables?</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><P
>See also <A
HREF="internalvariables.html#BASHPIDREF"
>$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
></P
><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
><P
></P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="SUBSHNLEVREF"
></A
></P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>While the <A
HREF="internalvariables.html#BASHSUBSHELLREF"
>$BASH_SUBSHELL</A
>
internal variable indicates the nesting level of a
subshell, the <A
HREF="internalvariables.html#SHLVLREF"
>$SHLVL</A
>
variable <EM
>shows no change</EM
> within
a subshell.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>echo " \$BASH_SUBSHELL outside subshell = $BASH_SUBSHELL" # 0
( echo " \$BASH_SUBSHELL inside subshell = $BASH_SUBSHELL" ) # 1
( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
# ^ ^ *** nested *** ^ ^
echo
echo " \$SHLVL outside subshell = $SHLVL" # 3
( echo " \$SHLVL inside subshell = $SHLVL" ) # 3 (No change!)</PRE
></FONT
></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"
><A
NAME="ALLPROFS"
></A
><P
><B
>Example 21-2. List User Profiles</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# allprofs.sh: Print all user profiles.
# This script written by Heiner Steven, and modified by the document author.
FILE=.bashrc # File containing user profile,
#+ was ".profile" in original script.
for home in `awk -F: '{print $6}' /etc/passwd`
do
[ -d "$home" ] || continue # If no home directory, go to next.
[ -r "$home" ] || continue # If not readable, go to next.
(cd $home; [ -e $FILE ] &#38;&#38; less $FILE)
done
# When script terminates, there is no need to 'cd' back to original directory,
#+ because 'cd $home' takes place in a subshell.
exit 0</PRE
></FONT
></TD
></TR
></TABLE
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>COMMAND1
COMMAND2
COMMAND3
(
IFS=:
PATH=/bin
unset TERMINFO
set -C
shift 5
COMMAND4
COMMAND5
exit 3 # Only exits the subshell!
)
# The parent shell has not been affected, and the environment is preserved.
COMMAND6
COMMAND7</PRE
></FONT
></TD
></TR
></TABLE
>
As seen here, the <A
HREF="internal.html#EXITREF"
>exit</A
>
command only terminates the subshell in which it is running,
<EM
>not</EM
> 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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>if (set -u; : $variable) 2&#62; /dev/null
then
echo "Variable is set."
fi # Variable has been set in current script,
#+ or is an an internal Bash variable,
#+ or is present in environment (has been exported).
# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
# or [[ ${variable-x} != x$variable ]]
# or [[ ${variable+x} = x ]]
# or [[ ${variable-x} != x ]]</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
>Another application is checking for a lock file:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>if (set -C; : &#62; lock_file) 2&#62; /dev/null
then
: # lock_file didn't exist: no user running the script
else
echo "Another user is already running that script."
exit 65
fi
# Code snippet by St<53>phane Chazelas,
#+ with modifications by Paulo Marcel Coelho Aragao.</PRE
></FONT
></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"
><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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> (cat list1 list2 list3 | sort | uniq &#62; list123) &#38;
(cat list4 list5 list6 | sort | uniq &#62; list456) &#38;
# Merges and sorts both sets of lists simultaneously.
# Running in background ensures parallel execution.
#
# Same effect as
# cat list1 list2 list3 | sort | uniq &#62; list123 &#38;
# cat list4 list5 list6 | sort | uniq &#62; list456 &#38;
wait # Don't execute the next command until subshells finish.
diff list123 list456</PRE
></FONT
></TD
></TR
></TABLE
></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"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
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
<EM
>not</EM
> launch a subshell.</P
><P
>{ command1; command2; command3; . . . commandN; }</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>var1=23
echo "$var1" # 23
{ var1=76; }
echo "$var1" # 76</PRE
></FONT
></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"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>An external command invoked with an <A
HREF="internal.html#EXECREF"
>exec</A
> does <EM
>not</EM
>
(usually) fork off a subprocess / subshell.</P
></TD
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