Advanced Bash−Scripting Guide

Chapter 20. Subshells
Running a shell script launches another instance of the command processor. Just as your commands are
interpreted at the command line prompt, similarly does a script batch process a list of commands in a file.
Each shell script running is, in effect, a subprocess of the parent shell, the one that gives you the prompt at the
console or in an xterm window.
A shell script can also launch subprocesses. These subshells let the script do parallel processing, in effect
executing multiple subtasks simultaneously.
Command List in Parentheses
( command1; command2; command3; ... )
A command list embedded between parentheses runs as a subshell.
Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible
to the parent process, to the shell that launched the subshell. These are, in effect, local variables.
Example 20−1. Variable scope in a subshell
#!/bin/bash
# subshell.sh
echo
outer_variable=Outer
(
inner_variable=Inner
echo "From subshell, \"inner_variable\" = $inner_variable"
echo "From subshell, \"outer\" = $outer_variable"
)
echo
if [ −z "$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 uninitialized because
# variables defined in a subshell are "local variables".
echo
exit 0
See also Example 32−1.
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Chapter 20. Subshells 267