Advanced Bash-Scripting Guide: A complete guide to shell scripting
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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.

Directory changes made in a subshell do not carry over to the parent shell.

Example 20-2. List User Profiles

#!/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 ] && 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

A subshell may be used to set up a "dedicated environment" for a command group.

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

One application of this is testing whether a variable is defined.

if (set -u; : $variable) 2> /dev/null
then
  echo "Variable is set."
fi

# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
# or                    [[ ${variable-x} != x$variable ]]
# or                    [[ ${variable+x} = x ]])

Another application is checking for a lock file:

if (set -C; : > lock_file) 2> /dev/null
then
  echo "Another user is already running that script."
  exit 65
fi   

# Thanks, S.C.

Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently.

Example 20-3. Running parallel processes in subshells

       (cat list1 list2 list3 | sort | uniq > list123) &
        (cat list4 list5 list6 | sort | uniq > list456) &
        # Merges and sorts both sets of lists simultaneously.
        # Running in background ensures parallel execution.
        #
        # Same effect as
        #   cat list1 list2 list3 | sort | uniq > list123 &
        #   cat list4 list5 list6 | sort | uniq > list456 &
        
        wait   # Don't execute the next command until subshells finish.
        
        diff list123 list456

Redirecting I/O to a subshell uses the "|" pipe operator, as in ls -al | (command).

A command block between curly braces does not launch a subshell.

{ command1; command2; command3; ... }

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