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.

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.

#!/bin/bash
# subshell.sh

echo

echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new         $BASH_SUBSELL variable.
echo

outer_variable=Outer

(
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner
echo "From subshell, \"inner_variable\" = $inner_variable"
echo "From subshell, \"outer\" = $outer_variable"
)

echo
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 uninitialized because
# variables defined in a subshell are "local variables".

echo

exit 0

See also Example 32-2.

+

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

#!/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 # 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 ]]

Another application is checking for a lock file:

if (set -C; : > lock_file) 2> /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 Stephane Chazelas, #+ with modifications by Paulo Marcel Coelho Aragao.

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

	(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; ... }