What is a Subshell?

From Linux Shell Scripting Tutorial - A Beginner's handbook
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  • Whenever you run a shell script, it creates a new process called subshell and your script will get executed using a subshell.
  • A Subshell can be used to do parallel processing.
  • If you start another shell on top of your current shell, it can be referred to as a subshell. Type the following command to see subshell value:

<syntaxhighlight lang="bash" >echo $BASH_SUBSHELL</source> OR <syntaxhighlight lang="bash" >echo "Current shell: $BASH_SUBSHELL"; ( echo "Running du in subshell: $BASH_SUBSHELL" ;cd /tmp; du 2>/tmp/error 1>/tmp/output)</source>

  • Any commands enclosed within parentheses are run in a subshell.

Exporting Functions and Variables

A subshell does not inherit a variable's setting. Use the export command to export variables and functions to subshell: <syntaxhighlight lang="bash" >WWWJAIL=/apache.jail export WWWJAIL die() { echo "$@"; exit 2; } export -f die

  1. now call script that will access die() and $WWWJAIL

/etc/nixcraft/setupjail -d cyberciti.com</source>

  • However, environment variables (such as $HOME, $MAIL etc) are passed to subshell.

Use exec command to avoid subshell

You can use the exec command to avoid subshell. The exec command replaces this shell with the specified program without swapping a new subshell or proces. For example, <syntaxhighlight lang="bash" >exec command

  1. redirect the shells stderr to null

exec 2>/dev/null</source>

The . (dot) Command and Subshell

The . (dot) command is used to run shell scripts as follows: <syntaxhighlight lang="bash" >. script.sh</source> The dot command allows you to modify current shell variables. For example, create a shell script as follows called /tmp/dottest.sh: <syntaxhighlight lang="bash" >#!/bin/bash echo "In script before : $WWWJAIL" WWWJAIL=/apache.jail echo "In script after : $WWWJAIL"</source> Close and save the file. Run it as follows: <syntaxhighlight lang="bash" >chmod +x /tmp/dottest.sh</source> Now, define a variable called WWWJAIL at a shell prompt: <syntaxhighlight lang="bash" >WWWJAIL=/foobar echo $WWWJAIL</source> Sample outputs:

/foobar

Run the script: <syntaxhighlight lang="bash" >/tmp/dottest.sh</source> Check the value of WWWJAIL: <syntaxhighlight lang="bash" >echo $WWWJAIL</source> You should see the orignal value of $WWWJAIL (/foobar) as the shell script was executed in a subshell. Now, try the dot command: <syntaxhighlight lang="bash" > . /tmp/dottest.sh echo $WWWJAIL</source> Sample outputs:

/apache.jail

The value of $WWWJAIL (/apache.jail) was changed as the script was run in the current shell using the dot command.

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