What is a Subshell?

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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:

echo $BASH_SUBSHELL

OR

echo "Current shell: $BASH_SUBSHELL"; ( echo "Running du in subshell: $BASH_SUBSHELL" ;cd /tmp; du 2>/tmp/error 1>/tmp/output)

• 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:

WWWJAIL=/apache.jail
export WWWJAIL
die() { echo "$@"; exit 2; } 
export -f die
# now call script that will access die() and $WWWJAIL
/etc/nixcraft/setupjail -d cyberciti.com

• 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,

exec command
# redirect the shells stderr to null
exec 2>/dev/null

The . (dot) Command and Subshell

The . (dot) command is used to run shell scripts as follows:

. script.sh

The dot command allows you to modify current shell variables. For example, create a shell script as follows called /tmp/dottest.sh:

#!/bin/bash
echo "In script before : $WWWJAIL"
WWWJAIL=/apache.jail
echo "In script after : $WWWJAIL"

Close and save the file. Run it as follows:

chmod +x /tmp/dottest.sh

Now, define a variable called WWWJAIL at a shell prompt:

WWWJAIL=/foobar
echo $WWWJAIL

Sample outputs:

/foobar

Run the script:

/tmp/dottest.sh

Check the value of WWWJAIL:

echo $WWWJAIL

You should see the orignal value of $WWWJAIL (/foobar) as the shell script was executed in a subshell. Now, try the dot command:

 . /tmp/dottest.sh
echo $WWWJAIL

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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