Command substitution

From Linux Shell Scripting Tutorial - A Beginner's handbook
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Command substitution means nothing more but to run a shell command and store its output to a variable or display back using echo command. For example, display date and time: <syntaxhighlight lang="bash" >echo "Today is $(date)"</source> OR <syntaxhighlight lang="bash" >echo "Computer name is $(hostname)"</source>

Syntax

You can use the grave accent (`) to perform a command substitution. The syntax is:

`command-name`

OR

$(command-name)

Command substitution in an echo command

<syntaxhighlight lang="bash" >echo "Text $(command-name)"</source> OR <syntaxhighlight lang="bash" > echo -e "List of logged on users and what they are doing:\n $(w)"</source> Sample outputs:

List of logged on users and what they are doing:
  09:49:06 up  4:09,  3 users,  load average: 0.34, 0.33, 0.28
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
vivek    tty7     :0               05:40     ?     9:06m  0.09s /usr/bin/gnome-
vivek    pts/0    :0.0             07:02    0.00s  2:07m  0.13s bash
vivek    pts/2    :0.0             09:03   20:46m  0.04s  0.00s /bin/bash ./ssl

Command substitution and shell variables

You can store command output to a shell variable using the following syntax:

var=$(command-name)

Store current date and time to a variable called NOW: <syntaxhighlight lang="bash" >NOW=$(date) echo "$NOW"</source> Store system's host name to a variable called SERVERNAME: <syntaxhighlight lang="bash" >SERVERNAME=$(hostname) echo "Running command @ $SERVERNAME...."</source> Store current working directory name to a variable called CWD: <syntaxhighlight lang="bash" >CWD=$(pwd) cd /path/some/where/else echo "Current dir $(pwd) and now going back to old dir .." cd $CWD</source>

Command substitution and shell loops

Shell loop can use command substitution to get input: <syntaxhighlight lang="bash" >for f in $(ls /etc/*.conf) do

  echo "$f"

done</source> However, a recommend syntax is as follows for file selections: <syntaxhighlight lang="bash" >for f in /etc/*.conf do

  echo "$f"

done</source> ← Continue statementHomeChapter 5 Challenges →