Command substitution

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:

echo "Today is $(date)"

OR

echo "Computer name is $(hostname)"

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

echo "Text $(command-name)"

OR

 echo -e "List of logged on users and what they are doing:\n $(w)"

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:

NOW=$(date)
echo "$NOW"

Store system's host name to a variable called SERVERNAME:

SERVERNAME=$(hostname)
echo "Running command @ $SERVERNAME...."

Store current working directory name to a variable called CWD:

CWD=$(pwd)
cd /path/some/where/else
echo "Current dir $(pwd) and now going back to old dir .."
cd $CWD

Command substitution and shell loops

Shell loop can use command substitution to get input:

for f in $(ls /etc/*.conf)
do
   echo "$f"
done

However, a recommend syntax is as follows for file selections:

for f in /etc/*.conf
do
   echo "$f"
done