Null command. Returns an exit status of 0. Sometimes used as the first character in a file to denote a Bourne shell script. See example below and under case. In the Korn shell, shell variables can be placed after the : to expand them to their values.
Check whether someone is logged in:
if who | grep $1 > /dev/null then : # do nothing # if pattern is found else echo "User $1 is not logged in" fi