:

Null command. Returns an exit status of 0. Sometimes used on older systems as the first character in a file to denote a Bourne shell script. See this Example and under case. The line is still processed for side effects, such as variable and command substitutions.

Example

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