Linux in a Nutshell, 6th Edition

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Null command. Returns an exit status of 0. The line is still processed for side effects, such as variable and command substitutions, or I/O redirection. See the following Example and the Example under case.

Example

Check whether someone is logged in:

if who | grep $1 > /dev/null
then :   # Do nothing if user is found
else echo "User $1 is not logged in"
fi