Unix in a Nutshell, 4th Edition

Name

!

Synopsis

    ! pipeline

Not ksh88. Negate the sense of a pipeline. Returns an exit status of 0 if the pipeline exited nonzero, and an exit status of 1 if the pipeline exited zero. Typically used in if and while statements.

Example

This code prints a message if user jane is not logged on:

    if ! who | grep jane > /dev/null
    then
            echo jane is not currently logged on
    fi