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Linux in a Nutshell, 6th Edition by Robert Love, Stephen Figgins, Ellen Siever, Arnold Robbins

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Name

!

Synopsis

! pipeline

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

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