How do I create an if or while loop without "test" and use a "not" operator?

Question

If I do:

if grep someexpression somefile >/dev/null; then
   ...
fi

Things work fine.

To do a "not", I know I can do this (using [] which is the test command):

if [ '!' somecondition ] ; then
   ...
fi

A simple workaround:

grep someexpression somefile >/dev/null
if [ '!' $? ]; then
   ...
fi

But the above workaround won't work with a while loop, so then I need to write a function instead, which is annoying. How do I do a not with the grep thing above, the proper way?

Mat · Accepted Answer · 2012-04-20 11:40:08Z

3

Try:

if ! grep someexpression somefile >/dev/null; then
   ...
fi

Note that the space after ! is mandatory, otherwise it will (try to) invoke history expansion.

edited Apr 20, 2012 at 11:40

answered Apr 20, 2012 at 11:27

Mat

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4 Comments

Peter Over a year ago

Interesting. Last time I had this problem, I tried exactly that, and it said something like "no such event". But your example worked fine.

Mat Over a year ago

tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html - it's even an example in the docs. (Works in ksh too, but I not sure if it is valid POSIX shell syntax.)

ormaaj Over a year ago

You had history expansion enabled. That shouldn't be a problem in a script. I disable it interactively because it causes more problems than it's worth. @Mat It's POSIX. It hasn't been around forever but I doubt there are serious users of such shells still around (way pre-bash/korn bourne shells).

camh Over a year ago

@Peter: You probably did not put a space between the ! and grep. Without a space (on the command line), !grep will search your history for the last command starting with grep. With a space, it will work as a negation operator.

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