Using ! operator on a shell command

Question

I'm trying to write a script which does something if a particular string is not present in a file.

I know that in order to check if string is available, we can do something like:

if grep -qi "sms" $FILE; then

But how do I combine a ! operator with a shell command?

or see http://stackoverflow.com/questions/10552711/how-to-make-if-not-true-condition/10552775 . ;-) Good luck. — shellter, Nov 19 '14 at 15:13
@anubhava `grep -v` returns true if any output is produced not if any lines match the inverted filter and are dropped. — Etan Reisner, Nov 19 '14 at 15:13

score 2 · Accepted Answer · answered Nov 19 '14 at 15:11

2

Just add it in front of your condition to make it evaluate on the contrary:

if ! grep -qi "sms" $FILE; then echo "yes"; fi
   ^

Test

$ cat a
hello
bye
$ if ! grep -qi "sms" a; then echo "sms not found"; fi
sms not found

answered Nov 19 '14 at 15:11

fedorqui

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@fedorqui Thanks. It works. I was trying !grep earlier. Apparently I have to leave a space between ! and grep. – drunkenfist Nov 19 '14 at 18:08
Nice to read that! Note that `!` is a keyword itself, so to make it work you need to make it be differentiated by the other keywords in action. That is, it needs a space around. – fedorqui Nov 20 '14 at 09:27

score 0 · Answer 2 · answered Nov 19 '14 at 15:16

The ! makes "success" to become "failure" and vice versa from shell points of view. In shell you can also use && (logical and operation), || (logical or operation) and group expressions using curly braces { ... }. For example:

{ grep root /etc/passwd && ! grep user /etc/passwd; } && echo ok || echo not ok

Remember to put ; before }. See man sh for more details.

Note that above is a plain shell expression with if-else-like logic. If you want to use test command or [ or [[ then you can also use other type of grouping, boolean arithmetics and other functions. See man test.

Using ! operator on a shell command

2 Answers2

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