How to make "if not true condition"?

Question

I would like to have the echo command executed when cat /etc/passwd | grep "sysa" is not true.

What am I doing wrong?

if ! [ $(cat /etc/passwd | grep "sysa") ]; then
        echo "ERROR - The user sysa could not be looked up"
        exit 2
fi

Solution 1

try

if ! grep -q sysa /etc/passwd ; then

grep returns true if it finds the search target, and false if it doesn't.

So NOT false == true.

if evaluation in shells are designed to be very flexible, and many times doesn't require chains of commands (as you have written).

Also, looking at your code as is, your use of the $( ... ) form of cmd-substitution is to be commended, but think about what is coming out of the process. Try echo $(cat /etc/passwd | grep "sysa") to see what I mean. You can take that further by using the -c (count) option to grep and then do if ! [ $(grep -c "sysa" /etc/passwd) -eq 0 ] ; then which works but is rather old school.

BUT, you could use the newest shell features (arithmetic evaluation) like

if ! (( $(grep -c "sysa" /etc/passwd) == 0 )) ; then ...`

which also gives you the benefit of using the c-lang based comparison operators, ==,<,>,>=,<=,% and maybe a few others.

In this case, per a comment by Orwellophile, the arithmetic evaluation can be pared down even further, like

if ! (( $(grep -c "sysa" /etc/passwd) )) ; then ....

OR

if (( ! $(grep -c "sysa" /etc/passwd) )) ; then ....

Finally, there is an award called the Useless Use of Cat (UUOC). :-) Some people will jump up and down and cry gothca! I'll just say that grep can take a file name on its cmd-line, so why invoke extra processes and pipe constructions when you don't have to? ;-)

I hope this helps.

Solution 2

I think it can be simplified into:

grep sysa /etc/passwd || {
    echo "ERROR - The user sysa could not be looked up"
    exit 2
}

or in a single command line

$ grep sysa /etc/passwd || { echo "ERROR - The user sysa could not be looked up"; exit 2; }

Solution 3

This one

if [[ !  $(cat /etc/passwd | grep "sysa") ]]
Then echo " something"
exit 2
fi

Solution 4

What am I doing wrong?

$(...) holds the value, not the exit status, that is why this approach is wrong. However, in this specific case, it does indeed work because sysa will be printed which makes the test statement come true. However, if ! [ $(true) ]; then echo false; fi would always print false because the true command does not write anything to stdout (even though the exit code is 0). That is why it needs to be rephrased to if ! grep ...; then.

An alternative would be cat /etc/passwd | grep "sysa" || echo error. Edit: As Alex pointed out, cat is useless here: grep "sysa" /etc/passwd || echo error.

Found the other answers rather confusing, hope this helps someone.

Solution 5

simply:

if ! examplecommand arg1 arg2 ...; then
    #code block
fi

without any brackets.

View more solutions

Comments

[0zkr PM]

Nice, but I prefer the Mr. shellter answer because is "self documented", is more "readable" the programmer intention.

[Orwellophile]

It's all rather silly really, from my answer to a much harder (question)[stackoverflow.com/a/30400327/912236] grep "^$user:" /etc/passwd would be the more correct way to search /etc/passwd incidently – grep -v where -v inverts the search if you wanted to avoid the mess of ||

[shellter]

yes, well there's solving a problem most efficiently, and then there is answering specific question. I've tried to answer the specific question. Thanks for your ideas. Good luck to all.

[Orwellophile]

not picking on your answers, quite enjoyed them. i just through i would throw in a properly bounded check on username, else if the OP really does search on "sys" or somesuch, he'll get quite the surprise. one more for the road? (( $( cat file | grep regex | wc -l ) ? 0 : 1 ))

[Julien]

I like this version. What about adding 1>&2 at the end of your echo to print on stderr ?

[ceving]

@0zkrPM But the shellter version does not work in Bourne shell. You will get !: not found

[tbc0]

Avoid output redirection when using 'grep like this. -q suppresses output.

[Kusalananda]

@SDsolar Your code is probably not executed by bash in that case.

[DocWeird]

Great! For some reason reqular "! grep -qs..." did not work with /proc/mounts and trying to find out if a regularly dropping USB-disk was mounted on Raspbian 4.9 kernel. This one did the job perfectly!

[FluffyKitten]

Welcome to Stack Overflow. Code-only answers are discouraged on Stack Overflow because they don't explain how it solves the problem. Please edit your answer to explain what this code does and how it improves on the other upvoted answers this question already has, so that it is useful to other users with similar issues.