Bash "not": inverting the exit status of a command

Question

I know I can do this...

if diff -q $f1 $f2
then
    echo "they're the same"
else
    echo "they're different"
fi

But what if I want to negate the condition that I'm checking? i.e. something like this (which obviously doesn't work)

if not diff -q $f1 $f2
then
    echo "they're different"
else
    echo "they're the same"
fi

I could do something like this...

diff -q $f1 $f2
if [[ $? > 0 ]]
then
    echo "they're different"
else
    echo "they're the same"
fi

Where I check whether the exit status of the previous command is greater than 0. But this feels a bit awkward. Is there a more idiomatic way to do this?

The canonical may be *[How can I negate the return-value of a process?](https://stackoverflow.com/questions/367069/)*? — Peter Mortensen, Sep 16 '21 at 18:14

William Pursell · Accepted Answer · 2019-12-03T23:39:11.087

13

if ! diff -q "$f1" "$f2"; then ...

edited Dec 03 '19 at 23:39

answered Feb 25 '13 at 17:45

William Pursell

204,365
48
270
300

score 2 · Answer 2 · answered Feb 25 '13 at 17:44

If you want to negate, you are looking for ! :

if ! diff -q $f1 $f2; then
    echo "they're different"
else
    echo "they're the same"
fi

or (simplty reverse the if/else actions) :

if diff -q $f1 $f2; then
    echo "they're the same"
else
    echo "they're different"
fi

Or also, try doing this using cmp :

if cmp &>/dev/null $f1 $f2; then
    echo "$f1 $f2 are the same"
else
    echo >&2 "$f1 $f2 are NOT the same"
fi

score 0 · Answer 3 · answered Feb 25 '13 at 17:50

0

To negate use if ! diff -q $f1 $f2;. Documented in man test:

! EXPRESSION
      EXPRESSION is false

Not quite sure why you need the negation, as you handle both cases... If you only need to handle the case where they don't match:

diff -q $f1 $f2 || echo "they're different"

answered Feb 25 '13 at 17:50

Karoly Horvath

94,607
11
117
176

1

Is this actually invoking test though? I'm not using "test" nor "[". – Coquelicot Feb 25 '13 at 18:00
1

It's not invoking it, it's a shell builtin (for performance reasons), but the syntax is the same – Karoly Horvath Feb 25 '13 at 18:19
2

`man test` is irrelevant here. From `man bash`: _ If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as described above._ – Pawel Veselov May 10 '18 at 17:17

Bash "not": inverting the exit status of a command

3 Answers3