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?

Answer 1

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

Answer 2

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