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

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

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

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