As I replied to your other comment, you cannot just replace [[ with [ and get the same behaviour. If you use the actual bash builtin [[ which we're discussing,

As I replied to your other comment, you cannot just replace [[ with [ and get the same behaviour. If you use the actual bash builtin [[ which we're discussing, it works as expected. Also, please use == when comparing, = is an assignment operator (in almost every programming language).