when to use ( ), [ ] and [[ ]] with " if "
 

when to use ( ), [ ] and [[ ]] with " if "

I'm assuming BASH, in which case try this resource:

http://www.tldp.org/LDP/abs/abs-guide.pdf

Once again, now that you have asked a question and found that telling people you are running FreeBSD which is actually ash, this needs to be the first piece of information you provide
so the people here do not go off on a wild goose chase trying to figure out what is different about your system.

So as it is not bash, I suggest you start looking at guides for ash and you will probably find some of the symbols you have just asked about are not used by ash.