If you use -r, -w, -x etc. in your programm, be sure not to get race conditions. These are often misused functions. The right way is: Try to do what you want (= read file), and print an error message if that fails. The other way round is: Can I do this, can I do that? and then perform your action. This is the race condition, because between your tests and your action many things can change (i. e. file permissions).

-- Frank