Many thanks for the reply.

No, I wasn't aware about the necessity to change group before ID. That's fixed it.

I still note, however, that setuid and setgid do not affect the effective ID. This is despite what the documentation says (just over halfway down). I've adjusted the script to only send setgid a single number as opposed to a space-separated list, but there's still no effect to the effective.

I'm certain it's down to my understanding. I would rather use the POSIX calls than the special variables, so if anyone has any more comments, I'd welcome them.

J.