How to detect your script is started using su...

I wrote a script that had a problem when started via su command. Actually, this is a script within /etc/profile.d so it is executed when new login shell is executed. I'll write about that problem in another post, but here I'll concetrate on how to detect su command.

But before continuing let me clarify that this is a bit of a misnomer. Namely, the goal is to detect whether current environment is a consequence of user ID switching after login, but since this is almost exclusively done using su command, then I think I can put this title. There is also one more "problem". Namely, all user IDs currently having running processes descended from user id 0. But, we are not going so far with philosophy. :)

I started by thinking/hoping that id command could identify originating user, i.e. real user. But that was not possible since the distinction between real and effective user ids is preserved only via setuid flag on files. So, another approach has to be used. There are three possibilities, each one with its own advantages and shortcomings.