[Carpetsmoker]

Is there a straightforward manner?

I know about su -c user, but that will try to start the login shell. If the login shell is set to /sbin/nologin (As it often is for cron/daemon users) it won't work.

I actually solved this problem already using a very simple C program using setgid(), setuid(), and system(). But I find it hard to believe that I'm the first one with this issue and that this is the easiest way ...