Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!umd5!uvaarpa!virginia!scl
From: scl@virginia.acc.virginia.edu (Steve Losen)
Newsgroups: comp.unix.questions
Subject: Re: System V newgrp
Message-ID: <535@virginia.acc.virginia.edu>
Date: 29 Dec 87 21:26:20 GMT
References: <4718@well.UUCP> <6866@ncoast.UUCP> <1014@bakerst.UUCP> <722@dolqci.UUCP> <3301@umn-cs.cs.umn.edu>
Reply-To: scl@virginia.EDU
Distribution: na
Organization: University of Va., Charlottesville, VA
Lines: 20
Keywords: group

newgrp is really a shell builtin command that does this:

exec /bin/newgrp group

hence the newgrp process replaces the current shell process.  

Newgrp is a setuid to root program that simply does this:
1) setgid(2) to the desired group (after checking the /etc/group file of course),
2) setuid(2) back to the real user, (we can't let you be superuser, can we?)
3) execs another shell for you.

Thus you are left with the same process but it has exec'ed two executables
while you were wondering, "Why the hell is newgrp so slow??"
Your shell now has a new group id, but you lost your environment
in the process.

By the way, you can never use newgrp in a shell script because of the
exec.  The script will never get to the command after the newgrp.
-- 
Steve Losen
University of Virginia Academic Computing Center