newgrp versus sg

[mullen%nrl-css at sri-unix.UUCP]

From:  Preston Mullen <mullen at nrl-css>

It took just a few minutes to transform newgrp.c into sg.c, where "sg"
behaves like "su", except that it changes group id instead of user id
and leaves the environment alone.

This makes all sorts of applications of groups possible that are just
too cumbersome with only "newgrp" to switch groups.  In fact, since it
is generally the case that no password is needed, it's easy to use "sg"
to do some trivial thing in a shell file without creating a special
set-gid command for that purpose.

My question is, why does such a program not already appear in V7 or 4.1bsd?

Other notes:

1. There is also a bug in newgrp.c (4.1bsd 10/1/80) --
   what should be the 2nd argument to execl is omitted.  Because
   the third (now second) argument "-i" begins with "-", the new
   shell is taken for a login shell, which is ridiculous, as the
   shell that invoked newgrp need not have been a login shell.

2. The 4.1bsd and V7 manual pages for "su" neglect to state
   that su also changes to the default group id for the new user id
   (as determined by the /etc/passwd entry for the user id).

P.S.	I know 4.2bsd obviates "sg", but not everybody has or will
	get 4.2bsd.

(end of message)