Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!cwjcc!hal!ncoast!allbery
From: allbery@ncoast.ORG (Brandon S. Allbery)
Newsgroups: comp.unix.wizards
Subject: Re: (login) behaviour.
Message-ID: <13683@ncoast.ORG>
Date: 25 May 89 23:25:13 GMT
References: <89@<BRISTER> <22200001@ux1.cso.uiuc.edu>
Reply-To: allbery@ncoast.UUCP (Brandon S. Allbery)
Followup-To: comp.unix.wizards
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 53

As quoted from <22200001@ux1.cso.uiuc.edu> by phil@ux1.cso.uiuc.edu:
+---------------
| (echo hey;login);echo test
| 
| Saw "hey" displayed, and went through login.  "ps ux" shows an extra csh.
| I logged out, and then saw "test" displayed.  So I guess I really got a
| new login shell from the subshell.  Apparently doing (login) all by itself
| doesn't really fork a new process for shell commands such as login and cd,
| but rather does them and then restores everything, such as connected
| directory.
+---------------

Nope.  The shells recognize "login" (and often "newgrp") as special commands,
and automagically maps them to "exec login" (and "exec newgrp").  When you
said "(login)", you said "start a subshell, which should exec /bin/login".
When you exited the resulting "login shell", you got the original shell back.

This can be more than slightly disorienting, by the way:

% who am i
allbery  tty15   May 25 18:25
% date
Thu May 25 19:18:02 EDT 1989
% (login root)
Password: ########
# ^D
% who am i
root     tty15   May 25 19:18
% _

(The above is NOT a typescript; I didn't actually do it.  But I have done so
before, on other systems; I can't do it on ncoast.)  <--"covering my *ss" ;-)

AT&T System V prohibits this, by the way:  since /etc/utmp contains the
login shell's pid, login can determine that it wasn't execed from the login
shell:

% who am i
bsa      tty15   May 25 18:25
% date
Thu May 25 19:18:02 EDT 1989
% (login root)
Login must be execed from the top-level shell.
% who am i
bsa      tty15   May 25 19:18
% _

++Brandon
-- 
Brandon S. Allbery, moderator of comp.sources.misc	     allbery@ncoast.org
uunet!hal.cwru.edu!ncoast!allbery		    ncoast!allbery@hal.cwru.edu
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