Newsgroups: comp.unix.ultrix
Path: utzoo!utgpu!watserv1!watcgl!idallen
From: "Ian! D. Allen [CGL]" <idallen@watcgl.waterloo.edu>
Subject: Ultrix 4.1 (etc.) "who am i" wrong or null output
Message-ID: <1991Apr14.193523.14523@watcgl.waterloo.edu>
Sender: idallen@watcgl.waterloo.edu (Ian! D. Allen [CGL])
Organization: Computer Graphics Laboratory, University of Waterloo, Ontario, Canada
Date: Sun, 14 Apr 1991 19:35:23 GMT
Lines: 66

The Ultrix 4.1 who (and all previous versions) suffers from the following
ancient BSD bugs.

Ultrix who bug #1:

    The first one will hit you if you run programs that use ptys without
    making utmp entries, as in "script" or "screen" or "xterm":

    Sample output:

	Script started on Sun Apr 14 14:54:35 1991
	% /bin/who am i
	watcgl!         ttype   Apr 12 18:00             <- nonsense

	Script started on Sun Apr 14 14:55:23 1991
	% /bin/who am i
	%                                                <- no output!
    
    The first nonsense output comes from who finding a utmp entry that
    matches your tty, left over from a previous user of the tty.  (When
    someone logs out, the utmp userid field is zeroed, but not the tty
    field.)  Who blindly prints the userid field without checking it, and
    since you have no userid entered in the utmp when you run script, who
    blindly prints a null string.  We fixed that in our BSD source in 1986.

Ultrix who bug #2:

    If you understand the above, then you'll realize that since Ultrix
    "who am i" only searches for a match on the tty field, without regard
    for a non-null userid field, the output can be wrong because of old
    entries in the utmp file.  The entries in the utmp file are made in
    one-to-one correspondence with the ttys in the /etc/ttys file, so if
    you run your system with a particular set of ttys you get a utmp file
    that has entries for a lot of the ttys (the userid fields being
    zeroed out when people log out).  If you then add or delete ttys in
    /etc/ttys, the correspondence between ttys and utmp shifts and "who
    am i" prints the wrong thing again.  We fixed that in 1988:

     * Revision 1.3  88/06/18  14:50:00  idallen
     * No longer prints wrong utmp entry for "who am i".
     * Used to find the first entry that matched the tty name, even if the
     * userid was null; but, null-name entries can get created by changes
     * in the /etc/ttys file causing a tty's ttyslot() number to shift.
     * Must have a non-null name to get a match.
     [...]
    /* "who am i" mode:
     * Have to have a non-null name on the line.
     * As /etc/ttys changes, we can end up with
     * duplicate ut_line entries for ttys (since
     * the ttyslot() number of a tty may change).
     * We must ignore all entries with null names.
     * -IAN!
     */

Ultrix (and BSD) who inconsistency:

    Look!  No host name in the first example:

    % rsh localhost /bin/who am i
    idallen  tty??   Apr 14 15:27                   <- why no host name?

    % /bin/who am i
    watcgl!idallen  tty01   Apr 14 15:28
-- 
-IAN! (Ian! D. Allen) idallen@watcgl.uwaterloo.ca idallen@watcgl.waterloo.edu
 [129.97.128.64]  Computer Graphics Lab/University of Waterloo/Ontario/Canada