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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!shamash!sialis!icus!lenny
From: lenny@icus.islp.ny.us (Lenny Tropiano)
Newsgroups: unix-pc.general,comp.sys.att
Subject: Funny thing happened to me ... Beware of "$_" and the shell you run
Keywords: /bin/sh, chmod, ooops.
Message-ID: <704@icus.islp.ny.us>
Date: 8 May 89 02:18:07 GMT
Distribution: usa
Organization: ICUS Software Systems, Islip, New York
Lines: 64

Doing some permission changing today, I goofed .. it had me going for a
while ... I figured I would relay this information, so if someone else goofs
like I did (and it's bound to happen), they'll know what to look for.

Here's what happened ...

I was creating a file in my /usr/mail directory called "LENNY", basically it's
for those BITNET-weirdo sites that like to return mail to ICUS!LENNY, and
then smail barfs on this end and returns the mail.  I place "Forward to lenny"
in that file so that it gets to me ... well, this doesn't have much to do
with my story except for what happened when I was changing permissions.

Normally I run /bin/ksh... so for those ksh people they know what "$_" does.
It basically returns the last argument in the previous command line.  It's
normally used like so...

# chown lenny /usr/mail/LENNY
# chgrp mail $_
# chmod 660 $_

In the last two commands, $_ is substituted with /usr/mail/LENNY, it saves
on those keystokes ...

Well that's normal and dandy for anyone running /bin/ksh, but what happens
if your shell is /bin/sh ...  If you echo $_ while running /bin/sh, you'll
notice it returns /bin/sh.  Now what happened to me...  /bin/sh's group was
changed  to mail, and the permission was 660.  Ut oh.   Well I didn't quite
notice this right away... I just logged out normally, thinking everything
was set.

I returned about 2 hours later and noticed that my screen was blank (that
green dot pattern) [for those wondering, I have an UNIX pc] ... Only thing
was that my phone manager was running, the status manager was running (and
the clock was correct) and the window manager was also running... No login
at all ... this was real weird (I thought at first).  There was no DTR 
(uugetty) on my Trailblazer, and my remote terminal line didn't have a 
getty either.

I figured something got zapped, so I took drastic measures and depressed
the RESET button.  The machine booted up to the point of ...

Version 3.51 ...
Real Memory = 
Available Memory =  
...

Then started doing some massive disk I/O.  I inserted the Floppy UNIX
and RESET again.  Well to make a long story short (I guess it's too late
for that) ... I checked obvious things like /etc/inittab, /lib/shlib,
/etc/rc, missing ... nada.   Well after a few reboots, and testing ... 
I noticed that nothing in /etc/rc was being executed ... Ahhh, I thought
it's a shell script, maybe /bin/sh is munged ... Sure was, at least
permissions were.  

Well that's what happens when you make /bin/sh non-executable ... Oooops.
No flames please, we're only human ...

								-Lenny

-- 
Lenny Tropiano             ICUS Software Systems         [w] +1 (516) 582-5525
lenny@icus.islp.ny.us      Telex; 154232428 ICUS         [h] +1 (516) 968-8576
{talcott,decuac,boulder,hombre,pacbell,sbcs}!icus!lenny  attmail!icus!lenny
        ICUS Software Systems -- PO Box 1; Islip Terrace, NY  11752