[ILUG] /bin/dash - lessons from the front line

[Chris Higgins]

Extract from apt-cache show dash

" It can be usefully installed as /bin/sh (because it executes scripts
 somewhat faster than "bash"), or as the default shell either of root ..."

I hit a weird problem recently upgrading the desktop systems we use
here for our staff. All but one went through the 
 "apt-get update, apt-get -u upgrade, apt-get -u dist-upgrade" commands
with no problems at all. 

One machine however ended up with all of the postrm scripts that
had anything to do with X ( xfree86, fonts, xlibs ) fail. So none
of the new packages would upgrade. They always failed with an 
"found end of file, expecting ) " syntax error. 

I've been keeping all the systems practically identical so that
I wouldn't see these problems. 

The scripts worked perfectly on my laptop, the line numbers where the
errors were listed didn't make any sense. 

When I hand ran the scripts I got the same error messages, and 
/bin/sh -x showed things dying shortly after an external
call to expr

I eventually tracked it down - the machine had /bin/sh linked
to dash. Switched it over to bash, ran apt-get -u upgrade
and it worked perfectly ( as it had on the other machines ).

I haven't had a chance to go into exactly how dash made it's
way in as /bin/sh ( but all machines have recently been
upgraded to 2.6.x - and dash seems to be 'required' for
2.6.x ). 

I also haven't had a opportunity to debug the dash process 
more completely - so I don't know if it's dash or something
in the root users profile which may be impacting on dash
(but I doubt this - as dash isn't an app that the
systems would either use or really care about so there
shouldn't have been any conf in place for dash).


So - if /bin/sh starts to behave strangely for you, 
just double check which shell you are actually using.

Chris. 

--

Chris Higgins                              Cisco Learning Partner
Darach Technology Ltd                      tel: +353-1-6204370
email: chris.higgins at darach.ie          fax: +353-1-6204371
http://www.darach.ie