Re: [MLUG] basic Unix pipe/exit code problem

[Jonathan King <EMAIL:PROTECTED>]

On Thu, 3 Mar 2005 14:05:52 -0600, Mark Rages <EMAIL:PROTECTED> wrote:
> Ok, so here's one solution from some Perl guys:
> 
> http://use.perl.org/~cog/journal/18948
> 
> yuck.  does everybody do their shell scripts this way?

Probably not, but this is a well-known Unix shell "gotcha". Things are
even worse in csh, I think.  The perl solution given here is an
attempt to plug the problems with sh using perl, which is a
historically ancient use of that tool.  There are shell idioms to get
around this stuff, but I agree that it really sucks when an intuitive
use of piping stuff around gives you obnoxious results.

But as for your specific example:

> > > > ( /bin/false | gzip > /dev/null ) || echo fail

Clearly shouldn't do what you want.  You told gzip to take its
standard input from a pipe, and even if a non-zero exit code is
raised, I can't see how what you did should prevent gzip from sucking
up it's stdin and compressing the result, which gives some trivial
file of minimum length  So check this out:

jonathan% false | gzip> dorky.gz
jonathan% ls -l dorky.gz
-rw-r--r--  1 jonathan  staff  20  3 Mar 14:49 dorky.gz
jonathan% gzip > dorky2.gz  # I hit cntrl-d to close stdin
jonathan% ls -l dorky2.gz
-rw-r--r--  1 jonathan  staff  20  3 Mar 14:49 dorky2.gz

Now, if you want my opinion, this (under tcsh) is much nastier:

jonathan% echo "" | gzip > dorky3.gz
jonathan% ls -l dorky3.gz
-rw-r--r--  1 jonathan  staff  21  3 Mar 14:51 dorky3.gz

Echo can never keep its mouth completely shut.  Grr...

jking
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