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

[Mark Rages <EMAIL:PROTECTED>]

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?

Regards,
Mark
EMAIL:PROTECTED

On Thu, 3 Mar 2005 13:08:02 -0600, Mark Rages <EMAIL:PROTECTED> wrote:
> On Thu, 3 Mar 2005 12:54:35 -0600 (CST), Mike Miller
> <EMAIL:PROTECTED> wrote:
> > On Thu, 3 Mar 2005, Mark Rages wrote:
> >
> > > I have reduced my problem to this:
> > >
> > > ( /bin/false | gzip > /dev/null ) || echo fail
> > >
> > > I want it to echo "fail" because /bin/false gives a nonzero exit code.
> > > But it doesn't.  What am I missing?
> >
> > Maybe I'm not understanding this.  If you are gzipping from stdin, don't
> > you need to do it this way:
> >
> >      | gzip -c - >
> >
> > Maybe that isn't your intention.  I'm not sure how gzip is used in a path
> > when it isn't reading stdin and writing to stdout.
> >
> > I don't quite understand 'false'.  Where is the exit code detected?  It
> > doesn't go to stderr or to stdout.
> 
> A little more explanation, then.
> 
> Unix programs return a numeric code to the shell when they finish.
> This code is 0 when the program finishes successfully, and an error
> code when the program fails.
> 
> /bin/false exits immediately with a nonzero exit code.  ( /bin/true
> exits with 0 )
> 
> I was using /bin/false only as an example of a failing command.  I was
> trying to explain my situation where I'm executing a string of
> commands piped together, and I want to know if one of the commands
> failed or not.  (this is a script for making backups, so I can't just
> let it silently fail!)
> 
> What I thought (appears to be wrong) is that if a command at the
> beginning of a pipe fails, it will send SIGPIPE to the next command,
> which will kill it with an error, and so on, until the shell will
> receive an error code.  This is obviously incorrect.  Can some Unix
> guru explain this to me?
> 
> Regards,
> Mark
> EMAIL:PROTECTED
> --
> You think that it is a secret, but it never has been one.
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> 


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