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Re: return from function doesn't work when used in tail of pipeline


From: Martin Schwenke
Subject: Re: return from function doesn't work when used in tail of pipeline
Date: Fri, 13 Aug 2010 09:49:19 +1000

On Thu, 12 Aug 2010 08:16:21 -0400, Greg Wooledge <address@hidden>
wrote:

> You already know the reason it behaves the way it does, so I'm not
> quite sure what answer you expect to receive.  It's not a bug -- it's
> the normal behavior of every shell.
> 
> imadev:~$ sh -c 'f() { (return 0); return 1; }; f; echo $?'
> 1
> [...]
> A return that's run in a subshell doesn't cause the parent shell to
> return.

I want it to either obey the principle of least surprise or be more
clearly documented!  return is syntax so I expect it to do what its
documentation says it does...

I've been programming in shell for 20 years and this one surprised me.
I have a very strong Unix background and I clearly understand things
like not being able to set variables in subshells.  It also surprised
other people, with similar backgrounds, who I showed it to.  It is much
less obvious than many other shell subtleties.

There are some obvious exceptions to the strict Unix subprocess model in
shell programming (e.g. I can set a variable without exporting it and
see it in a subshell) and I expected this to be one of them.

Adding something like this to the documentation for return would
probably be enough:

  Note that many compound commands that might be used in a function
  cause subshells to be created.  In this subshells return will behave
  as though it is being used outside a function.

> Use a process substitution in bash:
> 
> f() {
>   while IFS= read -r somevar; do
>     [[ $somevar = *quit* ]] && return 0
>     printf '%s\n' "$somevar"
>   done < <(grep foo bar)
>   return 1
> }
[...] 
> The process substitution doesn't require cmd to finish before it
> starts producing data.  It's equivalent to a pipeline, except that
> the reader doesn't need to be in a subshell.

Thanks, I'll use that unless I can come up with something more
portable!  :-)

peace & happiness,
martin



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