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Newsgroups: comp.unix.shell
From: Stephane CHAZELAS <this.addr...@is.invalid>
Subject: Re: piping stdout and stderr to different processes?
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Date: Fri, 27 Jul 2007 09:08:23 GMT
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2007-07-26, 20:44(-05), Neil Cherry:
[...]
>> {
>>   {
>>     cm1 3>&- |
>>       cmd2 2>&3 3>&-
>>   } 2>&1 >&4 4>&- |
>>     cmd3 3>&- 4>&-
>> } 3>&2 4>&1
>
> Thanks.
>
> OK now I see why I didn't get it to work. I didn't try that far.
> But I have to say I'm not quite sure what I'm reading just yet.
> I'll have to hit the man pages as I'm not used to the >&-
> syntax.

3>&- is for closing fd 3. It's not necessary, but it's for tidy
up. None of the commands will ever try (not should they) to
access the fd 3 and 4, so it's best to close them before
executing those commands so that they can use those fds for
something else. 

{
  {
    cm1 |
      cmd2 2>&3
  } 2>&1 >&4 |
    cmd3
} 3>&2 4>&1

is functionnaly equivalent.

if cmd2 doesn't output anything on its stdout nor stderr, it can
even be simplified to:
{ cm1 | cmd2; } 2>&1 | cmd3

Or if you want to be sure:

{ cm1 | cmd2 > /dev/null 2>&1; } 2>&1 | cmd3

-- 
Stéphane