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Reply-To: moss AT cs DOT umass DOT edu
Subject: Re: Pipe behavior clarification?
References: <CAPJ9Yc9BJXrvrFz3pKQiEzzQ8a8f0_9rPuLn+o+mc5_Zo2wdmg AT mail DOT gmail DOT com>
To: cygwin AT cygwin DOT com
From: Eliot Moss <moss AT cs DOT umass DOT edu>
Message-ID: <2e4799d9-e81f-fabb-9f31-3c569a70efc9@cs.umass.edu>
Date: Sun, 22 Jan 2017 17:23:47 -0500
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On 1/22/2017 3:19 PM, David Balažic wrote:
> Hi!
>
> Is this a correct pipe behavior?
>
> $ echo booo | tee >(md5sum --tag) >/dev/null
> MD5 (-) = 9c8b79bdf79ef0ee73a77b8d36d27a2d
>
> $ echo booo | tee >(md5sum --tag) | cat >/dev/null

Here's what I think happens, even if it is a bit counter-intuitive:

 >(...) creates a subprocess, whose input comes from some kind
of pipe or socket, and tee is presented with a filename it can
use to write to that socket.

The *output* of the >(...) subprocess is hooked up to what is
known to be the output of tee *at the time the subprocess is
created*.  This happens *before* any > redirections are done.

However, in the case of the | pipe, that plumbing is set up
*before* the >(...) construct is acted on.

Note that you could do >(md5sum --tag >whatever) if you want
to specifically control the output of md5sum.

I am sure someone more knowledgeable will correct me if I've
missed anything important here :-) ...

Regards - Eliot Moss

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