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Subject: Re: Pipe behavior clarification?
To: cygwin AT cygwin DOT com
References: <CAPJ9Yc9BJXrvrFz3pKQiEzzQ8a8f0_9rPuLn+o+mc5_Zo2wdmg AT mail DOT gmail DOT com> <2e4799d9-e81f-fabb-9f31-3c569a70efc9 AT cs DOT umass DOT edu>
From: Eric Blake <eblake AT redhat DOT com>
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Date: Mon, 23 Jan 2017 08:32:46 -0600
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From: Eric Blake <eblake AT redhat DOT com>
To: cygwin AT cygwin DOT com
Message-ID: <841dff2b-65d9-9d24-2b88-6742f1ea92cb AT redhat DOT com>
Subject: Re: Pipe behavior clarification?
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 <2e4799d9-e81f-fabb-9f31-3c569a70efc9 AT cs DOT umass DOT edu>
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On 01/22/2017 04:23 PM, Eliot Moss wrote:
> On 1/22/2017 3:19 PM, David Bala=C5=BEic wrote:
>> Hi!
>>
>> Is this a correct pipe behavior?
>>
>> $ echo booo | tee >(md5sum --tag) >/dev/null
>> MD5 (-) =3D 9c8b79bdf79ef0ee73a77b8d36d27a2d
>>
>> $ echo booo | tee >(md5sum --tag) | cat >/dev/null
>=20
> Here's what I think happens, even if it is a bit counter-intuitive:
>=20
>>(...) 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.
>=20
> 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.

Rather, all >() and > redirections are performed in left-to-right order.
 But you are correct that the second >/dev/null is overwriting the
stdout that was originally given by >(md5sum), and therefore tee is NOT
writing to the md5sum process.

>=20
> However, in the case of the | pipe, that plumbing is set up
> *before* the >(...) construct is acted on.

Also correct. Mixing >() and | is usually not what you want, as you are
no longer writing to the pipeline.

>=20
> Note that you could do >(md5sum --tag >whatever) if you want
> to specifically control the output of md5sum.
>=20
> I am sure someone more knowledgeable will correct me if I've
> missed anything important here :-) ...

You got the gist of it.  Order matters, and specifying more than one
stdout (by any mix of >, >(), or |) is generally not what you want.

--=20
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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