Re: Pipe behavior clarification?
- Date: Mon, 23 Jan 2017 09:42:08 -0500
- From: Eliot Moss <moss@xxxxxxxxxxxx>
- Subject: Re: Pipe behavior clarification?
On 1/23/2017 9:32 AM, Eric Blake wrote:
On 01/22/2017 04:23 PM, Eliot Moss wrote:
On 1/22/2017 3:19 PM, David Balažic wrote:
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.
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.
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.
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 :-) ...
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.
Dear Eric (et al.) -- I *mostly* agree with this, with the exception
that >(...) is *not* an output redirection. It will present tee with
the name of a file -- either a named pipe or a /dev/fdnnn file name.
By experimentation I discovered that this particular replacement
is not done in the same left-to-right pass as I/O redirections ...
Regards - EM
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