Pipe behavior clarification?

Eliot Moss moss@cs.umass.edu
Mon Jan 23 14:42:00 GMT 2017


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:
>>> 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.
>
> 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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