Pipe behavior clarification?

David Balažic xerces9@gmail.com
Mon Jan 23 17:37:00 GMT 2017

On 23 January 2017 at 15:32, Eric Blake <eblake@redhat.com> 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.

The last part is incorrect. tee is always writing to the md5sum process.
It can be verified by redirecting the md5sum output to a file, like
>(md5sum > file1),
or redirecting the final output to a file instead of /dev/null. In
both cases the (correct)
md5 hash will be there.


PS: I verified on SLES 11 that the behavior is the same as in cygwin,
so the thread is "technically closed".

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