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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> <2e4799d9-e81f-fabb-9f31-3c569a70efc9 AT cs DOT umass DOT edu> <841dff2b-65d9-9d24-2b88-6742f1ea92cb AT redhat DOT com> |
| To: | cygwin AT cygwin DOT com |
| From: | Eliot Moss <moss AT cs DOT umass DOT edu> |
| Message-ID: | <e0f341fe-dc2e-8d27-5d04-6e0b3fe91086@cs.umass.edu> |
| Date: | Mon, 23 Jan 2017 09:42:08 -0500 |
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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 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
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