X-Recipient: archive-cygwin AT delorie DOT com DomainKey-Signature: a=rsa-sha1; c=nofws; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:reply-to:subject:references:to:from:message-id :date:mime-version:in-reply-to:content-type :content-transfer-encoding; q=dns; s=default; b=ZzGAa7+u5fix05WO N5aZMrErNgG3x6pfWJn1hWi0k3T4XuYngK4+ekPHdAE7NdY9Sj+WMl+gpqwP1+cy ApkVFQtgGFkkdFlGuscne92un7CCmEhW3UPtEQ8dUUuxghYwGXY3vJXVuS/lxhZ4 Hm7kvGgtOL3dzryCQL1aGF4rNk4= DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:reply-to:subject:references:to:from:message-id :date:mime-version:in-reply-to:content-type :content-transfer-encoding; s=default; bh=pPfKynCEMk7iPvsl07Y6N2 oUy7E=; b=dEM0FL8LZsNSmX7q8sGJYqwzdPSa+wUGk6vtLDQpRGd06D8XlAspdp 04rTKD6qlNwI1s7yFU48xQRqfjaXSO2PGOtIza9T5/sJf5ATvFMTYojJW3QQp6Ux VbaAS+niGSxOBeJNQLlf+uIHqdk7DE7AqHzbmBEUOFZx5SxvRGsFs= Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner AT cygwin DOT com Mail-Followup-To: cygwin AT cygwin DOT com Delivered-To: mailing list cygwin AT cygwin DOT com Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.0 required=5.0 tests=AWL,BAYES_00,KAM_LAZY_DOMAIN_SECURITY,RP_MATCHES_RCVD,UNSUBSCRIBE_BODY autolearn=ham version=3.3.2 spammy=Hx-languages-length:2011, dear, Pipe X-HELO: mailsrv.cs.umass.edu Reply-To: moss AT cs DOT umass DOT edu Subject: Re: Pipe behavior clarification? References: <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 Message-ID: Date: Mon, 23 Jan 2017 09:42:08 -0500 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 MIME-Version: 1.0 In-Reply-To: <841dff2b-65d9-9d24-2b88-6742f1ea92cb@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-IsSubscribed: yes 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