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NAMEsem - semaphore for executing shell command lines in parallelSYNOPSISsem [--fg] [--id <id>] [--semaphoretimeout <secs>] [-j <num>] [--wait] commandDESCRIPTIONGNU sem is an alias for GNU parallel --semaphore.GNU sem acts as a counting semaphore. When GNU sem is called with command it starts the command in the background. When num number of commands are running in the background, GNU sem waits for one of these to complete before starting the command. GNU sem does not read any arguments to build the command (no -a, :::, and ::::). It simply waits for a semaphore to become available and then runs the command given. Before looking at the options you may want to check out the examples after the list of options. That will give you an idea of what GNU sem is capable of. OPTIONS
UNDERSTANDING A SEMAPHORETry the following example:sem -j 2 'sleep 1;echo 1 finished'; echo sem 1 exited sem -j 2 'sleep 2;echo 2 finished'; echo sem 2 exited sem -j 2 'sleep 3;echo 3 finished'; echo sem 3 exited sem -j 2 'sleep 4;echo 4 finished'; echo sem 4 exited sem --wait; echo sem --wait done In toilet analogy this uses 2 toilets (-j 2). GNU sem takes '1' to a toilet, and exits immediately. While '1' is sleeping, another GNU sem takes '2' to a toilet, and exits immediately. While '1' and '2' are sleeping, another GNU sem waits for a free toilet. When '1' finishes, a toilet becomes available, and this GNU sem stops waiting, and takes '3' to a toilet, and exits immediately. While '2' and '3' are sleeping, another GNU sem waits for a free toilet. When '2' finishes, a toilet becomes available, and this GNU sem stops waiting, and takes '4' to a toilet, and exits immediately. Finally another GNU sem waits for all toilets to become free. EXAMPLE: Gzipping *.logRun one gzip process per CPU core. Block until a CPU core becomes available.for i in *.log ; do echo $i sem -j+0 gzip $i ";" echo done done sem --wait EXAMPLE: Protecting pod2html from itselfpod2html creates two files: pod2htmd.tmp and pod2htmi.tmp which it does not clean up. It uses these two files for a short time. But if you run multiple pod2html in parallel (e.g. in a Makefile with make -j) there is a risk that two different instances of pod2html will write to the files at the same time:# This may fail due to shared pod2htmd.tmp/pod2htmi.tmp files foo.html: pod2html foo.pod --outfile foo.html bar.html: pod2html bar.pod --outfile bar.html $ make -j foo.html bar.html You need to protect pod2html from running twice at the same time. sem running as a mutex will make sure only one runs: foo.html: sem --id pod2html pod2html foo.pod --outfile foo.html bar.html: sem --id pod2html pod2html bar.pod --outfile bar.html clean: foo.html bar.html sem --id pod2html --wait rm -f pod2htmd.tmp pod2htmi.tmp $ make -j foo.html bar.html clean BUGSNone known.REPORTING BUGSReport bugs to <bug-parallel@gnu.org>.AUTHORCopyright (C) 2010-2022 Ole Tange, http://ole.tange.dk and Free Software Foundation, Inc.LICENSEThis program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or at your option any later version.This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <https://www.gnu.org/licenses/>. Documentation license IPermission is granted to copy, distribute and/or modify this documentation under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the file LICENSES/GFDL-1.3-or-later.txt.Documentation license IIYou are free:
Under the following conditions:
With the understanding that:
A copy of the full license is included in the file as LICENCES/CC-BY-SA-4.0.txt DEPENDENCIESGNU sem uses Perl, and the Perl modules Getopt::Long, Symbol, Fcntl.SEE ALSOparallel(1)
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