GNU semNAME
sem - semaphore for executing shell command lines in parallelSYNOPSIS
sem [--fg] [--id <id>] [--semaphoretimeout <secs>] [-j <num>] [--wait] command

DESCRIPTION
GNU sem is an alias for GNU parallel --semaphore.
GNU sem acts as a counting semaphore. When GNU sem is called with command it starts thecommand 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
commandCommand to execute. The command may be followed by arguments for thecommand.
--bg
Run command in background thus GNU sem will not wait for completion of thecommand before exiting. This is the default.
In toilet analogy: GNU sem waits for a toilet to be available, gives the toilet to aperson, and exits immediately.
See also: --fg--jobs N-j N--max-procs N-P N
Run up to N commands in parallel. Default is 1 thus acting like a mutex.
In toilet analogy: -j is the number of toilets.
--jobs +N-j +N--max-procs +N-P +N
Add N to the number of CPU cores. Run up to this many jobs in parallel. For
compute intensive jobs -j +0 is useful as it will run number-of-cpu-cores jobs
simultaneously.
--jobs -N-j -N--max-procs -N-P -N
Subtract N from the number of CPU cores. Run up to this many jobs in parallel. If theevaluated number is less than 1 then 1 will be used. See also--use-cpus-instead-of-cores.
Page 1GNU sem--jobs N%-j N%--max-procs N%-P N%
Multiply N% with the number of CPU cores. Run up to this many jobs in parallel. If
the evaluated number is less than 1 then 1 will be used. See also--use-cpus-instead-of-cores.
--jobs procfile-j procfile--max-procs procfile-P procfile
Read parameter from file. Use the content of procfile as parameter for -j. E.g. procfile
could contain the string 100% or +2 or 10.
--pipePass stdin (standard input) to command.
If command read from stdin (standard input), use --pipe.
--semaphorename name--id name
Use name as the name of the semaphore. Default is the name of the controlling tty(output from tty).
The default normally works as expected when used interactively, but when used in ascript name should be set. $$ or my_task_name are often a good value.
The semaphore is stored in ~/.parallel/semaphores/
In toilet analogy the name corresponds to different types of toilets: e.g. male, female,customer, staff.
--fg
Do not put command in background.
In toilet analogy: GNU sem waits for a toilet to be available, takes a person to thetoilet, waits for the person to finish, and exits.
--semaphoretimeout secs--st secs
If secs > 0: If the semaphore is not released within secs seconds, take it anyway.
If secs < 0: If the semaphore is not released within secs seconds, exit.
In toilet analogy: secs > 0: If no toilet becomes available within secs seconds, pee onthe floor. secs < 0: If no toilet becomes available within secs seconds, exit without
doing anything.
--waitWait for all commands to complete.
In toilet analogy: Wait until all toilets are empty, then exit.

UNDERSTANDING A SEMAPHORE
Try the following example:
sem -j 2 'sleep 1;echo 1 finished';sem -j 2 'sleep 2;echo 2 finished';sem -j 2 'sleep 3;echo 3 finished';

echo sem 1 exited
echo sem 2 exited
echo sem 3 exited
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GNU semsem -j 2 'sleep 4;echo 4 finished';sem --wait; echo sem --wait doneecho sem 4 exited

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 *.log
Run one gzip process per CPU core. Block until a CPU core becomes available.
for i in *.log ; doecho $i
sem -j+0 gzip $i ";" echo done
done
sem --wait

EXAMPLE: Protecting pod2html from itself
pod2html 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 makesure 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

BUGS
None known.

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GNU sem

REPORTING BUGS
Report bugs to <bug-parallel@gnu.org>.

AUTHORCopyright (C) 2010-2024 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 theLicense, or at your option any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; withouteven 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 theGNU 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 II
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Share AlikeIf you alter, transform, or build upon this work, you may distribute the resulting work
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WaiverAny of the above conditions can be waived if you get permission from the copyrightholder.
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Other Rights
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Your fair dealing or fair use rights, or other applicable copyright exceptions andlimitations;Page 4

GNU sem
The author's moral rights;
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NoticeFor any reuse or distribution, you must make clear to others the license terms of thiswork.
A copy of the full license is included in the file as LICENCES/CC-BY-SA-4.0.txt

DEPENDENCIES
GNU sem uses Perl, and the Perl modules Getopt::Long, Symbol, Fcntl.

SEE ALSO
parallel(1)Page 5