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SEM(1)				    parallel				  SEM(1)

NAME
     sem - semaphore for executing shell command lines in parallel

SYNOPSIS
     sem  [--fg]  [--id  <id>]	[--semaphore-timeout <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  the  command in the background. When num number of commands are
     running in the background, GNU sem waits for one of these to  complete  be-
     fore 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
     command  Command  to  execute. The command may be followed by arguments for
	      the command.

     --bg     Run command in background thus GNU sem will not wait  for  comple-
	      tion of the command before exiting. This is the default.

	      In  toilet  analogy:  GNU  sem waits for a toilet to be available,
	      gives the toilet to a person, 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 the evaluated number is less than 1 then 1 will
	      be used.	See also --use-cpus-instead-of-cores.

     --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.

     --pipe   Pass stdin (standard input) to command.

	      If command read from stdin (standard input), use --pipe.

     --semaphore-name name
     --id nameUse 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 a script 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 toi-
	      lets: 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 the toilet, waits for the person to finish, and
	      exits.

     --semaphore-timeout secs
     --st secsIf 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 on the floor. secs < 0:  If  no  toilet  becomes
	      available within secs seconds, exit without doing anything.

     --wait   Wait 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';   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 toi-
     let, 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 wait-
     ing, 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 wait-
     ing, 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 avail-
     able.

       for i in *.log ; do
	 echo $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 run-
     ning 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

BUGS
     None known.

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

AUTHOR
     Copyright	(C)  2010-2026	Ole Tange, http://ole.tange.dk and Free Software
     Foundation, Inc.

LICENSE
     This program is free software; you can redistribute it and/or modify it un-
     der 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 FIT-
     NESS 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 I
     Permission  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 II
     You are free:

     to Share to copy, distribute and transmit the work

     to Remix to adapt the work

     Under the following conditions:

     Attribution
	      You  must attribute the work in the manner specified by the author
	      or licensor (but not in any way that suggests  that  they  endorse
	      you or your use of the work).

     Share Alike
	      If you alter, transform, or build upon this work, you may distrib-
	      ute  the resulting work only under the same, similar or a compati-
	      ble license.

     With the understanding that:

     Waiver   Any of the above conditions can be waived if  you  get  permission
	      from the copyright holder.

     Public Domain
	      Where  the work or any of its elements is in the public domain un-
	      der applicable law, that status is in no way affected by	the  li-
	      cense.

     Other Rights
	      In no way are any of the following rights affected by the license:

	      * Your  fair dealing or fair use rights, or other applicable copy-
		right exceptions and limitations;

	      * The author's moral rights;

	      * Rights other persons may have either in the work  itself  or  in
		how the work is used, such as publicity or privacy rights.

     Notice   For  any	reuse or distribution, you must make clear to others the
	      license terms of this work.

     A	copy  of  the  full  license  is   included   in   the	 file	as   LI-
     CENCES/CC-BY-SA-4.0.txt

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

SEE ALSO
     parallel(1)

20260122			   2026-01-31				  SEM(1)

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