Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
SEM(1)				   parallel				SEM(1)

NAME
       sem - semaphore for executing shell command lines in parallel

SYNOPSIS
       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 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
       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
		completion 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.

       --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	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
		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 the toilet, 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 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
       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	; 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
       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

BUGS
       None known.

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

AUTHOR
       Copyright (C) 2010,2011,2012,2013,2014,2015,2016,2017 Ole Tange,
       http://ole.tange.dk and Free Software Foundation, Inc.

LICENSE
       Copyright (C) 2010,2011,2012,2013 Free Software Foundation, Inc.

       This 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 <http://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 fdl.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
		distribute the resulting work only under the same, similar or
		a compatible 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
		under applicable law, that status is in	no way affected	by the
		license.

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

		o Your fair dealing or fair use	rights,	or other applicable
		  copyright exceptions and limitations;

		o The author's moral rights;

		o 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 cc-by-sa.txt.

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

SEE ALSO
       parallel(1)

20161222			  2017-01-15				SEM(1)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | UNDERSTANDING A SEMAPHORE | EXAMPLE: Gzipping *.log | EXAMPLE: Protecting pod2html from itself | BUGS | REPORTING BUGS | AUTHOR | LICENSE | DEPENDENCIES | SEE ALSO

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=sem&manpath=FreeBSD+12.0-RELEASE+and+Ports>

home | help