Go to main content

man pages section 1: User Commands

Exit Print View

Updated: Thursday, March 14, 2019
 
 

sem (1)

Name

sem - semaphore for executing shell command lines in parallel

Synopsis

sem [--fg] [--id <id>] [--semaphoretimeout <secs>] [-j <num>] [--wait]
command

Description

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.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | shell/parallel   |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       parallel(1)



NOTES
       This software was built from source available at
       https://github.com/oracle/solaris-userland.  The original community
       source was downloaded from
       https://ftp.gnu.org/gnu/parallel/parallel-20170722.tar.bz2

       Further information about this software can be found on the open source
       community website at https://www.gnu.org/software/parallel.



20170522                          2017-01-15                            SEM(1)