Provided by: parallel_20210822+ds-2_all bug

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) 2010-2021 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 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 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 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:

                • Your fair dealing or fair use rights, or other applicable copyright 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 LICENCES/CC-BY-SA-4.0.txt

DEPENDENCIES

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

SEE ALSO

       parallel(1)