Provided by: parallel_20210822+ds-2_all 
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
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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)