Provided by: parallel_20231122+ds-1_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.
--pipe Pass stdin (standard input) to command.
If command read from stdin (standard input), use --pipe.
--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) 2010-2023 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)
20231122 2023-12-14 SEM(1)