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+#!/usr/bin/perl -w
+
+# SPDX-FileCopyrightText: 2021-2022 Ole Tange, http://ole.tange.dk and Free Software and Foundation, Inc.
+# SPDX-License-Identifier: GFDL-1.3-or-later
+# SPDX-License-Identifier: CC-BY-SA-4.0
+
+=head1 NAME
+
+sem - semaphore for executing shell command lines in parallel
+
+=head1 SYNOPSIS
+
+B<sem> [--fg] [--id <id>] [--semaphoretimeout <secs>] [-j <num>] [--wait] command
+
+=head1 DESCRIPTION
+
+GNU B<sem> is an alias for GNU B<parallel --semaphore>.
+
+GNU B<sem> acts as a counting semaphore. When GNU B<sem> is called
+with command it starts the command in the background. When I<num>
+number of commands are running in the background, GNU B<sem> waits for
+one of these to complete before starting the command.
+
+GNU B<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
+B<sem> is capable of.
+
+=head1 OPTIONS
+
+=over 9
+
+=item I<command>
+
+Command to execute. The command may be followed by arguments for the
+command.
+
+
+=item B<--bg>
+
+Run command in background thus GNU B<sem> will not wait for
+completion of the command before exiting. This is the default.
+
+In toilet analogy: GNU B<sem> waits for a toilet to be available,
+gives the toilet to a person, and exits immediately.
+
+See also: B<--fg>
+
+
+=item B<--jobs> I<N>
+
+=item B<-j> I<N>
+
+=item B<--max-procs> I<N>
+
+=item B<-P> I<N>
+
+Run up to N commands in parallel. Default is 1 thus acting like a
+mutex.
+
+In toilet analogy: B<-j> is the number of toilets.
+
+
+=item B<--jobs> I<+N>
+
+=item B<-j> I<+N>
+
+=item B<--max-procs> I<+N>
+
+=item B<-P> I<+N>
+
+Add N to the number of CPU cores.  Run up to this many jobs in
+parallel. For compute intensive jobs B<-j> +0 is useful as it will run
+number-of-cpu-cores jobs simultaneously.
+
+
+=item B<--jobs> I<-N>
+
+=item B<-j> I<-N>
+
+=item B<--max-procs> I<-N>
+
+=item B<-P> I<-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 B<--use-cpus-instead-of-cores>.
+
+
+=item B<--jobs> I<N>%
+
+=item B<-j> I<N>%
+
+=item B<--max-procs> I<N>%
+
+=item B<-P> I<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 B<--use-cpus-instead-of-cores>.
+
+
+=item B<--jobs> I<procfile>
+
+=item B<-j> I<procfile>
+
+=item B<--max-procs> I<procfile>
+
+=item B<-P> I<procfile>
+
+Read parameter from file. Use the content of I<procfile> as parameter
+for I<-j>. E.g. I<procfile> could contain the string 100% or +2 or
+10.
+
+
+=item B<--pipe>
+
+Pass stdin (standard input) to I<command>.
+
+If I<command> read from stdin (standard input), use B<--pipe>.
+
+
+=item B<--semaphorename> I<name>
+
+=item B<--id> I<name>
+
+Use B<name> as the name of the semaphore. Default is the name of the
+controlling tty (output from B<tty>).
+
+The default normally works as expected when used interactively, but
+when used in a script I<name> should be set. I<$$> or I<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.
+
+
+=item B<--fg>
+
+Do not put command in background.
+
+In toilet analogy: GNU B<sem> waits for a toilet to be available,
+takes a person to the toilet, waits for the person to finish, and
+exits.
+
+
+=item B<--semaphoretimeout> I<secs>
+
+=item B<--st> I<secs>
+
+If I<secs> > 0: If the semaphore is not released within I<secs>
+seconds, take it anyway.
+
+If I<secs> < 0: If the semaphore is not released within I<secs>
+seconds, exit.
+
+In toilet analogy: I<secs> > 0: If no toilet becomes available within
+I<secs> seconds, pee on the floor. I<secs> < 0: If no toilet becomes
+available within I<secs> seconds, exit without doing anything.
+
+
+=item B<--wait>
+
+Wait for all commands to complete.
+
+In toilet analogy: Wait until all toilets are empty, then exit.
+
+
+=back
+
+=head1 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 (B<-j 2>). GNU B<sem> takes '1'
+to a toilet, and exits immediately. While '1' is sleeping, another GNU
+B<sem> takes '2' to a toilet, and exits immediately.
+
+While '1' and '2' are sleeping, another GNU B<sem> waits for a free
+toilet. When '1' finishes, a toilet becomes available, and this GNU
+B<sem> stops waiting, and takes '3' to a toilet, and exits
+immediately.
+
+While '2' and '3' are sleeping, another GNU B<sem> waits for a free
+toilet.  When '2' finishes, a toilet becomes available, and this GNU
+B<sem> stops waiting, and takes '4' to a toilet, and exits
+immediately.
+
+Finally another GNU B<sem> waits for all toilets to become free.
+
+
+=head1 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
+
+=head1 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.
+B<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
+
+=head1 BUGS
+
+None known.
+
+
+=head1 REPORTING BUGS
+
+Report bugs to <bug-parallel@gnu.org>.
+
+
+=head1 AUTHOR
+
+Copyright (C) 2010-2022 Ole Tange, http://ole.tange.dk and Free
+Software Foundation, Inc.
+
+
+=head1 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/>.
+
+=head2 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.
+
+=head2 Documentation license II
+
+You are free:
+
+=over 9
+
+=item B<to Share>
+
+to copy, distribute and transmit the work
+
+=item B<to Remix>
+
+to adapt the work
+
+=back
+
+Under the following conditions:
+
+=over 9
+
+=item B<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).
+
+=item B<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.
+
+=back
+
+With the understanding that:
+
+=over 9
+
+=item B<Waiver>
+
+Any of the above conditions can be waived if you get permission from
+the copyright holder.
+
+=item B<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.
+
+=item B<Other Rights>
+
+In no way are any of the following rights affected by the license:
+
+=over 2
+
+=item *
+
+Your fair dealing or fair use rights, or other applicable
+copyright exceptions and limitations;
+
+=item *
+
+The author's moral rights;
+
+=item *
+
+Rights other persons may have either in the work itself or in
+how the work is used, such as publicity or privacy rights.
+
+=back
+
+=back
+
+=over 9
+
+=item B<Notice>
+
+For any reuse or distribution, you must make clear to others the
+license terms of this work.
+
+=back
+
+A copy of the full license is included in the file as
+LICENCES/CC-BY-SA-4.0.txt
+
+
+=head1 DEPENDENCIES
+
+GNU B<sem> uses Perl, and the Perl modules Getopt::Long,
+Symbol, Fcntl.
+
+
+=head1 SEE ALSO
+
+B<parallel>(1)
+
+=cut