SEM(1)

💖 SEM(1) — sem 💖

okay bestie, welcome to the sem manual, and yes it's a whole vibe ✨ This is the page for sem from the parallel family of tools — she's the semaphore girlie who runs your shell command lines in parallel without breaking a sweat. 🌸

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✨ NAME ✨

sem — semaphore for executing shell command lines in parallel 🎀

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🦋 SYNOPSIS 🦋

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

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👑 DESCRIPTION 👑

GNU sem</purl> is literally an alias for GNU parallel --semaphore. Same girl, different name, no cap. 💅

GNU sem acts as a counting semaphore. When GNU sem is called with command it starts that command in the background. Once num number of commands are already running in the background, GNU sem just waits for one of those to finish before it starts the new command. ✨

GNU sem does not read any arguments to build the command (so no -a, no :::, no ::::). She simply waits for a semaphore to become available and then runs the command you gave her. periodt. 🫶

Before you go scrolling through the options, you might wanna check out the examples down after the options list — that'll give you the whole vibe of what GNU sem is capable of. 🌈

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💅 OPTIONS 💅

• command — the command to execute. The command may be followed by arguments for the command. 👑
• --bg — run command in background, so 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, so she's 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 reads from stdin (standard input), use --pipe. 🍓
• --semaphore-name 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. 🚽
• --semaphore-timeout 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. 🚽✨

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🌸 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. 👑

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🧚 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

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🌈 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

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💀 BUGS 💀

None known. iconic behaviour honestly ✨

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🙈 REPORTING BUGS 🙈

Report bugs to <bug-parallel@gnu.org>. 📧

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🎀 AUTHOR 🎀

Copyright (C) 2010-2026 Ole Tange, http://ole.tange.dk and Free Software Foundation, Inc. 👑

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💜 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. 📄

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🍓 DEPENDENCIES 🍓

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

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⭐ SEE ALSO ⭐

• parallel(1)

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and that's the whole page bestie, you ate it, no crumbs 💅✨ This was SEM(1) from the parallel source, version dated 20260122 and last updated 2026-01-31 — mwah, go run some commands in parallel, ily 🫶🎀