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.
- Command to execute. The command may be followed by arguments for the command.
- 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.
- 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 for all commands to complete.
In toilet analogy: Wait until all toilets are empty, then exit.
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.
for i in *.log ; do echo $i sem -j+0 gzip $i ";" echo done done sem --wait
# 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
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 http://www.gnu.org/licenses/.
- to Share
- to copy, distribute and transmit the work
- to Remix
- to adapt the work
Under the following conditions:
- 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:
- 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.
- 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 cc-by-sa.txt.