Semaphore interface for the command line

Copyright © 2012, 2013 Stefan Klinger


The sem command offers a versatile, yet simple frontend to named system semaphores. Using semaphores instead of a construct with fifos avoids problems with concurrent initialisation.

To allow for the protection of multiple commands in shell scripts, the level of operation is more primitive than GNU sem. This also allows synchronisation of tasks across different scripts and even users.

sem comes with some auxilliary shell scripts to further facilitate semaphore management on Linux. The provided barrier script implements the eponymous synchronisation pattern.

The operations provided are to initialise, wait, post, and remove named semaphores. Blocking and nonblocking operation is supported, as well as timeout. A command may be run in between wait and post operations, optionally in the background. Most naming restrictions are liberated, names may even contain slashes. Global and per-user semaphores reside in different namespaces.

Getting started

Download the sources, unpack, make, and enjoy. sem comes with extensive documentation, and it's free.


Imagine you use the GraphicsMagick package to resize a lot of images. The compute intensive part is the command gm.

gm convert -resize x600 image.jpeg small-image.jpeg;

It produces a low-resolution version named “small-image.jpeg” of an image file “image.jpeg”. Typically, you would run this on hundreds of files, as in the following loop:

find ~/pics -iregex '.*\.jpe?g' | while read f; do
    gm convert -resize x600 "$f" "small-$f";

However, only one gm-process is running at any time. On a multicore machine with sufficient memory, it might be fine to run a couple of them concurrently. But putting them all in the background simultaneously will lead to a high load and probably consume all memory. So maybe just running four of them would be great.

To this end, simply prefix the gm command with a call of sem. The whole loop then reads

find ~/pics -iregex '.*\.jpe?g' | while read f; do
    sem foo -i4 -f -x -- gm convert -resize x600 "$f" "small-$f";

By semaphore magic, you will have exactly four instances of gm running for most of the time — exceptions being when there are less than four images left to convert, and the small lapse of time when one gm process has just ended and the next one has not yet been launched.