sem - semaphore for executing shell command lines in parallel
sem [--fg] [--id <id>] [--timeout <secs>] [-j <num>] [--wait] command
GNU sem is an alias for GNU parallel --semaphore.
It works as a tool for executing shell commands in parallel. GNU sem acts as a counting semaphore. When GNU sem is called with command it will start the command in the background. When num number of commands are running in the background, GNU sem will wait for one of these to complete before starting another command.
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 up to N commands in parallel. Default is 1 thus acting like a mutex.
Use id 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 id should be set. $$ is often a good value.
Do not put command in background.
If the semaphore is not released within secs seconds, take it anyway.
Wait for all commands to complete.
Run one gzip process per CPU core. Block until a CPU core becomes available.
for i in `ls *.log` ; do
echo $i
sem -j+0 gzip $i ";" echo done
done
sem --wait
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) you need to protect pod2html from running twice at the same time. sem running as a mutex will do just that:
sem --fg --id pod2html pod2html foo.pod > foo.html sem --fg --id pod2html rm -f pod2htmd.tmp pod2htmi.tmp
Quoting and composed commands are not working.
Report bugs to <bug-parallel@gnu.org>.
Copyright (C) 2010 Ole Tange, http://ole.tange.dk and Free Software Foundation, Inc.
Copyright (C) 2010 Free Software Foundation, Inc.
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GNU sem uses Perl, and the Perl modules Getopt::Long, Symbol, Fcntl.
parallel(1)