On Fri, May 8, 2015 at 2:42 PM, Giuseppe Aprea <giuseppe.ap...@gmail.com> wrote:
> I read again the first part of my message about semaphores and it doesn't > really express what I meant. Taking the two commands (or any other couple > you think is appropriate to show the differences): > > sem --fg --no-notice -j 2 echo ::: 1 2 3 4 > parallel --no-notice -j 2 echo ::: 1 2 3 4 Note that ::: is ignored for sem. If you read the SYNOPSIS of the man page, you will see that sem does not read arguments from :::, ::::, -a, or standard input. Instead you have to give the full command to sem: sem --fg -j 2 echo 1 sem --fg -j 2 echo 2 sem --fg -j 2 echo 3 sem --fg -j 2 echo 4 A more realistic example is: 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. - o - One of the situations in which this is useful is in 'crontab': If you have a command that runs every minute, but that it will fail, if two are running at the same time, you need a way to postpone the next run if one instance takes longer than 1 minute. If you cannot allow a certain command to be run twice at the same time, then you can simply prepend 'sem --id myid' to the command, and the second instance will not be allowed to run before the first is finished. - o - If the concept of a semaphore is still hazy please read: http://en.wikipedia.org/wiki/Semaphore_%28programming%29 /Ole
