Re: Is there any mutex/semaphore mechanism in shell scripts?

[Greg Wooledge]

On Wed, May 20, 2009 at 12:34:42PM -0700, jjjaime wrote:
> FUNCTION_1 &
> FUNCTION_2
> FUNCTION_3
> 
> So, to speed up the execution of the script, I want FUNCTION_1 and
> FUNCTION_2 in parallel. But the script fails when FUNCTION_2 ends before
> FUNCTION_1.

Why?

> Is there any mechanism for synchronization (i.e. semaphores/mutes) or any
> other suggestion for simulating it?

You haven't really shown a need for it, but I'll assume your example is
simply misleading, and disregard it.

If what you're really asking for is a way to do mutual exclusion (mutexes)
then there are a few choices:

 * mkdir is atomic, and you can use it for locking:

    while ! mkdir $HOME/mylockdir; do
      sleep 3
    done
    # Or something fancier.

   Just be sure to remove it when you're done.  (Set a 'trap', etc.)

 * ln -s is also atomic and can be used similarly.

 * There's "setlock" in daemontools (http://cr.yp.to/daemontools.html)
   if you want third-party stuff.

If what you're really asking for is "How do I keep two jobs running in
the background at all times, starting another job when one finishes?"
then there are a couple recipes at

http://mywiki.wooledge.org/ProcessManagement#advanced

I mention this only because we get that question pretty often in #bash,
and since your example was not helpful, I'm not sure exactly what it is
you wanted.