This is the way to get a lock:
system('exec gpu_lock.py --id')

As used by:
    obtain_gpu_lock_id.m

Matlab's system command spawns a shell process. Without the use of exec, the
lock would be freed when the shell terminates, which is immediately after
gpu_lock.py returns. The exec means that gpu_lock.py sees Matlab as the parent
process.

It is tempting to do:
    system('gpu_lock.py --id $PPID')
but this doesn't work in Matlab if the user's login shell is set to /bin/tcsh
(it would work in Octave, which always uses /bin/sh).