How do I write a bash script to restart a process if it dies?

Question

I have a python script that\'ll be checking a queue and performing an action on each item:

# checkqueue.py
while True:
  check_queue()
  do_something()
         


        

Answer 1

The easiest way to do it is using flock on file. In Python script you'd do

lf = open('/tmp/script.lock','w')
if(fcntl.flock(lf, fcntl.LOCK_EX|fcntl.LOCK_NB) != 0): 
   sys.exit('other instance already running')
lf.write('%d\n'%os.getpid())
lf.flush()

In shell you can actually test if it's running:

if [ `flock -xn /tmp/script.lock -c 'echo 1'` ]; then 
   echo 'it's not running'
   restart.
else
   echo -n 'it's already running with PID '
   cat /tmp/script.lock
fi

But of course you don't have to test, because if it's already running and you restart it, it'll exit with 'other instance already running'

When process dies, all it's file descriptors are closed and all locks are automatically removed.