How can my shell script test to see if it's already running?

I think this text in general is usefull, however, be very carefull (or actualy do not do it at all!) kill processes without checking if it is the correct process.

Another process might have taken your process ID and you may kill the wrong process, a check if the process is the program you are running should be done at all times!)

Do you mean "echo $$" rather than "cat $$"?

Ken, you're right! That should indeed be "echo $$" not "cat $$". Mea culpa! :-)

Well, a rather different problem is the following: We want a program, say "perl /home/me/mydir/myscript.pl" to run continously, and to be restarted should it be stopped by a system failure
, a reboot, a power outage or whatever.
So we put in /etc/inittab a line like
myscr:3456:respawn:/home/me/myscr

and myscr reads
"cd /home/me/mydir;
perl /home/me/mydir/myscript.pl > /dev/null &"

however, this will respawn when myscr dies, not when the perl script dies. Consequently, it will
respawn zillions of perl /home/me/mydir/myscript.pl processes and
eventually put the system to a standstill.
What needs to be done is to check if myscript.pl
is running and only if it is not call it.
In this case I doubt we can use the solution posted above and hence avoid doing a ps -ef | grep myscript.pl and need to excluded the grep from there), and I am not sure what the most elegant way to do the checking would be

Alternative, script around pidof. Should man pidof to verify behavior on your system.

I prefer never to /dev/null any program, but pidof doesn't like to be quiet.

Example (bashisms intended!):

#!/bin/bash

me=$(basename $0)

pidof -x -o '%PPID' $me > /dev/null 2>&1 && exit 1

echo $me

sleep 300