Locking daemon in bash example

lockdemo

#!/bin/bash

# As detailed at http://blog.didenko.com/2010/08/locking-for-daemons-in-bash.html

pidf=/tmp/$(basename ${0}).pid
exec 221>${pidf}
flock --exclusive --nonblock 221 ||
{
     echo "Another instance is apparently running."
     exit 1
}
echo ${$}>&221

echo "Running as process ID "${$}
echo "The process ID is stored in the file "${pidf}
echo "About to get buzy for about 20 seconds"
for (( i=0 ; i < 10; i=i+1 ))
do
	echo -n "z-"
	sleep 2
done
echo "Oh!"

echo "Done all the hard work. Exiting."

Maybe I didn't fully understand it, but how do we avoid file descriptor collision with other scripts?

EDIT: Nevermind, just tried it :) The actual pid file still matters.

As I understand it, line 6: exec 221>${pidf} clobbers the pid file, so if you run a second instance, the pid file is erased.

I tried exec 221>>${pidf} instead, but echo ${$}>&221 did not erase the file as I thought it might, so I end up with a list of past pid's. Suggestions?