Quick-and-dirty way to ensure only one instance of a shell script is running at a time

Question

What\'s a quick-and-dirty way to make sure that only one instance of a shell script is running at a given time?

Answer 1

Answered a million times already, but another way, without the need for external dependencies:

LOCK_FILE="/var/lock/$(basename "$0").pid"
trap "rm -f ${LOCK_FILE}; exit" INT TERM EXIT
if [[ -f $LOCK_FILE && -d /proc/`cat $LOCK_FILE` ]]; then
   // Process already exists
   exit 1
fi
echo $$ > $LOCK_FILE

Each time it writes the current PID ($$) into the lockfile and on script startup checks if a process is running with the latest PID.