Preventing duplicate shell script executions

run_cronjob_solo.sh

#!/bin/sh
# WIP prevent duplicate cron job shell executions.
# refer:  http://bencane.com/2015/09/22/preventing-duplicate-cron-job-executions/

do_function() {
	# do something...
}

PIDFILE=/path/to/<YOUR_PID_FILE>.pid
if [ -f $PIDFILE ]  ## if pid file exist and it is not a directory
then
	## if there really is another instance of this script running
	PID=$(cat $PIDFILE)
	ps -p $PID > /dev/null 2>&1
	if [ $? -eq 0 ]  ## if exit code is equal to 0 with ps command
	then
		echo "Process already running"
		exit 1
	else
		## process not found assume not running
		echo $$ > $PIDFILE
		if [ $? -ne 0 ]  # if exit code is not equal to 0 with echo command
		then
			echo "Could not create PID file"
			exit 1
		fi
	fi
else  ## if pid file doesn't exist
	echo $$ > $PIDFILE
	if [ $? -ne 0 ]  ## if exit code is not equal to 0 with echo command
	then
		echo "Could not create PID file"
		exit 1
	fi
fi

do_function
rm $PIDFILE