Run bash script forever with CRON with concurrency 1

daemon.sh

#!/bin/bash

# Original: http://bencane.com/2015/09/22/preventing-duplicate-cron-job-executions/
#
# This script executes ./cmd_loop.sh and save process ID (PID) in file ./forever.pid.
# Everytime it's executed it checks for PID in ./forever.pid and if process is still
# running then exists with nonzero code. Otherwise it executes ./cmd_loop.sh.

PIDFILE=./forever.pid

if [ -f $PIDFILE ]
then
  PID=$(cat $PIDFILE)
  ps -p $PID > /dev/null 2>&1
  if [ $? -eq 0 ]
  then
    echo "Process already running"
    exit 1
  else
    echo "Process not found assume not running"
    echo $$ > $PIDFILE
    if [ $? -ne 0 ]
    then
      echo "Could not create PID file"
      exit 1
    fi
  fi
else
  echo $$ > $PIDFILE
  if [ $? -ne 0 ]
  then
    echo "Could not create PID file"
    exit 1
  fi
fi

echo "Executing cmd_loop.sh"
./cmd_loop.sh

rm $PIDFILE