Avoid multiple instances of the script - Sample script [Linux] [Bash]

locksleep.sh

#!/usr/bin/env bash

# Avoid multiple instances of the script
# Sample script
# Based on description of
#   man 1 flock
# Your code go inside of "main" function and other fuctions

function main {

  echo $1 start
  sleep $1
  echo $1 end

}

# ----------------

script_file="$(/bin/readlink -f $0)"
lock_file=${script_file////_}

function already_executing_ {
  echo "'${script_file}' already executing"
  exit 1
}

(
  flock -n 9 || already_executing_

  main $@

) 9> /var/lock/${lock_file}