This example script will check to make sure that an earlier instance isn't already running before executing again. Useful for cronjobs that run every 5 minutes, and you don't want a backlog of processes piling up because of a slowness issue

checkIfRunning.sh

#!/bin/sh

# This example script will check to make sure that an earlier instance isn't already running before executing again.
# Useful for cronjobs that run every 5 minutes, and you don't want a backlog of processes piling up because of a slowness issue
# Borrowed from http://stackoverflow.com/questions/1440967/how-do-i-make-sure-my-bash-script-isnt-already-running
# Modified to work on FreeBSD as well as Linux

# Example usage:
#$ ./checkIfRunning.sh 
#Sleeping for 600 seconds.
#^Z
#[1]+  Stopped                 ./checkIfRunning.sh
#$ bg
#[1]+ ./checkIfRunning.sh &
#./checkIfRunning.sh 
#[4214] NOTICE: Script is already running. Exiting.
#$ ./checkIfRunning.sh 
#[4219] NOTICE: Script is already running. Exiting.
#$ 


ME=`basename $0`
PATH=$PATH:/usr/local/bin

checkIfRunning () {
  # Check to see if we are already running:
  # http://stackoverflow.com/questions/1440967/how-do-i-make-sure-my-bash-script-isnt-already-running
  # Modified for FreeBSD
  
  # Use a lockfile containing the pid of the running process
  # If script crashes and leaves lockfile around, it will have a different pid so
  # will not prevent script running again.
  lockfile=/tmp/$ME.lockfile
  # Create empty lock file if none exists
  touch $lockfile
  read lastPID < $lockfile
  # If lastPID is not null and a process with that pid exists, then exit.
  [ ! -z "$lastPID" -a `pgrep -F $lockfile 2> /dev/null` ] && { echo "[$$] NOTICE: Script is already running. Exiting." && exit 0 ; }
  # save my pid in the lock file
  echo $$ > $lockfile
}

# Wrap everything else into a {} block, for easier control and logging
doit () {

  checkIfRunning
  
  # Your code here
  echo "Sleeping for 600 seconds."
  sleep 600
  echo "Hello world!"

}

# Do it!
doit