#!/bin/ksh

PID=$$

if [[ -f /var/run/script.pid ]]
then
  OLDPID=`cat /var/run/script.pid`
  RUNNING=`ps -e | grep $OLDPID`
  if [[ ! -z "$RUNNING" ]]
  then
    exit 1
  fi
fi

echo $PID > /var/run/script.pid

# your code here

if [[ -f /var/run/script.pid ]]
then
  rm /var/run/script.pid
fi