title: How to ensure a rake task only running a process at a time
body: <p>I use crontab to invoke rake task at some time for example: every 3 hour </p>
  
  <p>I want to ensure that when crontab ready to execute the rake task
  it can check the rake task is running. if it is so don't execute. </p>
  
  <p>how to do this. thanks.</p>