title: Loop through hours from a specific time to another time
body: <p>I would like to write a bash/shell script to loop through 2016-07-27_00:00:00 to 2016-07-29_00:00:00 in 3 hourly interval. My problem is with the hours. I'm a new user and I want to loop through hours.</p>
  
  <p>So that it can look like following:</p>
  
  <pre><code>2016-07-27_00:00:00
  2016-07-27_03:00:00
  2016-07-27_06:00:00
  ...and so on.
  </code></pre>
  
  <p>Can any one kindly help me with that?</p>