title: Changing part of a line in a text file in bash
body: <p>I have a line in a text file, for example:</p>
  
  <blockquote>
    <p>abcde:jhdfj$jhbsfiy/.hghi$jh:12345:dgve::</p>
  </blockquote>
  
  <p>and I want to replace the 3rd field, 12345, with a variable. Without touching the rest of it and without copying anything to a variable... help? Yes, it's a uni question essentially :)</p>