title: Print every n lines from a file
body: <p>I'm trying to print every <code>n</code>th line from file, but <code>n</code> is not a constant but a variable.</p>
  
  <p>For instance, I want to replace <code>sed -n '1~5p'</code> with something like <code>sed -n '1~${i}p'</code>.</p>
  
  <p>Is this possible?</p>