title: sort by multiple columns using awk
body: <p>I have the following <code>sort</code> code: </p>
  
  <pre><code>sort -t, -k18,18n -k21,21nr
  </code></pre>
  
  <p>which sort a file with <code>,</code> delimiter, first ascending by column <code>$18</code>, then descending by column <code>$21</code>. </p>
  
  <p>I am looking for an equivalent of this command using <code>awk</code></p>
  
  <p>Please advice </p>