title: add expr within awk
body: <p>I am trying to increment record number variable (NR) within awk script like below but getting syntax errors. </p>
  
  <pre><code>awk '{print "dm" `expr $NR + 225` "," $0}'  test
  </code></pre>
  
  <p>where test is a text file with numerous records.</p>
  
  <p>Appreciate if someone can help me with the correct syntax ! </p>