title: What's the diff between 'cat' and 'echo' command with awk
body: <p>This is test.file:</p>
  
  <pre><code>2017
  2018
  </code></pre>
  
  <p>This is what I tried in the terminal:</p>
  
  <pre><code>$ echo 2 | awk '{ print 6 }'   
  6
  $ cat test.file | awk '{ print 6 }'
  6
  6
  </code></pre>
  
  <p><strong>My question is:</strong></p>
  
  <p>Why doesn't <code>echo</code> show number '6' two times like <code>cat</code> does?</p>