title: bash text search: find if the content of one file exists in another file
body: <p>Say we have two files: a.txt and b.txt. Each file has multiple lines of text.</p>
  
  <p>How do I write a shell script to check if all of the content of a.txt exists in b.txt?</p>
  
  <hr>
  
  <p>Thx for the hints guys, i didn't noticed -q will output 0 if successfully matched. </p>
  
  <p>I end up with:</p>
  
  <p>if <code>grep a.txt -q -f b.txt</code> ; then</p>
  
  <p>else</p>
  
  <p>fi</p>