title: Usage of dollar and tilde
body: <p>May i know why "<code>~</code>" and "<code>+$</code>" is used in this shell script.This is a part of script deleting files older than specific days.</p>
  
  <pre><code>if ! [[ $FILE_AGE =~ "^[0-9]+$" ]]; then
    usage
    exit 1
  </code></pre>
  
  <p>I am able to understand that this portion of the script is used to check whether we are entering numbers or alphabets.</p>