title: bash script variable substitution not happening
body: <p>In my bash script I have:</p>
  
  <pre><code>HOME_DIR="/root"
  EXE_DIR="${HOME_DIR}/exe"
  (crontab -l 2&gt;/dev/null; echo '30 3 * * * "${EXE_DIR}/backup"') | crontab -
  </code></pre>
  
  <p>This results in:</p>
  
  <pre><code># crontab -l
  30 3 * * * "${EXE_DIR}/backup"
  </code></pre>
  
  <p>How do I get the <code>EXE_DIR</code> variable to be substituted into the crontab command?</p>