title: Shell: insert variable into a command
body: <p>I'm trying to find a way the use a variable in this command to replace -10 with n_days var:</p>
  
  <pre><code>   n_days= -10
     date_prefix=$(date -d '-10 day' +%Y/%m/%d)
  </code></pre>
  
  <p>I tried this way but it didn't work:</p>
  
  <pre><code>   date_prefix=$(date -d '${n_days} day' +%Y/%m/%d)
  </code></pre>