title: How to restart a service if its dependent service is restarted
body: <p>A service (say bar.service) is dependent on another service (say foo.service), like below</p>
  
  <p>bar's service file:</p>
  
  <pre><code>[Unit]
  After=foo.service
  Requires=foo.service
  ...
  </code></pre>
  
  <p>If foo.service is restarted (either manually or due to a bug), how can bar.service be automatically restarted as well?</p>