title: What is a drop-in file? What is a drop-in directory? How to edit systemd service
body: <p>I want to change how a custom systemd service works.</p>
  <pre><code>systemctl edit serviceName
  </code></pre>
  <p>This edits /etc/systemd/system/serviceName.d/override.conf file. Override.conf is called a drop-in file and serviceName.d/ is a drop-in directory. I want to put a drop-in file definition and drop-in folder definition on the internet.</p>