docker daemon -H fd:// error out in shell
<p>I start the docker daemon with <code>-H fd://</code> in shell, and it error out. The OS is CentOS7.</p>

<pre><code># docker daemon -H fd://
FATA[0000] No sockets found
</code></pre>

<p>I can start with <code>systemctl start docekr</code>.</p>

<pre><code># cat /etc/systemd/system/docker.service.d/docker.conf 
[Service]
ExecStart=
ExecStart=/usr/bin/docker daemon -H fd://
# ps -ef|grep docker
/usr/bin/docker daemon -H fd://
</code></pre>

<p>In the <a href="https://docs.docker.com/engine/reference/commandline/daemon/" rel="nofollow">admin guide</a>, it says</p>

<p>On Systemd based systems, you can communicate with the daemon via Systemd socket activation, use docker daemon -H fd://. Using fd:// will work perfectly for most setups but you can also specify individual sockets: docker daemon -H fd://3. If the specified socket activated files aren’t found, then Docker will exit. You can find examples of using Systemd socket activation with Docker and Systemd in the Docker source tree.</p>

<p>So I can't use <code>-H fd://</code> in shell?</p>