What does this "-H fd:/" option mean in this docker.json file?

General
1

On Ubuntu 18.04.2

I wanted to set my Docker root dir so I followed the instructions given in https://github.com/IronicBadger/til/blob/master/docker/change-docker-root.md

From the page, the /etc/systemd/system/docker.service.d/docker.root.conf contains

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -g /new/docker/root -H fd://

What does fd:// represent?

I’m trying to debug this error.

Creating Dockerfile from file /data/workspace/jenkins-project/deploy/docker/webapp/Dockerfile.
Determining image tag: my-image:2019-06-14-1
Using the native docker binary.
Daemon vm is shutting down... The daemon has exited normally or was terminated in response to a user interrupt.
:Project:buildDocker FAILED
:Project:buildDocker (Thread[Daemon worker,5,main]) completed. Took 59.389 secs.
----- End of the daemon log -----

Thanks,
Chris

2

I’m guessing it means “file descriptor.”

3

Yep it does:

On Systemd based systems, you can communicate with the daemon via Systemd socket activation, use dockerd -H fd:// . Using fd:// will work perfectly for most setups but you can also specify individual sockets: dockerd -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.

(source: dockerd | Docker Docs)

4

meyay’s answer is right. If you like to understand the detail of this, I recommend to read the following Stackoverflow answer and the blog article:

sockets - what does fd:// mean exactly in dockerd -H fd:// - Stack Overflow

That explains how docker daemon and systemd handle this parameter in detail.