I would like to go the systemd override way to let dockerd to listen to port 2376. So I followed this instruction.
On the other hand, I would like to dig into systemd to know what's going on under the hood.
So I tried to inspect the unit file of docker by this command:
systemctl cat docker.service
According to the output of the command, two files are involved.
I believe the first one is default unit file for docker and the second one is the I created.
My problem is:
Both files include sentances - ExecStart= and twice in the second file like:
ExecStart=
ExecStart=/usr/bin/dockerd -H fd://
Is it necessary to assign empty to ExecStart= before setting meaningful value ExecStart=/usr/bin/dockerd -H fd:// ?
I have spilit this post into two questions and the other one here.
When you add entries to an override file, they are by default appended to any existing entries. That is, if your service example.service has:
[Service]
EnvironmentFile=/etc/foo.env
And you create /etc/systemd/system/example.service.d/override.conf
with:
[Service]
EnvironmentFile=/etc/bar.env
Then the effective configuration is:
[Service]
EnvironmentFile=/etc/foo.env
EnvironmentFile=/etc/bar.env
That's fine for many directives, but a service can have only one
ExecStart (unless it's a Type-oneshot service), so if you try to create an override file like this:
[Service]
ExecStart=/new/command/line
That will fail with an error along the lines of:
systemd: example.service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.
By specifying an empty ExecStart, you are "clearing out" all
previous entries. So if your example.service has:
[Service]
ExecStart=/bin/foo
And you create an override like:
[Service]
ExecStart=
ExecStart=/bin/bar
The effective configuration is:
[Service]
ExecStart=/bin/bar