I have a service that depends on another:
[Unit]
After=main.service
Requires=main.service
[Service]
ExecStart=/usr/local/bin/dependent
Restart=always
The kool things:
systemctl stop main causes this dependent service to also stopsystemctl restart main causes dependent service to also get startedBut, running systemctl main start does not cause this dependent service to get started.
How do I do it?
The After= and Before= parameters are strictly for ordering the startup (and maybe the shutdown) of two services if both are asked to be started. Having a service listed there doesn't mean that those services will also be started up.
If you want to control whether or not another service gets started, you need to use Requires=, Wants=, or Conflicts= (which go in the [Unit] section) or WantedBy= (which goes into the [Install] section).
In your case, you want to start a service when something it's dependent on is started (which doesn't happen automatically, as there is no reason to do so). You'll want to add the following to the file and then run systemctl enable dependent:
[Install]
WantedBy=main.service
This says that when main is started, to also start this service, and that if this service happens to fail, then don't stop main.
WantedBy= as suggested won't get the other service to start with systemctl start main.service. It's not until the main.service is enabled that happens. See the associated docs for WantedBy= here...