I want systemd to start a service when a USB device is plugged in and stop it when i remove it.
Use systemctl to get a list of units:
$ systemctl
UNIT LOAD ACTIVE SUB DESCRIPTION
sys-subsystem-net-devices-gamelink0.device loaded active plugged PL25A1 Host-Host Bridge
<snip>
There's no configuration required here - the .device unit just appears in response to udev events without any configuration. I've previously set up udev rules so that my USB host-to-host cable is consistently named gamelink0, but even without that it would show up under its default name.
The simplest way is just to take advantage of WantedBy. In gamelink.service:
$ cat /etc/systemd/systemd/gamelink.service
[Unit]
Description = Gamelink cable autoconf
After=sys-subsystem-net-devices-gamelink0.device
BindsTo=sys-subsystem-net-devices-gamelink0.device
[Service]
Type=simple
Environment=PYTHONUNBUFFERED=1
ExecStart=/usr/bin/python3 /home/john/gamelink.py
[Install]
WantedBy = sys-subsystem-net-devices-gamelink0.device
And then install it:
$ systemctl enable gamelink
Created symlink from /etc/systemd/system/sys-subsystem-net-devices-gamelink0.device.wants/gamelink.service to /etc/systemd/system/gamelink.service.
The WantedBy directive tells systemctl enable to drop the symlink in a .wants directory for the device. Whenever a new unit becomes active systemd will look in .wants to see what other related services needs to be started, and that applies to .device units just as much as .service units or .target units. That behaviour is all we need to start our daemon on hotplug.
The BindsTo directive lets us stop the service when the .device unit goes away (i.e. the device is unplugged). Used in conjunction with After we ensure that the service may never be in active state without a specific device unit also in active state.