How to write a systemd service that depends on a device being present?

I would like to write a systemd service that depends on USB WiFi dongle being present. It doesn't need to start when device is plugged in as long as it doesn't start when device is not plugged in. How can I achieve this?

The systemd configruation file is in [email protected] format. It uses the the %i placeholder. So I tried adding this to the unit file:

[Path]
PathExists = /dev/%i

Where %i would be something like wlan0. Unfortunately, this gets completely ignored, so I might not understand the proper use of this section.

I've also tried:

[Unit]
....
BindsTo=sys-subsystem-net-devices-%i.device
After=sys-subsystem-net-devices-%i.device

I it picked up from somewhere (I forgot where), and that doesn't work either.

You need to combine udev and systemd.

Add a new rule for udev, in /etc/udev/rules.d/95-mywifi.rules, as follows:

 ACTION=="add", ATTRS{idVendor}=="...", ATTRS{idProduct}=="...", SYMLINK=="mywifi", TAG+="systemd", ENV{SYSTEMD_WANTS}="[email protected]"

(You must substitute for the dots the Vendor and Product codes appropriate to your dongle).

The exisiting [email protected] is:

  # cat /lib/systemd/system/netctl-auto\@.service 
  [Unit]
  Description=Automatic wireless network connection using netctl profiles
  Documentation=man:netctl.special(7)
  BindsTo=sys-subsystem-net-devices-%i.device
  After=sys-subsystem-net-devices-%i.device
  Before=network.target
  Wants=network.target

  [Service]
  ExecStart=/usr/bin/netctl-auto start %I
  ExecStop=/usr/bin/netctl-auto stop %I
  RemainAfterExit=yes
  Type=forking

  [Install]
  WantedBy=multi-user.target

For this to work, you will have to setup a netctl profile, see the Arch Linux Wiki to see how to do that.