udev2dbus

What?

This is a simple script and udev rules that, when used together, generate dbus messages when a device is plugged in or unplugged.

Why?

I wanted to run a script in my X session when a keyboard was attached. All the solutions I could find for this involved custom-built udev rules which need your username and display number hard-coded. This seemed ridiculous to me; there had to be a way to listen for udev events through dbus. I couldn't find such a thing, so I created one.

How to install it?

Put 99-udev2dbus.rules in /etc/udev/rules.d/
Put udev2dbus.sh in /usr/local/bin/

You will need root/sudo privileges to do both. On modern systems that's all there is to it. On older systems you may need to restart udev or your computer.

How to use it?

Now, Whenever a device is added to or removed from your system, a dbus message will be generated. You don't need root in order to listen for these messages. You can watch for these messages with the dbus-monitor command:

dbus-monitor --system "interface='org.udev.event'"

Most programming and scripting languages nowadays can listen for and act on dbus messages. Here's a simple python example using pydbus:

#!/usr/bin/env python3

import os
import subprocess
import pydbus
from gi.repository import GLib


def handle_signal(sender, objPath, iface, signal, paramList):
    params = {}
    for i in paramList:
        (k, v) = i.split('=', 1)
        params[k] = v
    if (signal == 'add' and params['SUBSYSTEM'] == 'input'
            and 'keyboard' in params['MODEL_NAME'].lower()):
        subprocess.run(os.environ['HOME']+'/bin/fixkeys')


bus = pydbus.SystemBus()
dev = bus.subscribe(iface="org.udev.event", signal_fired=handle_signal)

GLib.MainLoop().run()

Tell your window manager to run that script on startup, and it'll handle the rest.

Notes, Gotchas, and Room for Improvement

Spaces in the MODEL_NAME and VENDOR_NAME strings are given by udev with spaces escaped as \x20. (Other characters may also be escaped.) This script should escape those, but it currently doesn't.
Several similar/identical messages are generated for each individual add/remove event. This should be prevented.

Author

David Perry, aka Boolean263. Inspired by dimonomid/my-udev-notify.