[Howto] Find matches for udev rules

Recently I bought a new keyboard and wondered if I can trigger actions via udev when the keyboard is plugged in. For that, however, it must be known how to create udev rules matching the keyboard in the first place.

To write udev rules the most important part is to match the device you want to handle with the rules. While hardware fixed in the computer might be easy, the rest usually isn’t. For example, you can trigger something when keyboard xyz of manufacturer abc is plugged in – but for that you must now where to find which information and how to present them to udev.

This can be done with the program udevmonitor. It informs you of newly plugged in hardware detected by udev:

# udevmonitor
...
UEVENT[1217720129.155386] add      /devices/pci0000:00/0000:00:1d.0/usb5/5-1 (usb)
UEVENT[1217720129.155440] add      /class/usb_endpoint/usbdev5.9_ep00 (usb_endpoint)
UEVENT[1217720129.157973] add      /devices/pci0000:00/0000:00:1d.0/usb5/5-1/5-1:1.0 (usb)
UDEV  [1217720129.166782] add      /devices/pci0000:00/0000:00:1d.0/usb5/5-1 (usb)
UDEV  [1217720129.171948] add      /class/usb_endpoint/usbdev5.9_ep00 (usb_endpoint)
...

The next step is to use these info to gather even more information to actually create a rule: the devices path is interesting an contains more information. So we have a look at it with yet another udev tool: udevinfo

# udevinfo -a -p /sys/devices/pci0000:00/0000:00:1d.0/usb5/5-1
...
  looking at device '/devices/pci0000:00/0000:00:1d.0/usb5/5-1':
    KERNEL=="5-1"
    SUBSYSTEM=="usb"
    DRIVER=="usb"
    ATTR{dev}=="189:522"
    ATTR{configuration}==""
    ATTR{bNumInterfaces}==" 2"
    ATTR{bConfigurationValue}=="1"
    ATTR{bmAttributes}=="a0"
    ATTR{bMaxPower}=="100mA"
    ATTR{urbnum}=="1541"
    ATTR{idVendor}=="046a"
    ATTR{idProduct}=="0023"
    ATTR{bcdDevice}=="0032"
...

This shows us several attributes we can use in the udev-rules. The data also match with the data given by lsusb, btw.:

# lsusb
...
Bus 006 Device 001: ID 1d6b:0001
Bus 005 Device 011: ID 046a:0023 Cherry GmbH Cymotion Master Linux Keyboard
Bus 005 Device 001: ID 1d6b:0001
...

So now a udev rule can be created with the data above. In case of the keyboard the rule could for example be:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="046a",ATTRS{idProduct}=="0023",ACTION=="add",RUN+="/usr/local/bin/mykeyboard.sh"

In this case udev launches the program mykeyboard.sh when a the usb subsystem recognizes a device from the vendor 046a with the product id 0023. This can of course be further fine grained, depending on the needs.

The rule itself must afterwards be copied to the right folder and must be given a number according to the priority regarding other rules. On a Fedora system the udev folder is /etc/udev/rules.d, and a typical udev rule name could be 62-keyboard.rules.