USB keyboards, xmodmap and udev
Tags:  hardware, linux,  first published on Jan 2, 2016

For many years I have run a PS/2 keyboard switch between my two computers with no issues whatsoever, however PS/2 ports can be in fairly short supply these days, and after I last upgraded I had to move to a USB switch to be able to connect to my computer, and use a USB converter for my PS/2 keyboard to connect to the switch (I am typing this on an original Microsoft Natural Keyboard, still going strong since I bought it in 1995)

After switching from PS/2 to USB I noticed that every time I toggled the keyboard between computers X11 would forget my xmodmap mappings, evidently the USB switch “switches” by simply unplugging / replugging the USB devices attached to it, and from an X11/computer perspective when it’s replugged it is a completely different device, and consequently without the xmodmap mappings I had on it before it was disconnected.

In order to fix this the first order of business is to figure out where the keyboard is connected from a USB perspective, to do that let’s first find out which event id it’s using: ls -l /dev/input/by-id here you should be able to find the name of your keyboard (in my case I see usb-CHESEN_PS2_to_USB_Converter-event-kbd there pointing to ../event15)

With this information you can run udevadm info -q all -a -n /dev/input/event15, where event15 is the input event name you just found. The first parent device listed (the second device overall, the first where you can see “looking at parent device”) will be the actual device that you want to use (in my case this device has an ATTRS{name} set to CHESEN PS2 to USB converter)

The other information in the parent device’s ATTRS are all things you can use for your udev rule to identify the correct device that you are plugging / unplugging, for example in my case the attributes look like

looking at parent device '/devices/pci0000:00/0000:00:1d.0/usb8/8-1/8-1.4/8-1.4.3/8-1.4.3:1.0/0003:0A81:0205.0005/input/input19':
   ATTRS{name}=="CHESEN PS2 to USB Converter"

and the udev rule I created looks like this (note the rule is all on one line)

luser@andromeda:~$ cat /etc/udev/rules.d/83-xmodmap.rules
ACTION=="add", KERNEL=="event[0-9]*", SUBSYSTEM=="input", ATTRS{name}=="CHESEN PS2 to USB Converter", ATTRS{phys}=="usb-0000:00:1d.0-1.4.3/input0", RUN+="/usr/local/bin/udev-keyboard"

After creating the udev rule sudo udevadm control --reload needs to be run for it to be activated.

With this, when the device you specify connects the /usr/local/bin/udev-keyboard script gets invoked.

Debian stretch and nouveau

I am not sure why, but since reinstalling with debian stretch and running nouveau rather than the proprietary NVidia drivers (although the latter shouldn’t make a difference really) I cannot find a way to get the udev script to work correctly. The script is executed by udev but even with daemonize, XAUTHORITY and so on, the xmodmap / xset lines will not persist the settings after the script end, and the keyboard will remain unremapped.

A different approach that still works is to launch a daemon as part of .xsession that waits for a signal triggered by udev and executes the script: an implementation of this is described in the following post on the Arch linux message board which works by having a file being watched by this process, and udev triggering the file modification.

The original post follows

I originally had the udev-keyboard script above call xmodmap directly but for some reason despite no errors being returned and the script being run correctly, xmodmap did not seem to have any effect.

After some time I started wondering if the issue was that X11 was not notified of the keyboard reconnecting until after the script returned as part of the udev connection process, a way to see if this is the case would be to detach the script and have udev continue and see what happens.

After some trial and error the only way I was able to get xmodmap to work reliably has been to have udev call a “daemon” script which detaches and in turn calls the script that will actually do the xmodmap calls.

This intermediate daemonization script can be written in any language, a simple python implementation is available here for example 1, it can be put in /usr/local/bin/udev-keyboard to act as the daemon, with a separate shell script called, say, /usr/local/bin/udev-doit that will contain the actual xmodmap and xset calls.

This udev-keyboard daemonization script is simply the python daemon base class from the site linked above, with this simple instantiation at the end of the file containing it


class KBDDaemon(Daemon):
    def run(self):

if __name__ == "__main__":
    daemon = KBDDaemon('/tmp/kbddaemon.pid')

With this setup the udev-doit script will be executed detached from the terminal, which seems to make it correctly interact with X11.

The script I am using follows, it has a hardcoded username since I am only ever logging in as my user on to start X, however it could easily be made more generic by having it look at, for example, what user the window manager is running as.

export XAUTHORITY=/home/luser/.Xauthority
export DISPLAY=:0

/usr/bin/xset r rate 350 35
/usr/bin/xmodmap /home/luser/.config/i3/xmodmap
sudo -u luser /usr/bin/notify-send -i info "Remapping done"

as you can see I am using the X11 auth token for the calls, otherwise xmodmap and xset would not work, and sudoing to the user to send a notification (notify-send does not seem to send one unless run as the actual user).

If you are having issues getting the script to run, the first thing would be to turn on udev debugging via sudo udevadm control –log-priority=debug and test the rule with udevadm test –action=add /devices/pci…. where the /devices/pci/… string is what is printed after the ‘parent device’ line above.

For example a successful invocation would show as something like this

sudo udevadm test --action=add /devices/pci0000........input97/event17
run: 'kmod load input:b000.......'
run: '/usr/local/bin/udev-keyboard'
unload module index
Unloaded link configuration context.

If your rule is not being run the first thing to try would probably be to make it more and more generic by removing == qualifiers, you can also see in the udevadm test output variables you can use to make sure the script is going to be run when the right device is plugged in.

If the xmodmap does not seem to be taking effect, you might investigate adding a sleep() to the script, but in that case note you might also want to change the daemon script to remove the pidfile logic, as in our case it doesn’t really matter if multiple copies of the script are invoked by udev (at one point for some reason udev was calling my script twice, never been able to reproduce it after it disappeared).

As an aside, if you are looking for a lightweight notification daemon, I can suggest dunst, as it’s fairly minimalistic and fits in well with an i3-based environment, on Debian you would install it with something like

luser@andromeda:~$ sudo aptitude -t jessie-backports install dunst libnotify-bin gnome-icon-theme gnome-icon-theme-extras

in order to get the daemon as well as some common icons to use for the notifications, which can be activated by changing the icon_position = off line to icon_position = left in your dunstrc (don’t forget to also add the /usr/share/icons/gnome/… icon directories to icon_folders as well so they can be found)

  • Initial release - 2016-01-01
  • Stretch issues - 2018-06-21