How to tell which keyboard was used to press a key?

Question

I frequently work on pairing stations where there are multiple keyboards installed. I can use setxkbmap with -device <ID> to set the layout for a specific keyboard (using an ID from xinput), but often it's not obvious which keyboard I'm at. It would be better to avoid the back-and-forth of trying both keyboards, so I'd like to write a quick tool to get this information for setxkbmap. I'd expect a typical use case like the following:

$ setxkbmap -device "$(get-keyboard-id)" -layout gb
Press Enter to detect keyboard ID

Which interface provides this information on Linux? Ideally it should work without X, but that's not a requirement (there doesn't seem to be many tools which support this without X).

---

Findings so far:

• Linux must know which keyboard I'm typing on to support different layouts for multiple keyboards simultaneously.
• xinput → list.c → list_xi2 → XIQueryDevice provides device IDs usable by setxkbmap.
• showkey and xev don't print keyboard IDs.
• xinput list-props $ID shows where keyboard events are sent. However, using code from another answer it seems this device doesn't print anything to identify the keyboard.

• One almost possible solution is to run xinput --test <ID> & for each keyboard ID and see which one returns something first. The problem with that is figuring out which "keyboards" are actually keyboards:

  $ xinput | grep keyboard
  ⎣ Virtual core keyboard                         id=3    [master keyboard (2)]
      ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
      ↳ Power Button                              id=6    [slave  keyboard (3)]
      ↳ Video Bus                                 id=7    [slave  keyboard (3)]
      ↳ Power Button                              id=8    [slave  keyboard (3)]
      ↳ Sleep Button                              id=9    [slave  keyboard (3)]
      ↳ WebCam SC-13HDL10931N                     id=10   [slave  keyboard (3)]
      ↳ AT Translated Set 2 keyboard              id=11   [slave  keyboard (3)]
  

Answer 1

Disable device

Here's one idea towards identifying which keyboard is which. You can use the command xinput to enable and disable devices.

Example

$ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad                id=12   [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=13   [slave  pointer  (2)]
⎜   ↳ Logitech USB Receiver                     id=9    [slave  pointer  (2)]
⎜   ↳ Logitech USB Receiver                     id=10   [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Sleep Button                              id=8    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=11   [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                    id=14   [slave  keyboard (3)]

The above output shows the various devices that I have on my Thinkpad laptop. I only have 1 keyboard attached, this one:

    ↳ AT Translated Set 2 keyboard              id=11   [slave  keyboard (3)]

Now take a look at the properties available through this device:

$ xinput list-props "AT Translated Set 2 keyboard"
Device 'AT Translated Set 2 keyboard':
    Device Enabled (124):   1
    Coordinate Transformation Matrix (126): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.

From the above you can see that it's enabled, so let's disable it:

$ xinput set-prop "AT Translated Set 2 keyboard" "Device Enabled" 0

To enable it:

$ xinput set-prop "AT Translated Set 2 keyboard" "Device Enabled" 1

The idea?

You could enable disable one of the keyboards using this command to determine which one you're on.

References

• second keyboard to run commands

Answer 2

More digging revealed another solution using plain Bash and a normal user account. Script:

#!/usr/bin/env bash

set -o errexit -o nounset -o noclobber -o pipefail

# Remove leftover files and processes on exit
trap 'rm --recursive -- "$dir"; kill -- -$$' EXIT
dir="$(mktemp --directory)"
cd "$dir"

# Log key presses to file
xinput --list --id-only | while read id
do
    # Only check devices linked to an event source
    if xinput --list-props "$id" | grep --quiet --extended-regexp '^\s+Device Node.*/dev/input/event'
    then
        xinput test "$id" > "$id" &
    fi
done

# Check for key presses
while sleep 0.1
do
    for file in *
    do
        if [[ -s "$file" ]]
        then
            echo "$file"
            exit
        fi
    done
done

Answer 3

The question sounds a bit contradictory since you're citing X tools but ask for a solution that "ideally should work without X".

About your 4^th finding: xinput will give you the correspondence

$ xinput list-props 11
Device 'AT Translated Set 2 keyboard':
    Device Enabled (145):   1
    Coordinate Transformation Matrix (147): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
    Device Product ID (266):    1, 1
    Device Node (267):  "/dev/input/event0"

at least with the following version

$ xinput --version
xinput version 1.6.1
XI version on server: 2.3

---

First step: detecting the keyboard event device in C ---

#include <stdio.h>
//#include <unistd.h>
#include <fcntl.h>
#include <linux/input.h>

// typical use : sudo ./a.out /dev/input/event*
int main (int argc, char *argv[])
{
  struct input_event ev[64];
  int fd[argc],rd,idev,value, size = sizeof (struct input_event);
  char name[256] = "Unknown";

  if(argc==1) return -1;

  int ndev=1;
  while(ndev<argc && (fd[ndev] = open (argv[ndev], O_RDONLY|O_NONBLOCK)) != -1){
    ndev++;
  }
  fprintf (stderr,"Found %i devices.\n", ndev);
  if(ndev==1) return -1;

  while (1){
    for(idev=1; idev<argc; idev++){
      if( (rd=read (fd[idev], ev, size * 64)) >= size){
      value = ev[0].value;
      if (value != ' ' && ev[1].value == 1 && ev[1].type == 1){
        ioctl (fd[idev], EVIOCGNAME (sizeof (name)), name);
        printf ("%s\n", name);
        return idev;
      }
      }
    }
//    sleep(1);
  }
  return -1;
}

Many thanks to this page. I've stripped most safety checks from the code I borrowed there, for clarity, in real code you probably want them.

Note that key presses are echoed, so you may indeed want to kindly ask the user to hit a modifier key (Shift, Control...) rather than any key.

Second step: use xinput to get the X ID from the device name

Compile the above C source and use this way:

xinput list --id-only "keyboard:$(sudo ./a.out /dev/input/event*)"