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

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)]

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 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

Method 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

Method 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 ("%sn", 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*)"

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0