Howto remap keyboard keys in Linux

Almost two years since I wrote a blog post, but learnt a lot in these past two years. I’ll be writing everything I can remember in the coming days, but first, I need to record down something I learnt recently.

I came across one challenge to generate a key press event where the key is not at all in the normal US keyboard. That special key (Hangul/English Toggle key) is only available in Korean Keyboard. So, here is how I did

We need to choose any one key available in the normal US keyboard and remap it to produce Hangul/English Toggle Key’s code. For my purpose I’m going to remap F7 key to produce Hangul/English keypress events. First, I need to know what F7 produces when it gets pressed. There is a tool called “evtest” which will show the code generated by physical keyboard

Event: time 1496043435.462606, type 4 (EV_MSC), code 4 (MSC_SCAN), value 41
Event: time 1496043435.462606, type 1 (EV_KEY), code 65 (KEY_F7), value 0

Here, the MSC_SCAN value of “41” (hexadecimal) is the code generated by the physical keyboard. Using this value. we need to create one file (/usr/lib/udev/hwdb.d/63-keyboard.hwdb) with the following content

keyboard:name:AT Translated Set 2 keyboard:dmi:*
 KEYBOARD_KEY_41=hangeul

Here, The first line starting with “keyboard:” indicates the name of the keyboard. There are lot of way to match the keyboard (see /usr/lib/udev/hwdb.d/60-keyboard.hwdb file for more details). The second line which starting with “KEYBOARD_KEY_” should have one space before “KEYBOARD”. The “hangeul” string at the end of the second line is a suffixed lowercase substring of a macro defined in /usr/include/linux/input-event-codes.h file. You can use more than one “KEYBOARD_KEY_=” lines to remap the keys in your physical keyboard.

Once you create the 63-keyboard.hwdb file. Run the following command to update /etc/udev/hwdb.bin file which is used by udev

sudo udevadm hwdb --update

Once the hwdb.bin file updated, Either reboot your machine or trigger udev events for the particular keyboard (here, /dev/input/event1 may vary depend on your keyboard devfile mapping)

sudo udevadm trigger /dev/input/event1

Once you have the remapping, you can test it again with evtest.

Event: time 1496046275.166879, type 4 (EV_MSC), code 4 (MSC_SCAN), value 41
Event: time 1496046275.166879, type 1 (EV_KEY), code 122 (KEY_HANGUEL), value 0

In the above output, MSC_SCAN value “41” generates “KEY_HANGUEL” instead of “KEY_F7”.

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