Swapping Keyboard Keys

Some weeks ago, my laptop k key just stopped to work. By chance, the k letter is not used to much in Portuguese and I could wait some days to see whether it returns to life or not. With its death declared, I started looking for some ways of mapping another key to the k key in Ubuntu, when I found about the xmodmap utility.

In Ubuntu, and all Linux systems using the X11 window system, the keyboard configuration is completely accessible in the form a customizable keymap that can be modified through xmodmap. So I decided to use the xmodmap utility to map the AltGr + j to the k key in my laptop. These are the instructions that I followed:

1. Keymap

Find the current keymap table by typing the following command:

$ xmodmap -pke

keycode  42 = g G g G
keycode  43 = h H h H
keycode  44 = j J j J
keycode  45 = k K k K
keycode  46 = l L l L

Each keycode is followed by the keysym it is mapped to. Each keysym column in the table corresponds to a particular combination of modifier keys:

1. Key
2. Shift + Key
3. Mode_switch + Key
4. Mode_switch + Shift + Key
5. AltGr + Key
6. AltGr + Shift + Key

Therefore, the above example indicates that the keycode 44 is mapped to the lowercase j, while the uppercase J is mapped to keycode 44 plus Shift, and so on.

2. AltGr keycode

Looking to the keymap table I could not identify any keycode mapped to the AltGr key, so I used the xev command to find the keycode that correspond to the AltGr key in my laptop.

The xev is a program that displays X events. For any key that you hit, the console where you started xev will display an event that includes the keycode for that key.

This was what I got when pressed the AltGr key in my laptop:

$ xev

KeyRelease event, serial 40, synthetic NO, window 0x4200001,
    root 0x90, subw 0x0, time 41801531, (829,465), root:(879,517),
    state 0x80, keycode 108 (keysym 0xfe03, ISO_Level3_Shift), same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

So the AltGr key on my keyboard has keycode 108 and it’s mapped to the ISO_Level3_Shift:

$ xmodmap -pke | grep 108

keycode 108 = ISO_Level3_Shift NoSymbol ISO_Level3_Shift NoSymbol

3. ISO_Level3_Shift vs Mode_switch

I tried to change the keycode 44 to map the AltGr + j to the letter k through the following command:

$ xmodmap -e "keycode 44 = j J j J k k"

But this did not work. In fact, I noticed that there is no keycode in my keymap where the 5th and 6th column of the keysym are used. That is, there is no value mapped to combinations that use the AltGr modifier.

I also did not find any keycode mapped to the Mode_switch modifier. Then I tried to change the AltGr key to be mapped to the Mode_switch instead of the ISO_Level3_Shift, and to change the keycode 44 to map the Mode_switch + j to k:

$ xmodmap -e "keycode 108 = Mode_switch NoSymbol Mode_switch NoSymbol Mode_switch"
$ xmodmap -e "keycode 44 = j J k K"

This worked, and now in my laptop the AltGr + j outputs k, while AltGr + Shift + j generates the uppercase K.

However, what exactly is this Mode_switch key? And what is its difference to the ISO_Level3_Shift?

After google a lot, the best explanation that I found was this one that I pasted here:

Mode_switch is the old-style (pre-XKB) name of the key that is called AltGr on many keyboard layouts. It is similar to Shift, in that when you press a key that corresponds to a character, you get a different character if Shift or AltGr is also pressed. Unlike Shift, Mod_switch is not a modifier in the X11 sense because it normally applies to characters, not to function keys, so applications only need to perform a character lookup to obtain the desired effect.

ISO_Level3_Shift is the XKB version of this key. Generally speaking, XKB is a lot more complicated and can do some extra fancy stuff. XKB’s mechanism is more general as it allows keyboard layouts to vary in which keys are influenced by which modifiers, it generalizes sticky (CapsLock-style) and simultaneous-press (Shift-style) modifiers and so on.

So take care before change your AltGr keymap. In my case, I did not notice any change in the keyboard operation and all keys are working correctly.

4. Activating the .Xmodmap at startup

After restart the laptop, I noticed that my changes have not take effect. If you want your xmodmap changes to run each time you log in, you need to create a file called ~/.Xmodmap with your modifications to the default keymap. For example, in my case the ~/.Xmodmap file looks like this:

keycode 108 = Mode_switch NoSymbol Mode_switch NoSymbol Mode_switch
keycode 44 = j J k K

You can check if you ~/.Xmodmap file is correct by calling:

$ xmodmap ~/.Xmodmap

And that’s it! :)