A simple, humble but comprehensive guide to XKB for linux

  • Are you a linux user?
  • Would you like to remap some keys of your keyboard? (any keyboard)
  • Would you like to quickly switch between different keyboard layouts?
  • Are you “serverfault-fed” whenever you need to touch your keyboard layout?
  • Are you chocking under the zillion of different solutions Google have found for your remapping issue but none of them seems to work?
  • Would you like to write a custom layout for your Kinesis or Ergodox keyboard?

If you answered yes to some of my questions then this article is definitely for you. I’m not a guru but I’m here to share what I’ve learned so far.

Under the hood

Linux, during the booting process, goes throw several stages called init and they are numbered from 1 to 5.

You can picture those stages as NASA missile stages:

Each init launches the next one and enables new features like file system decryption, users support, network support, network services …

Init 5 is the point where you see the graphical environment, usually displaying the login window. 
Init 3 is the place where, among many other things, the X service is loaded. The X service is responsible for providing the basic Graphical User Interface which is then completed and managed by the Windows Manager (Gnome, Kde, Xfce, I3 …)

The X service is tightly coupled with XKB the software responsible for handling your keyboard settings and layouts.

/etc/X11/xorg.conf is the configuration file for the X service in which it’s possible to set some configuration settings for XKB in this way:

Section "InputClass"
Identifier "system-keyboard"
MatchIsKeyboard "on"
Option "XkbLayout" "cz,us"
Option "XkbModel" "pc104"
Option "XkbVariant" ",dvorak"
Option "XkbOptions" "grp:alt_shift_toggle"
EndSection

You will find millions of this examples on the internet but look at my /etc/X11 directory on Linux Mint 17.1

$ tree
.
├── app-defaults
│ ├── Bitmap
│ ├── Bitmap-color
│ ├── Bitmap-nocase
│ ├── Clock-color
│ ├── Editres
│ ├── Editres-color
│ ├── Viewres
│ ├── Viewres-color
│ ├── XCalc
│ ├── XCalc-color
│ ├── XClipboard
│ ├── XClock
│ ├── XClock-color
│ ├── XConsole
│ ├── Xditview
│ ├── Xditview-chrtr
│ ├── Xedit
│ ├── Xedit-color
│ ├── Xfd
│ ├── XFontSel
│ ├── Xgc
│ ├── Xgc-color
│ ├── XLoad
│ ├── XLogo
│ ├── XLogo-color
│ ├── Xmag
│ ├── Xman
│ ├── Xmessage
│ ├── Xmessage-color
│ ├── XMore
│ ├── XSm
│ └── Xvidtune
├── default-display-manager
├── fonts
│ ├── misc
│ │ └── xfonts-base.alias
│ └── Type1
│ ├── gsfonts-x11.alias
│ ├── gsfonts-x11.scale
│ ├── xfonts-mathml.scale
│ └── xfonts-scalable.scale
├── rgb.txt
├── X -> /usr/bin/Xorg
├── xinit
│ ├── xinitrc
│ ├── xinputrc
│ └── xserverrc
├── xkb
├── Xreset
├── Xreset.d
│ └── README
├── Xresources
│ └── x11-common
├── Xsession
├── Xsession.d
│ ├── 00upstart
│ ├── 20x11-common_process-args
│ ├── 30x11-common_xresources
│ ├── 35x11-common_xhost-local
│ ├── 40x11-common_xsessionrc
│ ├── 50_check_unity_support
│ ├── 50x11-common_determine-startup
│ ├── 55cinnamon-session_gnomerc
│ ├── 60x11-common_localhost
│ ├── 60x11-common_xdg_path
│ ├── 60xdg-user-dirs-update
│ ├── 70gconfd_path-on-session
│ ├── 70im-config_launch
│ ├── 75dbus_dbus-launch
│ ├── 90consolekit
│ ├── 90gpg-agent
│ ├── 90qt-a11y
│ ├── 90x11-common_ssh-agent
│ ├── 98vboxadd-xclient
│ ├── 99upstart
│ └── 99x11-common_start
├── Xsession.options
├── xsm
│ └── system.xsm
└── Xwrapper.config

There is no xorg.conf and no trace, in the whole X11 directory, of any XKB option. This is because most of the recent distributions don’t use it anymore. I assume they make it on the fly accordingly to xrandr output but I might be wrong.

You, old school, can surely write your own xorg.conf and write there your XKB settings, but I don’t see any good reason to do so if X is working. 
I know by experience how hard and frustrating it can be writing a good xorg.conf file, especially when there is complicated monitor setup.

Anyway, good news: there is no need to write your own xorg.conf.

Where does the “magic” happen?

In two places:

  • /etc/default/keyboard
  • /usr/share/x11/xkb

This is my /etc/default/keyboard

XKBMODEL="kinesis"
XKBLAYOUT="us,us"
XKBVARIANT="kinesis_adv_dvorak_it,rus"
XKBOPTIONS="caps:none,shift:both_capslock,lv3:rwin_switch,grp:alt_space_toggle"
#XKBOPTIONS="terminate:ctrl_alt_bksp"

This is where the X service looks for the options to pass to XKB. Don’t get lost right now, keep reading. I’ll come back to this later.

The directory /usr/share/X11/xkb hosts the configuration files for XKB :

$ cd /usr/share/X11/xkb
$ tree -L 2
.
├── compat
├── geometry
├── keycodes
├── LICENSE
├── README.md
├── rules
├── symbols
└── types

Your windows manager uses the information stored there to populate the options of their own keyboard-manager application.

Cinnamon’s (the windows manager I sometimes use in Linux Mint) keyboard-manager application can be launched with the command :

cinnamon-settings keyboard

When you click the + button to add a new layout, the shown list of layouts comes from parsing the configuration files stored in /usr/share/X11/xkb/

Some terminology

Before going ahead let’s get clear on some terms:

home row:
the central row of your keyboard where you place your fingers before typing.

keyboard layout
it’s your set of keys on the keyboard and how they are arranged.
QWERTY is a layout, the most common. The name comes from reading the first six keys appearing on the top left letter row of the keyboard.

DVORAK is a layout: the name comes from its inventor. Its main feature is to have all the vowels placed on the left of the home row and it’s meant to speed up your typing rate and reduce the stress on your fingers.

There are many other layouts like this (colemak, for instance) and I would call them macro-layout because for each macro-layout there are plenty of layout variants. And you can add yours.

layout variant:
it represents the changes made to a “macro-layout” to adjust it to a different language. English and Russian have a quite a different charset: one is Latin the other one Cyrillic. English and Italian have a the same Latin charset but (modern) English lacks some diacritics (accents) that are still present in Italian.

QWERTY US layout
QWERTY Italian layout
DVORAK US layout

Each language has its own layout and of course, a user might want a customized layout. Aren’t we all here for this?

key mapping:
when you press a key on the keyboard, the operating system gets a key-code. Which is a number.
You can see this by using the xev program included in the x11-utils package.
From a terminal launch “xev -event keyboard” to see it in action. For instance by hit «a» you will see:

KeyPress event, serial 28, synthetic NO, window 0x2a00001,
root 0x29a, subw 0x0, time 88951134, (191,-3), root:(195,993),
state 0x0, keycode 38 (keysym 0x61, a), same_screen YES,
XLookupString gives 1 bytes: (61) "a"
XmbLookupString gives 1 bytes: (61) "a"
XFilterEvent returns: False
KeyRelease event, serial 28, synthetic NO, window 0x2a00001,
root 0x29a, subw 0x0, time 88951326, (191,-3), root:(195,993),
state 0x0, keycode 38 (keysym 0x61, a), same_screen YES,
XLookupString gives 1 bytes: (61) "a"
XFilterEvent returns: False

38 is the key-code for the «a» letter.

modifier keys:
as you can see from the previous experience, the operating system is informed of two events: when the key is pressed and when it’s released.
If you keep holding the key you will see those two blocks repeating quite fast. This does not happen with the so called modifier keys like «CTRL», «SHIFT», «ALT», «ALT_GR», «ESC», «CAPS LOCK«, «INSERT», «NUM LOCK» etc.
All the keys look alike but they are not (it’s not what it looks like :-)). 
Some are special.

Modifiers are used to perform alterations on regular characters:

  • with the SHIFT you can temporary make a capital letter
  • with the CAPS LOCK you make just capital letters
  • with the «3rd level modifier» (an arbitrary key chosen among the modifiers set, usually the «ALT GR» or the «Windows key») you have access to the letters written on the top and bottom right of some keys of your keyboard. 
    Have a look at the Italian QWERTY layout: 
    see the «[» character? that’s «3rd level modifier» + «è»
    see the «{» character? that’s «3rd level modifier» + «SHIFT» + «è»

The configuration directory

The configuration files contained in /usr/share/X11/xkb can be divided in 3 groups accordingly to their aim:

  1. files to graphically display the keyboard layout
  2. files to configure the key mapping and the keyboard layout
  3. files to enable the configuration
├── geometry      # this is group 1
├── keycodes      # this is group 2

├── rules # this is group 3
├── symbols       # this is group 2
I have no clear idea what’s the role of the compat and types directories but I never had the need to touch them.

Group 1:
In the geometry directory there are files giving instruction to the keyboard-manager application about the physical appearance of the keyboard. 
For the fun of it, I adjusted a geometry file found on the internet for my Kinesis Advantage
Now when I click on the keyboard icon of the keyboard-manager application I see this:

Kinesis geometry for Gnome — Cinnamon

Group 2:
In the keycodes and symbols directories there are files to modify to design a custom layout.

In the keycodes/evdev file you can map a binding between the key-code and the key-symbol. The key-symbol will be interpreted by XKB accordingly to the mapping table written in the file symbols/us.

Let me show you better.

This is a chunk from keycodes/evdev :

 <TLDE> = 49;
<BKSP> = 22;

It’s binding:

  • the key-code 49 to the TLDE (tilde «~») symbol
  • the key-code 22 to the BKSP (backspace) symbol

This is a chunk from symbols/us

key <TLDE> { [ grave, asciitilde ] };
key <BKSP> { [ BackSpace, BackSpace ] };[

It’s binding:

  • the TLDE symbol to 2 characters: «`» and «~».
  • the BKSP symbol to «Backspace» and «Backspace»

The latter of the 2 characters is always triggered with the combination «SHIFT» + KEY. This means that BACKSPACE works as BACKSPACE even if the SHIFT is pressed. It’s invariant, try it.
The symbols/us file hosts many different layouts. Each layout can be written from scratch or it can inherit from a parent layout and modify something.

Ex.:

I named this layout variant kinesis_adv_dvorak_it_custom. In cinnamon-settings it will be displayed as Kinesis Advantage (Dvorak US Custom) and it includes a parent layout variant named us(kinesis_adv_dvorak_it) which is, again, child of the us layout. And it performs no modification to the parent behavior.

xkb_symbols "kinesis_adv_dvorak_it_custom" {
name[Group1] = "Kinesis Advantage (Dvorak US Custom)";
include "us(kinesis_adv_dvorak_it)"
// add here below whatever customization you like
};

Group 3:
The files contained in the rules directory inform XKB about the available layouts and their children (layout-variants).

These are:

  • base.lst
  • base.xml
  • evdev.lst
  • evdev.xml

base.lst and evdev.lst are identical, same as base.xml and evdev.xml. 
I don’t know yet why they are identical and why they are not symlinks.

Anyway let’s focus on the QWERTY US layout:
in base.lst, look for “English (US)” and you will see these lines of which the first one is introducing the layout identifier “us”:

! layout
us English (US)
...
...
! variant
chr us: Cherokee

in base.xml, look for “English (US)” and you will see these lines:

<layoutList>
<layout>
<configItem>
<name>us</name>
<shortDescription>en</shortDescription>
<description>English (US)</description>
<languageList>
<iso639Id>eng</iso639Id>
</languageList>
</configItem>
<variantList>
<variant>
<configItem>
<name>chr</name>
<shortDescription>chr</shortDescription>
<description>Cherokee</description>
<languageList>
<iso639Id>chr</iso639Id>
</languageList>
</configItem>
</variant>
<variant>
...
<variant>

It goes ahead with many variants.
Basically the base.lst file is listing the layouts and their children (layout-variants) and the base.xml enriches the information about the parent layout and the layout variants.

Putting pieces together

Once XKB is configured to load the specific “us” layout and the «`» key is hit, this happens:

  1. the keyboard sends the key-code (49)
  2. XKB applies the symbol found in the “keycodes/evdev” file for the key-code 49 which is <TLDE>
  3. XKB reads from the “us” layout rules written in “symbols/us” that <TLDE> corresponds to «`» (or «~» if «SHIFT» is pressed) and returns the character.

How to change XKB configuration

Whenever you modify any of the XKB files or when you want to try a different layout among those already available the best option is to open a terminal and use setxkbmap to load the changes on the fly.

The simplest way is to run:

setxkbmap -layout us

but if you want the QWERTY US layout and the DVORAK US variant you can use:

setxkbmap -layout us,us -variant ,dvorak

Careful here, setxkbmap is picky about the syntax: the number of layouts and variants must match. This is why “us” is repeated and “dvorak” is preceded by the comma. It’s like: -layout us,us -variant «null»,dvorak

You can load any layout variant this way. 
To have a list of the layout variants currently available in your system, look for “xkb_symbolsin the “symbol/us” file: the following name is the variant name.

rgrep “xkb_symbols” symbols/us

Check your current XKB configuration with this:

setxkbmap -print -verbose 10

You should see something resembling this:

Setting verbose level to 10
locale is C
Trying to load rules file ./rules/evdev...
Success.
Applied rules from evdev:
rules: evdev
model: kinesis
layout: us,us
variant: ,kinesis_adv_dvorak_it
options: lv3:rwin_switch,grp:alt_space_toggle
Trying to build keymap using the following components:
keycodes: evdev+aliases(qwerty)
types: complete
compat: complete
symbols: pc+us+us(kinesis_adv_dvorak_it):2+inet(evdev)+group(alt_space_toggle)+level3(rwin_switch)
geometry: kinesis(model100)
xkb_keymap {
xkb_keycodes { include "evdev+aliases(qwerty)" };
xkb_types { include "complete" };
xkb_compat { include "complete" };
xkb_symbols { include "pc+us+us(kinesis_adv_dvorak_it):2+inet(evdev)+group(alt_space_toggle)+level3(rwin_switch)" };
xkb_geometry { include "kinesis(model100)" };
};

See the next paragraph to understand how to quickly switch between a different layouts.

XKB options

When running setxkbmap you can pass some options that can be very useful. For instance you might want to specify that the «3rd level modifier» is the right Windows key.

setxkbmap -layout us,us -variant ,dvorak -option "lv3:rwin_switch"

If you have loaded more than one layout you will definitely need a shortcut (let’s say «ALT»+«SPACE») to loop through them, so:

setxkbmap -layout us,us -variant ,dvorak -option "lv3:rwin_switch,grp:alt_space_toggle"

Note: somehow XKB options are sticky. If you run again “setxkbmap -print -verbose 10” you will see that the option “lv3:rwin_switch” is duplicated:

options:    lv3:rwin_switch,grp:alt_space_toggle,lv3:rwin_switch

To reset the options you need to run first the command:

setxkbmap -layout us,us -variant ,dvorak -option

Then:

setxkbmap -layout us,us -variant ,dvorak -option "lv3:rwin_switch,grp:alt_space_toggle"

Remapping keys

There are a number of ways to remap one or more keys of the keyboard.

Long way:
If you plan to review the whole layout of your keyboard and master it then you should follow the logic explained in this guide and create your own layout-variant. It will take some time and a lot of patience. And you will become a good friend of xev.
Start with the “keycodes/evdev” file, if necessary, and continue with the appropriate “symbols/«file»” where «file» is the name of the parent layout you want to work with.
I suggest to have a look at my github repository in which I store all the configuration files for my custom DVORAK layout and compare the original files with the updated ones to see what I did.

Once you are done with all your changes and you have tested everything with setxkbmap you are ready to edit the /etc/default/keyboard file to make the changes permanent. (for inspiration, see my file above)

Short way:
If you just need to remap few keys and you are in a rush, xmodmap is the most convenient way.
Ex.: this will remap your Caps Lock into the Escape button:

xmodmap -e "keycode 66 = Escape"

to make the remapping persistent you have to add the command in your ~/.Xmodmap file

Watch out!

If you are a Gnome or Cinnamon user, be aware that these two window managers have their own way to handle the keyboard settings and layouts which override the XKB configuration. I’m not sure what KDE and others do but I can tell that I3 windows manager definitely sticks to XKB.

No worries though, there is a way to force Gnome or Cinnamon to use XKB instead. Adjust these commands to your needs and run them in the terminal.

gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'us+«variant_name»'), ('xkb', 'us+«variant_name»')]"
# Update: 2017-10-25
# Ex.:
# gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'us+kinesis_adv_dvorak_it'), ('xkb', 'us+rus')]"
# This command would load the US layout with "kinesis_adv_dvorak_it" custom variant AND the US layout with the "russian" standard phonetic variant
# If you check the menù "Input sources" in "Gnome Settings -> Region & Language" after runnig this command you'll see the layout/variants listed
gsettings set org.gnome.desktop.input-sources xkb-options "['lv3:caps_switch,grp:alt_space_toggle']"
# "grp:alt_space_toggle" means that the shortcut "Alt+Space" (anywhere in the Desktop Environment) will sequentially load one of the available layouts/variants

Check your Gnome or Cinnamon settings with the utility dconf-editor (sudo apt-get install dconf-editor)

Update: 2016–06–18

In this tweet I’ve been asked a smart question which goes like this:

If a keyboard has programmable firmware, what’s your reason to deal with XKB?

From my experience, for these reasons:

  1. multiple layouts for multilingual writers using different charsets: if you speak, say, English and Russian (I’m slowly studying Russian and Bulgarian) if you will need to switch between Latin charset and Cyrillic. I’m pretty sure that some programmable keyboards can have different lawyers and one charset for each layer but not all the programmable keyboards can be that flexible.
  2. specific XKB features: XKB offers some features that a keyboard firmware can hardly provide and it can be more granular that a keyboard firmware. Conside these XKB options: 
    * caps:none: disables the CAPS LOCK 
    * shift:both_capslock: activates CAPS LOCK by pressing the two SHIFTS together
    * lv3:rwin_switch: sets which key activates the 3rd level modifier is used to print the symbols displayed on the right side of some keyboard keys like “ ° € ™ ® Á ù ” (see the Italian QWERTY layout above). In this case lv3 is the Right Window key but you can set something else.
  3. Remapping 3rd level symbols. Take English and Italian: they share the same Latin charset but English doesn’t use (anymore as far as I know) vowel with diacritics like: à ò è ù ì or á ó è ú í . Italian does. With XKB I was capable to remap these frequent vowels “à ò è ù ì” in a more comfortable position. Check out this file and look for “kinesis_adv_dvorak_it”
  4. Consistency across Windows managers. As mentioned above each Window Manager handles the keyboard its own way. I like to have the freedom to try new Window Managers but to keep control over my keyboard settings so I force the WM to use XKB and everything always works as expected.