Keyboard Layout Customization

20.09 and later

In 20.09 there's services.xserver.extraLayouts for this.

Using xkbcomp

Simple

The easiest way to customize your keyboard layout on NixOS is with these options:

services.xserver.layout: Keyboard layout, or multiple keyboard layouts separated by commas.
services.xserver.xkbVariant: X keyboard variant or multiple variants separated by commas (a variant can be empty).
services.xserver.xkbModel: Keyboard model.
services.xserver.xkbOptions: X keyboard options; layout switching goes here.

Example:

For desktop:

services.xserver = {
  xkb.layout = "us,ru";
  xkbVariant = "workman,";
  xkbOptions = "grp:win_space_toggle";
};

For console:

console.keyMap = "us";

You can find valid values for these options in $(nix-build --no-out-link '<nixpkgs>' -A xkeyboard_config)/etc/X11/xkb/rules/base.lst

Advanced

If the above options aren't enough, you can instead create your own keyboard layout by going through xkb. To get started, install xorg.xkbcomp and run setxkbmap -print > layout.xkb to get an initial file. This corresponds to your current layout. Use xkbcomp layout.xkb $DISPLAY to load the file as your new layout. Refer to https://wiki.archlinux.org/index.php/X_KeyBoard_extension on how to edit this file.

Lots of examples can be found in $(nix-build --no-out-link '<nixpkgs>' -A xorg.xkeyboardconfig)/etc/X11/xkb/. For available key symbols, see $(nix-build --no-out-link '<nixpkgs>' -A xorg.xproto)/include/X11/keysymdef.h.

To load this file at the start of the X session, add the following to your configuration.nix. The extra compilation step (xkbcomp) helps catching layout errors at build time.

let
  compiledLayout = pkgs.runCommand "keyboard-layout" {} ''
    ${pkgs.xorg.xkbcomp}/bin/xkbcomp ${./path/to/layout.xkb} $out
  '';
in
  services.xserver.displayManager.sessionCommands = "${pkgs.xorg.xkbcomp}/bin/xkbcomp ${compiledLayout} $DISPLAY";

If you are using home-manager, you also need to prevent home-manager from managing the keyboard by having home.keyboard = null; in your home-manager configuration.

Relevant other options

services.xserver.exportConfiguration: Makes it so the above mentioned xkb directory (and the xorg.conf file) gets exported to /etc/X11/xkb, which is useful if you have to often look stuff up in it.
services.xserver.xkbDir: Allows you to set a different xkb directory altogether. All the above mentioned things will use this instead of the default one in regards to xkb stuff.
console.useXkbConfig: Makes it so the tty console has about the same layout as the one configured in the services.xserver options.

Configs

https://github.com/infinisil/system/blob/94852ed690fccfdda27c2e3985be84c51f1eac8e/new-modules/keylayout.nix

Advanced configuration with xmodmap

Some users have found xmodmap to be a helpful tool although reports of successful implementation are varied.

cat /etc/nixos/configuration.nix

services.xserver.displayManager.sessionCommands =
  ${pkgs.xorg.xmodmap}/bin/xmodmap "${pkgs.writeText  "xkb-layout" ''
    ! Map umlauts to RIGHT ALT + <key>
      keycode 108 = Mode_switch
      keysym e = e E EuroSign
      keysym c = c C cent
      keysym a = a A adiaeresis Adiaeresis
      keysym o = o O odiaeresis Odiaeresis
      keysym u = u U udiaeresis Udiaeresis
      keysym s = s S ssharp
    
      ! disable capslock
      ! remove Lock = Caps_Lock
  ''}"

Works after boot and after suspend/resume.

You may need to add some delay to make xmodmap command work.

  services.xserver.displayManager.sessionCommands = "sleep 5 && ${pkgs.xorg.xmodmap}/bin/xmodmap -e 'keycode 43 = h H Left H' &";

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