Keyd

Keyd is a key remapping daemon, similar to kmonad.

Activate Keyd in NixOs

Note that the keyd module is relatively experimental (as of 2024), and still subject to changes. To active keyd in NixOs, you can simply add in the rest of your /etc/nixos/configuration.nix

  services.keyd = {
    enable = true;
  };

  # Optional, but makes sure that when you type the make palm rejection work with keyd
  # https://github.com/rvaiya/keyd/issues/723
  environment.etc."libinput/local-overrides.quirks".text = ''
    [Serial Keyboards]
    MatchUdevType=keyboard
    MatchName=keyd virtual keyboard
    AttrKeyboardIntegration=internal
  '';

After rebuilding your configuration, you should have keyd daemon running.

Configure Keyd: the quick and dirty way

In order to configure your system, you have two options: if you want to quickly test your configuration (avoids you to wait a full nixos-rebuild switch every time you change one option), you can simply write your configuration files in a file like /etc/keyd/test.conf containing something like:

[ids]
*

[main]
capslock = layer(control);

and apply your change via

$ systemctl keyd restart

This is practical to quickly test the configuration but in not very "Nixy" as the configuration is not contained in your beloved /etc/nixos/configuration.nix… hence the second option:

Configure Keyd in your configuration.nix

You can then move your configuration to /etc/nixos/configuration.nix by adding to your configuration something like:

services.keyd = {
  enable = true;
  keyboards = {
    # The name is just the name of the configuration file, it does not really matter
    default = {
      ids = [ "*" ]; # what goes into the [id] section, here we select all keyboards
      # Everything but the ID section:
      settings = {
        # The main layer, if you choose to declare it in Nix
        main = {
          capslock = "layer(control)"; # you might need to also enclose the key in quotes if it contains non-alphabetical symbols
        };
        otherlayer = {};
      };
      extraConfig = ''
        # put here any extra-config, e.g. you can copy/paste here directly a configuration, just remove the ids part
      '';
    };
  };
};

Note that for now, ids MUST be present in the file, so it means that layouts cannot be defined this way as reported in https://github.com/NixOS/nixpkgs/issues/284797. Yet, you can still use `environment.etc` to create them as usual.

Dealing with layouts

If you want to define a layout (like you custom qwerty), you can also do it directly in keyd, but keep in mind that keyd can manipulate keycodes (basically the position of the key on the keyboard that is named based on a qwerty layout by convention), but not keysyms (what actual symbol you will type), since keyd they basically recreates a fake keyboard that will be, later in the chain, processed by X11/ibus/… So if you want to use a layout, you need to be sure that the layout used by the user is set to US (e.g. in X11 via setxkbmap us). If you want to produce symbols not accessible by the US keyboard map (e.g. emoji etc), then keyd will basically create for you a XCompose file that you need to include (you may also need to enable Ibus for this), that is located in "${keyd}/share/keyd/keyd.compose". See keyd documentation for more details.