Custom home-manager installation with NixOS

Recently I hear about home-manager-template. The project rationale resonates with me. In fact, I did something similiar on my NixOS configuration.nix.

Apart of trying to make the home-manager installation more reproducible, my goals are:

• Install home-manager from my system configuration.nix.
• Use home-manager from a git checkout. That makes easier for me to test new ideas or patches.
• Don't repeat my self. I don't want to define the same variable at the system level and in home-manager, e.g.: my username or my hostname.

To accomplish that, I have the following function in my configuration.nix:

let
  user = "john";
  userHome = "/home/${user}";
  hostName = "laptop";

  home-manager = { home-manager-path, config-path }:
    assert builtins.typeOf home-manager-path == "string";
    assert builtins.typeOf config-path == "string";
    (
      pkgs.callPackage
        (/. + home-manager-path + "/home-manager") { path = "${home-manager-path}"; }
    ).overrideAttrs (old: {
      nativeBuildInputs = [ pkgs.makeWrapper ];
      buildCommand =
        let
          home-mananger-bootstrap = pkgs.writeTextFile {
            name = "home-manager-bootstrap.nix";
            text = ''
              { config, pkgs, ... }:
              {
                # Home Manager needs a bit of information about you and the
                # paths it should manage.
                home.username = "${user}";
                home.homeDirectory = "${userHome}";
                home.sessionVariables.HOSTNAME = "${hostName}";
                imports = [ ${config-path} ];
              }
            '';
          }; in
        ''
          ${old.buildCommand}
          wrapProgram $out/bin/home-manager --set HOME_MANAGER_CONFIG "${home-mananger-bootstrap}"
        '';
    });
in
{
  users.users.${user} = {
    home = userHome;
    packages = [
      (home-manager {
        home-manager-path = "${userHome}/home-manager";
        config-path = builtins.toString ../home-manager + "/${hostName}.nix";
      })
    ];
  };
}

Let's break that code to understand what is happening.

let
  user = "john";
  userHome = "/home/${user}";
  hostName = "laptop";

First I define some variables I'll use later. It is possible to define a function where you pass them as arguments, but for now I'm just using a let block.

  home-manager = { home-manager-path, config-path }:
    assert builtins.typeOf home-manager-path == "string";
    assert builtins.typeOf config-path == "string";

Now I define a function responsible for installing home-manager . It takes 2 arguments, home-manager-path (path to my local home-manager git checkout) and config-path (path to my main home-manager configuration file, usually called home.nix). I'm also type-checking the arguments.

    (
      pkgs.callPackage
        (/. + home-manager-path + "/home-manager") { path = "${home-manager-path}"; }
    ).overrideAttrs (old: {

I'm calling the derivation provided by home-manager, it is defined here:

https://github.com/rycee/home-manager/blob/master/home-manager/default.nix

Notice that it takes one argument, path, the path to my local home-manager checkout. Since I need further customization, I use the overrideAttrs function to produce a new derivation based on the original one.

      nativeBuildInputs = [ pkgs.makeWrapper ];
      buildCommand =
        let
          home-mananger-bootstrap = pkgs.writeTextFile {
           # ...
          }; in
        ''
          ${old.buildCommand}
          wrapProgram $out/bin/home-manager --set HOME_MANAGER_CONFIG "${home-mananger-bootstrap}"
        '';

These are the arguments for overrideAttrs. Since I want to wrap home-manager, I need pkgs.makeWrapper.

I'm extending the build command. I'm calling the original home-manager build command, and then wrapping it to set the environment variable HOME_MANAGER_CONFIG to a file that I'm generating with the writeTextFile helper function. More details on that later. Notice that HOME_MANAGER_CONFIG is the entrypoint for home-manager, usually that file is home.nix.

          home-mananger-bootstrap = pkgs.writeTextFile {
            name = "home-manager-bootstrap.nix";
            text = ''
              { config, pkgs, ... }:
              {
                # Home Manager needs a bit of information about you and the
                # paths it should manage.
                home.username = "${user}";
                home.homeDirectory = "${userHome}";
                home.sessionVariables.HOSTNAME = "${hostName}";
                imports = [ ${config-path} ];
              }
            '';

This is how I'm generating the main the entrypoint file for home-manager. It's a .nix file itself, which is added to the nix store. When I generate my NixOS configuration, with nixos-rebuild build, I know the value for some of the variables needed by home-manager, like my username or the hostname. I'm taking advange of that to generate a minimal home.nix file, where the values for that variables are injected, and then I'm importing my real home-manager configuration (defined by the config-path variable)

in
{
  users.users.${user} = {
    home = userHome;
    packages = [
      (home-manager {
        home-manager-path = "${userHome}/home-manager";
        config-path = builtins.toString ../home-manager + "/${hostName}.nix";
      })
    ];
  };
}

The last step, this is how I install home-manager from my NixOS configuration.nix. I'm just adding the package returned by my home-manager function. The 2 arguments to the functions are the path to my local home-manager checkout and the path to my home.nix file.

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