nixos-generate-config: add --flake option (#383033)

    [](#ch-options). A minimal example is shown in

    [Example: NixOS Configuration](#ex-config).

    This command accepts an optional `--flake` option, to also generate a

    `flake.nix` file, if you want to set up a flake-based configuration.

    The command `nixos-generate-config` can generate an initial

    configuration file for you:

    from the NixOS binary cache), you can re-run `nixos-install` after

    fixing your `configuration.nix`.

    If you opted for a flake-based configuration, you will need to pass the

    `--flake` here as well and specify the name of the configuration as used in

    the `flake.nix` file. For the default generated flake, this is `nixos`.

    ```ShellSession

    # nixos-install --flake 'path/to/flake.nix#nixos'

    ```

    As the last step, `nixos-install` will ask you to set the password

    for the `root` user, e.g.

- `nixos-rebuild-ng`, a full rewrite of `nixos-rebuild` in Python, is available for testing. You can enable it by setting [system.rebuild.enableNg](options.html#opt-system.rebuild.enableNg) in your configuration (this will replace the old `nixos-rebuild`), or by adding `nixos-rebuild-ng` to your `environment.systemPackages` (in this case, it will live side-by-side with `nixos-rebuild` as `nixos-rebuild-ng`). It is expected that the next major version of NixOS (25.11) will enable `system.rebuild.enableNg` by default.

- The `nixos-generate-config` command now supports a optional `--flake` option, which will generate a flake.nix file alongside the `configuration.nix` and `hardware-configuration.nix`, providing an easy instroduction into flake-based system configurations.

- A `nixos-rebuild build-image` sub-command has been added.

  It allows users to build platform-specific (disk) images from their NixOS configurations. `nixos-rebuild build-image` works similar to the popular [nix-community/nixos-generators](https://github.com/nix-community/nixos-generators) project. See new [section on image building in the nixpkgs manual](https://nixos.org/manual/nixpkgs/unstable/#sec-image-nixos-rebuild-build-image). It is also available for `nixos-rebuild-ng`.

.Op Fl -force

.Op Fl -root Ar root

.Op Fl -dir Ar dir

.Op Fl -flake

.

.

.

.It Fl -force

Overwrite

.Pa /etc/nixos/configuration.nix

if it already exists.

(and

.Pa /etc/nixos/flake.nix

if --flake is passed) if already present.

.

.It Fl -flake

Also generate

.Pa /etc/nixos/flake.nix Ns

\&.

.

.It Fl -no-filesystems

Omit everything concerning file systems and swap devices from the hardware configuration.

my $rootDir = ""; # = /

my $force = 0;

my $noFilesystems = 0;

my $flake = 0;

my $showHardwareConfig = 0;

for (my $n = 0; $n < scalar @ARGV; $n++) {

    elsif ($arg eq "--show-hardware-config") {

        $showHardwareConfig = 1;

    }

    elsif ($arg eq "--flake") {

        $flake = 1;

    }

    else {

        die "$0: unrecognized argument ‘$arg’\n";

    }

    mkpath($outDir, 0, 0755);

    write_file($fn, $hwConfig);

    $fn = "$outDir/flake.nix";

    if ($flake) {

        if ($force || ! -e $fn) {

            print STDERR "writing $fn...\n";

            mkpath($outDir, 0, 0755);

            write_file($fn, <<EOF);

            @flake@

EOF

        } else {

            print STDERR "warning: not overwriting existing $fn\n";

        }

    }

    # Generate a basic configuration.nix, unless one already exists.

    $fn = "$outDir/configuration.nix";

    if ($force || ! -e $fn) {

# This module generates nixos-install, nixos-rebuild,

# nixos-generate-config, etc.

{ config, lib, pkgs, ... }:

{ config, lib, pkgs, options, ... }:

let

  makeProg = args: pkgs.replaceVarsWith (args // {

      hostPlatformSystem = pkgs.stdenv.hostPlatform.system;

      detectvirt = "${config.systemd.package}/bin/systemd-detect-virt";

      btrfs = "${pkgs.btrfs-progs}/bin/btrfs";

      inherit (config.system.nixos-generate-config) configuration desktopConfiguration;

      inherit (config.system.nixos-generate-config) configuration desktopConfiguration flake;

      xserverEnabled = config.services.xserver.enable;

    };

    manPage = ./manpages/nixos-generate-config.8;

    withReexec = true;

  };

  defaultFlakeTemplate = ''

    {

      inputs = {

        # This is pointing to an unstable release.

        # If you prefer a stable release instead, you can this to the latest number shown here: https://nixos.org/download

        # i.e. nixos-24.11

        # Use `nix flake update` to update the flake to the latest revision of the chosen release channel.

        nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

      };

      outputs = inputs\@{ self, nixpkgs, ... }: {

        # NOTE: '${options.networking.hostName.default}' is the default hostname

        nixosConfigurations.${options.networking.hostName.default} = nixpkgs.lib.nixosSystem {

          modules = [ ./configuration.nix ];

        };

      };

    }

  '';

  defaultConfigTemplate = ''

    # Edit this configuration file to define what should be installed on

    # your system. Help is available in the configuration.nix(5) man page, on

in

{

  options.system.nixos-generate-config = {

    flake = lib.mkOption {

      internal = true;

      type = lib.types.str;

      default = defaultFlakeTemplate;

      description = ''

        The NixOS module that `nixos-generate-config`

        saves to `/etc/nixos/flake.nix` if --flake is set.

        This is an internal option. No backward compatibility is guaranteed.

        Use at your own risk!

        Note that this string gets spliced into a Perl script. The perl

        variable `$bootLoaderConfig` can be used to

        splice in the boot loader configuration.

      '';

    };

    configuration = lib.mkOption {

      internal = true;

      type = lib.types.str;