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

nixos/doc/manual/installation/installing.chapter.md

···
     [](#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:
 
···
     other issue (such as a network outage while downloading binaries
     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/doc/manual/release-notes/rl-2505.section.md

···
 
 - `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`.
 

nixos/modules/installer/tools/manpages/nixos-generate-config.8

···
 .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.

nixos/modules/installer/tools/nixos-generate-config.pl

···
 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";
···
     print STDERR "writing $fn...\n";
     mkpath($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";

nixos/modules/installer/tools/tools.nix

···
 # 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;
···
     withNgSuffix = false;
     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
···
 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;
···
     desktopConfiguration = lib.mkOption {
       internal = true;
       type = lib.types.listOf lib.types.lines;
-      default = [];
+      default = [ ];
       description = ''
         Text to preseed the desktop configuration that `nixos-generate-config`
         saves to `/etc/nixos/configuration.nix`.

nixos/tests/nixos-generate-config.nix

···
       machine.wait_for_unit("multi-user.target")
       machine.succeed("nixos-generate-config")
 
+      machine.succeed("nix-instantiate --parse /etc/nixos/configuration.nix /etc/nixos/hardware-configuration.nix")
+
       # Test if the configuration really is overridden
       machine.succeed("grep 'OVERRIDDEN' /etc/nixos/configuration.nix")
 
···
       machine.succeed(
           "grep 'services\\.xserver\\.desktopManager\\.gnome\\.enable = true;' /etc/nixos/configuration.nix"
       )
+
+      machine.succeed("rm -rf /etc/nixos")
+      machine.succeed("nixos-generate-config --flake")
+      machine.succeed("nix-instantiate --parse /etc/nixos/flake.nix /etc/nixos/configuration.nix /etc/nixos/hardware-configuration.nix")
     '';
   }
 )