nixosConfiguration: preSwitchCkeck: nvd

Allow to specify a list of classes in the host/<hostname>.nix (by
default the class <hostname> is used) that will automatically load
specified outputs of inputs.

export/nixosConfigurations.nix

@@ -14,12 +14,14 @@ let
     { pkgs, lib, config, ... }:
     {
       config.system.preSwitchChecks.nvd = ''
-        ${lib.getExe pkgs.nvd} --nix-bin-dir ${config.nix.package}/bin diff /run/current-system "''${1}"
-        if [[ -t 0 ]]; then
-          printf 'really %s to %s? [y]es/[N]o ' "''${2}" "''${1}"
-          read -r nvd_check
-        else
-          nvd_check=y
+        nvd_check=y
+        if [[ -d /run/current-system ]]
+        then
+          ${lib.getExe pkgs.nvd} --nix-bin-dir ${config.nix.package}/bin diff /run/current-system "''${1}"
+          if [[ -t 0 ]]; then
+            printf 'really %s to %s? [y]es/[N]o ' "''${2}" "''${1}"
+            read -r nvd_check
+          fi
         fi
         [[ "''${nvd_check}" == y ]]
       '';
@@ -30,6 +32,7 @@ let
     {
       namespace ? "fbs42",
       nixpkgs ? "nixpkgs",
+      classes ? [ hostname ],
     }:
     inputs.${nixpkgs}.lib.nixosSystem {
       specialArgs = { inherit inputs flake namespace; };
@@ -39,6 +42,9 @@ let
             base: builtins.attrValues (flake.lib.scan { base = "${base}/host/${hostname}"; })
           ) flake.lib.schema.base
         )
+        ++ map (class: {
+          imports = map (input: input.${namespace}.${class} or { }) (builtins.attrValues inputs);
+        }) classes
         ++ [ pre-switch-check ]
         ++ [ { config.networking.hostName = hostname; } ]
         ++ [ nixosModules.default ]