init

modules/nixos/default.nix

@@ -0,0 +1,3 @@
+{
+  shells = import ./shells;
+}

modules/nixos/shells/default.nix

@@ -0,0 +1,8 @@
+{ ...
+}:
+
+{
+  imports = [
+    ./fish.nix
+  ];
+}

modules/nixos/shells/fish.nix

@@ -0,0 +1,34 @@
+{ config
+, lib
+, pkgs
+, ...
+}:
+
+let
+  cfg = config.custom.shells.fish;
+in
+{
+  options.custom.shells.fish = {
+    enable = lib.mkEnableOption "Enable the Fish shell.";
+
+    defaultFor = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [];
+      description = "Users to set Fish as their default shell. If set the shell must be enabled.";
+    };
+  };
+
+  config = {
+    # Actually enable and assign the shell
+    programs = lib.mkIf cfg.enable {
+      fish.enable = true;
+    };
+
+    users.users = lib.mkIf cfg.enable (
+      lib.genAttrs cfg.defaultFor (user: {
+        shell = lib.mkOverride 900 "${pkgs.fish}/bin/fish";
+      })
+    );
+  };
+}
+