add: more system modules

system/modules/user/shell/default.nix

@@ -0,0 +1,40 @@
+{ lib, config, ... }:
+
+let
+  cfg = config.systemModules.user.shell;
+in
+
+{
+  imports = [
+    ./fish
+    # ./bash
+    # ./zsh
+  ];
+
+  options.systemModules.user.shell = {
+    fish = {
+      enable = lib.mkEnableOption "Enable fish as the user shell";
+    };
+    zsh = {
+      enable = lib.mkEnableOption "Enable zsh as the user shell";
+    };
+    bash = {
+      enable = lib.mkEnableOption "Enable bash as the user shell";
+    };
+
+  };
+
+   config = { 
+    assertions = [
+      {
+        assertion = 
+          (lib.length (lib.filter (x: x) [
+            cfg.fish.enable or false
+            cfg.zsh.enable or false
+            cfg.bash.enable or false
+          ]) <= 1); 
+        message = "Only one user shell can be active in the configuration";
+      }
+    ];
+  };
+}