Use shell packages to select the user's shell

The string type is still available for backward-compatiblity.

lib/types.nix

@@ -100,6 +100,10 @@ rec {
         in if isDerivation res then res else toDerivation res;
     };
 
+    shellPackage = package // {
+      check = x: (package.check x) && (hasAttr "shellPath" x);
+    };
+
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.

nixos/lib/utils.nix

@@ -8,4 +8,10 @@ rec {
    replaceChars ["/" "-" " "] ["-" "\\x2d" "\\x20"]
     (if hasPrefix "/" s then substring 1 (stringLength s) s else s);
 
+  # Returns a system path for a given shell package
+  toShellPath = shell:
+    if types.shellPackage.check shell then
+      "/run/current-system/sw${shell.shellPath}"
+    else
+      shell;
 }

nixos/modules/config/shells-environment.nix

@@ -1,7 +1,7 @@
 # This module defines a global environment configuration and
 # a common configuration for all shells.
 
-{ config, lib, pkgs, ... }:
+{ config, lib, utils, pkgs, ... }:
 
 with lib;
 
@@ -135,13 +135,13 @@ in
 
     environment.shells = mkOption {
       default = [];
-      example = [ "/run/current-system/sw/bin/zsh" ];
+      example = literalExample "[ pkgs.bashInteractive pkgs.zsh ]";
       description = ''
         A list of permissible login shells for user accounts.
         No need to mention <literal>/bin/sh</literal>
         here, it is placed into this list implicitly.
       '';
-      type = types.listOf types.path;
+      type = types.listOf (types.either types.shellPackage types.path);
     };
 
   };
@@ -158,7 +158,7 @@ in
 
     environment.etc."shells".text =
       ''
-        ${concatStringsSep "\n" cfg.shells}
+        ${concatStringsSep "\n" (map utils.toShellPath cfg.shells)}
         /bin/sh
       '';
 

nixos/modules/config/users-groups.nix

@@ -1,9 +1,8 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, utils, pkgs, ... }:
 
 with lib;
 
 let
-
   ids = config.ids;
   cfg = config.users;
 
@@ -103,7 +102,7 @@ let
       };
 
       home = mkOption {
-        type = types.str;
+        type = types.path;
         default = "/var/empty";
         description = "The user's home directory.";
       };
@@ -118,8 +117,10 @@ let
       };
 
       shell = mkOption {
-        type = types.str;
-        default = "/run/current-system/sw/bin/nologin";
+        type = types.either types.shellPackage types.path;
+        default = pkgs.nologin;
+        defaultText = "pkgs.nologin";
+        example = literalExample "pkgs.bashInteractive";
         description = "The path to the user's shell.";
       };
 
@@ -359,11 +360,12 @@ let
 
   spec = pkgs.writeText "users-groups.json" (builtins.toJSON {
     inherit (cfg) mutableUsers;
-    users = mapAttrsToList (n: u:
+    users = mapAttrsToList (_: u:
       { inherit (u)
-          name uid group description home shell createHome isSystemUser
+          name uid group description home createHome isSystemUser
           password passwordFile hashedPassword
           initialPassword initialHashedPassword;
+        shell = utils.toShellPath u.shell;
       }) cfg.users;
     groups = mapAttrsToList (n: g:
       { inherit (g) name gid;
@@ -373,6 +375,12 @@ let
       }) cfg.groups;
   });
 
+  systemShells =
+    let
+      shells = mapAttrsToList (_: u: u.shell) cfg.users;
+    in
+      filter types.shellPackage.check shells;
+
 in {
 
   ###### interface
@@ -478,6 +486,9 @@ in {
       };
     };
 
+    # Install all the user shells
+    environment.systemPackages = systemShells;
+
     users.groups = {
       root.gid = ids.gids.root;
       wheel.gid = ids.gids.wheel;

nixos/modules/programs/bash/bash.nix

@@ -200,7 +200,7 @@ in
     # Configuration for readline in bash.
     environment.etc."inputrc".source = ./inputrc;
 
-    users.defaultUserShell = mkDefault "/run/current-system/sw/bin/bash";
+    users.defaultUserShell = mkDefault pkgs.bashInteractive;
 
     environment.pathsToLink = optionals cfg.enableCompletion [
       "/etc/bash_completion.d"

nixos/modules/programs/shadow.nix

@@ -1,6 +1,6 @@
 # Configuration for the pwdutils suite of tools: passwd, useradd, etc.
 
-{ config, lib, pkgs, ... }:
+{ config, lib, utils, pkgs, ... }:
 
 with lib;
 
@@ -43,13 +43,13 @@ in
     users.defaultUserShell = lib.mkOption {
       description = ''
         This option defines the default shell assigned to user
-        accounts.  This must not be a store path, since the path is
+        accounts. This can be either a full system path or a shell package.
+
+        This must not be a store path, since the path is
         used outside the store (in particular in /etc/passwd).
-        Rather, it should be the path of a symlink that points to the
-        actual shell in the Nix store.
       '';
-      example = "/run/current-system/sw/bin/zsh";
-      type = types.path;
+      example = literalExample "pkgs.zsh";
+      type = types.either types.path types.shellPackage;
     };
 
   };
@@ -60,7 +60,9 @@ in
   config = {
 
     environment.systemPackages =
-      lib.optional config.users.mutableUsers pkgs.shadow;
+      lib.optional config.users.mutableUsers pkgs.shadow ++
+      lib.optional (types.shellPackage.check config.users.defaultUserShell)
+        config.users.defaultUserShell;
 
     environment.etc =
       [ { # /etc/login.defs: global configuration for pwdutils.  You
@@ -74,7 +76,7 @@ in
             ''
               GROUP=100
               HOME=/home
-              SHELL=${config.users.defaultUserShell}
+              SHELL=${utils.toShellPath config.users.defaultUserShell}
             '';
           target = "default/useradd";
         }