Add a command ‘nixos-help’ that opens the NixOS manual in a browser

modules/services/misc/nixos-manual.nix

@@ -16,6 +16,17 @@ let
     inherit pkgs options;
   };
 
+  entry = "${manual.manual}/share/doc/nixos/manual.html";
+
+  help = pkgs.writeScriptBin "nixos-help"
+    ''
+      #! ${pkgs.stdenv.shell} -e
+      if ! ''${BROWSER:-w3m} ${entry}; then
+        echo "$0: unable to start a web browser; please set \$BROWSER or install ‘w3m’"
+        exit 1
+      fi
+    '';
+
 in
 
 {
@@ -69,7 +80,7 @@ in
 
     system.build.manual = manual;
 
-    environment.systemPackages = [ manual.manpages ];
+    environment.systemPackages = [ manual.manpages help ];
 
     boot.extraTTYs = mkIf cfg.showManual ["tty${cfg.ttyNumber}"];
 
@@ -78,7 +89,7 @@ in
         { description = "NixOS Manual";
           wantedBy = [ "multi-user.target" ];
           serviceConfig =
-            { ExecStart = "${cfg.browser} ${manual.manual}/share/doc/nixos/manual.html";
+            { ExecStart = "${cfg.browser} ${entry}";
               StandardInput = "tty";
               StandardOutput = "tty";
               TTYPath = "/dev/tty${cfg.ttyNumber}";