Make `tools/default.nix` more easily extendable.

tools/default.nix

@@ -13,22 +13,37 @@ let
       echo ${lib.escapeShellArg ./pylib} > $out/${lib.escapeShellArg python.sitePackages}/tools.pth
     '';
   };
+  python-commands = [
+    ./update-nixpkgs
+  ];
 in
-# This derivation creates a package that wraps our tools to setup an environment
-# with there dependencies available.
+  # This derivation creates a package that wraps our tools to setup an environment
+  # with there dependencies available.
 pkgs.runCommand "ps_tools" {
   nativeBuildInputs = [ makeWrapper ];
   shellHook = ''
+    # Only display the help if we are running an interactive shell.
     if [[ $- == *i* ]]; then
       cat <<MOTD
     Tools (pass --help for details):
-    - update-nixpkgs
+    ${lib.concatStringsSep "\n" (map (path:
+        "- ${baseNameOf path}"
+    ) python-commands)}
     MOTD
     fi
   '';
-} ''
-  mkdir -p $out/bin
-  # makeWrapper <executable> <wrapperfile> <args>
-  # See https://nixos.org/manual/nixpkgs/stable/#fun-makeWrapper
-  makeWrapper ${python-env}/bin/python $out/bin/update-nixpkgs --add-flags ${toString ./update-nixpkgs}
-''
+  } ''
+    mkdir -p $out/bin
+    ${lib.concatStringsSep "\n" (map (path:
+      let
+        baseName = baseNameOf path;
+        # We use toString so that we wrap the in-tree scripts, rather than copying
+        # them to the nix-store. This means that we don't need to run nix-shell again
+        # to pick up changes.
+        sourcePath = toString path;
+      in
+      # makeWrapper <executable> <wrapperfile> <args>
+      # See https://nixos.org/manual/nixpkgs/stable/#fun-makeWrapper
+      "makeWrapper ${python-env}/bin/python $out/bin/${baseName} --add-flags ${sourcePath}"
+      ) python-commands)}
+  ''