Add help for interactive nix-shell use.

docs/source/dev/README.rst

@@ -49,7 +49,7 @@ To update the version of NixOS we deploy with, run:
 
 .. code: shell
 
-   nix-shell --command 'update-nixpkgs'
+   nix-shell --run 'update-nixpkgs'
 
 That will update ``nixpkgs-2015.json`` to the latest release on the nixos-21.05 channel.
 

shell.nix

@@ -9,6 +9,8 @@ pkgs.mkShell {
   shellHook = ''
     export NIX_PATH="nixpkgs=${pkgs.path}";
   '';
+  # Run the shellHook from tools
+  inputsFrom = [tools];
   buildInputs = [
     tools
     pkgs.morph

tools/default.nix

@@ -18,6 +18,14 @@ in
 # with there dependencies available.
 pkgs.runCommand "ps_tools" {
   nativeBuildInputs = [ makeWrapper ];
+  shellHook = ''
+    if [[ $- == *i* ]]; then
+      cat <<MOTD
+    Tools (pass --help for details):
+    - update-nixpkgs
+    MOTD
+    fi
+  '';
 } ''
   mkdir -p $out/bin
   # makeWrapper <executable> <wrapperfile> <args>