dave@host ~$ sudo nix-shell -p cowsay
warning: the group 'nixbld' specified in 'build-users-group' does not exist
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «string»:1:25:

            1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (cowsay) ]; } ""
             |