example in the manual: https://nixos.org/manual/nixpkgs/stable/#chap-trivial-builders https://nixos.org/manual/nixpkgs/stable/#chap-trivial-builde... shows how

example in the manual: https://nixos.org/manual/nixpkgs/stable/#chap-trivial-builders https://nixos.org/manual/nixpkgs/stable/#chap-trivial-builde... shows how it creates a file `$out/message`. If you want to write to nix store with stdenv use $out.