nix-book/source/recipes/build/nixpkg/flake.nix

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
      in
      {
        packages = rec {
          hello = pkgs.stdenv.mkDerivation rec {
            name = "hello-with-cow";

            src = ./.;

            unpackPhase = "true";

            buildPhase = ":";

            installPhase =
              ''
                mkdir -p $out/bin
                cp $src/hello-with-cow $out/bin
                chmod +x $out/bin/hello-with-cow

                # modify the hello-again script so it can find hello-nix
                COWSAY=$(type -p cowsay)
                sed "s_cowsay_"$COWSAY"_" --in-place $out/bin/hello-with-cow
              '';

            buildInputs = [ pkgs.cowsay ];
          };
          default = hello;
        };

        apps = rec {
          hello = flake-utils.lib.mkApp { drv = self.packages.${system}.hello; };
          default = hello;
        };
      }
    );
}