nix-book/source/new-flake/bash-flake/flake-3.nix

{
  description = "what does the cow say";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
  };

  outputs = { self, nixpkgs }: {

    devShells = {
      x86_64-linux.default =
        let
          pkgs = import nixpkgs { system = "x86_64-linux"; };
        in
          pkgs.mkShell {
            packages = [ pkgs.cowsay ];
          };
    }; # devShells

    packages = {
      x86_64-linux.default =
        let
          pkgs = import nixpkgs { system = "x86_64-linux"; };
        in
          pkgs.stdenv.mkDerivation {
            name = "cow-hello.sh";
            src = ./.;
            unpackPhase = "true";
            buildPhase = ":";
            installPhase =
              ''
                mkdir -p $out/bin
                cp $src/cow-hello.sh $out/bin
                chmod +x $out/bin/cow-hello.sh

                # modify the cow-hello.sh script so it can find cowsay
                COWSAY=$(type -p cowsay)
                sed "s_cowsay_"$COWSAY"_" --in-place $out/bin/cow-hello.sh
              '';
            buildInputs = [ pkgs.cowsay ];
          }; # mkDerivation
    }; # packages

  }; # outputs
}