{
  # Packages a shell script as a Nix derivation.
  # Try: `nix build`, then `./result/bin/greet`
  # Or:  `nix run`
  description = "Package a script with stdenv.mkDerivation";

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

  outputs =
    { self, nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };
    in
    {
      packages.${system}.default = pkgs.stdenv.mkDerivation {
        pname = "greet";
        version = "0.1.0";

        # `src = ./.` copies the flake directory into the store.
        # The build then runs inside that copy.
        src = ./.;

        # Skip the default unpack/build phases; this example has no C code.
        dontUnpack = true;
        dontBuild = true;

        # installPhase is a shell snippet. `$out` is the output store path.
        installPhase = ''
          mkdir -p $out/bin
          cp ${./greet.sh} $out/bin/greet
          chmod +x $out/bin/greet

          # Patch the shebang so the script uses a store-path bash,
          # not /bin/sh (which may not exist on a pure NixOS system).
          substituteInPlace $out/bin/greet \
            --replace-fail "#!/usr/bin/env bash" "#!${pkgs.bash}/bin/bash"
        '';
      };

      # `nix run` looks here. `program` must be a store path to an executable.
      apps.${system}.default = {
        type = "app";
        program = "${self.packages.${system}.default}/bin/greet";
        meta.description = "Run the packaged greet script.";
      };
    };
}