thing.nix

{
  programs.fish = {
    enable = true;
    interactiveShellInit =
      let
        shellderSrc = pkgs.fetchurl {
          url =
            "https://github.com/oh-my-fish/theme-agnoster/raw/master/fish_prompt.fish";
          sha256 = "1d8dmqb757fi1crhwriq9vl77yg079zdjs53r2naiym02wdhkl4a";
        };
      in
      ''
        source ${shellderSrc}
        echo 'all other stuff goes here too'
      '';
  };
}