let
  contentAddressedByDefault = builtins.getEnv "NIX_TESTS_CA_BY_DEFAULT" == "1";
  caArgs = if contentAddressedByDefault then {
    __contentAddressed = true;
    outputHashMode = "recursive";
    outputHashAlgo = "sha256";
  } else {};
in

rec {
  shell = "@bash@";

  path = "@ls@";

  system = "@system@";

  shared = builtins.getEnv "_NIX_TEST_SHARED";

  hasSuffix = ext: fileName:
    let lenFileName = builtins.stringLength fileName;
        lenExt = builtins.stringLength ext;
    in !(builtins.lessThan lenFileName lenExt) &&
       builtins.substring (builtins.sub lenFileName lenExt) lenFileName fileName == ext;  id = x: x;

  mkDerivation = args:
    derivation ({
      inherit system shell;
      builder = shell;
      args = ["-e" args.builder or (builtins.toFile "builder-${args.name}.sh" ''
        if [ -e "$NIX_ATTRS_SH_FILE" ]; then source $NIX_ATTRS_SH_FILE; fi;
        eval "$buildCommand"
      '')];
      PATH = path;
    } // caArgs // removeAttrs args ["builder" "meta"])
    // { meta = args.meta or {}; };
}