foo.nix

with (import <nixpkgs> {});

stdenv.mkDerivation {
  name = "testing";
  foo = ["a" "b" "c"];
  bar = "a b c";

  prePatch = ''
    echo "$foo"
    echo "$bar"
    for x in $foo; do echo $x; done
    for x in $bar; do echo $x; done
    exit 1
  '';
  unpackPhase = ":";

  src = ./test.nix;
}

these derivations will be built:
  /nix/store/5azzcpcsr0306k27flj74lbwlgsa5j9m-testing.drv
building '/nix/store/5azzcpcsr0306k27flj74lbwlgsa5j9m-testing.drv'...
unpacking sources
patching sources
a b c
a b c
a
b
c
a
b
c
builder for '/nix/store/5azzcpcsr0306k27flj74lbwlgsa5j9m-testing.drv' failed with exit code 1
error: build of '/nix/store/5azzcpcsr0306k27flj74lbwlgsa5j9m-testing.drv' failed