run-elixir-umbrella-projects.ex

defmodule MixTests do
  @commands [
    "git diff --name-only",
    "git diff --cached --name-only",
    "git diff --name-only ..develop",
    "git ls-files --others --exclude-standard",
  ]

  def files_to_run do
    @commands
    |> Enum.flat_map(&watch/1)
    |> Enum.map(&transform/1)
    |> Enum.reject(&is_nil/1)
    |> Enum.sort
    |> Enum.uniq
    |> Enum.join(" ")
    |> IO.puts
  end

  defp watch(command) do
    [head|tail] = String.split(command)
    {result, 0} = System.cmd(head, tail)
    String.split(result)
  end

  defp transform("apps/" <> file), do: file |> String.replace(~r/^\w+\//, "") |> transform
  defp transform("lib/" <> file), do: file |> String.replace(~r/(.*)\.ex$/, "test/\\g{1}_test.exs") |> transform
  defp transform("test/"<> _rest = file), do: file
  defp transform(_file), do: nil
end

MixTests.files_to_run