System.cmd "elixir", ["-e", "IO.puts(:user, 'stdout');IO.puts(:standard_error, 'stderr');"]
stderr
      {"stdout\n", 0}
iex(3)> System.cmd "elixir", ["-e", "IO.puts(:user, 'stdout');IO.puts(:standard_error, 'stderr');"], stderr_to_stdout: true
{"stdout\nstderr\n", 0}

  def escape(string) do
    for <<c <- string>>, into: "", do: escape_char(c)
  end

  defp escape_char(?\\), do: ~S|\\|
  defp escape_char(?/), do: ~S|\/|
  defp escape_char(?"), do: ~S|\"|
  defp escape_char(c), do: <<c>>

  def shell_encode(string) do
    string
    |> String.replace(~s(\\), ~s(\\\\))
    |> String.replace(~s(\"), ~s(\\"))
    |> String.replace(~s(\n), ~s(\\n))
  end

iex(1)> {stream, 0}= System.cmd "elixir", ["-e", "IO.puts(:user, 'stdout');IO.puts(:standard_error, 'stderr');"], into: IO.stream(:stdio, :line), stderr_to_stdout: true
stdout
stderr
{%IO.Stream{device: :standard_io, line_or_bytes: :line, raw: false}, 0}
iex(2)> {stream, 0}= System.cmd "elixir", ["-e", "IO.puts(:user, 'stdout');IO.puts(:standard_error, 'stderr');"], stderr_to_stdout: true
{"stdout\nstderr\n", 0}