In iex redirect stdio to file.

console_helpers.ex

defmodule Console.Helpers do
  defmacro capture_io(filename, do: block) do
    quote do
      filename = unquote(filename)
      current_leader = Process.group_leader()
      {:ok, fh} = File.open(filename, [:write])
      Process.group_leader(self(), fh)
      unquote(block)
      Process.group_leader(self(), current_leader)
    end
  end
end