(In tests) how to capture another process's IO

gen_server_that_uses_stdio.ex

defmodule GenServerThatUsesStdio do
  use GenServer

  def start_link() do
    GenServer.start_link(__MODULE__, [])
  end
  
  def handle_cast(message, state) do
    IO.puts message
    {:noreply, state}
  end
end

set_processes_group_leader_test.exs

defmodule SetProcessesGroupLeaderTest do
  use ExUnit.Case

  test "capturing another process's output on stdio" do
    {:ok, pid} = GenServerThatUsesStdio.start_link()

    {:ok, capture_gl} = StringIO.open("")
    Process.group_leader(pid, capture_gl)

    GenServer.cast pid, 42

    # Give the process time to handle the cast before it gets killed!
    :timer.sleep 100

    {:ok, {_, output}} = StringIO.close(capture_gl)

    assert output == "42\n"

    Process.exit(pid, :normal)
  end
end