Capture IO.puts in ExUnit tests

Whenever writing a CLI you often have to communicate something to the user via IO.puts, and while some languages make it complicated to capture output in a test, Elixir makes it extremely straight-forward.

All we have to do is import the ExUnit.CaptureIO module into our test which exposes the capture_io method.

Example

Implementation:

defmodule MyApp.CLI do
  def main(argv), do
    # parse args ...
    # return :help if the -h switch is supplied
    # pass to process
    argv
    |> parse_args
    |> process
  end
  
  def process(:help) do
    IO.puts "usage: my_app <arg1> <arg2>"
  end
end

IO capturing test:

defmodule CliTest do
  use ExUnit.Case
  
  import ExUnit.CaptureIO
  
  test "prints usage instructions when the help switch is supplied" do
    execute_main = fn ->
      MyApp.CLI.main(["-h"])
    end
    
    assert capture_io(execute_main) =~ "usage:"
  end
end

For more information read the ExUnit.CaptureIO documentation