Capture output from stdout

capture.rb

require "tempfile"


# Captures the given stream and returns it:
  #
  #   stream = capture(:stdout) { puts 'notice' }
  #   stream # => "notice\n"
  #
  #   stream = capture(:stderr) { warn 'error' }
  #   stream # => "error\n"
  #
  # even for subprocesses:
  #
  #   stream = capture(:stdout) { system('echo notice') }
  #   stream # => "notice\n"
  #
  #   stream = capture(:stderr) { system('echo error 1>&2') }
  #   stream # => "error\n"

def capture(stream)
  stream = stream.to_s
  captured_stream = Tempfile.new(stream)
  stream_io = eval("$#{stream}")
  origin_stream = stream_io.dup
  stream_io.reopen(captured_stream)

  yield

  stream_io.rewind
  return captured_stream.read
ensure
  captured_stream.unlink
  stream_io.reopen(origin_stream)
end

output = capture(:stdout) do
  puts "This goes to standard out"
  STDOUT.puts "This tries to bypass capture"
  system('echo "output from a subprocess"')
end

puts output.split('\n')

capture_output.rb

def capture_output
  old_stdout, old_stderr = STDOUT.clone, STDERR.clone
  pipe_r, pipe_w = IO.pipe
  pipe_er, pipe_ew = IO.pipe

  pipe_r.sync  = true
  pipe_er.sync = true

  output = ""
  err_output = ""

  reader = Thread.new do
    begin
      loop do
        output << pipe_r.readpartial(1024)
      end
    rescue EOFError
    end
  end

  reader2 = Thread.new do
    begin
      loop do
        err_output << pipe_er.readpartial(1024)
      end
    rescue EOFError
    end
  end

  STDOUT.reopen(pipe_w)
  STDERR.reopen(pipe_ew)

  yield
ensure
  STDOUT.reopen(old_stdout)
  STDERR.reopen(old_stderr)

  pipe_w.close
  pipe_ew.close

  reader.join
  reader2.join

  return "#{output} #{err_output}"
end

output = capture_output do
  puts "This goes to standard out"
  STDOUT.puts "This tries to bypass STDOUT"
  system 'echo "output from a subprocess"'
  STDERR.puts "This tries to bypass STDERR"
end

puts output.inspect