capturing stdout and stderr

capture.rb

def capture(options={})
  out = options[:out]
  err = options[:err] || :out

  stdout_old = $stdout.dup
  stderr_old = $stderr.dup

  $stdout.reopen out

  err = $stdout if err == :out
  $stderr.reopen err
  yield
ensure
  $stdout.flush
  $stderr.flush
  $stdout.reopen stdout_old
  $stderr.reopen stderr_old
end

if __FILE__ == $0
  capture(:out => open('a.txt', 'w')) do
    STDOUT.puts 'STDOUT.puts'
    system 'echo "system"'
    STDERR.puts 'STDERR.puts'
  end
end

__END__
$ cat a.txt
STDOUT.puts
system
STDERR.puts