Execute a command, return its output, error stream and exit status

cmd.rb

# returns [output string, err string, exit code]
def cmd(args, input = nil)
  parent_read, child_write = IO.pipe
  err_read, err_write = IO.pipe
  child_read, parent_write = IO.pipe if input

  pid = fork do
    if input
      parent_write.close
      $stdin.reopen(child_read)
    end
    $stdout.reopen(child_write)
    $stderr.reopen(err_write)
    exec(*args)
  end

  if input
    parent_write.write input
    parent_write.close
  end
  Process.wait(pid, 0)

  child_write.close
  err_write.close

  [parent_read.read, err_read.read, $?.exitstatus]
end

# stdout, stderr, no input
p cmd('echo foo 1>&2; echo bar')
# with input
p cmd('cat -', 'hello kitty')
# stdout, stderr with input
p cmd('ruby -e "puts STDIN.read; warn \'magic\'; exit 37"', 'hello ruby')