code: func (s *Session) RequestSubsystem(subsystem string) error {
  	msg := subsystemRequestMsg{
  		Subsystem: subsystem,
  	}
  	ok, err := s.ch.SendRequest("subsystem", true, Marshal(&msg))
  	if err == nil && !ok {
  		err = errors.New("ssh: subsystem request failed")
  	}
  	return err
  }
docs: // RequestSubsystem requests the association of a subsystem with the session on the remote host.
  // A subsystem is a predefined command that runs in the background when the ssh session is initiated