说明：  在实际操作中会遇到会话不能关闭，但是长时间不操作会话会关闭的问题，比如通过终端会话导入数据库数据，但是如果时间一长会话就关闭了，导致数据库导入失败，这是很危险的事情。screen可以保持会话，而且能唤醒会话，保证会话一直存在
(In the actual operation will encounter session cannot be closed, but not for a long time operating session will close, such as through the terminal session data into the database, but for a long time if the session is closed, resulting into the database failure, this is a very dangerous thing. Screen can keep conversations, and can wake up sessions and make sure the session is always there)