linux下的access()函数判断文件是否存在、打印时间:https://www.iteye.com/blog/hualang-1152055
C shell :https://blog.csdn.net/lu_embedded/article/details/78669939
open():https://www.cnblogs.com/fengbohello/p/4262961.html
http://c.biancheng.net/cpp/html/238.html
延时:https://blog.csdn.net/fireroll/article/details/8651593