shutdown -h now 关闭系统(1)
init 0 关闭系统(2)
telinit 0 关闭系统(3)

shutdown -h hours:minutes & 按预定时间关闭系统
shutdown -c 取消按预定时间关闭系统

shutdown -r now 重启(1)
reboot 重启(2)

logout 注销
time 测算一个命令（即程序）的执行时间