shutdown命令安全地将系统关机。 有些用户会使用直接断掉电源的方式来关闭linux，这是十分危险的。因为linux与windows不同，其后台运行着许多进程，所以强制关机可能会导致进程的数据丢失﹐使系统处于不稳定的状态﹐甚至在有的系统中会损坏硬件设备。可用man shutdown来了解命令的详细使用。
SHUTDOWN(8) shutdown SHUTDOWN(8) NAME shutdown - Halt, power-off or reboot the machine SYNOPSIS shutdown [OPTIONS...] [TIME] [WALL...] DESCRIPTION shutdown may be used to halt, power-off or reboot the machine. The first argument may be a time string (which is usually "now"). Optionally, this may be followed by a wall message to be sent to all logged-in users before going down. The time string may either be in the format "hh:mm" for hour/minutes specifying the time to execute the shutdown at, specified in 24h clock format. Alternatively it may be in the syntax "+m" referring to the specified number of minutes m from now. "now" is an alias for "+0", i.e. for triggering an immediate shutdown. If no time argument is specified, "+1" is implied. Note that to specify a wall message you must specify a time argument, too. If the time argument is used, 5 minutes before the system goes down the /run/nologin file is created to ensure that further logins shall not be allowed. OPTIONS The following options are understood: --help Print a short help text and exit. -H, --halt Halt the machine. -P, --poweroff Power-off the machine (the default). -r, --reboot Reboot the machine. -h Equivalent to --poweroff, unless --halt is specified. -k Do not halt, power-off, reboot, just write wall message. --no-wall Do not send wall message before halt, power-off, reboot. -c Cancel a pending shutdown. This may be used cancel the effect of an invocation of shutdown with a time argument that is not "+0" or "now". EXIT STATUS On success, 0 is returned, a non-zero failure code otherwise. SEE ALSO systemd(1), systemctl(1), halt(8), wall(1) systemd 229 SHUTDOWN(8)
从man page可知道：shutdown被用来暂停、关闭、重启机器。shutdown后面有3个参数，都是有[]，说明都是可选参数。在DESCRIPTION里了解到，如果未指定TIME参数，那就意味着默认配置“+1”，单位为分钟。如果TIME参数为now，那就意味着时间配置为“+0”，单位为分钟。时间也可设置为"hh:mm" = hour/minutes进行操作。
命令参数说明：
--help：帮助 -H/--half：关闭机器 -P/--poweroff：关闭机器 -r/--reboot：重启机器 -h：与--poweroff一样 -k：不暂停、不关闭、不重启机器，只发送消息给在线的用户 --no-wall：暂停、关闭、重启机器前，不发送消息给在线的用户 -c：取消shutdown命令操作
实例：
特定时间执行关机命令
shutdown –h now //立即关机 shutdown –h 12:00 //在12:00关机
指定五分钟后关机，并发出警告信息
shutdown +5 “This System will be shutdown in 5 minutes” Broadcast message from root@CentOS6.8(/dev/pts/0) at 13:33 … The system is going down for maintenance in 5 minutes! //系统提醒 This System will be shutdown in 5 minutes //用户自定义提醒
指定3分钟后重启，并发出警告信息
shutdown –r +3 “3分钟后关机重启” Broadcast message from root@CentOS6.8(/dev/pts/0) at 13:53 … The system is going down for reboot in 3 minutes! //系统提醒 3分钟后关机重启 //用户自定义提醒
给所有登录用户发送提醒，不关闭重启。
[root@CentOS6 桌面]# shutdown –k 5 “Waring:maybe the system will be shutdown” [root@CentOS6 桌面]# Broadcast message from root@CentOS6.8(/dev/pts/0) at 14:26 … The system is going down for maintenance in 5 minutes! Waring:maybe the system will be shutdown