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() shutdown SHUTDOWN() 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, 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, is returned, a non-zero failure code otherwise. SEE ALSO
systemd(), systemctl(), halt(), wall() systemd SHUTDOWN()
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 : //在12:00关机
指定五分钟后关机，并发出警告信息
shutdown + “This System will be shutdown in minutes”
Broadcast message from root@CentOS6.(/dev/pts/) at : … The system is going down for maintenance in minutes! //系统提醒
This System will be shutdown in 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分钟后关机重启”
Broadcast message from root@CentOS6.(/dev/pts/) at : … The system is going down for reboot in 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 “Waring:maybe the system will be shutdown”
[root@CentOS6 桌面]#
Broadcast message from root@CentOS6.(/dev/pts/) at : … The system is going down for maintenance in minutes!
Waring:maybe the system will be shutdown