Linux关机命令详解
在Linux下一些常用的关机/重启命令有shutdown、halt、reboot、及init，它们都可以达到重启系统的目的，但每个命令的内部工作过程是不同的，通过本文的介绍，希望你可以更加灵活的运用各种关机命令。
1、shutdown
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。
而在系统关机前使用shutdown命令，系统管理员会通知所有登录的用户系统将要关闭。并且login指令会被冻结，即新的用户不能再登录。直接关机或者延迟一定的时间才关机都是可能的，还可能重启。这是由所有进程(process)都会收到系统所送达的信号(signal)决定的。这让像vi之类的程序有时间储存目前正在编辑的文档，而像处理邮件(mail)和新闻(news)的程序则可以正常地离开等等。
shutdown执行它的工作是送信号(signal)给init程序，要求它改变runlevel。
runlevel 0被用来停机(halt)，runlevel 6是用来重新激活(reboot)系统，而runlevel 1则是被用来让系统进入管理工作可以进行的状态﹔这是预设的，假定没有-h也没有-r参数给shutdown。要想了解在停机(halt)或者重新开机(reboot)过程中做了哪些动作，你可以在这个文件/etc/inittab里看到这些runlevels相关的资料。
shutdown 参数说明：
[-t] 在改变到其它runlevel之前，告诉init多久以后关机。
[-r] 重启计算器。
[-k] 并不真正关机，只是送警告信号给每位登录者(login)。
[-h] 关机后关闭电源(halt)。
[-n] 不用init，而是自己来关机。不鼓励使用这个选项，而且该选项所产生的后果往往不总是你所预期得到的。
[-c] cancel current process取消目前正在执行的关机程序。所以这个选项当然没有时间参数，但是可以输入一个用来解释的讯息，而这信息将会送到每位使用者。
[-f] 在重启计算器(reboot)时忽略fsck。
[-F] 在重启计算器(reboot)时强迫fsck。
2、halt——最简单的关机命令
其实halt就是调用shutdown -h。halt执行时，杀死应用进程，执行sync系统调用，文件系统写操作完成后就会停止内核。
参数说明:
[-n] 防止sync系统调用，它用在用fsck修补根分区之后，以阻止内核用老版本的超级块(superblock)覆盖修补过的超级块。
[-w] 并不是真正的重启或关机，只是写wtmp(/var/log/wtmp)纪录。
[-d] 不写wtmp纪录(已包含在选项[-n]中)。
[-f] 没有调用shutdown而强制关机或重启。
[-i] 关机(或重启)前，关掉所有的网络接口。
[-p] 该选项为缺省选项。就是关机时调用poweroff。
3、reboot
reboot的工作过程差不多跟halt一样，不过它是引发主机重启，而halt是关机。它的参数与halt相差不多。
4、init
init是所有进程的祖先，它的进程号始终为1，所以发送TERM信号给init会终止所有的用户进程﹑守护进程等。shutdown 就是使用这种机制。init定义了8个运行级别(runlevel)，init 0为关机，init 1为重启。关于init可以长篇大论，这里就不再叙述。另外还有telinit命令可以改变init的运行级别，比如，telinit -iS可使系统进入单用户模式，并且得不到使用shutdown时的信息和等待时间。
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