Reboot/halt/shutdown command on CentOS 7

2004

　　

　　和Windows相比，Linux提供了多种关机、重启的命令，包括reboot/halt/shutdown等，而如果在服务器上，BMC还会提供ipmitoolchassis power reset/ipmitool  chassis power cycle/ipmitool chassispower off等命令，那么面对这么多可行的命令，该如何选择呢？
　　由于Linux内核和OS版本众多，不同的OS和内核版本上的reboot/halt/shutdown等命令实现方式可能会有差异，这就需要基于具体的OS和内核进行分析。下面基于CentOS7，分享一下认识reboot/halt/shutdown差异的过程。

　　

　　1.确定是否都是链接到同一个命令
　　通过下面的命令可以看到，在CentOS7上所有的reboot命令都链接到systemctl：

[root@localhostetc]# which shutdown

　　/usr/sbin/shutdown

[root@localhostetc]# which poweroff

　　/usr/sbin/poweroff

[root@localhostetc]# which reboot

　　/usr/sbin/reboot

[root@localhostetc]# cd /usr/sbin/

[root@localhostsbin]# ls -alrt shutdown

　　lrwxrwxrwx.1 root root 16 Apr  5 13:07 shutdown -> ../bin/systemctl

[root@localhostsbin]# ls -alrt poweroff

　　lrwxrwxrwx.1 root root 16 Apr  5 13:07 poweroff -> ../bin/systemctl

[root@localhostsbin]# ls -alrt reboot

　　lrwxrwxrwx.1 root root 16 Apr  5 13:07 reboot -> ../bin/systemctl

[root@localhostsbin]# which systemctl

　　/usr/bin/systemctl
　　

　　

　　2.认识怎样关机保证不造成数据丢失
　　除了要求能按照期望实现开关机、重启之外，在服务器尤其是存储服务器的工作环境下，用户对关机和重启能否造成数据丢失也特别关心。为此，弄清楚哪种使用方式不会造成数据丢失很有意义。还是以CentOS为例，根据systemctl的说明，reboot、halt与poweroff一样，只要不带–force--force后缀，就不会造成数据丢失。下面是关于这些命令的具体说明：
　　

　　halt
　　Shut downand halt the system. This is mostly equivalent to start halt.target--irreversible, but also prints a wall message to all users. Ifcombined with --force, shutdown of all running services is skipped, however all processes are killed and all file systems are unmountedor mounted read-only, immediately followed by the system halt. If--force is specified twice, the operation is immediately executedwithout terminating any processes or unmounting any file systems.This may result in data loss.
　　

　　poweroff
　　Shut downand power-off the system. This is mostly equivalent to startpoweroff.target --irreversible, but also prints a wall message to allusers. If combined with --force, shutdown of all running services isskipped, however all processes are killed and all file systems areunmounted or mounted read-only, immediately followed by the poweringoff. If --force is specified twice, the operation is immediatelyexecuted without terminating any processes or unmounting any filesystems. This may result in data loss.
　　

　　reboot[arg]
　　Shut downand reboot the system. This is mostly equivalent to startreboot.target --irreversible, but also rints a wall message to allusers. If combined with --force, shutdown of all running services isskipped, however all processes are killed and all file systems areunmounted or mounted read-only, immediately followed by the reboot.If --force is specified twice, the operation is immediately executedwithout terminating any processes or unmounting any file systems.This may result in data loss.
　　

　　Shutdown
　　如果后面什么参数都不加，效果和shutdown-P +1相同，即先调用wall向所有登录的用户发出即将关机的信息，然后组织新的用户登录，接着等待60s，最后执行poweroff。因此，
　　区别于halt/poweroff,shutdown更安全，在不带任何参数的情况下，shutdown本质是delayedshutdown，当前我们的CentOS上默认是延迟60s后关机，具体请参考下面的命令和日志文件：

[root@localhost~]# cat /var/log/messages | tail -n 2

　　Apr28 22:50:30 localhost systemd: Started Session 195 of user root.
　　Apr28 22:50:30 localhost systemd-logind: New session 195 of user root.

[root@localhost~]# date

　　ThuApr 28 22:51:25 CST 2016

[root@localhost~]# shutdown -k

　　Shutdownscheduled for Thu 2016-04-28 22:52:30 CST, use 'shutdown -c' tocancel.

[root@localhost~]#

　　Broadcastmessage from root@localhost.localdomain (Thu 2016-04-28 22:51:30CST):
　　

　　Thesystem is going down for power-off at Thu 2016-04-28 22:52:30 CST!
　　

　　

[root@localhost~]# cat /var/log/messages | tail -n 4

　　Apr28 22:51:30 localhost systemd: Starting Delayed Shutdown Service...
　　Apr28 22:51:30 localhost systemd: Started Delayed Shutdown Service.
　　Apr28 22:51:30 localhost systemd-shutdownd: Shutting down at Thu2016-04-28 22:52:30 CST (poweroff)...
　　Apr28 22:51:30 localhost systemd-shutdownd: Creating /run/nologin,blocking further logins...
　　

　　关于shutdown和poweroff/halt/reboot的关系可参考下面的说明：
　　SYNOPSIS
　　shutdown[OPTIONS...] [TIME] [WALL...]
　　

　　DESCRIPTION
　　shutdown maybe used to halt, power-off or reboot the machine.
　　

　　The firstargument may be a time string (which is usually "now").Optionally, this may be followed by a wall
　　message to besent to all logged-in users before going down.
　　

　　The timestring may either be in the format "hh:mm" for hour/minutesspecifying the time to execute the
　　shutdown at,specified in 24h clock format. Alternatively it may be in the syntax"+m" referring to the
　　specifiednumber of minutes m from now.  "now" is an alias for "+0",i.e. for triggering an immediate
　　shutdown. Ifno time argument is specified, "+1" is implied.
　　

　　Note that tospecify a wall message you must specify a time argument, too.
　　

　　If the timeargument is used, 5 minutes before the system goes down the/run/nologin file is created to ensure
　　that furtherlogins shall not be allowed.
　　

　　OPTIONS
　　The followingoptions are understood:
　　

　　--help
　　Print ashort help text and exit.
　　

　　-H, --halt
　　Halt themachine.
　　

　　-P,--poweroff
　　Power-offthe machine (the default).
　　

　　-r, --reboot
　　Reboot themachine.
　　

　　-h
　　Equivalentto --poweroff, unless --halt is specified.
　　

　　-k
　　Do nothalt, power-off, reboot, just write wall message.
　　

　　--no-wall
　　Do not sendwall message before halt, power-off, reboot.
　　

　　-c
　　Cancel apending shutdown. This may be used cancel the effect of an invocationof shutdown with a time
　　argumentthat is not "+0" or "now".
　　

　　3.利用IPMI命令开关机
　　如果机器支持BMC，可以通过BMC使用ipmitool去开关机。这种情况下,BMC直接关掉给host系统的DC电源，如果是chassispower reset/chassis power cycle还会间隔一会重新上电，因此在host系统上运行的应用程序和IO数据都来不及killed掉并同步到硬盘，这就会造成数据丢失。当是它的好处是：快捷，并且可以基于BMCLan 接口去远程开关、重启机器。
　　这些命令包括：
　　ipmitoolchassis power off ＃通过BMC关必DC
　　ipmitoolchassis power on # 通过BMC给DC上电
　　ipmitoolchassis power cycle # 先把DC掉店，间隔约1s后给DC上电
　　ipmitoolchassis power reset # 先把DC掉店，然后给DC上电