What is the centos shutdown command?

centos shutdown command: 1. "halt" command, you can shut down immediately; 2. "poweroff" command, you can shut down immediately; 3. "shutdown -h now" command, you can shut down immediately; 4. "shutdown" -h number" command, you can specify the minutes to automatically shut down; 5. "init 0".

#The operating environment of this tutorial: CentOS 6 system, Dell G3 computer.

Linux centos restart command:

• reboot
• ##shutdown -r now Immediately Restart (for root user)
• shutdown -r 10 Automatically restart after 10 minutes (for root user)
• shutdown -r 20:35 Restart when the time is 20:35 (for root users)

If the restart is set through the shutdown command, you can use the shutdown -c command to cancel the restart

Linux centos Shutdown command:

• halt Shut down immediately

• poweroff Shut down immediately

• shutdown -h now Shut down immediately (for root users)

• shutdown -h 10 Automatically shut down after 10 minutes

• init 0 Shutdown
  

If you set the shutdown through the shutdown command, you can use

shutdown - c command cancel

1. shutdown

shutdown command safely shuts down the system. Some users will shut down Linux by directly cutting off the power supply, which is very dangerous. Because Linux is different from Windows, there are many processes running in the background, so forced shutdown may cause the data of the process to be lost, put the system in an unstable state, and even damage the hardware equipment in some systems.

When using the shutdown command before shutting down the system, the system administrator will notify all logged-in users that the system will be shut down.

And the login command will be frozen, that is, new users can no longer log in. It is possible to shut down directly or delay shutting down for a certain period of time. It may also restart. This is determined by the fact that all processes will receive signals sent by the system. This gives programs like vi time to save the document currently being edited, and programs like mail and news can exit normally, etc.

shutdown performs its job by sending a signal [signal] to the init program, asking it to change the runlevel.

Runlevel 0 is used to shut down [halt], runlevel 6 is used to reactivate [reboot] the system,

and runlevel 1 is used to allow the system to enter management work. Status; This is the default, assuming neither -h nor -r parameters are given to shutdown. To understand what actions were taken during the shutdown (halt) or restart (reboot) process, you can see the runlevels-related information in this file /etc/inittab.

shutdown Parameter description:

　[-t] Tell init how long to shut down before changing to other runlevels.

　[-r] Restart the calculator.

　[-k] does not actually shut down, but only sends a warning signal to each login [login].

　[-h] Turn off the power after shutdown [halt].

　[-n] No need to init, but shut down by yourself. Using this option is discouraged, and the consequences of this option are often not always what you expect.

　[-c] cancel current processCancel the shutdown program currently being executed. So of course this option has no time parameter, but you can enter a message to explain it, and this message will be sent to each user.

　[-f] Ignore fsck when restarting the calculator [reboot].

　[-F] Force fsck when restarting the calculator [reboot].

　[-time] Set the time before shutdown [shutdown].

2. Halt—the simplest shutdown command

In fact, halt is to call shutdown -h. When halt is executed, the application process is killed, the sync system call is executed, and the kernel is stopped after the file system write operation is completed.

Parameter description:

　[-n] Prevent sync system calls. It is used after patching the root partition with fsck to prevent the kernel from overwriting the patched one with an old version of the superblock. Super block.

　[-w] is not a real restart or shutdown, it just writes wtmp [/var/log/wtmp] records.

　[-d] Do not write wtmp records [included in option [-n]].

　[-f] Force shutdown or restart without calling shutdown.

　[-i] Before shutting down (or restarting), turn off all network interfaces.

　[-p] This option is the default option. Just call poweroff when shutting down.

3. init

init is the ancestor of all processes. Its process number is always 1, so sending the TERM signal to init will terminate all user processes and daemons. Process etc. shutdown uses this mechanism.

init defines 8 runlevels (runlevel),

init 0 is shutdown, init 1 is restart.

I can talk about init in a long way, but I won’t go into it here. There are also telinit commands that can change the running level of init. For example, telinit -iS can make the system enter single-user mode, and the information and waiting time when using shutdown cannot be obtained.

Related recommendations: "Linux Video Tutorial"

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