Linux shutdown command details

In Linux, some common shutdown/restart Commands include shutdown, halt, reboot, and init. They can all restart the system, but the internal working process of each command is different.

I. Common commands

Restart command
1. Reboot
2. shutdown-R now restarts immediately (used by the root user)
3. shutdown-r restarts automatically after 10 minutes (used by root users)
4. shutdown-r restart at (for root users)
If you use the shutdown command to set the restart, you can use the shutdown-C command to cancel the restart.

 

Shutdown command
1. Shut down halt immediately
2. Power off immediately
3. shutdown-H now shut down immediately (used by the root user)
4. shutdown-H: shutdown automatically 10 minutes later
If you use the shutdown command to set shutdown, you can use the shutdown-C command to cancel the restart.

 

Ii. Command details

1 Shutdown
The shutdown command safely shuts down the system. It is very dangerous for some users to disable Linux by directly disconnecting the power supply. Because Linux and Windows run many processes in the background, forced shutdown may lead to data loss of the process, make the system unstable, or even damage hardware in some systems. When the shutdown command is used before the system is shut down, the system administrator will notify all logged-on users that the system will be shut down. The LOGIN command is frozen, meaning that new users cannot log on again. It is possible to directly shut down or wait for a certain period of time to shut down, or restart. This is determined by the signal that all processes receive from the system. This gives programs like Vi time to store the files being edited, while programs like mail and news can leave normally.
Shutdown is to send a signal to the INIT program and require it to change the runlevel. Runlevel 0 is used to stop halt, runlevel 6 is used to re-activate (reboot) the system, and runlevel 1 is used to bring the system into a State that can be done in management; this is the default, assume that no-H or-R parameter is set to shutdown. You can view runlevels related information in the/etc/inittab file if you want to know what actions are taken during the halt or reboot process.

Shutdown parameter description:
[-T] before changing to another runlevel, tell init how long it will take to shut down
[-R] restart Calculator
[-K] does not actually shut down, but sends a warning signal to each login)
[-H] power off after Shutdown (halt)
[-N] shut down by yourself instead of init. You are not encouraged to use this option, and the consequences of this option are often not always pre-expected.
[-C] cancel current process cancels the currently running Shutdown program. Therefore, this option certainly does not have a time parameter, but you can enter a message for explanation, and this information 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

2 halt (simplest shutdown command)
In fact, halt calls shutdown-H. When halt is executed, the application process is killed and the sync system call is executed. After the file system write operation is completed, the kernel is stopped.
Parameter description:
[-N] prevents the sync system from calling. It is used after the root partition is repaired with fsck to prevent the kernel from overwriting the repaired super block with the old version of superblock.

[-W] does not actually restart or shut down, but only writes the wtmp (/var/log/wtmp) record.
[-D] Do not write the wtmp record (included in Option [-N)
[-F] Force shutdown or restart without calling Shutdown
[-I] disable all network interfaces before shutting down (or restarting ).
[-P] this option is the default option. Is to call poweroff when Shutdown

3 reboot
The reboot process is almost the same as that of halt, but it causes host restart while halt is shutdown. Its parameters are similar to those of halt.

4 init
Init is the ancestor of all processes, and its Process number is always 1. Therefore, sending a term signal to init will terminate all user processes and daemon processes. Shutdown uses this mechanism. Init defines eight runlevels. init 0 indicates shutdown, and init 1 indicates restart. I will not describe the init here. In addition, the telinit command can change the init running level. For example, telinit-Is can enable the system to enter the single-user mode, and the information and waiting time when shutdown is not available.

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