Use it flexibly to learn more about the shutdown commands in Linux systems

　Some commonly used shutdown/restart commands under Linux include shutdown, halt, reboot, and init. They can all achieve the purpose of restarting the system, but the internal working process of each command is different. Through the introduction of this article, I hope you can Use various shutdown commands more flexibly.

　1.shutdown

　The 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. If you use 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 the shutdown for a certain period of time, and it is also possible to 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.

　The job of shutdown is to send 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 put the system into a state where management work can be performed; this is the default, assuming there is no -h There is no -r parameter 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 process Cancel the shutdown process 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].

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