Slackware Linux Essentials
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12.3 Shutting Down Properly

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It is very important that you shut down your system properly. Simply turning the power -off with the power switch can cause serious filesystem damage. While the system is on, -files are in use even if you aren't doing anything. Remember that there are many -processes running in the background all the time. These processes are managing the system -and keep a lot of files open. When the system's power is switched off, these files are -not closed properly and may become corrupted. Depending on what files become damaged, the -system might be rendered completely unusable! In any case, you'll have to go through a -long filesystem check procedure on the next reboot.

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If you configured your system with a journalling filesystem, like ext3 or reiserfs, -you'll be partially protected from filesystem damage, and your filesystem check on reboot -will be shorter than if you had used a filesystem without journalling, like ext2. -However, this safety net is no excuse for improperly shutting down your system! A -journalling FS is meant to protect your files from events beyond your control, not from -your own laziness.

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In any case, when you want to reboot or power down your computer, it is important to -do so properly. There are several ways of doing so; you can pick whichever one you think -is the most fun (or least amount of work). Since a shutdown and a reboot are similar -procedures, most of the ways for powering off the system can also be applied to -rebooting.

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The first method is through the shutdown(8) program, and it -is probably the most popular. shutdown can be used to reboot or -turn off the system at a given time, and can display a message to all the logged-in users -of the system telling them that the system is going down.

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The most basic use of shutdown to power down the computer is:

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-# shutdown -h now
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In this case, we are not going to send a custom message to the users; they will see -shutdown's default message. “now” is the time that we want to shutdown, and the “-h” means to halt the system. This is not a very friendly way -to run a multi-user system, but it works just fine on your home computer. A better method -on a multiuser system would be to give everyone a little advance warning:

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-# shutdown -h +60
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This would shutdown the system in one hour (60 minutes), which would be just fine on a -normal multiuser system. Vital systems should have their downtime scheduled far in -advance, and you should post warnings about the downtime in any appropriate locations -used for system notifications (email, bulletin board, /etc/motd, whatever).

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Rebooting the system uses the same command, but substitutes “-r” for “-h”:

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-# shutdown -r now
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You can use same time notation with shutdown -r that you -could with shutdown -h. There are a lot of other things that you -can do with shutdown to control when to halt or reboot the -machine; see the man page for more details.

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The second way of shutting down or powering off the computer is to use the halt(8) and reboot(8) commands. As the -names indicate, halt will immediately halt the operating system, -and reboot will reboot the system. (reboot is actually just a symbolic link to halt.) They are invoked like so:

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-# halt
-# reboot
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A lower-level way to reboot or shutdown the system is to talk directly to init. All the other methods are simply convenient ways to talk to -init, but you can directly tell it what to do using telinit(8) (note that it only has one “l”). Using telinit will tell init what runlevel to -drop into, which will cause a special script to be run. This script will kill or spawn -processes as needed for that runlevel. This works for rebooting and shutting down because -both of those are special runlevels.

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-# telinit 0
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Runlevel 0 is halt mode. Telling init to enter runlevel 0 -will cause all processes to be killed off, the filesystems unmounted, and the machine to -be halted. This is a perfectly acceptable way to bring down the system. On many laptops -and modern desktop computers, this will also cause the machine to be turned off.

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-# telinit 6
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Runlevel 6 is reboot mode. All processes will be killed off, the filesystems will be -unmounted, and the machine will be rebooted. This is a perfectly acceptable method of -rebooting the system.

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For the curious, when switching to runlevel 0 or 6, whether by using shutdown, halt, or reboot, the script /etc/rc.d/rc.6 is run. -(The script /etc/rc.d/rc.0 is another symbolic link, to /etc/rc.d/rc.6.) You can customize this file to your tastes--but be -sure to test your changes carefully!

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There is one last method of rebooting the system. All the other methods require you to -be logged in as root. However, it is possible to reboot the -machine even if you aren't root, provided that you have physical access to the keyboard. -Using Control+Alt+Delete (the "three-fingered salute") will cause the machine to -immediately reboot. (Behind the scenes, the shutdown command is -called for you when you use Control+Alt+Delete.) The salute doesn't always work when using X Windows--you may -need to use Control+Alt+F1 (or another Function key) to switch to a non-X Windows terminal -before using it.

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Finally, the file that ultimately controls every aspect of startup and shutdown is the -/etc/inittab(5) file. In general, you should not need to modify -this file, but it may give you insight into why some things work the way they do. As -always, see the man pages for further details.

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