Shutting down the linux system

Shutting Down the Linux System

If you look at the scripts in runlevel 0, you'll find a number of services being shut down, followed by the killing of all active processes, and finally, the halt script in /etc/rc.d/init.d directory executing the shutdown.

The halt script is used to either halt or reboot your system, depending on how it is called. What happens during a shutdown? If you're familiar with other operating systems (such as DOS), you remember that all you had to do was close any active application and then turn off the computer. Although Linux is easy to use, shutting down your computer is not as simple as turning it off. (You can try this, but you do so at your own risk.) A number of processes must take place before you or Linux turns off your computer. The following sections take a look at some of the commands involved.

shutdown
Although many people use Linux as single users on a single computer, many of us use computers on either a distributed or shared network. If you've ever been working under a tight deadline in a networked environment, you know the dreadful experience of seeing a System is going down in 5 minutes! message from the system administrator. You might also know the frustration of working on a system on which the system administrator is trying to perform maintenance, suffering seemingly random downtimes or frozen tasks.

Luckily for most users, maintenance jobs are performed during off hours, when most people are home with their loved ones or fast asleep in bed. Unluckily for sysadmins, this is the perfect time for system administration or backups, and one of the top reasons for the alt.sysadmin.recovery newsgroup.

The primary command to stop Linux is the shutdown command. Like most UNIX commands, shutdown has a number of options. A man page for the shutdown command is included with Red Hat Linux, but you can quickly read its command-line syntax if you use an illegal option, such as -z. Thanks to the programmer, here it is:

Usage: shutdown [-akrhfnc] [-t secs] time [warning message]
-a: use /etc/shutdown.allow
-k: don't really shutdown, only warn.
-r: reboot after shutdown.
-h: halt after shutdown.
-f: do a 'fast'reboot (skip fsck).
-F: Force fsck on reboot.
-n: do not go through "init" but go down real fast.
-c: cancel a running shutdown.
-t secs: delay between warning and kill signal.
** the "time" argument is mandatory! (try "now") **
To properly shut down your system immediately, use the -h option, followed by the word now or the numeral 0:


# shutdown -h now

or


# shutdown -h 0

If you want to wait for a while, use the -t option, followed by a number (in seconds) before shutdown or reboot. If you want to restart your computer, use the -r option, along with the word now or the numeral 0:


# shutdown -r now

or


# shutdown -r 0

NOTE

You'll find two curious text strings embedded in the shutdown program:

"You don't exist. Go away."
"Oh hello Mr. Tyler - going DOWN?"
Both are found by executing this:


# strings /sbin/shutdown


To find out about "You don't exist. Go away", see Robert Kiesling's Linux Frequently Asked Questions with Answers. You should be able to find a copy at http://www.linuxdoc.org/FAQ.

You can also use linuxconf to shut down your computer. If you're logged in as the root operator, enter the following from the command line of your console or an X11 terminal window:


# linuxconf --shutdown

linuxconf presents a shutdown dialog box, as shown in Figure 9.4. To restart your system, press the Tab key until you highlight the Accept button, and then press the Enter key. You can also enter a time delay or halt your system immediately, and specify a message to broadcast to all your users when you execute the shutdown.


Figure 9.4 The linuxconf command will perform a system reboot or shutdown.

halt and reboot
Two other commands also stop or restart your system: halt and reboot. reboot is a symbolic link to halt, which notifies the kernel of a shutdown or reboot. Although you should always use shutdown to restart your system, you can use the "Vulcan neck pinch": Ctrl+Alt+Del.

If you use the keyboard form of this command, you'll find that Linux uses the following command:


# shutdown -t3 -r now

NOTE

This command is defined in your system's initialization table, /etc/inittab.

Restarting your computer with the shutdown command calls the sync command, which updates the inodes (structure representations)of each of your files. If you exit Linux without updating this information, Linux could lose track of your files on disk, and that spells disaster!

NOTE

The only time you'll want to risk shutting down Linux through a hard reset or the power-off switch on your computer is if you can't quickly kill a destructive process, such as an accidental rm -fr /*. Yet another reason to never run Linux as the root operator all the time!

By now you should know that exiting Linux properly can help you avoid problems with your system. What happens if something goes wrong? In the next section you learn preventive measures, how to maintain your filesystem, and how to recover and overcome problems.