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5.5. Shutting Down the System

Fortunately, shutting down the Linux system is much simpler than booting and startup. However, it's not just a matter of hitting the reset switch. Linux, like all Unix systems, buffers disk reads and writes in memory. This means disk writes are delayed until absolutely necessary, and multiple reads on the same disk block are served directly from RAM. This greatly increases performance as disks are extremely slow relative to the CPU.

The problem is that if the system were to be suddenly powered down or rebooted, the buffers in memory would not be written to disk, and data could be lost or corrupted. /sbin/update is a program started from /etc/rc.d/rc.sysinit on most systems; it flushes dirty buffers (ones that have been changed since they were read from the disk) back to disk every five seconds to prevent serious damage from occurring should the system crash. However, to be completely safe, the system needs to undergo a "safe" shutdown before rebooting. This will not only ensure that disk buffers are properly synchronized, but also allow all running processes to exit cleanly.

shutdown is the general, all-purpose command used to halt or reboot the system. As root, you can issue the command:

/sbin/shutdown -r +10
to cause the system to reboot in ten minutes. The -r switch indicates the system should be rebooted after shutdown, and +10 is the amount of time to wait (in minutes) until shutting down. The system will print a warning message to all active terminals, counting down until the shutdown time. You can add your own warning message by including it on the command line, as in:
/sbin/shutdown -r +10 "Rebooting to try new kernel"

You can also specify an absolute time to shutdown, as in:

/sbin/shutdown -r 13:00
to reboot at 1:00 pm. Likewise, you can say:
/sbin/shutdown -r now
to reboot immediately (after the safe shutdown process).

Using the -h switch, instead of -r, will cause the system to simply be halted after shutdown; you can then turn off the system power without fear of losing data. If you specify neither -h nor -r, the system will go into single-user mode.

As we saw in the section "Section 5.3.2, "init, inittab, and rc files"," you can have init catch the Ctrl-Alt-Del key sequence and execute a shutdown command in response to it. If you're used to rebooting your system in this way it might be good idea to check that your /etc/inittab contains a ctrlaltdel entry. Note that you should never reboot your Linux system by pressing the system power switch or the reboot switch on the front panel of your machine. Unless the system is flat-out hung (a rare occurrence), you should always use shutdown. The great thing about a multiprocessing system is that one program may hang, but you can almost always switch to another window or virtual console to recover.

shutdown provides a number of other options. The -c switch will cancel a currently running shutdown. (Of course, you can kill the process by hand using kill, but shutdown -c might be easier.) The -k switch will print the warning messages but not actually shut down the system. See the manual page for shutdown if you're interested in the gory details.

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