322 lines
6.7 KiB
HTML
322 lines
6.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>More about shutdowns</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux System Administrators Guide"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Boots And Shutdowns"
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HREF="boots-and-shutdowns.html"><LINK
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REL="PREVIOUS"
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TITLE="The boot process in closer look"
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HREF="boot-process.html"><LINK
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REL="NEXT"
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TITLE="Rebooting"
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HREF="rebooting.html"></HEAD
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CLASS="SECT1"
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BGCOLOR="#FFFFFF"
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><DIV
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SUMMARY="Header navigation table"
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><TR
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><TH
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COLSPAN="3"
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>Linux System Administrators Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="boot-process.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 8. Boots And Shutdowns</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="rebooting.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="SHUTDOWN"
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></A
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>8.3. More about shutdowns</H1
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><P
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>It is important to follow the correct procedures when you shut
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down a Linux system. If you fail do so, your filesystems probably
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will become trashed and the files probably will become scrambled.
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This is because Linux has a disk cache that won't write things
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to disk at once, but only at intervals. This greatly improves
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performance but also means that if you just turn off the power
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at a whim the cache may hold a lot of data and that what is on
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the disk may not be a fully working filesystem (because only
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some things have been written to the disk).</P
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><P
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>Another reason against just flipping the power switch is that
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in a multi-tasking system there can be lots of things going on
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in the background, and shutting the power can be quite
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disastrous. By using the proper shutdown sequence, you ensure
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that all background processes can save their data.</P
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><P
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>The command for properly shutting down a Linux system
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is <B
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CLASS="COMMAND"
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>shutdown</B
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>. It is usually used in one of
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two ways.</P
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><P
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>If you are running a system where you are the only user,
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the usual way of using <B
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CLASS="COMMAND"
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>shutdown</B
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> is to quit
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all running programs, log out on all virtual consoles, log
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in as root on one of them (or stay logged in as root if you
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already are, but you should change to root's home directory or
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the root directory, to avoid problems with unmounting), then
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give the command <B
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CLASS="COMMAND"
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>shutdown -h now</B
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> (substitute
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<TT
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CLASS="LITERAL"
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>now</TT
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> with a plus sign and a number in minutes
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if you want a delay, though you usually don't on a single user
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system).</P
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><P
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>Alternatively, if your system has many users, use the command
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<B
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CLASS="COMMAND"
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>shutdown -h +time message</B
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>, where
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<TT
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CLASS="LITERAL"
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>time</TT
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>
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is the
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time in minutes until the system is halted, and
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<TT
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CLASS="LITERAL"
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>message</TT
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>
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is a short explanation of why the system is shutting down.
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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><TT
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CLASS="PROMPT"
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>#</TT
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> <TT
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CLASS="USERINPUT"
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><B
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>shutdown -h +10 'We will install a new
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disk. System should
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> be back on-line in three hours.'</B
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></TT
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>
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<TT
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CLASS="PROMPT"
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>#</TT
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></PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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This will warn everybody that the system will shut down in
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ten minutes, and that they'd better get lost or lose data.
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The warning is printed to every terminal on which someone is
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logged in, including all <B
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CLASS="COMMAND"
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>xterm</B
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>s:
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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><TT
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CLASS="COMPUTEROUTPUT"
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>Broadcast message from root (ttyp0) Wed Aug 2 01:03:25 1995...
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We will install a new disk. System should
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be back on-line in three hours.
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The system is going DOWN for system halt in 10 minutes !!</TT
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></PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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The warning is automatically repeated a few times before the boot,
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with shorter and shorter intervals as the time runs out.</P
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><P
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>When the real shutting down starts after any delays, all
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filesystems (except the root one) are unmounted, user processes
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(if anybody is still logged in) are killed, daemons are shut down,
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all filesystem are unmounted, and generally everything settles
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down. When that is done, <B
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CLASS="COMMAND"
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>init</B
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> prints out a
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message that you can power down the machine. Then, and only then,
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should you move your fingers towards the power switch.</P
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><P
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>Sometimes, although rarely on any good system, it is
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impossible to shut down properly. For instance, if the kernel
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panics and crashes and burns and generally misbehaves, it might
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be completely impossible to give any new commands, hence shutting
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down properly is somewhat difficult, and just about everything
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you can do is hope that nothing has been too severely damaged
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and turn off the power. If the troubles are a bit less severe
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(say, somebody hit your keyboard with an axe), and the kernel
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and the <B
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CLASS="COMMAND"
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>update</B
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> program still run normally,
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it is probably a good idea to wait a couple of minutes to give
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<B
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CLASS="COMMAND"
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>update</B
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> a chance to flush the buffer cache,
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and only cut the power after that.</P
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><P
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>In the old days, some people like to shut down using the command
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<B
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CLASS="COMMAND"
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>sync</B
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> three times, waiting for the disk I/O to stop,
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then turn off the power. If there are no running programs, this is
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equivalent to using <B
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CLASS="COMMAND"
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>shutdown</B
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>. However, it
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does not unmount any filesystems and this can lead to problems
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with the ext2fs ``clean filesystem'' flag. The triple-sync
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method is <EM
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>not recommended</EM
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>.</P
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><P
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>(In case you're wondering: the reason for three syncs is
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that in the early days of UNIX, when the commands were
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typed separately, that usually gave sufficient time for most
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disk I/O to be finished.)
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="boot-process.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="rebooting.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
|
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><TR
|
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><TD
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WIDTH="33%"
|
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ALIGN="left"
|
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VALIGN="top"
|
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>The boot process in closer look</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="boots-and-shutdowns.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
|
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>Rebooting</TD
|
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></TR
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></DIV
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> |