239 lines
4.2 KiB
HTML
239 lines
4.2 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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>Booting in single user mode</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="init"
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HREF="init-intro.html"><LINK
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REL="PREVIOUS"
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TITLE="Special configuration in
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/etc/inittab"
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HREF="inittab.html"><LINK
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REL="NEXT"
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TITLE="Logging In And Out"
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HREF="log-in-and-out.html"></HEAD
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><TR
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>Linux System Administrators Guide: </TH
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WIDTH="10%"
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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 9. <B
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CLASS="COMMAND"
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>init</B
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></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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HREF="log-in-and-out.html"
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>Next</A
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></TD
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></TR
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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="BOOT-SINGLE-USER"
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></A
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>9.5. Booting in single user mode</H1
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><P
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>An important run level is <I
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CLASS="GLOSSTERM"
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>single user
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mode</I
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> (run level 1),
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in which only the system administrator is using the machine
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and as few system services, including logins, as possible are
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running. Single user mode is necessary for a few administrative
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tasks, such as running <B
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CLASS="COMMAND"
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>fsck</B
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> on a
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<TT
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CLASS="FILENAME"
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>/usr</TT
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> partition, since this requires that
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the partition be unmounted, and that can't happen, unless just
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about all system services are killed.</P
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><P
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>A running system can be taken to single user mode by using
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<B
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CLASS="COMMAND"
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>telinit</B
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> to request run level 1. At bootup,
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it can be entered by giving the word <TT
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CLASS="LITERAL"
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>single</TT
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>
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or <TT
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CLASS="LITERAL"
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>emergency</TT
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> on the kernel command line: the
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kernel gives the command line to <B
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CLASS="COMMAND"
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>init</B
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> as well,
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and <B
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CLASS="COMMAND"
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>init</B
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> understands from that word that it
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shouldn't use the default run level. (The kernel command line is
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entered in a way that depends on how you boot the system.)</P
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><P
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>Booting into single user mode is sometimes necessary so
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that one can run <B
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CLASS="COMMAND"
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>fsck</B
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> by hand, before anything
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mounts or otherwise touches a broken <TT
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CLASS="FILENAME"
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>/usr</TT
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>
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partition (any activity on a broken filesystem is likely to
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break it more, so <B
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CLASS="COMMAND"
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>fsck</B
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> should be run as soon
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as possible).</P
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><P
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>The bootup scripts <B
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CLASS="COMMAND"
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>init</B
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> runs
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will automatically enter single user mode, if the automatic
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<B
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CLASS="COMMAND"
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>fsck</B
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> at bootup fails. This is an attempt to
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prevent the system from using a filesystem that is so broken that
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<B
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CLASS="COMMAND"
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>fsck</B
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> can't fix it automatically. Such breakage
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is relatively rare, and usually involves a broken hard disk or an
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experimental kernel release, but it's good to be prepared.</P
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><P
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>As a security measure, a properly configured system
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will ask for the root password before starting the shell in
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single user mode. Otherwise, it would be simple to just enter
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a suitable line to LILO to get in as root. (This will break if
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<TT
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CLASS="FILENAME"
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>/etc/passwd</TT
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> has been broken by filesystem
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problems, of course, and in that case you'd better have a boot
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floppy handy.)</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="inittab.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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>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="log-in-and-out.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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>Special configuration in
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<TT
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CLASS="FILENAME"
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>/etc/inittab</TT
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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="init-intro.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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>Logging In And Out</TD
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></TR
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></TABLE
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> |