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