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<H1><A NAME="SECTION00850000000000000000">Booting in single user mode</A></H1>
<P>
<A NAME="secsingleusermode"> </A>
<P>
An important run level is <b>single user mode</b> (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,<A NAME="tex2html38" HREF="footnode.html#2315"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> such as running <tt>fsck</tt><A NAME="2475"> </A> on a <tt>/usr</tt><A NAME="2477"> </A>
partition--this requires that the partition be unmounted,
and that can't happen, unless just about all system services
are killed.
<P>
A running system can be taken to single user mode by using
<tt>telinit</tt><A NAME="2479"> </A> to request run level 1. At bootup, it can be
entered by giving the word <tt>single</tt> or <tt>emergency</tt>
on the kernel command line: the kernel gives the command line
to <tt>init</tt><A NAME="2481"> </A> as well, and <tt>init</tt><A NAME="2483"> </A> 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>
Booting into single user mode is sometimes necessary so that
one can run <tt>fsck</tt><A NAME="2485"> </A> by hand, before anything mounts or
otherwise touches a broken <tt>/usr</tt><A NAME="2487"> </A> partition (any activity
on a broken filesystem is likely to break it more, so <tt>fsck</tt><A NAME="2489"> </A>
should be run as soon as possible).
<P>
The bootup scripts <tt>init</tt><A NAME="2491"> </A> runs will automatically enter single
user mode, if the automatic <tt>fsck</tt><A NAME="2493"> </A> at bootup fails. This is an
attempt to prevent the system from using a filesystem that is so
broken that <tt>fsck</tt><A NAME="2495"> </A> 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>
As a security measure, a properly configured system will ask
for the <tt>root</tt> 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 <tt>root</tt>. (This will
break if <tt>/etc/passwd</tt><A NAME="2497"> </A> has been broken by filesystem problems,
of course, and in that case you'd better have a boot floppy handy.)
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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