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An important run level is single user
mode (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 fsck on a
/usr partition, since this requires that
the partition be unmounted, and that can't happen, unless just
about all system services are killed.
A running system can be taken to single user mode by using
telinit to request run level 1. At bootup,
it can be entered by giving the word
emergency on the kernel command line: the
kernel gives the command line to init as well,
and init 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.)
Booting into single user mode is sometimes necessary so
that one can run fsck by hand, before anything
mounts or otherwise touches a broken
partition (any activity on a broken filesystem is likely to
break it more, so fsck should be run as soon
The bootup scripts init runs will automatically enter single user mode, if the automatic fsck at bootup fails. This is an attempt to prevent the system from using a filesystem that is so broken that fsck 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.
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
/etc/passwd has been broken by filesystem
problems, of course, and in that case you'd better have a boot
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