Runlevel 6—This runlevel reboots the system.Runlevel 1 (also known as single-user mode or maintenance mode) is most
commonly used to repair file systems and change the root password on a
system when the password has been forgotten. Trespassers with physical
access to the machine can also use runlevel 1 to access your system.
CAUTION
Never forget that uncontrolled physical access is a virtual guarantee of
access to your data by an intruder.Booting into the Default Runlevel
Ubuntu boots into runlevel 5 by default, which means it starts the system as
normal and leaves you inside the X Window System, looking at the graphical
login prompt. It knows what runlevel 5 needs to load by looking in the
rc*.d directories in /etc. Ubuntu contains directories for rc0.d through
to rc5.d and rcS.d.
Assuming that the value is 1 , the rc script executes all the scripts under the
/etc/rc.1 directory and then launches the graphical login.
If Ubuntu is booted to runlevel 1, for example, scripts beginning with the
letter K followed by scripts beginning with the letter S under the
/etc/rc1.d directory are then executed:
Click here to view code image
matthew@seymour:~$ ls /etc/rc1.d/
K10jackd K20rsync K20vboxdrv K80cups
S70pppd-dns
K15pulseaudio K20saned K20winbind README
S90single
K20acpi-support K20saslauthd K74bluetooth S30killprocs
K20kerneloops K20speech-dispatcher K77ntp S70dns-clean
These scripts, as with all scripts in the rc*.d directories, are actually
symbolic links to system service scripts that reside in the /etc/init.d
directory.
The rc1.d links are prefaced with a letter and number, such as K15 or S10.
The K or S in these prefixes indicates whether a particular service should be
killed (K) or started (S) and pass a value of stop or start to the
appropriate /etc/init.d script. The number in the prefix executes the
specific /etc/init.d scripts in a particular order. The symlinks have
numbers to delineate the order in which they are started. Nothing is sacred