In a traditional SysV init system, differently configured runlevels can be

chosen at boot time. Some distributions, including ubuntu, are migrating to the "upstart" init system. Most services and daemons are still provided with the older SysV style startup scripts. Upstart maintains compatibility with these services by running their scripts at boot time as they would be under the init startup system.

In ubuntu systems, a casualty of this migration is that runlevels no longer work entirely as expected. Emmet Caulfield has an explanation and a solution on this blog page. Using the script he provides, the kernel command line is scanned at boot time for the expression init N, where N is a number signifying the desired runlevel.

Most distributions and some older versions of ubuntu will boot into a non-default runlevel by appending a single numerical digit (or 'S' for single user mode) to the end of the bootloader kernel command line. This /etc/event.d/rc-default can be used instead of the file provided in the above link to make ubuntu follow that convention.

/etc/event.d/rc-default

# rc - runlevel compatibility
#
# This task guesses what the "default runlevel" should be and starts the
# appropriate script.
#
# Edited to support booting to non-default runlevel by adding a
# single digit from [2345] as last option on kernel command line
# at boot. See elif statement below. lsw 10/4/2008
#

start on stopped rcS

script
	runlevel --reboot || true
	if grep -q -w -- "-s\|single\|S" /proc/cmdline; then
	    telinit S
	elif RL="$(grep -o "[[:blank:]][2345]$" /proc/cmdline || true)"; then
	    if [ -n "$RL" ]; then
		telinit $RL
	    else
		telinit 2
	    fi
	elif [ -r /etc/inittab ]; then
	    RL="$(sed -n -e "/^id:[0-9]*:initdefault:/{s/^id://;s/:.*//;p}" /etc/inittab || true)"
	    if [ -n "$RL" ]; then
		telinit $RL
	    else
		telinit 2
	    fi
	else
	    telinit 2
	fi
end script

If you want to try it, save a copy of your original /etc/event.d/rc-default file in some other directory on your system. Please read the warnings in Emmet Caulfields's blog. I won't repeat them all here, but make sure you have another way to boot your system and can edit the rc-default file if necessary. Copy and paste or download the file to your system. From the directory where you have saved the new file,

sudo cp rc-default /etc/event.d

In a default ubuntu installation, runlevels 2,3,4 and 5 are all configured alike. 2 is the default startup runlevel in ubuntu, so modify 3,4, or 5 for the configuration you will use less frequently. Add an appropriate boot stanza to /boot/grub/menu.lst to make the alternate runlevel a choice at boot time. The Illustrated Dual Boot site is a great reference for working with grub.

Probably the most common non-default runlevel is one configured to use text mode only, i.e. no X server or gui. See Editing Runlevels for more information.

*   *   *

If you would rather not modify
/etc/event.d/rc-default
and /boot/grub/menu.lst, and only rarely need a different runlevel, here is an alternate method.

*   *   *

The man pages for telinit and init and the readme files in /usr/share/doc/sysv-rc and /usr/share/doc/upstart provide some explanation of how runlevels, init, and upstart work.