Upstart

Ubuntu Upstart changed the basic way Linix boots the system. It's event based so applications can be loaded when an event happens vs loading via some numeric ordering. Upstart has been used in Ubuntu for a few releases now, but 9.10 Karmic Kola made significant changes and most application startup happens with upstart now.

Basic knowledge

The place to start with upstart is in its configuration directory

cd /etc/init

Here you will find the scripts that start the system.

runlevel

The default runlevel used to be in /etc/inittab, now the config file /etc/init/rc-sysinit.conf contains the default runlevel and /etc/init/rc.conf is called to start the apps in /etc/rcX.d/ where 'X' is the numeric runlevel selected.