Runlevel is the mode in which the operating system like Linux is running. Conventionally, seven runlevels from 0 to 6 existed. Where 0 meant shutdown and 6 meant Reboot.
In previous versions, Ubuntu used to the /etc/inittab file to manage runlevels, just like most of the Linux distributions. This file was based on traditional init daemon, which is used to perform system startup tasks. This was replaced in Ubuntu 6.10 (Release date: 26th-Oct-2006) with Upstart, an event based daemon. Now there are several files under the /etc/events.d/ directory.
The runlevels that Ubuntu handles by default are:
- Runlevel 0 – Halt
- Runlevel 1 – Multiuser text mode
- Runlevel 2 – Graphical multiuser mode
- Runlevel 6 – Reboot
For more details visit Ubuntu’s Upstart section.