Although most people consider a computer to be either on or off, in Ubuntu there are a number of states in between. Known as runlevels, they control what system services are started upon bootup. These services are simply applications running in the background that provide some needed function to your system, such as getting information from your mouse and sending it to the display; or a service could monitor the partitions to see whether they have enough free space left on them. Services are typically loaded and run (also referred to as being started) during the boot process, in the same way as Microsoft Windows services are. Internally, Ubuntu uses a system known as Upstart for fast booting, but this has a special backward compatibility layer that uses runlevels in the way that Linux veterans are accustomed.
You can manage nearly every aspect of your computer and how it behaves after booting via configuring and ordering boot scripts, as well as by using various system administration utilities included with Ubuntu. In this chapter, you learn how to work with these boot scripts and system administration utilities. This chapter also offers advice for troubleshooting and fixing problems that might arise with software configuration or the introduction or removal of various types of hardware from your system.
Source of Information : Sams Ubuntu Unleashed 2008 Edition
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