Startup

When the machine is turned on, it searches its own hardware for a bootable file. The exact search order is system dependent, but these days you typically see the machine looking at its floppy drive, then the CD-ROM, then the master hard disk. Check your BIOS configuration for information about your particular machine.

Usually, Linux systems are started by the LInux LOader (LILO), a program that can boot multiple operating systems. LILO works well when placed in the master boot record (MBR) of your hard drive and can be configured to invoke Windows or other operating systems in addition to Linux. However, if you are using Windows NT or OS/2, you may be forced to install LILO somewhere other than the MBR, as these operating systems demand total control of the MBR. In addition, Linux may be started from a boot floppy or CD-ROM.

Regardless of your boot method, the first thing that happens is your kernel is loaded. The kernel initializes its own internal data structures and loads its device drivers. If all goes well, the kernel then starts the init program, which, in turn, forks off clones to create the various user processes.

Linux uses the concept of runlevels to define which processes the system allows to run at a given time. A runlevel is a numeric system variable between 0 and 6 that has a set of allowable processes associated with it. When the system transitions between runlevels, it consults the /etc/inittab file to see what processes to start or stop.

Runlevel	Meaning
0	system halted
1	administrative runlevel
2, 3, 4, 5	user definable run levels
6	reboot runlevel

A great deal of boot-time system configuration is accomplished via script.The /etc/rc.d/rc{runlevel}.d directories contain scripts associated with the various runlevels. Upon entering that runlevel, these scripts are executed by init in the order of their appearance in the output of the ls command .