HOW A *NIX SYSTEM BOOTS.

1. System powered on and BIOS starts

2. BIOS checks the h/w components RAM,Keyboard,Mouse,HDD etc are connected and properly configured

3. BIOS sets up the environment and then checks the 1st bootable media (CDROM/LAN/HDD etc.)

4. BIOS searches for MBR (Master Boot Record ) in the bootable media

MBR - is of size 512 bytes which stores the bootable code with the partition table and then sets up the menu

5. BOOTCODE is  checked - STAGE 1 of GRUB

GRUB - Grand Unified Bootloader -The default boot loader in *NIX based systems

6. Grub stage 1.5 is consulted presenting the Grub menuto user

7. From the menu the OS is selected to boot

Q . Where the menu is stored ? ( /boot/grub/grub.conf)

FACTS : The boot files are stored in /boot/grub

Stage 1 file present here with 512 bytes which sits in bootable mediawhich consults with stage 1.5

Stage 1.5 sets up the appropriate file system i.e. wheather to boot from the

CDROM using iso9660 i.e. iso9660_stage1.5  or from the

FAT file system using fat_stage1.5 or from the

XFS file system using XFS_stage1.5 or from the

EXT2FS /EXT3FS using e2fs_stage1.5

Stage 1.5 provides the  drive information which is consulted with from stage 1 to access the File System

8. Grub stage 2 now sets up the environment and calls the INIT process ,which is present in the /sbin (System Binary )

INIT - A special process which sets up the linux environment

The parent process which starts first in a *NIX system with PID 1 and forks other processes .

9.  INIT reads a special file /etc/inittab (A Table file ) , which contains system initialization entries

INITTAB -  contains initdefault and runlevel entries , 

RUNLEVEL -

                  Runlevel is a system state in which some services are started and
some are killed according to the need . Runlevel is controlled by the
the parent process INIT . Like there are 6 run levels bydefault in
any Linux.Unix systems as follows ;

Runlevel 0 init 0 halt used to halt a system

Runlevel 1 - init 1 Single User Mode Used for
system maintainnance like backup etc. and in this the network services
are stopped and only single user can logged in to a system for the
maintainnance .

Runlevel 2 - init 2 Same as runlevel 3 but without NFS

Runlevel 3 init 3 Full multiuser mode with networking but without  X11 i.e. GUI  is absent .

Runlevel 4 init 4 Reserved for future purpose or u can edit this to create ur own runlevel

Runlevel 5 init 5 Full multiuser with X11 support

Runlevel 6 - init 6 Used for Reboot

This runlevel concept is usually used in all Linux machines as well as Unix machines .

In linux the file /etc/inittab stores the default runlevelin the line

10. Now INIT sets up and boot appropriate runlevel as defined in inittab file .