Boot Process of Linux

There are 6 process occurs during booting process of kernel

(1) BIOS
(2) MBR
(3) GRUB
(4) KERNEL
(5) INIT
(6) RUN LEVEL PROGRAMS


BIOS:- BIOS stands for Basic Input Output System
Bios search , loads & execute boot loader program.Once the boot loader program is detected & loaded into memory.Bios gives control to it.


MBR:- MBR stands for master boot record
 The first sector  of bootable device is MBR
Its size is 512 bytes

446 Bytes  (Primary Bootloader Info)

64 Bytes  (Partition table info)

2Bytes (validation check)

GRUB:- GRUB stands Grand Unified Bootloader 

It loads and execute the defaults kernel image as specified in grub configuration file /boot/grub/grub.conf

KERNEL:- Mount the root file system . Kernel execute the /sbin/init .INIT is the first prog executed in Linux kernel .Kernel contains device driver compiled inside which help to access the hardware.

INIT:- INIT decide the run levels. These run levels are mention in /etc/inittab file
There are 7 run levels in linux

0  -->  Halt
1  -->  Single user mode
2  -->  Multiuser mode without NFS
3  -->  Full multiuser mode
4  -->  Unused
5  -->  Graphical
6  -->  Reboot

RUN LEVEL PROGRAMS:- When the Linux system booting up various services getting started.
for example   httpd ---- OK
                     named----OK
                     vsftpd -----OK
Depending on your default init level setting , the system will execute the programs from one of the following directory.


Run level 0 – /etc/rc.d/rc0.d/
Run level 1 – /etc/rc.d/rc1.d/
Run level 2 – /etc/rc.d/rc2.d/
Run level 3 – /etc/rc.d/rc3.d/
Run level 4 – /etc/rc.d/rc4.d/
Run level 5 – /etc/rc.d/rc5.d/
Run level 6 – /etc/rc.d/rc6.d/