Linux boot up process | boot process in linux step by step

Linux boot up process | boot process in linux step by step : 6 Stages

In this topic we will discuss linux boot process, boot process in linux step by step. linux boot up process divided in 6 stages as below.

Boot process for linux : 6 Stages

 

BIOS	Basic Input/Output system executes MBR
MBR	Master Boot Record executes GRUB
GRUB	Grand Unified Bootloader executes kernel
Kernel	Kernel executes /sbin/init
init	Init executes runlevel programs
Runlevel	Runlevel programs are executed  from /etc/rc.d/rc*.d/

Here we will discuss all stages for boot process of linux step by step:


1. BIOS

• BIOS stands for Basic Input/Output System.

• Searches, loads, and executes the boot loader program and Performs some system integrity checks.

• BIOS loads and executes the MBR boot loader 

2. MBR

• MBR stands for Master Boot Record.

• located in /dev/hda, or /dev/sda (less than 512 bytes in size).

• MBR loads and executes the GRUB boot loader.

3. GRUB

• GRUB stands for Grand Unified Bootloader.

• Grub configuration file is /boot/grub/grub.conf.

• GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

• If you have multiple kernel images installed on your system, you can choose which one to be executed.

• GRUB just loads and executes Kernel and initrd images.

4. Kernel

• Kernel executes the /sbin/init 

5. Init

• Init finds the /etc/inittab file to decide the Linux run level.
• Available run levels in RHEL are as below.

              

0 – halt

1 – Single user mode

2 – Multiuser, without NFS

3 – Full multiuser mode

4 – unused

5 – X11

6 – reboot

• Init executes the default initlevel from /etc/inittab and loads all appropriate program.
• Execute ‘grep initdefault /etc/inittab’ to identify the default run level

• We acn set the default run level to either 3 or 5.