Detail Explain Linux Booting Process and Grub Stages

In this Article  we will discuss more  of Linux Boot process .How a Linux system boots?

How computer bootup?

Booting is a process that starts operating systems when the user power on a computer system

A boot sequence is the collection or set  of operations the computer to performs when it is powered on.

Booting sequence

• Power on


• CPU jumps to address of BIOS and initiate


• BIOS runs POST (Power-On Self Test)


• Find bootable devices


• Loads and executes boot loader form MBR


• GRUB (Grand Unified boot Loader) execute the Kernel


• Kernel execute the init process /service by checking default Runlevel


• Runlevel execute all service from /etc/rc.d....


• Load the OS

BIOS (Basic Input Output System)

BIOS is refers as the software code run by a computer when powered on The primary function of BIOS is code program embedded on a chip that recognizes and controls various devices that make up the computer.

Boot loader

MBR (Master Boot Record)

OS is booted from a hard disk, where the Master Boot Record (MBR) contains the primary boot loader

The MBR is a 512-byte sector, located in the first sector on the disk (sector 1 of cylinder 0, head 0)

After the MBR is loaded into RAM, the BIOS yields control to it. The first 446 bytes are the primary boot loader, which contains both executable code and error message text.

The 6 stage of  the booting process.

 1.BIOS (Basic Input/Output System)

 2.MBR(Master Boot Record)

3.LILO or GRUB

   LILO:-Linux Loader

   GRUB:-Grand Unified Bootloader

4.Kernel

 5.init

 6.Run Levels

1.BIOS:

 When we switched on the machine  BIOS performs a Power-On Self-Test (POST) for all of the different hardware components attached to the system to make sure everything is working fine.

2.MBR

MBR contains boot information which BIOS executes. MBR will search for a valid primary partition marked as bootable on the hard disk.

3.GRUB

GRUB  consists of 446 bytes of primary bootloader code and 64 bytes of the partition table.

4.Kernel

Once GRUB  transfers the control to Kernel,the Kernels does the following tasks

• Intitialises devices and loads initrd module


• mounts root filesystem 

5.Init

The kernel, once it is loaded, finds init in sbin(/sbin/init) and executes it.

6.Runlevel

There are seven  run levels in which the Linux OS runs and different run levels serves for different purpose. The descriptions are

given below.

• 0  – halt (shut down)


• 1  – Single user mode or maintenance mode  


• 2  – Multiuser, without NFS (without networking)


• 3  – Full multiuser mode


• 4  – unused


• 5  – X11 or Graphics mode  


• 6  – Reboot/ restart