Home > Linux > Step by step explanation of Linux boot sequence

Step by step explanation of Linux boot sequence

In this topic we will discuss indepth of Linux Boot Sequence.How a linux system boots?This will help administrators in

troubleshooting some bootup problem.Before discussing about  I will notedown the major component we need to know

who are responsible for 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:

      i.When we power on BIOS performs a Power-On Self-Test (POST) for all of the different hardware components in the system to make sure everything is working properly

     ii.Also it checks for whether the computer is being started from an off position (cold boot) or from a restart (warm boot) is
stored at this location.

     iii.Retrieves information from CMOS (Complementary Metal-Oxide Semiconductor) a battery operated memory chip on the motherboard that stores time, date, and critical system information.

     iv.Once BIOS sees everything is fine it will begin searching for an operating system Boot Sector on a valid master boot sector
on all available drives like hard disks,CD-ROM drive etc.

     v.Once BIOS finds a valid MBR it will give the instructions to boot and executes the first 512-byte boot sector that is the first
sector (“Sector 0”) of a partitioned data storage device such as hard disk or CD-ROM etc .

2.MBR

     i. Normally we use multi-level boot loader.Here MBR means I am referencing to DOS MBR.

     ii.Afer BIOS executes a valid DOS MBR,the DOS MBR will search for a valid primary partition marked as bootable on the hard disk.

     iii.If MBR finds a valid bootable primary partition then it executes the first 512-bytes of that partition which is second level MBR.

     iv. In linux we have two types of the above mentioned second level MBR known as LILO and GRUB

3.LILO

     i.LILO is a linux boot loader which is too big to fit into single sector of 512-bytes.

     ii.So it is divided into two parts :an installer and a runtime module.

     iii.The installer module places the runtime module on MBR.The runtime module has the info about all operating systems installed.

     iv.When the runtime module is executed it selects the operating system to load and transfers the control to kernel.

     v.LILO does not understand filesystems and boot images to be loaded and treats them as raw disk offsets

GRUB

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

     ii.GRUB locates all the operating systems installed and gives a GUI to select the operating system need to be loaded.

     iii.Once user selects the operating system GRUB will pass control to the karnel of that operating system.
see below what is the difference between LILO and GRUB

4.Kernel

     i.Once GRUB or LILO transfers the control to Kernel,the Kernels does the following tasks

  • Intitialises devices and loads initrd module
  • mounts root filesystem

5.Init

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

     ii.Hence the first process which is started in linux is init process.

     iii.This init process reads /etc/inittab file and sets the path, starts swapping, checks the file systems, and so on.

     iv.It runs all the boot scripts(/etc/rc.d/*,/etc/rc.boot/*)

     v.starts the system on specified run level in the file /etc/inittab

6.Runlevel

     i.There are 7 run levels in which the linux OS runs and different run levels serves for different purpose.The descriptions are
given below.

  • 0  – halt
  • 1  – Single user mode
  • 2  – Multiuser, without NFS (The same as 3, if you don’t have networking)
  • 3  – Full multiuser mode
  • 4  – unused
  • 5  – X11
  • 6  – Reboot

     ii.We can set in which runlevel we want to run our operating system by defining it on /etc/inittab file.

Now as per our setting in /etc/inittab the Operating System the operating system boots up and finishes the bootup process.

Below are given some few  important differences about LILO and GRUB

LILO

GRUB

LILO has no interactive command interface GRUB has interactive command interface
LILO does not support booting from a network GRUB does support booting from a network
If you change your LILO config file, you have to rewrite the LILO stage one boot loader to the MBR GRUB automatically detects any change in config file and auto loads the OS
LILO supports only linux operating system GRUB supports large number of OS

To know more about the booting process you can follow the link below
http://www.ibm.com/developerworks/linux/library/l-linuxboot/

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  1. Spatik
    October 16, 2011 at 6:21 pm

    Well explained, thank you ..;)

  2. smruti
    January 9, 2012 at 8:20 pm

    awesome documents…

  3. Anubhav
    January 27, 2012 at 5:50 pm

    Gd Job Kirti….

  4. Narayan Sahu
    January 31, 2012 at 4:31 pm

    THANK U………………….

  5. February 22, 2012 at 2:48 pm

    post server config..it wil helpful for me

  6. Renju Sebastian
    February 23, 2012 at 11:46 am

    very good

  7. rohit nair
    March 16, 2012 at 4:00 pm

    very informative !!! worth

  8. sai krishna
    April 17, 2012 at 4:11 pm

    Thanks, its very helpfull.

  9. vsha
    April 23, 2012 at 6:48 pm

    good explained

  10. Pravin Y
    May 15, 2012 at 5:51 pm

    really very nice………..
    I completely understood now………………….
    thank you so much…………….
    🙂

  11. Ramesh
    May 20, 2012 at 10:42 pm

    its very well explained and simple to understand..thanks

  12. Vijay santore
    June 11, 2012 at 3:12 pm

    Bahut BAdiya.. YE booting proceess bahut dimaag khati thi … good explanation

  13. Thamizharasan P
    June 17, 2012 at 2:50 am

    Great… Superb yaar , it will very helpful for me.
    Thanks a lot

    Regards
    Thamizharasan P

  14. lila varma
    June 17, 2012 at 11:31 pm

    good explaned thanks…..

  15. karthigairajan
    July 20, 2012 at 12:56 pm

    Super Explanation

  16. smartkugu
    July 23, 2012 at 8:17 pm

    Worthful

  17. Altair
    July 26, 2012 at 12:38 pm
  18. Pradyumna Kumar Dash
    September 8, 2012 at 1:43 am

    Good job if you add more regarding four files which execute after init .

  19. ajay
    October 11, 2012 at 8:42 am

    really… easy understanding way…………

  20. sandeep
    October 15, 2012 at 10:46 am

    its very good , and very helphull thanks

  21. vinoth
    October 29, 2012 at 6:35 pm

    really it’s very simple to understand .. thanks a lot ..

  22. ajas k a
    December 7, 2012 at 12:24 pm

    nice explanation in simple language…….

  23. March 10, 2013 at 12:21 pm

    good work

  24. Rabindra Jena
    May 26, 2013 at 12:47 pm

    Grate to see ur name kirti

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