linux booting process step by step explained in detail.

Linux Booting process / sequence in details

In this tutorial we will learn Linux Booting process or sequence.

When You press power button on your system, and after few moments you see the Linux login prompt.

Have you ever wonderd what happens behind the scenes from the time you press the power button until the Linux login prompt appears?
following are the 6 high level stages of a typical Linux boot process.


Basic input/output System
executes MBR


Master boot record executes


Grand Unified Bootloader
Executes Kernel


Kernel executes /sbin/init


Init executes runlevel programs


Runlevel programs are executed
from /etc/rc.d/rc*.d/



  • BIOS stands for Basic Input/Output System
  • This is the first thing which loads once you press
    power button on your machine.
  • When you press the power button of the machine, CPU
    looks out into ROM for further instruction.
  • The ROM contains JUMP function in the form of
    instrucion which tells the CPU to bring up the BIOS
  • BIOS Performs some system integrity checks
  • Searches, loads, and executes the boot loader
  • It looks for boot loader in floppy,cd-rom, or hard
    drive. You can press a key (typically F12 or F2, but it depends on your system)
    during the BIOS startup to chaange the boot sequence.
  • Once the boot loader program is detected and loaded
    into the memory, BIOS gives the control to it.
  • So, in simple terms Bios loads executes the MBR boot


  • MBR stands for Master Boot Record.
  • It is located in the 1st sector of the
    bootable disk. Typically /dev/had, or/dev/sda
  • MBR is less than 512 bytes in size. This has three


  1.              primary boot loader info in 1st 446 bytes
  2.              partition table info in next 64 bytes
  3.              MBR validation check in last 2 bytes


GRUB stands for Grand Unified Bootloader.
This loads the kernel into 3 stages
The primary boot loader takes up less than 512 bytes
of disk space in the MBR – too small a space to contain the instructions
necessary to load a complex operating system.
Instead the primary boot loader performs the
function of loading either the stage 1.5 or stage 2 boot loader.
Stage 1 can load the stage 2 directly, but it is
normally set up to load the stage 1.5.
This can happen when the /boot partition is situated
beyond the 1024 cylinder head of the hard drive.
GRUB Stage 1.5 is located in the first 30 KB of Hard
Disk immediately after MBR and before the first partition.
This space is utilized to store file system drivers
and modules.
This enabled stage 1.5 to load stage 2 to load from
any known location on the file system i.e. /boot/grub
This is responsible for loading kernel from
/boot/grub/grub.conf and any other modules needed
Loads a GUI interface i.e. splash image located at
/grub/splash.xpm.gz with list of available kernels where you can manually
select the kernel or else after the default timeout value the selected kernel
will boot
  • If you have multiple kernel images installed on your
    system, you can choose which one to executed.
  • 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.
  • Grub has the knowledge of the filesystem (the older
    linux loader LILO didn’t understand filesystem)
  • GRUB configuration file is /boot/grub/grub.conf
    (/etc/grub.conf is a link to this). The following is sample of grub.conf file.


grub.conf generated by anaconda
Note that you do not have to rerun grub after making changes to this file
NOTICE:  You have a /boot
partition.  This means that
#          all kernel and initrd paths are
relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro
#          initrd
Red Hat Enterprise Linux (2.6.32-71.el6.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-71.el6.x86_64
ro root=UUID=4afa457a-d662-4e89-b4e2-09303c44c3eb rd_NO_LUKS rd_NO_LVM
rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc
KEYTABLE=us crashkernel=auto rhgb quiet
  • As you notice from the above info, it contains
    kernel and initrd image.
  • So, in simple terms GRUB just loads and executes
    kernel and initrd images.


  • This can be considered the heart of operating system
    responsible for handling all system processes.
  • Mounts the root file system as specified in the “root=”
    in grub.conf
  • Kernel Executes the /Sbin/init program
  • Since init was the 1st program to be
    executed by Linux Kernel, It has the process id(PID) of 1. Do a `ps-ef |grep
    init’ and check the pid.
  • Initrd stands for initial RAM disk.
  • Kernel as soon as it is loaded configures hardware
    and memory allocated to the system.
  • Initrd is used by kernel as temporary root file
    system until kernel is booted and the real root file system is mounted. It also
    contains necessary drivers complied inside, which helps it to access the hard
    drive partitions, and other hardware.



  • Executes the system to boot into the run level as
    specified in /etc/inittab
  • Following are available run lavels inside
    /etc/inittab file


# Default runlevel. The runlevels used by RHS are:
#   0 – halt
(Do NOT set initdefault to this)
#   1 – Single
user mode
#   2 –
Multiuser, without NFS (The same as 3, if you do not have networking)
#   3 – Full
multiuser mode
#   4 – unused
#   5 – X11
#   6 – reboot
(Do NOT set initdefault to this)
  • Init identifies the default initlevel from /etc/inittab
    and uses that to load all appropriate program.
  • Execute ‘grep initdefault /etc/inittab’ on your
    system to identify the default runlavel.
  • Typically you would set the default run level to
    either 3 or 5.


  • Depending on your default init level setting, the
    system will execute the programs from one of the following directories.


           Runlevel       Directory
  • Runlevel 0 /etc/rc.d/rc0.d
  • Runlevel 1 /etc/rc.d/rc1.d
  • Runlevel 2 /etc/rc.d/rc2.d
  • Runlevel 3 /etc/rc.d/rc3.d
  • Runlevel 4 /etc/rc.d/rc4.d
  • Runlevel 5 /etc/rc.d/rc5.d
  • Runlevel 6 /etc/rc.d/rc6.d


  • Programs used for runlevels 0 to 6 are located in
    subdirectories /etc/rc.d/rc0.d through /etc/rc.d/rc6.d, respectively.
  • When the linux system is booting up, you might see
    various services getting started. For example, it might say “starting sendmail….OK”.
    Those are the run level programs, executed from the run level directory as
    defined by your run level.

I hope you enjoyed reading this article…

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