Linux Boot Process

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There are 6 stages of a typical Linux boot process

1.BIOS
2.MBR
3.GRUB/LILO
4.Kernel
5.Init
6.Runlevel Programs


1.BIOS:


  1. Basic Input Output System is the lowest level interface to peripherals andcontrols the first step of the boot process.
  2. BIOS performs some integrity checks and serches,loads, executes the bootable program. Its OS independent.
  3. It checks for a bootable media in CD-ROM,HDD,USB to use to boot the system. you can press a key(F2,F12,F10 depends on BIOS settings on the system) during the BIOS start-up to change the boot sequence. Now a days network bootà typically called PXE-boot got more familiar.
  4. BIOS looks for Master Boot Record(MBR) which starting at the first sector on the first hard drive, loads its contents into memory, and then passes control to MBR.

A Typical BIOS screen would look like as below.




Ø  The main Task BIOS performs is POST ( Power On Self Test) Its a quick test of memory , It does the basic check for fundamental hardwareon your computer.

Ø  In side the BIOS list of devices that it will check in order to see if there is a MBR present on the particular device or not?

Ø  If BIOS didn’t found the MBR i.e. Bootable Program any of the listed devices in BIOS then BIOS fails to load the MBR throws an error like bootable media not found !!! 

2.MBR(Master Boot Record):



MBR located in the 1st Sector of the bootable disk. Typically /dev/hda  (or) /dev/sda which is the size of 512 bytes.




v MBR Contains the instructions how to load the GRUB boot –loader and how to execute the GRUB as well. MBR typically contains the Partition table to boot over which describes the partitions on the disk and sets up the menu.

v In the default Red Hat Linux configuration GRUB uses the settings in the MBR to display boot options in a menu which provides access to system.
            
      MBR automatically launches the boot loader.



3.GRUB/LILO(GRand Unified Bootloader):



1.GRUB(LILO) Boot loader will then ask for the OS label which will identify which kernel to run and where it is located.

2.GRUB includes 3 stages
      Stage 1 : Typically includes MBR(512 bytes)
      Stage 1.5 : loads the FS drivers (ext3,ext4,FAT etc.)
      Stage 2  : Which loads the INIT.

3.GRUB displays the 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.

4.Grub configuration file:/boot/grub/grub.conf(RHEL)


[root@www grub]# pwd
/boot/grub
[root@www grub]# ls -l /boot/grub/grub.conf
-rw-------. 1 root root 769 Feb 11 02:13 /boot/grub/grub.conf
[root@www grub]# ls -l /etc/grub.conf
lrwxrwxrwx. 1 root root 22 Feb 11 02:13 /etc/grub.conf -> ../boot/grub/grub.conf
[root@www grub]#


[root@www grub]# pwd
/boot/grub
[root@www grub]# vim grub.conf

















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