Linux Boot Process:-

Linux boot process have six step these steps are mentioned below

Step1:- BIOS

Step2:- MBR

Step3:- Grub

Step4:- kernel

Step5:- init programs

Step6:- Run levels

(1)    BIOS:- Bios stands for basic input/output system, when we have power on any Linux machine it check some system integrity on self-test, it search, load and execute boot loader program. Once boot loader program detected  and loaded into memory, BIOS gives the control to it

(2)    MBR:- MBR stands for Master Boot Record, MBR size less than 512 Bytes and it has three partition, first partition size have 446 bytes and has master boot records info, second partition size have 64 bytes and has partition table info and third partition size has 2 bytes and has validation check. It has information about GRUB, so we can say MBR execute GRUB boot loader.

(3)    GRUB:- GRUB stands for Grand Unified Boot Loader, if we have install multiple kernel image installed in Linux system, we can choose which one to be executed. GRUB display a splash screen, wait for few seconds, if you don’t enter anyting, it loads default kernel image as specified in grub configuration file. Grub configuration file is /boot/grub/grub.conf it contains kernel and initrd image.

(4)    KERNEL:- Mount the root file system as specified in the “root=” in grub.conf, kernel executes the /sbin/init program, Since init is a first program to be executed by kernel, it has the process id of 1, initrd stands for Initial RAM Disk.

(5)    INIT:- Looks at /etc/inittab file to decide the Linux run level.

(6)    Runlevel Program:-  When the Linux system is booting up, you might see various service getting started for example it might say “starting sendmail ….ok” Those are run level directory as defined by your run level.