Boot process that begins with the BIOS executing a short program that is stored in the Master Boot Record (MBR) of the first physical drive.
- BIOS:
BIOS stands for Basic Input Output System, BIOS is the first process begins when you power on the systems, the most important use of BIOS during the booting process is POST. POST stands for Power on Self-Test. It’s a series of tests conducted by the bios, which confirms the proper functioning of different hardware components attached to the computer.
Bios determines all bootable devices available in system, prompts to select bootable device like hard disk which consists of MBR (primary boot loader). - MBR:
MBR(Master boot Record) is located in a first sector of bootable disk(hard disk ) with a size of 512KB and it consists of primary boot loader information and disk partition architecture like /dev/sda or /dev/had . And this loads primary boot loader GRUB - GRUB:
GRUB (GRand Unified Bootloader) related config file will be in /etc/grub/grub2.conf this will allow you select Linux kernel version and grub main objective is to load Linux kernel onto main memory. - KERNEL:
Once you select kernel it mounts root file system as specified in grub2.conf as read only and then it initialize /sbin/init program./sbin/init is called the “mother of all processes” in that it is process number 1 in all Unix and Linux systems, and all other non-kernel processes are spawned directly or indirectly by init. It is init that is responsible for starting the appropriate scripts
and terminals. - INIT:
init is the first program to be executed and assigned process ID is 1 and It will mount all partitions that are specified in the /etc/fstab. And executes the system to boot into runlevel as specified in /etc/inittab. - RUN LEVEL:
A no of run level scripts are defined inside /etc/rc.d/rx.d . scripts are used for run levels 0 to 6 and located in subdirectories of /etc/rc.d/rc0.d/ to /etc/rc.d/rc6.d respectively.
Init 0 , init 1 , init 3 , init 6