Linux Boot process

Let’s describe Linux Boot Process:

The Linux boot process involves several stages, in which the operating system performs various tasks to prepare the system for use.

When the system is powered on, the BIOS (Basic Input/Output System) or the UEFI (Unified Extensible Firmware Interface) performs a power-on self-test (POST) to check the hardware components and to load the bootloader.
The bootloader, such as GRUB (GRand Unified Bootloader), is responsible for loading the operating system kernel and transferring control to it.
The operating system kernel, which is the core of the operating system, initializes the system and starts the system services.
The system services, such as the device drivers, are loaded and initialized.
The operating system loads the user profile and starts the user interface, such as the desktop or the login screen.
The user can log in and start using the system.

This is a general overview of the Linux boot process. The exact sequence of events may vary depending on the specific distribution of Linux and the hardware configuration of the system.

BIOS	Basic INPUT/OUTPUT System. Executes MBR
MBR	Master Boot Record Executes GRUB
GRUB	Grand Unified Bootloader Executes kernel
KERNEL	Kernel Executes /sbin/init
INIT	Init Executes Run level programs
Run Level	Run Level Programs are executed from /etc/rc.d/rc*.d/

As power comes up the BIOS is given control
BIOS runs self tests, usually including cursory memory tests.
The BIOS then loads the first sector of the disk to be used for booting and transfers control to it.
The MBR code varies. One version will chain to the code in the first sector of the boot partition (Windows), another will load a bootloader. Windows boot proceeds from code and information in the boot partition.
The bootloader chooses kernel location and version
The bootloader prepares kernel and initrd image in memory, transfers control to kernel
Loading kernel modules
Discovering hardware and load additional kernel modules to support it
Looking for disks
R/O mount of / partition so that it can potentially be checked and repaired
init process spawn
/etc/inittab read and executing
Mounting all FSes from /etc/fstab
runlevels running (based on default runlevel in /etc/inittab) or another init method such as systemd or upstart
rc.local
login prompt