explain in great depth how does GRUB work

GRUB (GNU GRand Unified Bootloader) is a popular boot loader used by most of the Linux distributions. Its main function is to help users to select an operating system and kernel to boot from during the startup process of a computer system.

When a computer boots up, the BIOS (Basic Input/Output System) is the first code that executes. It is responsible for identifying hardware devices on the computer such as the hard disk, keyboard, and mouse. Once the BIOS completes its initial configuration and detection processes, it then looks for a boot loader to initiate the operating system.

GRUB is one such boot loader that is commonly used with Linux distributions. It is designed to have a modular structure so that different modules can be combined to support different file systems and boot configurations. For example, the menu interface and kernel modules are standard components of GRUB.

When GRUB is installed, it usually places its files in the root file system of the drive that contains the operating system. During the boot process, the BIOS loads the first sector of the boot device, called the Master Boot Record (MBR), and passes control to the bootloader. GRUB is typically installed in the MBR, allowing it to load and execute before any other operating system files.

Once control is passed to GRUB, it loads its configuration file, which is typically called "grub.cfg". This file contains configuration information for the menu interface used by GRUB, as well as options for the kernel and other modules. Once the configuration file is loaded, GRUB displays a menu of boot options.

Users can select the desired option using the arrow keys on the keyboard and pressing enter. Once the user has made a selection, GRUB loads the associated kernel and executes its initialization code. After the initialization process, the kernel loads the necessary modules and drivers to boot the operating system.

In summary, GRUB is a flexible and modular boot loader that is essential for booting Linux distributions. It offers a simple user interface, allowing users to select the desired operating system and kernel at boot time. GRUB performs various functions such as loading the kernel, loading necessary drivers and modules, and starting the operating system.