Linux Learner Group

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Linux Boot Process at High Level

1. Power on the system.

2. The first piece of code is executed = BIOS/UEFI.

3. BIOS = Basic Input Output System embedded on the motherboard by the manufacturer.

4. BIOS initializes the basic hardware and performs POST (Power On Self Test).

5. Then BIOS locates the MBR (Master Boot Record), which is placed on the first sector of the hard disk.

6. MBR contains the bootloader (GRUB2) and also the information about each partition of the HDD.

7. Then BIOS loads the bootloader and hands over control to the bootloader (GRUB2).

8. GRUB = Grand Unified Bootloader.

9. Bootloader at this stage loads the selected Kernel image from the boot directory (/boot) into memory.

10. GRUB2 bootloader also loads initramfs image, which is nothing but a compressed root file system, and later it is used by the Kernel stage to mount the root file system.

11. Then GRUB2 hands over control to the Kernel.

12. Kernel extracts the content of the initramfs and mounts the root file system as a temporary file system, before the actual root file system is mounted.

13. Kernel initializes different drivers and devices and starts the first process on the system, which is systemd with the process ID = 1.

14. Now the control is with the systemd process. systemd process starts the system with the desired target, and it also starts all other required processes to boot up the system.