Linux Boot Process

The process of booting up a Linux kernel can be view as 6 separate stages

  1. BIOS selects the boot device.
  2. BIOS loads the bootsector from the boot device.
  3. GRUB loader
  4. Kernel decompression
  5. Low-level initialization by init
  6. User prompt

Lets look at each of the stages, and see what they do

1. Bios Selects Boot Device

On startup, power is sent to the CPU and it starts executing instructions. The first instruction it runs passes control to the BIOS (Basic Input/Output System). The BIOS then does two things:

  1. Run POST (Power On Self Test) operation.
  2. Selecting first Boot device.

POST operation

The POST operation checks for hardware availability by sending electrical pulses to the devices. If no electrical pulses are sent back, it means the device is faulty or does not exists. These activities are outside the scope of Linux, as the Kernel is not even loaded in yet until later

Selecting the First Boot Device

After running POST is completed, the BIOS will have a list of available devices. It will select the first boot device and returns control to the CPU. If the first boot device cannot be found, it will continue to look for the next available boot device. If BIOS do not find any available boot devices it will alert user stating "No boot device found"

2. Bios Loads Boot Sector from the Boot Device

One the boot device has been identified, the BIOS passes control back to the CPU. The CPU will then read the first 512 bytes of the boot device to obtain the Master Boot Recorder (MBR). The MBR consists of the following information

  1. Primary boot loader code / Stage 1 Boot loader (446 Bytes)
  2. Partition table information (64 Bytes)
  3. Magic Number (2 Bytes)

Primary Boot Loader Code

This code provides information regarding the actual boot loader on the hard disk. It is used to load the actual boot loader

Partition Table Information

This information here stores the details of all the partitions in the hard disk. It includes information such as the start and end of the partition, and what type of partition it is. Each partition needs 16 Bytes to be represented.

Since the Partition Table Information section is only 64 Bytes long, we can store at most 64/16=4 partitions on a hard disk

Magic Number

This number is use for validating the MBR. It is used to recover the MBR when it gets corrupted.

3. GRUB Loader / Stage 2 boot Loader

If the CPU has a valid boot device, which has a valid MBR that contains the Primary boot loader code / Stage 1 Boot loader, we proceed to load the Stage 2 Boot loader, or the GRUB (Grand Unified Bootloader) into memory. The GRUB is located 30 KB away from the MBR and before the first partition.

The GRUB displays the available Kernels to the user. Once the user selects a kernel, the kernel image is then loaded into memory and control is passed to the kernel.

Nowadays GRUB2 is being used in place of GRUB. GRUB2 is the default boot loader for RHEL 7, Fedora, and Ubuntu. To read more about GRUB2, click here

4. Kernel Decompression

The kernel is a compressed image, and it is store in /boot along with an initial RAM disk image, and device maps of the hard drives.

Once the kernel is loaded into the main memory, it begins decompression. After decompression, the expanded image overwrite the initial uncompressed image. Once the kernel has extracted itself, it loads systemd, which is the replacement for the old SysV init program, and turns control over to it.

At this point, the Linux kernel and systemd (or in some systems, init) are running

5. Low-level initialization (init, Upstart and Systemd)

Once the kernel has been decompressed, it begins the init function, which is the low level initialization of the kernel. The kernel looks at the /etc/inittab file and decides the run level. It also looks into the folder /etc/rc*and runs the startup scripts there. The files for init are stored in /etc/init

init is a daemon process which starts as soon as the computer starts and continues running till it is shutdown. init has a pid of 1. If somehow the init daemon could not start, no process will be started and the system will enter Kernel Panic.

init is going to be replaced by systemd in the future, as systemd offers a cleaner way of booting. Ubuntu used to adoptupstart as their initialization, but switched to systemd as of 9 March 2015. To read more about systemd, click here

6. User Prompt

Once the above is completed, a shell is presented to the user to prompt for login

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