Linux Booting Process

Knowing Linux booting process is an essential part of every Linux user/administration which will give you a clear picture of how Linux Operating system works. In this post we will see what happens when a Linux OS boots i.e. after powering on the machine to the user log-in prompt. Below image will give you clear idea what will happen in Linux booting process.

A quick view of booting sequence:

Power on
CPU jumps to BIOS
BIOS runs POST
Finds first bootable device
Load and execute MBR
Load OS
User prompt

This a rough idea what happens in Linux booting. Below are the detailed stages in Linux Booting process.

Stages of Booting:

1)System startup(Hardware )
2)Boot loader Stage 1 (MBR loading)
3)Boot loader Stage 2 (GRUB loader)
4)Kernel
5)INIT
6)User prompt

Stage 1: System startup

This is the first stage of booting process. When you power on/Restart your machine the power is supplied to SMPS (switched-mode power supply) which converts AC to DC. The DC power is supplied to all the devices connected to that machine such as Motherboard HDD’s, CD/DVD-ROM, Mouse, keyboard etc. The most intelligent device in the computer is Processor(CPU), when supplied with power will start running its sequence operations stored in its memory. The first instruction it will run is to pass control to BIOS(Basic Input/Output System) to do POST(Power On Self Test). Once the control goes to BIOS it will take care of two things

• Run POST operation.
• Selecting first Boot device.

POST operation: POST is a processes of checking hardware availability. BIOS will have a list of all devices which are present in previous system boot. In order to check if a hardware is available for the present booting or not it will send an electric plus to each and every device in the list that it already have. If an electrical pulse is returned from that device it will come to a conclusion the hardware is working fine and ready for use. If it does not receive a single from a particular device it will treat that device as faulty or it was removed from the system. If any new hardware is attached to the system it will do the same operation to find if its available or not. The new list will be stored in BIOS memory for next boot.

Selecting First Boot Device: Once the POST is completed BIOS will have the list of devices available. BIOS memory will have the next steps details like what is the first boot device it has to select etc. It will select the first boot device and gives back the control to Processor(CPU). Suppose if it does not find first boot device, it will check for next boot device, if not third and so on. If BIOS do not find any boot device it will alert user stating “No boot device found”.

Stage 2: MBR loading

Once the BIOS gives control back to CPU, it will try to load MBR of the first boot device(We will consider it as HDD). MBR is a small part of Hard Disk with just a size of 512 Bytes, I repeat its just 512 Bytes. This MBR resides at the starting of HDD or end of HDD depending on manufacturer.

What is MBR?

MBR(Master Boot recorder) is a location on disk which have details about

• Primary boot loader code(This is of 446 Bytes)
• Partition table information(64 Bytes)
• Magic number(2 Bytes)

Which will be equal to 512KB(446+64+2)KB.

Primary Boot loader code: This code provides boot loader information and location details of actual boot loader code on the hard disk. This is helpful for CPU to load second stage of Boot loader.

Partition table: MBR contains 64 bytes of data which stores Partition table information such as what is the start and end of each partition, size of partition, type of partition(Whether it’s a primary or extended etc). As we all know HDD support only 4 partitions, this is because of the limitation of its information in MBR. For a partition to represent in MBR, it requires 16 Bytes of space in it so at most we will get 4 partitions. 

Magic Number: The magic number service as validation check for MBR. If MBR gets corrupted this magic number is used to retrieve it. What to take backup of your MBR.

Once your CPU knows all these details, it will try to analyse them and read the first portion of MBR to load Second stage of Boot loader

Stage 3: Boot loader Stage 2 (GRUB loader)

Once the Bootloader stage 1 is completed and able to find the actual bootloader location, Stage 1 bootloader start second stage by loading Bootloader into memory. In this stage GRUB(Grand Unified Bootloader) which is located in the first 30 kilobytes of hard disk immediately following the MBR is loaded into RAM for reading its configuration and displays the GRUB boot menu (where the user can manually specify the boot parameters) to the user. GRUB loads the user-selected (or default) kernel into memory and passes control on to the kernel. If user do not select the OS, after a defined timeout GRUB will load the default kernel in the memory for starting it.

Stage 4: Kernel

Once the control is given to kernel which is the central part of all your OS and act as a mediator of hardware and software components. Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown. Once the Kernel starts its operations the first thing it do is executing INIT process.

Stage 5: INIT

This is the main stage of Booting Process

init(initialization) process is the root/parent process of all the process which run under Linux/Unix. The first process it runs is a script at /etc/rc.d/rc.sysinit which check all the system properties, hardware, display, SElinux, load kernel modules, file system check, file system mounting etc. Based on the appropriate run-level, scripts are executed to start/stop various processes to run the system and make it functional. INIT process read /etc/inittab which is an initialization table which defines starting of system programs. INIT will start each run level one after the other and start executing scripts corresponds to that runlevel. The script information is stored in different folders in /etc/ folder

/etc/rc0.d/ –Contain Start/Kill scripts which should be run in Runlevel 0

/etc/rc1.d/ –Contain Start/Kill scripts which should be run in Runlevel 1

/etc/rc2.d/ –Contain Start/Kill scripts which should be run in Runlevel 2

/etc/rc3.d/ –Contain Start/Kill scripts which should be run in Runlevel 3

/etc/rc4.d/ –Contain Start/Kill scripts which should be run in Runlevel 4

/etc/rc5.d/ –Contain Start/Kill scripts which should be run in Runlevel 5

/etc/rc6.d/ –Contain Start/Kill scripts which should be run in Runlevel 6

Once the initialization process completes mandatory run level and reach to default runlevel set in /etc/inittab, init process run one more file /etc/rc.local which are the last commands run in initialization process or even booting process. Once everything is completed the control is given back to the kernel

Stage 6: User prompt

This is actually not part of booting process but thought of including it here for better understating. Once the Kernel get the control it start multiple instances of “getty” which waits for console logins which spawn one’s user shell process and gives you user prompt to login.