How does the Linux boot process work? - Quora

1.BIOS(Basic Input/Output System)

1. When we power on, BIOS performs a Power-On Self-Test (POST) for all of the different hardware components in the system to make sure everything is working properly.
2. Also it checks for whether the computer is being started from an off position (cold boot) or from a restart (warm boot) is stored at this location.
3. Retrieves information from CMOS (Complementary Metal-Oxide Semiconductor), a battery operated memory chip on the motherboard that stores time, date, and critical system information.
4. Once BIOS sees everything is fine, it will begin searching for an operating system Boot Sector on a valid master boot sector on all available drives like hard disks, CD-ROM drive etc.
5. Once BIOS finds a valid MBR, it will give the instructions to boot and executes the first 512-byte boot sector that is the first sector (“Sector 0″) of a partitioned data storage device such as hard disk or CD-ROM etc.
   BIOS find a valid MBR → boot and executes MBR

2.MBR(Master Boot Record)

1. Normally we use multi-level boot loader. Here MBR means I am referencing to DOS MBR.
2. Afer BIOS executes a valid DOS MBR, the DOS MBR will search for a valid primary partition marked as bootable on the hard disk.
   MBR → valid primary partition (Bootable)
3. If MBR finds a valid bootable primary partition then it executes the first 512-bytes of that partition which is second level MBR.
   MBR → valid bootable primary partition → execute first 512 bytes (Boot Loader)
4. In linux we have two types of the above mentioned second level MBR known as LILO and GRUB

3.LILO or GRUB

LILO - LInux LOader

1. LILO is a linux boot loader which is too big to fit into single sector of 512-bytes.
2. so it is divided into two parts an installer and a runtime module.
3. The installer module places the runtime module on MBR. The runtime module has the info about all operating systems installed.
   (installer module → runtime module → MBR)
4. When the runtime module is executed, it selects the operating system to load and transfers the control to kernel. (runtime module → select operating system to load → transfer control → kernel)
5. LILO does not understand file systems and boot images to be loaded and treats them as raw disk offsets

GRUB - GRand Unified Bootloader

1. GRUB MBR consists of 446 bytes of primary boot loader code and 64 bytes of the partition table.
2. GRUB locates all the operating systems installed and gives a GUI to select the operating system need to be loaded.
3. Once user selects the operating system, GRUB will pass control to the kernel of that operating system.see below what is the difference between LILO and GRUB

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Select operating system → grub will pass control → the kernel that operating system

 

4.Kernel

1. Once GRUB or LILO transfers the control to Kernel, the Kernels does the following tasks

• initializes devices and loads initrd module
• mounts root file system

5.init

1. The kernel, once it is loaded, finds init in sbin(/sbin/init) and executes it.
2. Hence the first process which is started in linux is init process.
3. This init process reads /etc/inittab file and sets the path, starts swapping, checks the file systems, and so on.
4. it runs all the boot scripts(/etc/rc.d/*, /etc/rc.boot/*)
5. starts the system on specified run-level in the file /etc/inittab

6.Run Levels

1. There are 7 run levels in which the linux OS runs and different run levels serves for different purpose. The descriptions are given below.

• 0 – halt
• 1 – Single user mode
• 2 – Multi-user, without NFS (The same as 3, if you don’t have networking)
• 3 – Full multi-user mode
• 4 – unused
• 5 – X11
• 6 – Reboot

1. We can set in which run-level we want to run our operating system by defining it on /etc/inittab file.