Sunday, 16 April 2017

Have you ever wondered what happens when you turn on your Linux machine? What processes are happening inside your hard drive, your RAM, BIOS and etc. Well, you can stop searching for answer on line, because here's an article that will explain Linux Booting process in 6 steps. I should note that this article is a part of my NDG Linux Course program that I'm enrolled in, so you can read it on my website.

Written by: Amar Tufo.
16 April, 2017

 1. BIOS - 

 BIOS seraches, loads and executes the boot loader.

2. MBR (Master Boot Record) - it is located in the 1st sector of the bootable disk /dev/sda. MBR is less than 512 bytes in size and consists of three components:

- primary boot loader 446 bytes
- partition table info 64 bytes
- magic numbers 2 bytes

MBR loads and executes the GRUB boot loader

3. GRUB (GRand Unified Bootloader) - It is a Multiboot boot loader which displays a splash screen, waits a few seconds, if you don't enter anything, it loads the default kernel image as specified in the GRUB conf file /etc/grub.conf. It contains kernel and initrd image. In case you don't know what initrd image is? Initrd stands for InitialRAMdisk which is initial root file system that is mounted prior to when the real root file system is available. The initrd is bound to the kernel and loaded as a part of the kernel boot procedure.

4. Kernel - Once the control is given to the kernel which is the central part of any OS and which act as a mediator between hardware and software. Kernel once loaded into RAM stays there until the machine is shutdown. Once the kernel starts it's operation the first thing it will do is to execute the INIT process.

5. INIT (Initialization) - Looks at the /etc/inittab file to decide the Linux run level. There are 6 major Linux run level every time Linux boots:

- 0 halt
- 1 single halt mode
- 2 multiuser NO NFS
- 3 full multiuser mode
- 4 unused
- 5 x11
- 6 reboot

6. Run Level Program - When the Linux is booting up, you might see various services getting started, such as 'starting sendmail . . . . OK'. There are called the run level programs. Depending on your default init kernel setting, system is going to execute the programs from one of the following directories: run level 0 - /etc/rc.d/rc0.d/ .

This is the 6 booting steps of Linux. If you like this article, don't forget to share on the social networks, be sure to visit my official LinkedIn page to find out about my work and the things I wrote and do. If you are willing to know in details about processes that are happening during Linux boot than follow the links bellow.

> 6 Stages of Linux Boot Process (Startup Sequence)

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