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Linux boot process. So in this lesson, we

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will learn how the system boots up. So

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I'm sure you probably wondered that when

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you press the power button on your

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laptop or your computer, then after

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a few minutes, a few seconds your

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computer come up, you get a login prompt

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and your system started. But have you

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ever wondered what exactly happens under

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the hood? Meaning you start up your car

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and ignites the spark and start up your

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car, right, but what happened exactly

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behind the scene? So I want to cover

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that because it's one of the very

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important lessons. And some of the

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things in this lesson are important in

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terms of interview, a lot of people ask

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you questions about what happen, or

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tell us the sequence of boot process. In

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this picture, you will see exactly the

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six steps that a system takes place in

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order to bring your system to a level

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where you will be able to log in. The

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first one is every time you press the

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power button on your computer, it goes

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to BIOS. What BIOS stands for is, basic

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input and output of a system. At this

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stage, it performs some system integrity

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checks, loads and execute bootloader

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program. It looks for bootloader like

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floppy disk or CD ROM or your hard drive,

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it tries to find out how to boot, where's

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it located, where do I have to go to

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bring the system up. The next thing that

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it does, it goes to MBR. Now MBR stands

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for master boot record. It is located in

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the first sector of the bootable disk,

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typically like DEV, HDA, or SDA Linux. So

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this is the first sector on your disk

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where it is located to start the computer.

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MBR is somehow like 512 megabytes in

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just bytes and sizes, very small. And

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this, it contains information about GRUB.

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The next step it does its goes to GRUB.

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GRUB stands for grand unified boot

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loader. If you have multiple kernel

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images installed on your system, you can

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choose which one to be executed. So

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there could be older version of Kernel,

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newer version of your Kernel, you could

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start it on any of the Kernel, and that

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option is given to you by GRUB. And GRUB

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configuration files are located in slash

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boot slash grub, and the file is

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grub.conf. You can modify it if you want

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to change the order of your system

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startup. The next one, after the GRUB

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comes the Kernel. The Kernel is the one

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that mounts the root filesystem as

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specified in the root equal in grub.conf

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file. That's exactly in the

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grub.conf file is, it actually execute the

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init program. Kernel executes the slash sbin,

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slash init program, then it starts

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the initial RAM disk, and initial RAM

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disk which is initrd is used by

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Kernel as a temporary root filesystem

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until Kernel is booted and the real

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root filesystem is mounted. It

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also contains necessary drives compiled

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inside which helps us to access the hard

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drive partition and other hardware. The

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next one goes through the the boot

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process is init. It looks like, the

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slash etc inittab. This is the file

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to decide the Linux run level. So if you

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wanted to boot your system in a

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different run levels, let's say there

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are six run levels, actually seven run

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levels. There is 0 for a halt, 1 for

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single user mode, 2 for multiuser,

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3 for full multiuser mode, and 5

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is unused if sorry, 4 is unused,

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5 is to boot the system in x11 and

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6 is to reboot. These are the run

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levels that are defined inside of the

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init, slash inittab file.

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Once it does that, then the runlevel program

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begins to start, and every

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run level program has its own directory,

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and that directory is /etc/rc.d/rc,

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enter digit, which is run level digit,

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and .d. Every file, every boot order

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is in these files. So here are the run

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levels. We could go over a few of them

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in our Linux environment. So let me log

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into my Linux machine, clear the screen.

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I want to become root, because some of

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these files can only be accessed through

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root. So the first one is BIOS, which is

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located on the disk. The second one is

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MBR, and let's go to the Kernel, which

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is sbin/init. So we'll go etc,

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sorry, sbin/init.

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[Author typing]

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Then you have other run programs, we

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could go to the other run programs like

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/etc/rc .d/, and here you see all

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the run levels. If you wanted to start your

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computer and run level five. So let's

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see which run level we are in?who minus

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r, we are in run level five. So we'll go

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into rc5.d. And here it's

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telling it to start the console, which

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is the GUI, and start the network, which

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is network. So this is what exactly run

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level five is. And that's how your

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computer is started, you get the login

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prompt, and there are also many many

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different documentation online you can

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find out about how the system boots up.

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And of course I will encourage you to go

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look them up and understand exactly how

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Linux boot process works.

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[No audio]