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Hey everyone and welcome back to another skill.

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So in this skill right here we have a lot of stuff to get to but the very first thing

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that we are going to focus on is the Linux boot process.

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Now the Linux boot process comprises of multiple different steps.

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We have to understand what those steps are and well the very first thing that we should

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do is talk about the first step in that process of course.

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So the very first process or rather step in the process should I say is what is called

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the BIOS.

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Now the BIOS stands for the basic input output system and the first thing to understand about

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the BIOS is that this is not related to Linux in fact it's not related to the operating

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system itself this is actually to do with the hardware on your computer.

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Now what is going to happen is that when you push the power button on your computer to

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turn it on BIOS is going to effectively take action.

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Now what the BIOS is going to do is it's going to perform something known as the post.

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This is the power on self test.

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Now all the post is going to do is it's going to just do some basic hardware checks.

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So this could be things like checking your keyboards are operational so on so forth just

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some very basic low level checks.

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Now crucially what the BIOS is going to do is it's going to be responsible for booting

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the operating system but the question is from where.

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So what do I mean when I say that we'll think about it.

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It may actually be not obvious that the operating system can be booted from different locations

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typically this is going to come in the form of your hard drive that is where the operating

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system is going to be installed but you can have things such as removable media say for

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example you could have an operating system stored on a USB stick.

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Now when the BIOS is engaged we can ultimately select whereabouts we want to try to find

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this operating system should be check the hard drive first or should be check for removable

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media should be check for CD ROMs whatever it may be.

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But the key thing here is that the BIOS is going to search whatever location you tell

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it to be at the hard drive or the USB whatever it may be.

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The thing that the BIOS is going to look for and this is very very crucial the BIOS is going

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to look for the presence of an MBR.

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This is the master boot records.

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Now so long as the BIOS can find the MBR everything is AOK the process can then continue.

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So really if we happen to have a particular order i.e. we want to search the hard drive

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first for an MBR if the BIOS finds an MBR we're going to load it from the hard drive.

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However if it does not find it from the hard drive it can then get down the chain to the

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next thing we have specified in our order of operations maybe that could be the USB

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or CD ROM or a network based drive.

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Nevertheless whatever it may be so long as we find this MBR which happens to be very

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tiny indeed this is only 512 bytes after all but these are very important bytes that happen

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to be located at the very first sector of the disk.

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And so long as the BIOS can find that MBR it will then attempt to load the operating

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system.

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So really once the MBR runs the next stage is the bootloader is going to take effect.

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Now to note there are different bootloaders available on Linux.

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The very first bootloader is one called Lilo and the thing to note about Lilo is that it's

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kind of old and antiquated you're not going to see it very much at all in modern based

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systems if at all in a modern based system and Lilo is Linux specific this is the Linux

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loader but ultimately this was replaced by the grand unified bootloader otherwise known

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as Grubb.

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Now Grubb itself has since been replaced with Grubb 2 which is really just the updated version

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of Grubb and this really is the one that you're going to see all over the place in modern

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Linux based systems.

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The thing here about Grubb is that Grubb is not just a Linux based bootloader the way

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Lilo is Grubb indeed can be used on other operating systems such as Windows.

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Now when the bootloader happens to run you may be happen to see a splash menu.

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Splash menu is just going to splash onto your screen and it's going to actually show you

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the different operating systems that you have available to you so let's maybe say you happen

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to have your hard disk and you partition it maybe you have a Linux operating system here

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let's maybe say Ubuntu and you also have a Windows operating system on the same disk

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this would be known as a dual boot scenario so let's maybe say you loved Linux for your

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system administration based stuff but you happen to really like the gaming ecosystem

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of Windows you wanted to have both the bootloader such as Grubb 2 is going to allow you to select

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which operating system we actually want to boot into now whatever bootloader you're using

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whether it is Lilo whether it is Grubb we have a primary purpose here that bootloader

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is going to load the kernel into memory now this is a very very important step the kernel

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as you may know is a very important part of the operating system it happens to be the

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core of the operating system and basically this kind of has control over absolutely everything

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so getting this to load into memory is a crucial component without this happening your system

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is not going to work now once the kernel has successfully loaded into memory we're then

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going to transition to the next phase this is called the kernel initialization phase so once

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the kernel has been loaded into memory what it's going to do it's going to load at least in modern

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systems system D now system D has a massive responsibility and modern Linux systems it's

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going to mount the file systems it's going to also handle the running and stopping of all the

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processes on the system but crucially to note is that system D is a newer implementation on older

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systems you may see different styles of programs such as sysv init now whether you happen to be

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running sysv init or system D essentially the same type of thing is going to happen they're going

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to be responsible for controlling those processes that are going to get your system fully up and

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running so really as a recap the process is going to be the very first step is going to be the

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BIOS which will do our basic checks for hardware the BIOS is then going to try to find the MBR on

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some type of media let's say a network drive or a hard drive or a USB the MBR is then going to run

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the bootloader so that could maybe be grub 2 and the bootloader is going to be responsible for

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loading the kernel into memory so then we have our kernel initialization and the kernel will load

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and modern systems at least system D which is going to mount your file system and begin running all

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of those essential processes to get your operating system up and running so that really is us for

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the basics of the Linux boot process I hope this has been informative for you and I'd like to thank

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you for viewing