The Linux Boot Process (Linux+ Objective 1.1.2)

hi i'm sean powers and i'm here to help you learn everything do what you love and most importantly be kind but today specifically we're continuing our linux plus series uh to get certified and we are on section 1.1.2 which is what i'm calling this part that i have in highlighted it's just the basic boot process and then also we're going to talk a little bit about kernel panics now again 1.1.2 is just something i made up this is just part of the 1.1 section objectives for the linux plus exam i'm going to put a link to the actual objectives in the description so you can download those and look at them but this is what we're covering today and the first thing we're going to talk about is bios versus uefi and honestly from a functionality standpoint both of them do pretty much the same thing ubfi and bios are both like firmware that is on the hardware of the computer that allows the software like the operating system to interface with the hardware of the motherboard and it's also the thing that determines how the system boots and all that sort of a thing they are different uh bios is older uefi is newer uefi stands for unified extensible firmware interface which is a mouthful but basically like i said they do the same thing this is when you start up the computer like press f12 to configure it and stuff whether it's bios or uefi you are configuring that firmware that comes on the motherboard of the computer now there are differences of course like uefi stores its configuration stuff on a partition of the hard drive as opposed to bios which doesn't necessarily store information on a hard drive in a partition so if you whether you're running running linux or windows if you have a system with uefi it's going to have a small partition the efi partition and you've probably seen that in windows and maybe in linux as well um there are some advantages uefi supports drives that are bigger than two terabytes uh it it's designed for gpt which is like the supports more partitions obviously it's newer now i put this last bullet point in green because even though a lot of new computers actually almost all new computers come with uefi they sometimes call it bios just so people know what it's talking about and so it'll be like you know press f11 to enter bios or enter setup and it'll just refer to things as bios even if it's actually uefi which is just super duper annoying but i guess i get it you know they want to use a common term and like oh i need to go into bios but all i have is uefi well you're going to configure stuff it's going to be in either uefi or bios anyway bios like i said is older it's still around on some systems especially older systems it just stands for basic input output system and again this is like the thing you configure like your drives and your peripherals that are on the motherboard and stuff like that it has limited uh support i don't know what i was going for with limited support it has limited support for a hard drive size you know two terabytes is the limit that it can support there are ways to get around that but by default it doesn't handle anything bigger than two terabytes uh the configuration is stored on the motherboard in a rom location uh and of course it is older but this that we're covering in this objective is the basics of linux like it's a summary of the fundamentals of linux and so from that standpoint just know that whether your system has uefi or bios that's what it's going to use to boot the system and how the operating system is going to interface in fact the operating system works fine with both linux is perfectly happy with bios or uefi it's just and it'll configure the system properly as you do the install so at the end of the day just know what they do and that you have one over the other and if it's newer you probably have that one now since this uefi or bios is the part that boots the system right i mean this is the interface between the hardware and the operating system it's booting we need to talk about boot sources and also boot sources as specifically mentioned and they mention a bunch of these so i want to talk about them but there's there's kind of a divide between hardware boot sources and software based boot sources and i'll explain what i mean basically the uefi or the bios is going to allow you to boot the computer to a number of locations now obviously a hard drive you know that's something that it boots to on a fully installed system uh you're probably familiar with like setting it to boot to a cd or dvd drive same thing with a usb stick a lot of times that will be one of the options you know like press f10 to choose the boot device or whatever or you can go into setup and choose which one it it starts from but there's a couple you may not have heard of pixi pxe which stands for pre-boot execution environment and i know that's kind of a stretch because it's like the second letter of execution but preview pre-boot execution environment and then uh some motherboards although i'll be honest i've never seen this uh have ipixi which is just uh you know i in front of the pxc initials and what these do is they allow you to boot from a network so thin clients will often pixie boot but you can also like do an install over the network by pixie booting or having a tft a combination of a dhcp server and a tftp server that will tell the system where to locate the boot files and then it will go to the tftp server which is just a very simplistic file server on your network and it will download the kernels to boot and rather than a local device it'll literally boot off the network which is just cool as heck the difference with ipxy is that instead of using tftp which is like i said is very simplistic it'll allow you to use an http stored kernel image and it's faster etc etc but functionally they are the same thing this is just to boot off of the network and so all of those are determined by bios or uefi however there are other boot sources that you can do but you kind of already start the process like you start to boot from the hard drive and then the linux grub 2 bootloader takes over and before it boots linux proper it will allow you to do a couple other things you can boot to an iso file so instead of burning a dvd to a disk and like hoping that you have a dvd drive it will actually allow you to boot directly from an iso file i've actually never done that but it's like a big feature of grub 2 so you know yay for yay for that um mem test is usually built in it's a it's a very stripped down operating system that will allow you to test ram in case you have a periodic kernel panics which we'll talk about later you could also boot other os's like let's say you have windows and linux grub2 could allow you to choose which thing it's going to boot so this is kind of like an interim it's like a it's like boots to this but then grub 2 decides where it's going to boot to now the bootloader that we have to learn about is grub 2 and you're probably wondering okay what about grub one well at the last iteration of the linux plus exam we did have to learn about grub one or grub legacy and grub two but they've eliminated any mention of grub legacy in this current iteration of the linux plus objectives so we don't have to learn that but just so you know grub 1 or just grub at the time or grub legacy is an older less flexible version grub 2 is obviously its successor and is more powerful it has a little bit of confusion that we're going to come into when we look at ubuntu and red hat separately but just know this is the software uh that starts the booting process and like we just learned it can actually either boot linux or boot something else or you know directly to boot from an iso file but grub 2 is the bootloader that we're going to learn about in linux there aren't other options grub 2 is it and now this is just a quick reminder i'm going to go through the entire boot process that linux takes i am purposefully going to just touch on a couple things because remember this is the very first section of the linux plus course we are just getting an overview of how stuff works you don't have to worry about configuring the nitty-gritty details of all of these processes you're just supposed to understand what actually happens when the linux system boots and so we start at the very beginning very good place to start with the bios or uefi depending on your system and it will you know if it doesn't boot to like a usb drive and it just goes right to the hard drive and starts grub 2 grub 2 will then load uh vm linux or vm linus now these are the actual kernel that ships with your distribution okay this is after install so we've installed the system uh grub 2 then we'll use this kernel and what's important to know is that first of all the difference here vm linux you probably won't see anymore because vm line is with a z is simply the same kernel just compressed to save space so you're going to see one of the two you're probably going to see it with the z but it's it is the kernel that is uh the the full kernel that ships with your uh ubuntu or with your linux system and it doesn't matter the hardware that you have this is a like a generic uh kernel compiled to work with a multitude of hardware and that's where things get tricky because they don't want to have you compile your own custom kernel every time we need to be able to load the specific things that your system needs and that's where init rd or init ram disk comes in what the init rd does is it loads just enough software like kernel modules to recognize your hard drive uh anything that it will need to load the full kernel so basically your hard drive like let's say you have an nvme drive or some special sata drive or something that needs kernel modules to load init rd will include that just so that it has enough to mount the whole system so that it can see all of the other installed files okay so this is like it it's kind of like bootstrapping the install uh process by just having a temporary init ram disk so it doesn't it only exists in ram like it uses the actual ram of your system to create this and those are the modules uh you actually create this init rd specifically for your computer and you don't do it the installer does it as you install your system it looks for what it needs and then it creates an init ram disk using the make init rd tool i only say that because it specifies that in the objectives and it creates this init ram disk that it uses once it it loads up the other stuff it will then mount the hard drive where all of the full kernel modules are because the the linux kernel is modular almost everything that you have to put into it are just modules that are external files that you like load into memory and so the full kernel is going to be on a hard drive once we can load that hard drive and mount that file system with the init rd then all of the modules that are on the actual physical hard drive can be loaded and so then the init rd you know it gets rid of that and it actually starts using just the fully mounted uh file system and so um that's after it boots now i do want to point out something let me i made a mess here so let me erase some of this um it's often confused when we say init rd versus the init ram fs and init ram fs this is this is getting down a rabbit hole and this is a little bit too deep maybe even for this video but i really want to touch on it the init ram fs is a file system that vm linux that the actual strip down bear kernel uses uh to it's kind of an in addition to the init ram disk and it uses um dynamic tools in fact drakut is the tool that you create the init ram fs with and what it does is it uses uh auto detection to dynamically uh create the devices required in order to mount the system to get all of the stuff loaded and i know that that's just you know that's just a whole nightmare of things but it's not the same thing as init rd that's the part i really wanted to point out init rd has all the stuff that your your kernel will need in order to load stuff init ram fs is a temporary file system that it creates and then dynamically mounts all the the devices that it needs in order to finally mount the full system and no longer depend on those things once the full system hard drive is mounted it has access to all of its kernels or all of its kernel modules it loads up and it works just fine all of this happens usually in a matter of seconds uh but that's the process that it goes through uh which is really chaotic but that that's how it goes so that was the boot process it's okay if it was like a little confusing you're like wait i thought i understood how linux boots and now i don't just look at that slide and those terms are things that you need to understand or not understand know about but not fully understand i'm not going to get into how we create the init ram fs using drag code and auto detected devices on the system during boot etc that's beyond the scope of probably even linux plus in general but that's the process it goes through when it boots now once it's booting again grub 2 is our bootloader that does all of those things and if you need to make changes to those it's important to know where the files live because knowing the name of the files is is just a nightmare and so i'm going to show you on both an ubuntu debian based system and also a red hat using rocky linux uh what to look for and it's going to be different all the time and it's so stinking annoying but first let's go to a bunch of because that's the simplest and i'll show you where you can figure changes in grub all right so first of all uh we're on ubuntu mata here i'm logging as rude i did sudo su just so i could b root because some things you need root access to c but if we look at the boot folder again that's where the kernels are stored we learned that in our last video you'll see that there are sure enough vm linus just like i said you know and this is just a symbolic link to the latest version of vm linus on our system and we have the init ram disc that is detected and created for our system there's actually a generic one long story a lot of hardware is detected and easily used inside a knit ram uh the init rd so that's perfectly fine there's like a system map the system map actually tells the system like okay once i can actually mount my hard drive where are all of my kernels that's in the system map files and so this and the config these are the configuration options that were used during the build of the kernel you got all that down all right what i wanted to show you though is inside here there's going to be so inside boot there's going to be efi for if it's a uefi system uh and if it's not uefi it's just going to be a grub folder it gets a little bit confusing here because a lot of times it will just be created inside the grub folder and then it'll be symbolically linked to the efi folder but anyway let's look really quick at the grub folder inside there we'll see this grub dot cfg this is where the configuration for what the system is supposed to do when it boots is located but it is not where you configure it do not mess with this grub config manually that's just not what you do okay the other thing if we look in the efi looks like there's going to be okay there is a grub 64.efi your installer is going to know the thing to create and it's going to put it in the right spot but again since this is not where we edit it this is just where they are eventually stored where we edit the configuration is going to be inside etc default and you'll see inside here there's a bunch of stuff but the one we want to look at is grub okay and let's just look at that vi grub this is where we can make changes so greet grub default like the first menu entry uh hide the system so it doesn't show you all the stuff as it scrolls by these are where we can make a bunch of different changes you're not going to need to make changes in here very often but if you need to make changes to grub it's going to be in this file so you make those changes and then on ubuntu all you have to do is say update dash grub uh oh it's either going to be update dash grub or update dash grub 2 and they're the same file right it's just one is a symbolic link to the other it's just update grub that you have to run and that will create all of the proper files it finds all of the init disks etc etc it creates it it creates that grub config file where it needs to be if it needs to be in the efi folder if it needs to be in the et cetera grub folder it finds all of those things and creates it just fine it's really nice that's all you have to do now the update grub actually uses a couple different command line tools some of those are let's see uh i'm just going to type grub dash and then hit tab a couple times to show us the few that are mentioned in the objectives are going to be grub install which is how you install the system like if it gets corrupt and you want to rub run grub install to get everything back to where it's supposed to be that's what the installer the operating system installer does at the beginning it will run that to put everything in the proper place the make config is actually what update grub does behind the scenes and i'll show you that more in depth on the red hat system because red hat doesn't have update grub you have to do it manually using grub mk config all right so that's that's important and then there's one other one that they mention oh grub 2 update but in ubuntu it's just update grub and again that's uh that's when you make a change you do that and it it creates it so that's how it works here i do want to point out that this is super annoying again because remember i said the grub and grub 2 thing well it used to be that these things were called grub 2 dash mate config but now since grub 1 isn't an option ubuntu decided why are we using the 2 why don't we just call it grub dash make config and have that point to the grub 2 program that we use so it's important to look at your particular system to see what the tools are called because like the objectives say grub 2 dash make config but on ubuntu it's grub dash mate config and it's the same thing it's all using grub 2. and rocky linux is even rocky linux or red hat linux is even more frustrating but the same we start in the same place first of all we're going to go into etc default and we're going to see there is grub if we mess with grub here we have fewer options here but we can make changes here make these changes and then because we don't have update grub which automatically creates the grub.config file in the right place we need to use grub 2 different than ubuntu this one still uses grub 2 dash and we need to use grub 2 dash make config but we need to know where to put that config file and so let's go into the boot folder and we'll see in here we're going to go into the grub 2 folder unlike a bunch where it was just grub so grub 2 and inside here we're going to see that's where the grub dot cfg file is okay so what we need to do if we make changes to that etc default grub command is run grub 2 dash make config dash 0 for output file and we're going to put it in boot grub 2 grub dot cfg it will generate that it will put it in there it will automatically put the uefi stuff in the appropriate place which is really nice right we don't have to like try to figure out if we have to put it in the efi folder it does that part automatically but we did have to figure out where exactly to put that configuration file and that's where we put it again you very rarely have to do this stuff but it's important to understand what happens and to add insult on top of injury on top of insult on top of frustration with the newest versions of red hat enterprise linux it's not clear if editing etc default grub and then running grub2-make config actually updates things they really really really want you to use their tool called grubby now grubby is not mentioned in linux plus so i won't even demonstrate that but basically what grubby does it's a command line tool that allows you to make the changes that you would normally edit inside the configuration file like the timeout stuff that you know we looked at that that grub file that we can make changes to like the timeout and the the default option and all that stuff instead of making configuration file changes they want you to use grubby and just make the changes with the grubby command and then the grubby command does stuff behind the scenes to activate those changes that you do again it's not listed in linux plus so you don't have to know that for the exam however if you're configuring red hat enterprise linux a newer version like version 8 or especially 9 just know that grubby is probably the way you're going to have to make changes not using the standard grub tool tools that you need to learn for the objectives and i know this video is getting long but the last thing i want to talk about is kernel panics because we've done a lot of stuff about kernels right like you've seen how you can like adjust things and there's multiple kernels vm lineath and like several kernels the nice thing about linux when you install a new kernel like an upgrade it keeps at least one of the previous versions in case something goes wrong with the new one you can roll back and rolling back isn't as obvious as it could be but i want to show you how you do that because a lot of times linux systems will have all that gr that grub menu stuff hidden so it's not ugly and you don't see a bunch of ugly text it just like keeps everything with like a nice little graphical thing in the middle so it's like oh i'm so fancy i'm a new operating system but you can't see options if you need to go through them so what you do is as you reboot the system hold down a shift key and that should reveal the grub menu which will allow you to pick a different way to boot the system remember it starts grub and then grub decides where to go from there one of the there's you can go to are a previous kernel so i'll show you on ubuntu we'll just restart it and i'll show you how you select that all right so i'm here on this ubuntu system i am just going to power it right down all right i'm going to power it up and as it starts i'm holding down the shift key so i'm holding it down and boom there we get the grub menu if you don't hold it down just as it's starting you're not going to see this menu but if you get to this menu you can go down to advanced options for ubuntu press enter and now look at this now there's recovery mode you can try to fix stuff but generally if you get a kernel panic right after an update uh you can go back to the previous version in our case this is 5.15 0-2-5 instead of 4-1 so this is the older one the one before the update happened just press enter and it will boot to that kernel and then you can like remove the new kernel and reinstall it and see what went wrong but you'll be able to boot your system without the kernel panic okay that's that's how you boot to a previous kernel hold down that shift key so you see this grub menu and that's really only if something drastically happens suddenly like you just made a change to a kernel or you just made grub updates or you know like i said the system updated to a new kernel and then just panics like when it restarts it like i'm not starting like killing init can't find init rd all those sorts of kernel panics that happened after an update that's how you would fix it unfortunately that that's not usually how kernel panics happen usually kernel panics happen periodically or randomly or like when the system gets like a lot of ram usage because usually kernel panics mean a hardware problem and it's so frustrating and hard to track down it could be one bad stick of ram and you could try running mem test which is one of those options in grub where it'll test your memory sticks you could try pulling out a stick of ram and see if it kernel panics and if it if it does kernel panic would that stick out well you know that stick's probably good and just keep swapping them to see if you can find the one that causes the kernel panic uh it could be uh your cpu's not seated it could be uh something on your motherboard is bad it could be uh it could be so many different hardware related things that it's very difficult to troubleshoot a kernel panic but if you know if if you get a kernel panic especially if it only happens sometimes it's probably hardware related and you need to track that down using just troubleshooting uh usually process of elimination but if it's kernel related something you just did you can roll back because that is uh it's usually obvious when something goes wrong like that anyway that's the boot process for linux i know it's crazy it's a little bit it's a lot of it confusing but if you followed along and and you kind of understood what i was saying that's going to get you to the place you need to be for the linux plus exam because again this is just a quick overview of the boot process and what uefi is and and how the init rd kind of fills in the gap so that it can fully boot and mount its hard drives uh to get access to all the kernel modules and all that sort of stuff anyway i hope i hope this is fun for you because this kind of stuff even though it's confusing it is fun and i always love delving deep into what linux actually does uh so if you found this interesting you're gonna love the rest of the course and if you found this overwhelming stick with it uh stick with me hopefully the rest of the course will uh make a little bit more sense you know come back and be like oh i get it i know what's going on anyway i'll see you at the next video i hope this was helpful bye

Channel: Shawn Powers

Views: 31,233

Keywords:

Id: esH6GUjVa8Y

Length: 26min 33sec (1593 seconds)

Published: Wed Jul 27 2022