Hello, and this lecture, we are going to cover the Linux bood process and especially the process that
has been utilized in the newer version of Linux operating systems, the boot sequence changes in S.O.S
and RedHat seven and above, and some of the other operating systems as well.
So if you're using RAHAD five as five or six, you probably going to notice that it has or its uses
a different process in the in the newer version of S.O.S or read, had it actually utilized a different
boot process.
And this is what we are going to cover.
The System D is a new service manager and sent to us and rels seven that actually manages the boot sequence.
It is backward compatible with System four and its scripts sorry.
System five in its scripts is used by previous versions of RedHat Linux, including Réal five as well.
So you could use this process to run that could run on the previous versions as well.
Now every system administrators needs to understand the process of an operating system.
I would say whether it's Linux, Windows or whatever the operating system is in order to troubleshoot
more effectively because what happens if the system doesn't boot?
Then you need to understand all the sequence where it it goes through, how it goes through, which
it reads, so you could troubleshoot and go through each of them to identify the problem.
So let's go into the next slide and I will show you in a graphical way how this process works.
The first thing first is as soon as you power on a machine or a computer, it actually has a fun way
in it.
And remember, that firmware has nothing to do with the operating system.
That firmware actually comes in with that machine preinstalled by that manufacture.
That firmware is called bios, which is referred to as basic input and output setting.
So it starts up it takes the power on, it starts up and it goes through the post-process and the post-process
is power on self-test started meaning it starts up and then it goes through all the hardware that is
attached that makes the entire computer system, it goes through every one of them and checks.
Hey, you OK?
Are you OK?
Are you up.
Are there any problems with the hardware?
It goes through that post check after that, the NBER is loaded, NBER stands for Master Board record.
The information is saved in the first sector of the hard disk that is referred to as MDR, which indicates
where the Grubbed two is located so it can be loaded in the computer ram.
So bias looks up, does the post and then it it goes to the first sector of the disk and finds out that
MVR, NBER has information of Grubert two and the Grubbed two stands for Grand Unified Bootloader and
it is as it is called, as two meaning version two and it predates version one, which doesn't have
one it on it, but it just says Grupp.
So basically Grubbed two job is to load the kernel, the Linux kernel, and that information is in slash
boot slash grub to slash grub dot CFT, which is the config file.
So you when you put up your Linux operating system, you probably don't see the bios and pull post because
it goes through your screen so fast and MVR is probably not noticed because that's the time your machine
is looking for that sector.
But you will see at one point on your screen, on your black screen, it's going to show up as Linux
kernel and sometimes you probably going to see one, two, three or five different kernels that it needs
to load, needs to boot from.
So you could at that time also move your cursor up and down and select the kernel you want to load or
let the system decide which kernel it's going to load it depending on the grub configuration.
All right.
So now the kernel comes up, Kernel says, OK, the grub asked me to to start, OK, the kernel is the
core of the operating system.
So what it does is it loads the required drivers and it ordered IMG file.
Now this is the colonel's job is to load the driver, meaning every hardware that is attached to your
machine.
It in order for it to work, the colonel has to communicate to that hardware, so the colonel, in order
for colonel to communicate, it has to has that driver that talks to the hardware, say loads up all
those hardware, and then it starts the first, very first OS, our operating system process that is
called System D now system D, as I said earlier, it's a new process introducing the new version of
Linux.
So it starts up the next step as the last of the system D starts up.
And of course this is the first process in the system and by default it is given a process ID of no
one.
As the other process starts, that gets the incremental numbers of the process.
It's then start all the required processes that is needed that were enabled at the time.
Then it reads the azzi system D system default, the target file this this to bring the system to the
run level.
Now every system, every Linux system has a total of seven run levels.
That goes from six through zero through six, which defines where it's going to start.
It needs a networking single user mode or if it's actually needs to shut down.
All those run levels are actually defined in default dot target file.
So it looks up that in the file sees which target file it has to solve, which run level it has to start,
and that's how your system becomes online.
So this is the Linux process.
And as I said, this is a newer process that's introduced lately in newer versions.
So definitely go through this.
This is one of the important things that every system administrator needs to know, needs to remember
while they are troubleshooting the issues.
All right.
Good luck.