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- All right, so previously we have talked

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about the different phases
in the boot procedure.

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Now we need to talk about
which tools you can use

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in case anything goes wrong.

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Well, if it goes wrong very early

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like before you see the boot loader prompt

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then there is only one option

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and that is the rescue disk.

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The rescue disk typically
is the installation disk

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of your Linux distribution
and it allows you

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to access a prompt from which
you can access your hard drive

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and actually fix it like reinstall grub 2.

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If you do see grub 2 but
you don't get to your kernel

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you might have an option to
get through the grub menu

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and on the grub menu you
might change parameters

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to tell your kernel how to load.

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If the kernel and the
initramfs are doing okay,

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well then still from the grub boot menu

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you can do a couple of things.

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You can use the option init
= slash bin slash bash.

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That is if you have a very
early problem in System d.

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It allows you to break into your system

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before system d is actually loading.

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Otherwise, there is the emergency target.

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Which is a target in System
d, and that allows you

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to get into the System d
procedure pretty early.

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And if the problem is a little bit later

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there's also the rescue.target.

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The rescue target is also
interrupting the boot procedure

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and that allows you to get into System d

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but a little bit later.

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So if you have a problem
loading your disk devices

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for instance in System d

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well then emergency target
is more appropriate.

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Rescue target is bringing you in

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a little bit further in the procedure.

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All of these options

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are all parameters that are also entered

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on the grub boot menu.

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The difference is that here
they are kernel arguments

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and here if something is preventing you

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from seeing the kernel, you
might use the grub boot menu

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to refer to another kernel.

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So that is what you can
do for troubleshooting.

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Let's go check it out.

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So I just pressed the shift
key to enter the grub boot menu

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on this CentOS Stream system

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and we are going to use e for edit.

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And I would like to get started
by using the most minimal

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of all init = slash bin slash bash,

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which is making sure that
we get into the system

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before system d.

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So what is so interesting about
init = slash bin slash bash,

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Do you see that?

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I have a root prompt.

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Who am I?

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I'm root and I did not enter a password.

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But the thing is that we
are in a very minimal mode.

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We don't even have system d.

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So system ctl list
units is giving me what?

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It's giving me fail to connect to bus,

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no such file or directory.

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Init = bin bash means don't give me init,

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which is the normal system d

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give me bash, and that's where we are.

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And if at this point

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I want to get to the system d environment,

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I need to use exec slash
usr slash lib slash system d

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slash system d.

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Now what is this thing?

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Well, exec, if you use it
in order to start a program

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you remove the current program from memory

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you replace it with a target program

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And that's exactly what we need

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because system d needs to be pid 1.

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If I would just start system d

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then system d would become a child

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of the current best
process that's not working

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and that's why I needed this

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exec to usr lib system d, system d

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which is the name of the system d binary.

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Now we are back

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and I can show you some more.

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So let me restart again.

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Shift key to enter the grub boot menu

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and again e for for edit

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where again I'm going
to the end of the line

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that reads Linux.

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I'm removing rhgb and quiet
and I'm using system d dot

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unit = emergency.target cause
that is where I wanna be.

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Now where init = bin
bash is letting you in

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without entering any passwords here

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you need to enter the root password.

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So system ctl,

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list units

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showing all the units,
we have 49 in total.

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This is a minimal troubleshooting mode

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as I've already shown you before.

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Even your root valve
system is in read-only mode

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So you need mount minus o
remount comma rw on slash

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to make it reach right ball
and then you can reboot.

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I'm going to leave CentOS for now

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and we are going to check out to Ubuntu.

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So here we are on Ubuntu

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pressing e to edit and looking
for the line that ends Linux.

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I am going to remove the quiet and splash.

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And I would like to show you

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system d dot unit = rescue dot target.

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And control X to boot with that option.

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So here we are in the in the rescue mode.

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Now in Ubuntu it's pretty interesting

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because in Ubuntu you don't
have to enter a root password.

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Why not? Well, Ubuntu root
user doesn't have a password.

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So we get free access, which
makes it a bit vulnerable

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but it's the way it is.

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So I just press enter
and I'm in a root shell

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where I can use all of
my troubleshooting skills

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like system ctl list units.

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That's not really troubleshooting

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but I want you to get an idea

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of the units that are
currently loaded, 104.

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On CentOS, it's about the same number.

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The thing here is that the rescue target

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is bringing you in a
quite complete system.

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An emergency target is only
giving you a minimal system.

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I think that's enough for troubleshooting.

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Let's reboot and in the next video

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you'll learn about using the rescue disk.