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The very first thing that happens while your Linux
system is booting is Grub, the grand unified bootloader.

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And this grand unified bootloader has
a Grub shell. And this Grub shell

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actually is very convenient to influence
the way how Linux is booting.

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Now, from Grub,
the kernel is loaded.

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A kernel comes with a helper component.
And this helper component is the initrd.

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It may also be known as init
RAMFS on your distribution, but the functionality

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is the same regardless. The name
initrd contains essential drivers that are required.

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Next component that
is loaded is systemd.

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Systemd is going through different phases. We really need to
use a different color here because we are in the

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different part of the boot procedure. I like to
distinguish between a phase that is called the early boot.

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In early boot you get all your essential system
components and after the early boot you get your services.

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Once you have had your services,
you're done and you're in a shell.

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Now, if things are going wrong,
you need to know that you

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can enter the Grub boot menu
and in the grub boot menu

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you can enter different parameters. The
essential parameters that you need to

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know about are systemd
dot unit is emergency target.

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That is booting you into the emergency target
and that jumps in right before the early boot.

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So you have really a minimum amount of
surfaces that is loaded and that makes troubleshooting easier.

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Now if you know that the
problem is not in early boot,

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it's always wise to boot a
little bit further and then you

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can use the grip boot
argument. Systemd unit is rescue target.

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That's another rescue mode. But in this rescue mode you
have already gone completely through the early boot, so have

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more of the normal operational parts of your system available
and that makes fixing your system a little bit easier.