The very first thing that happens while your Linux system is booting is Grub, the grand unified bootloader.
And this grand unified bootloader has a Grub shell. And this Grub shell
actually is very convenient to influence the way how Linux is booting.
Now, from Grub, the kernel is loaded.
A kernel comes with a helper component. And this helper component is the initrd.
It may also be known as init RAMFS on your distribution, but the functionality
is the same regardless. The name initrd contains essential drivers that are required.
Next component that is loaded is systemd.
Systemd is going through different phases. We really need to use a different color here because we are in the
different part of the boot procedure. I like to distinguish between a phase that is called the early boot.
In early boot you get all your essential system components and after the early boot you get your services.
Once you have had your services, you're done and you're in a shell.
Now, if things are going wrong, you need to know that you
can enter the Grub boot menu and in the grub boot menu
you can enter different parameters. The essential parameters that you need to
know about are systemd dot unit is emergency target.
That is booting you into the emergency target and that jumps in right before the early boot.
So you have really a minimum amount of surfaces that is loaded and that makes troubleshooting easier.
Now if you know that the problem is not in early boot,
it's always wise to boot a little bit further and then you
can use the grip boot argument. Systemd unit is rescue target.
That's another rescue mode. But in this rescue mode you have already gone completely through the early boot, so have
more of the normal operational parts of your system available and that makes fixing your system a little bit easier.