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Boot Procedure.md

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Boot Procedure

Grub

The location of Grub depends upon if you boot using BIOS system or en EFI system. To know if you are on a BIOS or EFI system. dmesg | grep "EFI". If you get nothing you booted using BIOS. You can dmesg | grep "BIOS" just to be sure. Another way is to use lsblk and if your boot disk is mounted on "/boot" than you are using BIOS. If it says "/boot/efi". You guessed it, you are using EFI. :)

You need to edit the configuration file in "/etc/default/grub" to make changes persistent. Afterwards you must compile the changes to grub.cfg. You cannot edit that file directly.

For BIOS/MBR

To make static changes to Grub edit "/etc/default/grub". Commit the changes you make. grub2-mkconfig -o /boot/grub2/grub.cfg

For EFI

To make static changes to Grub edit "/etc/default/grub". Commit the changes you make. grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

Systemd Targets

  • emergency.target
    • Used for troubleshooting in a minimal environment.
  • rescue.target
    • Also used for troubleshooting but not in a minimal environment.
  • multi-user.target
    • The default environment, without a GUI.
  • graphical.target
    • A fully functioning target like multi-user.target but with a GUI.

To know what target you are in do systemctl get-default. You can also do systemctl list-dependencies Since I'm in the grapical.target, it lists first the default.target and then underneath it is multi-user.target.

Change the default systemd target

Let's set the default to a non GUI target. systemctl set-default multi-user.target You must reboot the machine to enter the new default target.

Change a running target

Let's say we are running in a multi-user.target (GUI) and we want to switch to a non-graphical target we run this command. systemctl isolate multi-user.target Once we are done and we want to get back to the GUI we issuesystemctl set-default multi-user.target

Change a target during boot in Grub

Add this the end of the linux line. systemd.unit=emergency.target You can use any of the targets I listed before instead of emergency.

Debug Shell

How to use early boot debug shell:

  1. systemctl enable --now debug-shell.service
  2. During boot you can access the virtual terminal on TTY9. (Ctrl-Alt-F9)
  3. Enter a root shell without entering a password
  4. When you have finished using it you should disable the service right away. systemctl disable --now debug-shell.service