GRUB - Grand Unified Boot Loader

GRUB's responsibility in the boot process

GRUB2 Installation

There may come a time where you need to install or reinstall GRUB2 on a system. Depending on whether your system is booting via BIOS or UEFI, you'll need to use a slightly different method;

1. BIOS

   The below command can be used to install GRUB2 on a BIOS system (using MBR disks format)
   

   grub2-install [options] /dev/sdX

   Option	Function
   --modules modulenames	Reload specified kernel modules
   --install-modules modulenamess	Install only the specified modules and dependencies.

   
   

2. UEFI

   
   

GRUB2 Configuration, Customisation, and Options

/etc/grub.d - This directory stores configuration scripts that define boot entries for GRUB2. Scripts within this directory are used to build the grub.cfg file;

daniel@jump:~$ ls -l /etc/grub.d
total 136
-rwxr-xr-x 1 root root 10627 Dec 18  2022 00_header
-rwxr-xr-x 1 root root  6260 Dec  2  2022 05_debian_theme
-rwxr-xr-x 1 root root 18683 Dec 18  2022 10_linux
-rwxr-xr-x 1 root root 43031 Dec 18  2022 10_linux_zfs
-rwxr-xr-x 1 root root 14387 Dec 18  2022 20_linux_xen
-rwxr-xr-x 1 root root 13369 Dec 18  2022 30_os-prober
-rwxr-xr-x 1 root root  1372 Dec 18  2022 30_uefi-firmware
-rwxr-xr-x 1 root root   700 May 17  2023 35_fwupd
-rwxr-xr-x 1 root root   214 Dec 18  2022 40_custom
-rwxr-xr-x 1 root root   215 Dec 18  2022 41_custom
-rw-r--r-- 1 root root   483 Dec 18  2022 README

If you wanted to add a custom script or feature to grub, you could do that within this directory by either adding a new file, or updating an existing file. 

/boot/grub - This directory stores the actual GRUB2 configuration files used during boot;

daniel@jump:~$ ls -l /boot/grub
total 2380
drwxr-xr-x 2 root root    4096 Jan 27  2024 fonts
-rw-r--r-- 1 root root     712 Jan 27  2024 gfxblacklist.txt
-rw-r--r-- 1 root root    9028 Jun  4 06:18 grub.cfg
-rw-r--r-- 1 root root    1024 Jul  4  2024 grubenv
drwxr-xr-x 2 root root   12288 Jan 27  2024 i386-pc
drwxr-xr-x 2 root root    4096 Jan 27  2024 locale
-rw-r--r-- 1 root root 2392289 Jan 27  2024 unicode.pf2

/etc/default/grub - This file contains the GRUB2 display menu settings that are read by the /etc/grub.d scripts.

There are also 2 commands to note for GRUB configuration:

grubby

• Function: grubby is a command-line tool that allows you to directly manipulate individual GRUB2 menu entries on your system. It provides functionalities for:

  • Setting the default boot entry
  • Adding, removing, or modifying existing entries
  • Changing boot order
  • Viewing information about current entries

• Focus: grubby operates on individual boot entries within the existing GRUB2 configuration.

  
  

Generating or regenerating the grub.cfg file

grub2-mkconfig

grub2-mkconfig (or sometimes shortened to update-grub2 on some systems) is a utility used to regenerate the GRUB2 configuration file (grub.cfg) located in /boot/grub. It reads configuration scripts from the /etc/grub.d directory.