GRUB

GRUB (GRand Unified Bootloader) is a widely used bootloader that allows users to boot multiple operating systems on a computer. It provides a flexible and powerful way to control the boot process, making it a crucial component in many Linux distributions. Here’s a detailed explanation of GRUB, its configuration, and usage:

Key Features of GRUB

1. Multiboot Support: GRUB can boot various operating systems, including different versions of Linux, Windows, and other Unix-like systems.

2. Extensible and Modular: GRUB’s modular design allows for extensions and customizations, making it versatile.

3. Command-Line Interface: GRUB provides a command-line interface for advanced troubleshooting and configuration.

4. Graphical Menu Interface: GRUB can display a graphical menu to select from available boot entries.

GRUB Configuration Files

grub.cfg

The primary configuration file for GRUB is grub.cfg, usually located in /boot/grub or /boot/efi/EFI/<distro> on UEFI systems.

/boot/grub/grub.cfg

This file is auto-generated and should not be edited directly. Instead, make changes to the source files that generate grub.cfg and regenerate it.

Source Configuration Files

• /etc/default/grub: Contains main configuration settings for GRUB.

• /etc/grub.d/: Contains scripts that generate grub.cfg.

Configuring GRUB

/etc/default/grub

This file allows you to set various options for GRUB. Here are some common settings:

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

• GRUB_DEFAULT: Sets the default menu entry (by index or name).

• GRUB_TIMEOUT: Sets the timeout before the default entry is booted.

• GRUB_CMDLINE_LINUX_DEFAULT: Adds kernel parameters for the default boot.

• GRUB_CMDLINE_LINUX: Adds kernel parameters for all boot entries.

/etc/grub.d/

This directory contains scripts used to create grub.cfg. Common scripts include:

• 00_header: Sets up the initial configuration.

• 10_linux: Adds entries for Linux kernels.

• 30_os-prober: Adds entries for other operating systems.

Updating GRUB Configuration

After modifying the source configuration files, update grub.cfg by running:

sudo update-grub

This command regenerates grub.cfg based on the scripts in /etc/grub.d/ and settings in /etc/default/grub.

GRUB Commands

Booting from the Command Line

In case of boot issues, GRUB’s command-line interface can be used to manually boot an operating system.

1. Access GRUB Command Line: Press c at the GRUB menu.

2. Load the Kernel:

   linux /vmlinuz-<kernel-version> root=/dev/sda1

3. Load the Initial RAM Disk:

   initrd /initrd.img-<kernel-version>

4. Boot:

   boot

Adding a Boot Entry

You can add a custom boot entry to /etc/grub.d/40_custom:

menuentry 'Custom Linux' {
    set root='hd0,1'
    linux /vmlinuz-<kernel-version> root=/dev/sda1
    initrd /initrd.img-<kernel-version>
}

Then update GRUB:

sudo update-grub

Troubleshooting GRUB

Recovering GRUB

If GRUB is broken (e.g., after installing a new OS), you can recover it using a live CD/USB:

1. Boot from Live CD/USB.

2. Mount the Root Filesystem:

   sudo mount /dev/sda1 /mnt

3. Mount Other Necessary Filesystems:

   sudo mount --bind /dev /mnt/dev
   sudo mount --bind /proc /mnt/proc
   sudo mount --bind /sys /mnt/sys

4. Chroot into the Mounted Filesystem:

   sudo chroot /mnt

5. Reinstall GRUB:

   grub-install /dev/sda
   update-grub

Fixing Boot Entries

If an OS is not appearing in the GRUB menu, run:

sudo os-prober
sudo update-grub

Conclusion

GRUB is a powerful and flexible bootloader that provides a robust way to manage the boot process for multiple operating systems. Understanding its configuration files, commands, and troubleshooting techniques is essential for Linux system administration and ensuring smooth system boots.