When you power on your computer, what happens before you see your operating system’s logo?
Before the OS even begins to load, a crucial piece of software called the bootloader takes over. In the Linux ecosystem, the most widely used bootloader is known as GRUB.
What is GRUB?
GRUB (GNU GRUB, short for "GRand Unified Bootloader") is an essential utility that manages the boot process, allowing your machine to start one or several operating systems. It’s automatically installed during most Linux setup procedures and is the first program to take control after BIOS or UEFI finishes initializing your hardware. Depending on your setup, GRUB will either present a menu to choose between operating systems or directly launch the Linux kernel without needing user input.
What can GRUB do?
GRUB is more than just "start Linux." It can:
- Display a system selection menu in multi-boot setups (Linux + Windows, multiple Linux distributions, etc.),
- Allow you to manually specify the kernel and boot parameters,
- Boot from various file systems, including ext4, Btrfs, FAT, NTFS, and others,
- Support both classic BIOS and modern UEFI boot modes,
- Help recover a system when normal booting fails (via a command line interface).
How does GRUB work?
The computer’s boot process happens in several stages:
- BIOS or UEFI — Initializes hardware and searches for a bootloader.
- GRUB Stage 1 — Loads from the MBR (on older systems) or the EFI partition (on newer ones).
- GRUB Stage 2 — The main part of GRUB, which loads the configuration and the kernel.
- Operating System Kernel — The Linux (or other OS) kernel is loaded and takes over control.
Configuring and Recovering GRUB
Where is GRUB’s configuration stored?
The main GRUB configuration file is located at:
However, it is not recommended to edit this file directly because it is automatically generated by the system. Instead, you should modify:
After making changes, update the bootloader configuration using:
Example configuration of /etc/default/grub:
GRUB_DISTRIBUTOR=`lsb_release -d -s 2>/dev/null || echo Ubuntu`
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX="acpi=force"
How to recover GRUB if it stops working?
If after installing Windows or replacing a drive Linux no longer boots, GRUB might have been overwritten. You can recover it using a Live USB distribution.
Run the following commands:
sudo grub-install --boot-directory=/mnt/boot /dev/sdX
Then, enter the installed system's environment and update the configuration:
update-grub
GRUB and Multi-OS Selection
If you have multiple operating systems installed on your machine, GRUB will automatically create a boot menu allowing you to choose which OS to launch — for example:
- Ubuntu
- Windows 10
- Advanced options for Ubuntu
You can customize the order and appearance of this menu via the /etc/default/grub file and update it using update-grub. You can also set the default system and adjust the timeout before automatic boot.
GRUB 1 vs GRUB 2
Today, most systems use GRUB 2 — a more flexible and modular version that supports modern boot formats. The older GRUB (Legacy) is now obsolete and rarely used.
Conclusion
GRUB is not just a bootloader — it is a critical component of the Linux ecosystem, providing flexible control over operating system startup. Thanks to its wide range of features, it fits both simple user systems and complex setups with multiple OSs and non-standard boot conditions.
Proper GRUB configuration can not only speed up the boot process but also help prevent potential issues related to multi-boot setups, system updates, or recovery after failures.
While its configuration might seem complex at first glance, understanding the basic principles — like editing /etc/default/grub instead of directly modifying grub.cfg — significantly simplifies working with this powerful tool.
By mastering GRUB, you gain not only control over your system's boot process but also increase its resilience to unexpected changes and failures.
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