Can anyone explain me with fairly simple words and not too technical concepts what exactly is GRUB, what is its primary usage and how to use it efficiently for a dual boot Windows/Ubuntu installation?

I have already read a couple of resources but I am still confused.

  • Welcome to askubuntu. I feel your question is too broad. I suggest you start here - en.wikipedia.org/wiki/Linux_startup_process . At the final step Ubuntu uses upstart , which is discussed here - en.wikipedia.org/wiki/Upstart . After reading those overviews, if you have a more specific question about UBUNTU then come back here. If it is about booting in general, the kernel, or general linux you will be referred to alternate sites such as stackexchange.
    – Panther
    Sep 18, 2013 at 19:13
  • 4
    The question seems pretty specific to me.
    – psusi
    Sep 19, 2013 at 1:15

3 Answers 3


Basically, GRUB bootloader is the software that loads the Linux kernel. (It has other uses as well). It is the first software that starts at a system boot.

When the computer starts, BIOS first run a Power-on self-test (POST) to check hardware like memory, disk drives and that it works properly.

Then BIOS checks the Master Boot Record (MBR), which is a 512 byte section located first on the Hard Drive. It looks for a bootloader (like GRUB). The hard drive's partition tables are also located here.

enter image description here

Then you'll be prompted by the GRUB menu which can contain a list of the operating systems installed (in the case of dual boot), or perhaps the different kernels installed in a Linux distro.

When you choose which distro or kernel you want to use, GRUB loads the selected kernel. The kernel starts init (or systemd), which is the first process to start in Linux. init then starts other processes like network services and other that you might have configured to start at boot time.

enter image description here

The boot process contains alot of defined expressions and terms, way to much to add in a simple answer like this. Google is your friend!

Remember that this is a very basic explanation of the boot process and GRUB's role in it.

This youtube video provides a pretty nice introduction to the linux boot process, including basic information on the GRUB Bootloader:

Understanding the Linux Boot Process - CompTIA Linux+, LPIC-1

  • The youtube video is quite old and long and I found this itsfoss.com/what-is-grub very helpful and easy to read.
    – Rick
    Nov 29, 2021 at 15:06

Grub is the Grand Unified Boot Loader.

If you come from Windows the easiest explanation for GRUB is "This is the program that linux writes to your MBR (Master Boot Record)"

This program is responsible for detecting and loading any OS on your personal computer.

In common cases You have simply to install Ubuntu after you put window on your machine. The Ubuntu installer will automatically detect your windows installation and write the configuration file for you.

So When you turn on your PC GRUB comes up and shows you a menu to select if you like to boot either Windows or Ubuntu.

That's all.


GRUB is a multi boot boot-loader, is a programm who run at the system start and allow you to select what Operation system you want to start, like if you want ubuntu and windows on the same computer, with GRUB you can select those at system start

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