Aside from providing a fancy looking menu when a system starts, GRUB, or GRand Unified Bootloader, is a boot loader that allows us to choose which kernel we would like to boot into.
For a better explanation, here the excerpt from gnu.org:
Briefly, a boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel software (such as the Hurd or Linux). The kernel, in turn, initializes the rest of the operating system (e.g. GNU).
So now that the fancy definitions are out the way, let’s modify the standard GRUB configuration to enable this mysterious menu. First we should make a backup copy of the current file in case we mess things up:
sudo cp /etc/default/grub /etc/default/grub.bkup
Once that is done, we can then procedd to modifying the file. In this example, we will use vim as the editor:
sudo vim /etc/default/grub
Now, there may be references Online saying that there are two lines to be modified. In my experience, I have only had to modify one o them, which is this one:
and comment it out the following way:
Now, what cn be done is change the amount of time the menu will show before boting into the default kernel. For that, the line to modify is:
That 10 stands for ten seconds. Change it to how ever many seconds you wish is appropriate, maybe something like 3 should be enough time to intervene with the keyboard.
Once done editing that file, we can save an exit. For the changes to take effect, we need to update the configuration. For that, we issue the command:
And done! Now when reooting, a menu will appear with different boot options. To select a different kernel, simply go down to Advanced options for linux/ubuntu depending on your distribution.