Grub is the most commonly used boot loader in Linux systems. Grub : A program that calls a Unix/Linux operating system into memory. Officially GNU GRUB, GRUB is a popular boot loader due to its flexibility and configuration capabilities, allowing changes to be made at boot time and support for boot images from the network. Quite often you don't see the grub menu at all. And under most circumstances it is OK. But if you have some problems and you have to go to recovery mode, you have to see the menu and select an OS from the menu. So you love to see this grub menu when you boot your system and you are unable to see it. The culprit is the GRUB_TIMEOUT. If this value is set to 0, then you can't see the menu. The correct method to change its setting is through the file /etc/default/grub sudo vi /etc/default/grub $ cat /etc/default/grub # If you change this file, run 'update-grub' afterwards to update # /boot/grub/grub.cfg. # For full documentation of the op