On most Linux dedicated servers, grub is the bootloader that gets the operating system loaded when the system first boots. With grub, you can determine which installed kernel version will load, attach any startup options to the kernel, and even load other operating systems. In most cases, grub will do its work silently never showing much of itself, but if you need the grub menu to appear when you start your system, there are a couple of easy ways to do it.
Disclaimer: Incorrectly altering grub can cause your system to fail before booting. Therefore, I do not recommend this when you do not have physical access to your machine. If your machine will not boot, you will not have remote access.
When your system starts, immediately after it passes the initial BIOS screen, hold the Shift key. The grub menu should appear with a number of options for booting, depending on your Linux distribution.
Alternatively, if you need the grub menu to appear regularly, you can change the settings in /etc/default/grub. Change the line:
(or any number greater than zero – the interval is seconds)
In order to save the changes, you must then run “update-grub” as root (or sudo).
For more information about Grub2, see the online documentation.