Ubuntu – How to get the GRUB menu to display

You need to make:

GRUB_HIDDEN_TIMEOUT=5
GRUB_TIMEOUT=0 

Then, when you switch the computer on, pres ESC to enter to the boot menu, otherwise, the computer will boot in the default selection.

For sake of simplicity, you can change those values easily with Grub Customizer

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

In the tab General, click on advanced settings, then, select active the following keys and update the values to:

GRUB_HIDDEN_TIMEOUT = 5
GRUB_TIMEOUT = 0

Optional, to display a counter showing the remaining seconds while you can press ESC, set:

GRUB_HIDDEN_TIMEOUT_QUIET = false

OK folks, here's the answer... to obtain a hidden GRUB menu in dual-boot configurations... two edits... and a sudo update-grub...

Edit #1

To obtain a hidden GRUB menu in a multi-boot configuration, we first need to edit /etc/default/grub. Open this file using the below command:

sudo editor /etc/default/grub

Once the file is open, replace these lines

GRUB_HIDDEN_TIMEOUT_QUIET=true
#GRUB_HIDDEN_TIMEOUT=0
GRUB_TIMEOUT=10

with these:

GRUB_HIDDEN_TIMEOUT_QUIET=false
GRUB_TIMEOUT_STYLE=countdown
#GRUB_HIDDEN_TIMEOUT=0
GRUB_TIMEOUT=3

This will cause GRUB to display a 3 second countdown timer on the screen. By doing this, a user can hit the Esc key to bring up the default menu. Otherwise, the default OS will boot.

If you wish that the default OS should be set to the last-booted OS, add the below two lines under the "GRUB_TIMEOUT=3" shown above:

GRUB_DEFAULT=saved    # change an existing line to this
GRUB_SAVEDEFAULT=true # add this line

Edit #2

Next, the OS prober needs to be updated to disable the quick_boot feature. To do this, open /etc/grub.d/30_os-prober in your favorite editor and change the below line (line 23 in 17.04) by replacing the 1 with a 0:

quick_boot="1"

When you're done, the line should read like:

quick_boot="0"

Save the files and then run the below command to reconfigure the bootloader and to apply your changes:

sudo update-grub