In Ubuntu, Grub boot menu is hidden by default. You can display it by pressing “SHIFT” continuously at boot time. This is useful (among other cases) in case you want to boot in Recovery mode.
To make this change permanent:
With your favorite editor (nano in my case)
sudo nano /etc/default/grub
Comment this line
GRUB_HIDDEN_TIMEOUT=0
and save your changes
#GRUB_HIDDEN_TIMEOUT=0
Update grub
sudo update-grub2
Finally, reboot
systemctl reboot
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