sudo -H gedit /etc/default/grub)...
then save the file and quit the editor. Next run:
If this option is unset or set to
menu, then GRUB will display the menu and then wait for the timeout set by
GRUB_TIMEOUT to expire before booting the default entry. Pressing a key interrupts the timeout.
If this option is set to
hidden, then, before displaying the menu, GRUB will wait for the timeout set by
GRUB_TIMEOUT to expire. If ESC is pressed during that time, it will display the menu and wait for input. If a hotkey associated with a menu entry is pressed, it will boot the associated menu entry immediately. If the timeout expires before either of these happens, it will boot the default entry. In the
countdown case, it will show a one-line indication of the remaining time.