How to get to the GRUB menu at boot-time?

Menu will appear if you press and hold Shift during loading Grub, if you boot using BIOS. When your system boots using UEFI, press Esc.

For permanent change you'll need to edit your /etc/default/grub file -- place a "#" symbol at the start of line GRUB_HIDDEN_TIMEOUT=0.

Save changes and run sudo update-grub to apply changes.

Documentation: https://help.ubuntu.com/community/Grub2

I have tried both the Shift and Space keys but nothing works. Only the Esc key works for Ubuntu 14.04 and 16.04 to get Grub Menu at boot time.

In Ubuntu 18.04, there is no GRUB_HIDDEN_TIMEOUT=0 line in /etc/default/grub - instead there is GRUB_TIMEOUT_STYLE=hidden. According to info -f grub -n 'Simple configuration' :

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.

So if you either remove or comment out the line as:

#GRUB_TIMEOUT_STYLE=hidden

and run sudo update-grub, then the menu will show by default.

18.04 + Not dual boot, boots in grub 5 seconds:

GRUB_TIMEOUT_STYLE=false

GRUB_TIMEOUT= (no value). Works fine LVM.