How can I display the grub during boot?

Edit the file /etc/default/grub, and find this line:

 GRUB_TIMEOUT_STYLE=hidden

Change that line to:

GRUB_TIMEOUT_STYLE=menu

and most importantly, after making that change, save the file and run:

 sudo update-grub

sometimes if grub is not installed properly you may need

if boot is legacy

sudo grub-install /dev/sdx

sudo grub-install --force /dev/sdx

sdx being the boot drive sda,sdb or sdc If boot is EFI you need to specify where your EFI System partition is mounted:

 sudo grub-install --efi-directory=<address of efi-directory (eg. /mnt/efi)>

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