RHEL9 and value GRUB_TIMEOUT=0 that unit is second and it's so fast so we can't enter grub menu. Is there anyone like me? We need to update /etc/default/grub and grub2-mkconfig again, after all that we can enter grub menu and select any boot target.
$ vi /etc/default/grub.cfg
$ [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
if you're BIOS
$ sudo grub2-mkconfig -o /boot/grub2/grub.cfg
, ortherwise UEFI
$ sudo grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
and reboot.
Source: https://www.redhat.com/sysadmin/linux-change-boot-options-grub
