Selecting the default kernel to boot into

The GRUB default kernel is preset to be the number zero (via the GRUB_DEFAULT=0 directive). This will ensure that the "first kernel" – the most recently added one – boots by default (upon timeout). This may not be what we want; as a real example, on our Ubuntu 18.04.3 LTS guest VM, we set it to the default Ubuntu distro kernel by, as earlier, editing the /etc/default/grub file (as root, of course) like so:

GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 5.0.0-36-generic"

Of course, this implies that if your distro is updated or upgraded, you must again manually change the preceding line to reflect the new distro kernel that you wish to boot into by default, and then run sudo update-grub.

Right, our freshly edited GRUB configuration file is shown as follows:

$ cat /etc/default/grub
[...]
#GRUB_DEFAULT=0
GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 5.0.0-36-generic&quot...