On Wed, Jun 01, 2016 at 12:22:14PM +0200, Florian Zieboll wrote:
> On Tue, 31 May 2016 16:14:58 -0400
> Haines Brown <haines@???> wrote:
>
> > How (simply) do you tell GRUB to boot an earlier available kernel?
>
>
> If you want to leave the problematic kernel installed, you can make
> grub2 default to another boot entry by adding/editing the line
>
> GRUB_DEFAULT=0
>
> to /etc/default/grub and then running "update-grub".

A more user-friendly way is "grub-reboot" for one-shot, and
"grub-set-default" for changing the default permanently.

I got an impression, though, that the question asked was not "how to change
the default from an already booted system" but how to get it to boot in the
first place. Unlike lilo which leaves you screwed with no recourse other
than fetching some alternate boot media, with grub you either press anything
(other than Enter :þ) on the grub prompt to stop the countdown, or, if the
grub prompt is not enabled, press Shift when BIOS/EFI loads the bootloader.
You can then choose which of installed kernels to boot, pass additional
parameters, etc.

--
An imaginary friend squared is a real enemy.