Use grub to boot the target kernel after reboot

The key concept is that grub.cfg is the final product. The grub behavior will be performed according to grub.cfg (and the environment variables used by it). Thus please try to control everything with the tools of grub first. Let the tools control grub.cfg and environment variables for you.

Firstly, enable the default value is mutable: change the default value from 0 to be saved.

$ sudo vi /etc/default/grub
GRUB_DEFAULT=0

to be

GRUB_DEFAULT=0

and then give it a default value (0 here). The number "0" is the grub entry number. It could be 0, 1, 2, ...etc. depends on the order and number of your entries.

$ sudo grub-set-default 0

Update the grub.cfg and make all change effective.

$ sudo update-grub

Now your grub entries are ready to be selected. For example, select the 6th entry of a sub-menu (order number 1)

$ sudo grub-reboot "1>6"

And then reboot

$ sudo reboot

By the way titles should also work according to its GNU manual. https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html

Previously it was documented the way to use entry title. While this still works it’s not recommended since titles often contain unstable device names and may be translated