Enable a particular OS to update-grub

[DavidS]

I have my laptop set up for UEFI boot, and it is partitioned like this:

1. /dev/sda1 efi (Mounted as /boot/efi)
2. /dev/sda2 ext4 (UbuntuUnity 22.04)
3. /dev/sda3 ext4 (Ubuntu 20.04)
4. /dev/sda4 swap
5. /dev/sda5 ext4 (Mounted as /home)

Both Ubuntu versions mount partitions 1 and 5 as shown in the list.

If I am running the OS on partition 2 and run update-grub after making changes to /etc/default/grub, then the next time I reboot the machine I see my changes in the boot menu as expected. But if I run the OS on partition 3 (which was installed before the other) and make any such changes, they have no effect at all on the actual boot process.

Since the version on partition 3 is my "default" system at the moment, is there some way for me to enable it, rather than the other OS, to make changes to the grub boot menu?

[tea for one]

It looks like Ubuntu 22.04 on /dev/sda2 is controlling grub at the moment.

Boot into Ubuntu 20.04 on /dev/sda3
Open a terminal and enter:-

sudo grub-install /dev/sda

sudo update-grub

If there are errors in the terminal output, please post the command and output within code tags.
If no errors, reboot and see if your preferred grub menu appears.

[DavidS]

Thank you so much for that answer. I now have my system set up as I want it!

[oldfred]

Note that a major update to grub, will reinstall it.
So if 22.04 gets update, you may just have to redo the update in 20,04, if you still want it as the default.

[Bashing-om]

DavidS; Hey --

I too multi-boot 'buntus and my final solution to controlling grub is to disable /etc/grub.d/30_os-prober in my secondary systems; such then only my primary system controls booting.

but --- more than one way