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Say you have a "main" GRUB on /dev/sda, with several linux kernels, and you also have a secondary linux distribution on /dev/sdb2.

The computer boots into /dev/sda GRUB. From there, how to boot the kernels in /dev/sdb2 ?

When you update-grub in the distribution in /dev/sda, the OS-prober will detect those in /dev/sdb2 and populate the GRUB menu accordingly in (/dev/sda)/boot/grub/grub.cfg.

But this is inconvenient, as when you install a new kernel or change boot options in the /dev/sdb2 distribution, this will update (/dev/sdb2)/boot/grub/grub.cfg but not the main grub.cfg.

How to get the main GRUB to read and incorporate the entries from the secondary GRUB ?

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There is a GRUB command, not documented in the manual, that precisely parses a config file, extracting the entries and ignoring the remainder of the configuration : extract_entries_source. With this, any change to the secondary distribution's GRUB menu entries will be automatically taken into account in the main GRUB's menu.

In the main ("host") distribution

Add a custom entry in /etc/grub.d/40_custom :

# A dummy entry to mark the separation
menuentry "Extracted from /dev/sdb2" {
        true
}

# the following if for MBR disk, change to part_gpt if necessary
insmod part_msdos
# adapt with appropriate file system module
insmod ext2
# replace aaaa-bbbb-cccc with actual partition UUID
search --fs-uuid aaaa-bbbb-cccc --set slavegrub --no-floppy
extract_entries_source ($slavegrub)/boot/grub/grub.cfg

The partition's (file system's) UUID can be obtained with blkid.

You will also want to prevent GRUB to automatically populate the menu with OS-prober. This is configured in /etc/default/grub and can be disabled globally with GRUB_DISABLE_OS_PROBER or selectively for some partitions with GRUB_OS_PROBER_SKIP_LIST (see GRUB manual).

Then you can update-grub.

In the secondary distribution

There is nothing to do as the /boot/grub/grub.cfg file will only be read for its entries and will never be actually executed. You will probably want to disable the OS-prober completely there, as those entries would be redundant when integrated within the main GRUB's menu.

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