Re: Chainload grub to grub?

[Pascal Hambourg]

Hello Michael,

Le 06/03/2023 à 10:11, Michael Peters a écrit :
> Hi. I'm trying to figure out how to chainload one grub to another.
>
> bios boot, not UEFI.
> grub 2.06 built from source.
>
> What GNU/Linux identifies as /dev/sdc has a CentOS 7.9 install with its own
> /boot at /dev/sdc1 and the operating system root at /dev/sdc2. I have had
> too many bad experiences with a shared /boot between multiple Linux
> installs, especially when package managers like to rewrite the grub
> configuration file.
>
> What GNU/Linux identifies as /dev/sda has a "Linux From Scratch" install
> with its own /boot at /dev/sda1 and the operating system root at /dev/sda2.
>
> Switching the boot order in the bios every time I want to switch operating
> systems is a royal pain in the arse, but I also don't want a shared /boot
> partition.
>
> What I would like to do is keep the /dev/sda (the drive with Linux From
> Scratch) as the boot drive the bios sees, but have a menu entry that will
> chain load the CentOS managed grub.

The chainloader target must be a valid boot sector. It can be a GRUB 
boot sector or any other boot loader.
Here, I guess that Centos GRUB boot sector is in sdc, not sdc1 or sdc2. 
I do not know how to search for a whole partitioned drive or extract a 
whole drive id from a partition id.
Installing the GRUB boot sector in an ext* partition is not recommended 
because this uses unreliable blocklists.

Other options:
- swap the boot drive (hd0) and the target drive with "drivemap"
- load GRUB core image /grub/i386-pc/core.img with "multiboot"
- load GRUB config file /grub/grub.cfg with "configfile"

Some pathsnames may be different in your distro, grub2 instead of grub.