How to generate grub.cfg for different disk?

Question

I have 2 disks:
1-st disk contains base operating system (/dev/sda with many partitions and OSes)
2-nd disc should be setup (/dev/sdb1=/boot,/dev/sdb2=/)

I have grub2 package installed in the base operating system. I want to create grub.config file for the operating systems on the second disk (without installing grub2 package in chroot on second disk, because this disk is for different architecture. I just want to regenerate config)

if I use

mkdir -p /mnt/gentoo
mount /dev/sdb2 /mnt/gentoo
mount /dev/sdb1 /mnt/gentoo/boot
grub-mkconfig -o /mnt/gentoo/boot/grub/grub.cfg

then there is no way to specify /dev/sdb as a base disk for enumerating installed OSes for grub-mkconfig

If disk /dev/sda contains N partitions, then grub-mkconfig creates ~ N * number_of_kernels menu entries. But I want make it creating entries for partitions of disk /dev/sdb (which have one partition /dev/sdb2 with one installed OS).

I also tried grub-customizer utility, because it have disk selection in parameters dialog. But looks like grub-customizer uses all utilities and files which are relative to the choosen disk, and I don't have grub2 on /dev/sdb installed (see note above about different architecture)

Someone told me that it is possible to do this by temporary correcting fstab of base system, but I doubt this will help.

So, how to generate grub.conf for different disk? (without writing it by hands with texteditor)

UPD: feature request - https://savannah.gnu.org/bugs/index.php?44129

Solution

Can you elaborate on what your end goal is? It seems like you have an odd need and an even more curious need of a "widescale solution." Perhaps you're trying to streamline some mini-PC installation process?

Can you chroot into your sdb OS (maybe with qemu if a different architecture)? If so, that would be the best option. Otherwise, you can look at the source of the grub-mkconfig script and customize it for your needs. I see some relevant code that shouldn't be too hard to tweak.

Comments

[Admin]

just use sudo vim /mnt/gentoo/boot/grub/grub.cfg and delete the unused entrys. And DO NOT PLAY AROUND WITH FSTAB!

[Admin]

that is not widescale solution. Proper solution should add a new command line argument for grub-mkconfig i think. I just want a confirmation that there is no some tricks or other workarounds before filling feature request in bugtracker

[Admin]

I always install GRUB to the other disk (grub-install /dev/sdb) then unplug it (the pc must run during the complete process) and reinstall GRUB on the onboard device (grub-install /dev/sda)

[user1709408]

superuser.com/questions/615956/…