Re: grub: eval vars for linux command / write config agnostic to other disks

[Pascal Hambourg]

Le 28/03/2020 à 17:56, Toerless Eckert a écrit :
> I would like to have a grub config that will work even
> if the disk is re-plugged into a different SATA port
> and independent of whether other disks are plugged in.
>
> Lets say i today have this config:
>
>   set root='hd5,gpt2'
>   search --no-floppy --fs-uuid --set=root 4bdb3675-62ff-499f-9fc4-e5fd8c7ba777
>   linux   /vmlinuz root=/dev/sdf5 ro
>
> The problem really is the "root=/dev/sdf5" when starting linux.

Indeed.

> When the disk is plugged into a different slot, or if one of
> the prior disks is removed, then /dev/sdf5 will be wrong.

There are many other situations which may cause a change in drive names.
SCSI drive names are not persistent across reboots. Do not rely on them.

> I know, ideally inux should be able to solve the problem itself,
> but it seemingly can't: LABEL=<label> or similar are not available
> when choosing the root partition.

That's because you do not use an initramfs (formerly initrd). The kernel 
itself does not understand UUID or LABEL.
Why don't you use an initramfs ? This is quite common these days, and so 
much more convenient.

> Is there a way to evaluate grub2 variables being passed to a
> command like the linux command ?

Yes, but it is not reliable. There is no persistent mapping between GRUB 
device numbers and Linux device names.

You'd rather use a persistent identifier that the kernels kows about : 
PARTUUID. Get the value with blkid.

Can't remember whether the kernel knows about PARTLABEL too, which would 
be nicer. Guess not.