Re: Clarification of what grub-install does, versus grub-mkconfig/update-grub

[Chris Green]

On Fri, Nov 22, 2019 at 10:40:18PM +0100, Pascal Hambourg wrote:
> 
> 
> Le 22/11/2019 à 18:08, Chris Green a écrit :
> > 
> > grub-install installs the 'invisible' bits of grub like the MBR and
> > the code executed after the MBR.  These reside on parts of the disk
> > that one can't normally 'see'.
> 
> Yes. They are called the boot image and the core image, which must reside on
> the same disk. grub-install also installs GRUB modules in /boot (by default)
> or in the location specified with --boot-directory.
> 
> >  They are installed, by default, on the 'first' hard disk.
> 
> No, there is no default location for the boot image and core image. Anyway
> "first hard disk" is meaningless. First for what ? The BIOS ? Linux ?
> 
So how does it decide?  ... or at least how does, for example, an
Ubuntu install decide?


> > There only needs to be one set of these 'bits',
> 
> But there can be as many as you want, in every disk and every partition.
> 
Yes, I see that, one can install them on every drive but which gets
run at boot time then?


> > grub-mkconfig (or update-grub on ubuntu etc.) creates the grub
> > configuration file and other stuff that resides in the /boot
> > directory.
> 
> No, grub-mkconfig/update-grub generates only the configuration file
> /boot/grub/grub.cfg. Other GRUB files are installed by grub-install.
> 
OK


> > What I still don't quite understand is what happens with (as I have)
> > more than one OS installed.  Presumably only one grub.cfg gets used
> > even though there is more than one. How does the initial boot process
> > (starting with MBR etc.) and created by grub-install decide which
> > /boot it should use (and is it easy to change which /boot it goes to?).
> 
> The BIOS boots one drive. Its MBR contains a GRUB boot image which loads and
> runs the core image of the same installation, which loads modules and
> configuration files from the same installation.
> 
But someone said it doesn't matter which drive is marked as the boot
drive, how does the BIOS decide which drive to boot from?

-- 
Chris Green