Dual boot multiple fedora verions

[Chris Murphy]

On Sun, Nov 23, 2014 at 11:52 AM, Andy Campbell
<fedora at starsend.force9.co.uk> wrote:
> Is there an elegant way to install two versions of Fedora at the same
> time ?

Not automatically.

> p1  boot1  ( F19)
> p2  boot2  ( F20)

What you end up with in this case, is broken F19 right now because in
the F20 install environment, the F19 / LV isn't activated. So
grub2-mkconfig won't find F19 and thus won't make a boot entry for it.
If you boot F20, activate all the LVs, and re-run grub2-mkconfig -o
/boot/grub2/grub.cfg then you'll get a grub menu that has both Fedora
20 and 19 boot entries.

However, the F19 boot entries are a.) generic, they do not contain any
Fedora 19 specific boot parameters; b.) it's not updated from within
Fedora 19 when you get kernel updates.

So this is sort of brain dead simple for upstream grub folks to fix,
but they just don't give a crap near as I can tell. Or maybe they
think it's so brain dead simple that users should do this, or the
distributions should make grub work this way - I don't know. UX is not
GRUB's forte.

What you can do instead is, on Fedora 20, modify /etc/default/grub,
adding a line to disable os-prober:
GRUB_DISABLE_OS_PROBER="true"

That will prevent it from using os-prober to generate the Fedora 19 entries.

Then you'll use /etc/grub.d/40_custom to add your own boot menu entry,
which will forward to the Fedora 19 grub.cfg using something like
this:

menuentry 'Fedora 19' {
set root='hd0,msdos1' ## I'm assuming your F19 grub root is the first
/boot partition
search --no-floppy --fs-uuid --set=root <uuidforF19boot> ## You can
can this fs volume UUID from blkid and just copy/paste it in, this is
optional but more reliable if you have more than one drive attached
configfile /boot/grub2/grub.cfg
}

Save that and rerun grub2-mkconfig -o /boot/grub2/grub.cfg

So what this does is it makes the Fedora 20 grub.cfg contain a single
menu entry for Fedora 19. If you choose that Fedora 19 menu entry you
will get a new GRUB menu that contains only Fedora 19 boot entries.
Since you're now directly using the Fedora 19 grub.cfg that's updated
by grubby when new kernels are installed, you'll always have an up to
date grub menu for F20 and F19 once you've done all this work.

-- 
Chris Murphy