update grub + 2 ubtuntu installs with shared /boot

Gary W. Swearingen garys at opusnet.com
Fri Jun 16 19:28:36 UTC 2006

Philip Lawatsch <philip at lawatsch.at> writes:

> I have
> a amd64 system (dapper) installed into /foo, a
> a i386 system (dapper) installed into /bar and
> a partition mounted as /boot by both (!) installes

Please reconsider your way of saying where your OSes are installed.
Maybe the above makes sense in some context or with more explanation,
but it's seriously confusing to this long-time Unix user.  One usually
states the device names of the partitions where an OS mounts its

> Now my problem is that when update-grub is called on one system it will
> happily trash the other systems entries in menu.lst.
> What happens is that it seems to completely rewrite the entries and thus
> does not consider that the kernels have different root parameters.
> So, if update-grub is run on 32 bit then the next time i boot the 64 bit
> kernel I'll end up in my 32 bit system (with a 64 bit kernel though) and
> vice versa.
> Would someone know a fix for this problem _except_ creating seperate
> /boot dirs and install grub 3 (2 would work too I assume) times?

You can't be supprised that mounting one partition as the "/boot" of
two different OSes will give you grief.  It looks like to do what you
want, you'd need to customize the /sbin/update-grub script.  (The
script is apparently not smart enough to guess what root parameters
you want for each kernel nor allow you to tell it.) But that's
probably not worth the effort since editing menu.lst by hand is so
easy.  You'll probably have to read a bit of "info grub" to learn
how to specify the proper root partition for each OS entry in menu.lst.

I don't understand the comment about installing grub multiple times.
As long as you have installed the grub bootloader once, you should
only have to ensure that menu.lst reflects the kernels that you have
in your single "/boot" and then reboot.  And it shouldn't matter from
which OS you installed the bootloader.

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