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hydrapolic
n00b
Joined: 07 Feb 2008
Posts: 69
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 Posted: Wed Jul 04, 2012 12:02 pm Post subject: grub-2.00 fallback howto |
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This is a howto for those that would like to run grub2 with fallback. The problem is obvious: you administer a Gentoo machine that you don't have physical access to, or it takes too long to access it. However, as a good sysadmin, you want to keep your kernel up to date. It's not unusal to forget to compile in some driver and boooom ! Ping fails and you get nervous thinking about what you did wrong...
I'm happily using kernel 3.3.8 and I realy want to try out the new 3.4.4 release !
# /usr/sbin/grub2-set-default 0
# cat /boot/grub2/grub.cfg
| Code: |
load_env
function savedefault {
saved_entry=1
save_env saved_entry
}
set default="${saved_entry}"
set timeout=5
menuentry 'Gentoo 3.4.4' {
savedefault
root=hd0,1
linux /kernel-3.4.4-gentoo root=UUID=ad584938-9193-4cbc-8b87-3b74929205b4 panic=5
initrd /initramfs-3.4.4-gentoo.img
}
menuentry 'Gentoo 3.3.8' {
root=hd0,1
linux /kernel-3.3.8-gentoo root=UUID=ad584938-9193-4cbc-8b87-3b74929205b4 panic=5
initrd /initramfs-3.3.8-gentoo.img
}
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grub2-set-default stores the saved_entry variable in /boot/grub2/grubenv. load_env loads the variables (saved_entry) and sets the default entry to 0 (because we set it to 0). Grub2 tries to load Gentoo 3.4.4 but since it ends with a kernel panic (I "forgot" to include my sata drivers), the system reboots in 5 seconds (panic=5). However, before the system rebooted, the savedefault function stored saved_entry=1 in grubenv. After the reboot, grub2 loads the saved_entry of 1 and boots Gentoo 3.3.8. Don't forget to use your own UUID (use blkid).
Comments are welcome (pm). Thank you ! 
More info about grub2: http://wiki.gentoo.org/wiki/GRUB2_Quick_Start |
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