Re: When does fallback setting take effect?

[Jordan Uggla]

On Wed, Mar 7, 2012 at 12:44 PM, Hongming Xiao <address@hidden> wrote:
> All,
>
> The grub manual says:
>
> fallback
>
> If this variable is set, it identifies a menu entry that should be
> selected if the default menu entry fails to boot. Entries are
> identified in the same way as for ‘default’ (see default).
>
> My question is: how does grub know whether the system "fails to boot"?
> Let's say in grub.conf default is set to 0 and fallback is set to 1,

Just to be clear, this list is for grub2 which uses grub.cfg, not
grub.conf (used by some forks of grub legacy).

> will system boot into 1 in following scenarioes? If yes, from where
> grub learn what happened?
>
> a) kernel 0 crashed during startup
no
> b) kernel 0 failed to start some service during startup?
no
> c) kernel 0 crashed at any time
no
> d) cycling power while kernel 0 startup?
no

Only if the commands in the menu entry fail. For instance, if the
vmlinuz file cannot be read by grub and so the "linux" command fails.

More *could* be done to change to a fallback entry the next boot after
booting fails after grub has passed control to the kernel. Ubuntu for
instance has grub set a variable recordfail (which is stored in
/boot/grub/grubenv using the save_env command) and then has a service
at boot, /etc/init.d/grub-common, which clears this variable
indicating that booting was successful (successful in this case
meaning that booting got as far as the grub-common service being
"started"). If at boot the recordfail flag is still there, then
something failed between grub and /etc/init.d/grub-common and Ubuntu
unhides the menu and removes the timeout so that the user has an
opportunity to see the menu and change options themselves (Ubuntu
normally hides the menu by default). With grub's scripting
capabilities this could easily be changed to 'if [ "$recordfail" = 1
]; then default=foo; fi' if you wanted to set something like this up
yourself.

> ....
>
> Thanks,
> Huang Zhong
>
> _______________________________________________
> Help-grub mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/help-grub

-- 
Jordan Uggla (Jordan_U on irc.freenode.net)