On Mon, 2019-10-07 at 23:02 +0000, sixpack13 wrote:
> /etc/default/grub
> is used during kernel updates
>
> there GRUB_DEFAULT= should be 0 or saved (I guess it's the default
> after fresh OS install)
>
> GRUB_DEFAULT=saved
> reads /boot/grub2/grubenv
>
> - if I get that right -
The default = saved option would make the next boot use the last kernel
that you selected. However, it depends on each stanza saving itself as
the one to be used. Zero means to boot from the topmost stanza
(usually new kernels are written to the top of the list, so zero picks
the latest).
i.e. You have three kernels to choose from, each grub entry for that
kernel saves its choice when selected, then boots up. Next boot up,
grub looks to see what grub entry number was saved, and boots it.
Taking part of grub.conf from very old system of mine as an example:
default saved
title Fedora Core (2.6.17-1.2142_FC4)
root (hd0,0)
kernel /vmlinuz-2.6.17-1.2142_FC4 ro root=LABEL=/ acpi=force
initrd /initrd-2.6.17-1.2142_FC4.img
savedefault
title Fedora Core (2.6.17-1.2139_FC4)
root (hd0,0)
kernel /vmlinuz-2.6.17-1.2139_FC4 ro root=LABEL=/ acpi=force
initrd /initrd-2.6.17-1.2139_FC4.img
savedefault
title Fedora Core (2.6.16-1.2111_FC4)
root (hd0,0)
kernel /vmlinuz-2.6.16-1.2111_FC4 ro root=LABEL=/ acpi=force
initrd /initrd-2.6.16-1.2111_FC4.img
At the beginning is the "default saved" command that tells grub to boot
using the same kernel as last time. However, only the first two
entries save themselves as the default. If I elect to boot from the
third kernel (last on the list), I'm not sure what would happen next
boot. I'm guessing that whichever out of the first two I last used
would be booted, because one of them set themselves as default, and
nothing else unsets that.
NB: I'm not sure if "savedefault" has to be last, or can be typed
anywhere in the stanzas. And I'd be very surprised if the modern grub
didn't behave in the same way (having to deliberately save the chosen
entry as the default), but it installed no man file, nor a useful info
file on the PC that I'm looking at.
--
uname -rsvp
Linux 3.10.0-1062.1.2.el7.x86_64 #1 SMP Mon Sep 30 14:19:46 UTC 2019 x86_64
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