Thursday, March 09, 2017

grub2-set-default and submenus

At work, I was investigating why "grub2-set-default" apparently did not work for booting older kernels on a SLES12-SP1 system (openSUSE has the same configuration). A colleague found out, that it worked once he removed the "submenu" wrap around the additional installed kernels.

Today, after some googling, I found out that even though you have unique "menuentry" titles, a plain "grub2-set-default my\ menu\ entry" still does not work, unless you give the path to the submenu.

This is done in grub2 syntax like this:
grub2-set-default "1>openSUSE Leap 42.2, with Linux 4.4.46-11-default"
The "1>" tells grub2 to look for the menuentry in the submenu, which is the second toplevel item. For SUSE / openSUSE the second toplevel item is always, AFAICT, the "Advanced options for $VERSION" menu, where the additional kernels live.

An alternative for my case would have been
grub2-set-default "1>1"
Which would be "the second entry from the submenu which is the second toplevel item" (counting from zero). But you need to look at the config file and count the entries.

The entries have an additional ID that looks like it is costructed like:
gnulinux-$(uname -r)-advanced-${UUID_OF_ROOTFS}
 in my case:
Still looks unwieldy, but you might be able to automatically determine that from a script.