Want to be able to remotely, non-interactively reboot a computer from an operating system installed on one disk or partition to another? With legacy BIOS boot, GRUB, and chainloading, this is easy: simply set the GRUB as your universal boot decision point, and you can change which menu entry is the default for boot. Yeah, sure, this works if you want to permanently make a change… but what if you only want to temporarily boot into one operating system, and on next boot after, boot to your main operating system? Well, there must be a way using grubenv and scripting, but that is a tricker proposition.
The latter case is especially useful if you think you might boot into an operating system in a potentially bad configuration, then need to use a watchdog timer to force a reboot into your known good operating system.
But, that is only partially to the main point of this blog article. How do you do this with UEFI? At first sight, UEFI looks to throw a monkey wrench in the way of the old standard, it’s much trickier to do bootloader chainloading with EFI boot. It’s actually quite simple once you look into the details. EFI has primitives to support everything you could want here directly. Because you can edit EFI variables from within the booted operating system, you do not need physical access to the machine to mess around in the “BIOS configuration” screens.
If you want to make a permanent change to which operating system is booted by default, you edit the BootOrder variable to place your preferred operating system (or bootloader, for that matter) in front. But, if you want the case where you only want to try booting into a different operating system for one boot, then automatically switch back to the default operating system for subsequent boots, you set the BootNext variable.
So, if you were getting cold feet about chainloading in UEFI boot, rest assured, it is easy enough to accomplish exactly what you want with EFI directly.
The efibootmgr command is the name of the game. Check out its man
page for details.
Ah, yes, returning to the question about how to do BootNext it with
GRUB. Edit your grub.cfg to set GRUB_DEFAULT=saved, then use the
command grub-reboot $1 to set your default GRUB boot entry for the
next boot only. Other items of interest: the setting
GRUB_SAVEDEFAULT=false and the command grub-set-default. Another
name is grub2-once.
20201223/DuckDuckGo grub set temporarily next boot
20201223/https://askubuntu.com/questions/574295/how-can-i-get-grub2-to-boot-a-different-option-only-on-the-next-boot
20201223/https://wiki.debian.org/GrubReboot