Erik Simmler

Internaut, software developer and irregular rambler

A hardware boot switch

I recently added a spare SSD so I could play with desktop Linux again, and I again quickly grew tired of trying to catch the GRUB boot selection screen. A bit of searching turned up this project log for a Simplified hardware boot switch.

The basic idea came together smoothly enough, but I have a few minor additions I thought worth mentioning.

In the guide, you detect the presence of the “switch drive” and set the default to the index of the entry you want to boot into.

search --fs-uuid --set switch_drive C539-84FE
if [ "${switch_drive}" ] ; then
    # Boot Windows
    set default="2"
    # Boot Linux
    set default="0"

This works great! Until Fedora starts adding alternate/recovery kernel boot options after system updates. This causes the index of the Windows entry to shift down unpredictably.

A GRUB boot selection screen showing four Fedora entries (mostly differing only by kernel  version) and a Windows option.

The first time it happened I just bumped the set default up to "3". Then it happened again, and I began searching for a more robust way to select the default.

The context that these GRUB scripts runs in is unfamiliar and not extremely accessible. Most information I found was about how to customize the static config, rather than doing dynamic logic at boot time.

Finally, I stumbled across a promising scrap in an Ask Ubuntu answer. Apparently one of the ways to set a default (statically) is to set the GRUB_DEFAULT variable to the string label of the entry you want to use as the default (their example is GRUB_DEFAULT="Windows NT/2000/XP (loader) (on /dev/sda1)").

So, I wondered, could I also use the string label in my boot time set default? The answer turned out to be yes!

So here’s the full addition to my /etc/grub.d/40_custom file:

search --label --set switch_drive boot_windows
if [ "${switch_drive}" ] ; then
    set default="Windows Boot Manager (on /dev/nvme0n1p1)"
    set default="0"

I found the exact string label of the Windows entry with this commend:

$ sudo fgrep windows /boot/grub2/grub.cfg
menuentry 'Windows Boot Manager (on /dev/nvme0n1p1)' --class windows --class os $menuentry_id_option 'osprober-efi-CE17-A4E0' {

Also note that I’m looking up the switch_drive by --label instead of --fs-uuid. It’s a minor tweak, but it feels a little less coupled to the specific device.

Finally, wrap it up by generating the GRUB config (this is in Fedora 37):

$ sudo grub2-mkconfig -o /boot/grub2/grub.cfg

My computer case doesn’t lend itself well to direct modding, so I 3D printed a crude enclosure and leave it plugged on the end of a USB extension cable. I should hit a net time savings from this project in about ten years.

A picture of a thumbdrive in a 3D printed enclosure with an on/off switch on top.