Kubuntu 26.04 - Auto updating Grub fails on snapshot detection.

[p0b-UK]

Hi all,

I've been trying to get grub-btrfs working on my Kubuntu 26.04 installation, but running into an issue when it attempts to update Grub on automatic snapshot detection.

Installed gawk and inotify.
Cloned the repo.
Ran make install.
SystemD unit was created, so enabled and ran it.
Used Snapper to take a manual snapshot.

Unfortunately none of the snapshots appears in Grub, but if I run 'update-grub' manually the boot menu is populated correctly.

As I understand in Ubuntu, update-grub is just a script which runs the following:-

#!/bin/sh
set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"

Since a default installation didn't appear to work for my distribution I gave the grub-btrfs documentation a closer look. The section under 'Customization' suggests that different distributions update Grub in different ways and some host Grub in different folders. The advice is to update the GRUB_BTRFS_MKCONFIG variable under /etc/default/grub-btrfs/config with the command specific to your distribution.

So I edited the variable (restarting grub-btrfs between each attempt) with the following:-

GRUB_BTRFS_MKCONFIG=update-grub
GRUB_BTRFS_MKCONFIG='update-grub'
GRUB_BTRFS_MKCONFIG="update-grub"
GRUB_BTRFS_MKCONFIG=grub-mkconfig -o /boot/grub/grub.cfg
GRUB_BTRFS_MKCONFIG='grub-mkconfig -o /boot/grub/grub.cfg'
GRUB_BTRFS_MKCONFIG="grub-mkconfig -o /boot/grub/grub.cfg"
GRUB_BTRFS_MKCONFIG=/sbin/update-grub
GRUB_BTRFS_MKCONFIG='/sbin/update-grub'
GRUB_BTRFS_MKCONFIG="/sbin/update-grub"
GRUB_BTRFS_MKCONFIG=/sbin/grub-mkconfig -o /boot/grub/grub.cfg
GRUB_BTRFS_MKCONFIG='/sbin/grub-mkconfig -o /boot/grub/grub.cfg'
GRUB_BTRFS_MKCONFIG="/sbin/grub-mkconfig -o /boot/grub/grub.cfg"

I think that was all the variations I attempted. None of which helped.

Checking JournalD log and grepping for 'grub' I see the following error message when I manually take a snapshot:-

13/05/2026 22:38	grub-btrfsd	11672: Detected snapshot creation/ deletion, recreating Grub menu
13/05/2026 22:39	grub-btrfsd	[!] Error during grub menu creation (grub/ grub-btrfs error)

This doesn't provide much info, so again checked the documentation and seen I can create a logfile and add verbose output. So edited the grub-btrfs unit configuration with:-

sudo systemctl edit --full grub-btrfs.service

Restarted, ran a manual snapshot then viewed the output of the logfile:-

Tue 12 May 23:57:15 BST 2026 Arguments:
Tue 12 May 23:57:15 BST 2026 Snapshot directories: /.snapshots
Tue 12 May 23:57:15 BST 2026 Timestift autodetection: false
Tue 12 May 23:57:15 BST 2026 Timeshift old: false
Tue 12 May 23:57:15 BST 2026 Logfile: /home/frankie/grub-btrfs.log
Tue 12 May 23:57:15 BST 2026 Recursive: false
GRUB-BTRFSD log Tue 12 May 23:57:15 BST 2026
Tue 12 May 23:57:15 BST 2026 grub-btrfsd starting up...
Tue 12 May 23:57:15 BST 2026 starting daemon watching /.snapshots...
Tue 12 May 23:57:15 BST 2026 Subdaemon function started, PID: 41793
Tue 12 May 23:57:15 BST 2026 41793: Entering infinite while for /.snapshots
Tue 12 May 23:57:15 BST 2026 41793: Snapshot dir watchtimeout: 0
Tue 12 May 23:57:15 BST 2026 41793: Watching /.snapshots for new snapshots...
Wed 13 May 00:00:00 BST 2026 41793: Detected snapshot creation/ deletion, recreating Grub menu
Wed 13 May 00:00:05 BST 2026 error: [!] Error during grub menu creation (grub/ grub-btrfs error)
Wed 13 May 00:00:06 BST 2026 41793: Watching /.snapshots for new snapshots...
Wed 13 May 00:04:08 BST 2026 41793: Received SIGINT/ SIGTERM
Wed 13 May 00:04:08 BST 2026 Parent 41773: Received signal SIGINT/ SIGTERM

Again, not much to go off. So I'm a bit stuck.

Am I doing something incorrect?
Is the syntax I'm using for GRUB_BTRFS_MKCONFIG correct?

Any help or pointers would be much appreciated.

[mrahawkins]

Hi @fwilliamsgithub, I had the same issue trying to install on debian. My suspicion is that it relates to PR #428. What worked for me was removing this section from the config and then re-running make install:

I think the issue is that the if statement doesn't work because the config file isn't actually run it just uses the variables defined, but I'm not 100% sure on that

[p0b-UK]

Sorry for the late reply. Thanks very much, this worked. Chucked the issue at Claude as well, gave the following response:-

To recap what was actually wrong (two separate bugs stacked on top of each other):

GRUB_BTRFS_MKCONFIG in /etc/default/grub-btrfs/config had been set to "grub-mkconfig -o /boot/grub/grub.cfg" instead of just "grub-mkconfig", which caused a duplicated -o flag whenever the daemon ran a full regen.

The bigger one: the config file's Fedora-detection block (if [ -f /etc/fedora-release ]; then ... fi) is only meant to apply on Fedora, but systemd's EnvironmentFile= parser doesn't understand if/fi — it just reads every KEY=VALUE line regardless of context — so it was unconditionally setting GRUB_BTRFS_GRUB_DIRNAME="/boot/grub2" even on your Kubuntu box, where that directory doesn't exist. That's what was actually breaking grub.cfg regeneration.