Root subvolume UUID detection issue in grub-btrfs on Debian 13 (testing) with snapper

[q2g748r9h8-lab]

Hello,

first of all thank you for maintaining grub-btrfs.
It is a very useful project and greatly improves the usability of Btrfs snapshots.

While testing a Debian 13 (testing / trixie) installation with Btrfs and snapper, I encountered an issue where snapshots were not detected and therefore not added to the GRUB menu.

Environment:

• Distribution: Debian 13 (testing / trixie)
• Filesystem: Btrfs
• Snapshot tool: snapper
• Bootloader: GRUB
• Package: grub-btrfs

Observed behaviour:
Running „update-grub“ did not generate snapshot entries.
During execution the following message appeared:

"UUID of the root subvolume is not available"

Root cause:
The issue appears to be related to how the script determines the UUID of the root subvolume.

In the file:
/etc/grub.d/41_snapshots-btrfs

the UUID is determined using the following command:
root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null | awk -F: '/^\sUUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"

On Debian 13 this did not return a value, which caused the snapshot detection logic to fail.

Workaround
The system already correctly detects the filesystem UUID earlier in the script as:

root_uuid

Using this variable directly resolved the issue.
Example change:

before:
root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null | awk -F: '/^\sUUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"

after:
root_uuid_subvolume="$root_uuid"

After applying this change and running:
update-grub

all snapper snapshots appeared correctly in the GRUB boot menu.

Additional note
The underlying issue may be related to differences in how awk or btrfs-progs format the output on Debian, which caused the UUID detection to fail.

It might help if the detection logic were made more robust or if the already detected root UUID were reused instead of parsing the subvolume output.

Thank you again for the project and your work maintaining it.

Best regards
Ralph

[n-ncls]

Hi,

I ran into the same problem on LMDE 7/Debian-6.12.74-2.

The reason I found was the installed version of awk is mawk 1.3.4. This version doesn't handle \s cleanly and so there's no UUID found in btrfs subvolume show /. If replaced with [ \t] it works. Found in /etc/grub.d/41_snapshots-btrfs, lines 125 and 141.

If modified the script works correctly and
sudo update-grub
produces the needed entries into the boot menu.

Edit: I just noticed, there's already an open issue: #409

Cheers,
Nicolas

[q2g748r9h8-lab]

Hi Nicolas
Thanks, this makes a lot of sense.

That would explain why the UUID parsing failed in my case as well. I had worked around it by replacing:

root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null | awk -F: '/^\s*UUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"

with:

root_uuid_subvolume="$root_uuid"

But your note suggests the actual root cause is that \s is not portable in awk, especially with mawk.

So replacing \s with [ \t] in the relevant expressions seems like the cleaner fix, while reusing $root_uuid could still be a useful fallback for robustness.

Thanks for identifying that.
Best regards
Ralph

[LCBM-Frank]

Tried it on Mint and had the same issue. Installing "gawk" solved the problem