UUID of the root subvolume is not available on Debian 13

[ahmetaksit]

https://github.com/Antynea/grub-btrfs/blob/master/41_snapshots-btrfs

grub-btrfs/41_snapshots-btrfs

Line 125 in 14fa71c

awk -F':' '/^\s*UUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"

Changed => awk '$1=="UUID:" {print $2}')"

It's not reading the UUID code and the snapshot list isn't appearing in the GRUB menu. I made changes after compiling on Debian 13 and now it's working. Here's the necessary change.
Root subvolume UUID
AA aksit ERROR => root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null |
AA aksit ERROR => awk -F':' '/^\s*UUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"
root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null |
awk '$1=="UUID:" {print $2}')"
[ -z "$root_uuid_subvolume" ] && print_error "UUID of the root subvolume is not available"

AA thanks

[eljfe]

Hi,
I found this post troubleshooting a first time install on a Debian Trixie machine.

The Problem:

Snapshots menu didn't appear in grub menu. The message "UUID of the root subvolume is not available" appeared when I ran update-grub.

Unfortunately this patch by @ahmetaksit didn't work on my system.

grub-btrfs version:

Version 4.13-yabsnap_info_support-2024-03-06T13:43:57+00:00

A fix

I made the following change which solved the issue on my machine.

root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null | \
	grep -E '^\s+UUID' | awk -F' ' '{print $2}')"

I noticed a preference for sed and awk in the code. If there is some reason grep is not preferred, the following also works,

root_uuid_subvolume="$(btrfs subvolume show / 2>/dev/null | \
	sed -n 's/^\s*UUID.*/&/p' | awk -F' ' '{print $2}')"

It's rare I dig into opensource code. Miraculously I found this post. Thanks to @ahmetaksit!

[pmarcop]

My two cents on the matter. I am looking at the main problem in a broad sense, trying to change as little as possible in the code. I hope this doesn't sound like a repeat.

TL;DR

\t \s are not POSIX-compliant and not recognized in regexes by mawk implementation of awk. Use [[:blank:]] or [[:space:]] instead.

Summary

41_snapshots-btrfs currently uses non‑standard \s and \t escape in an awk regular expression when parsing the output of btrfs subvolume show / to determine the value of root_uuid_subvolume. This breaks on implementations like mawk, which follow POSIX ERE and do not support \s and \t in regexes as a shorthand for whitespace or tabs, respectively.

grub-btrfs/41_snapshots-btrfs

Line 125 in 14fa71c

awk -F':' '/^\s*UUID/ {gsub(/^[ \t]+/, "", $2); print $2}')"

Details

The script calls:

btrfs subvolume show /

and pipes the output to awk to extract the UUID of the root subvolume. In a recent change, the awk script started using \s and t inside the regular expressions to match whitespaces.

However, in POSIX extended regular expressions (which awk uses), \s and \t are not defined as whitespace/tab classes. Instead, they are treated as a literal \s or '\t'. Only a limited set of escapes are standard, and shorthand classes like \s are non‑portable extensions found in some regex engines (e.g. PCRE, some GNU tools), but not in mawk and other strictly POSIX‑compliant awk implementations.

As a result, on systems where /usr/bin/awk is mawk, the regular expression does not behave as intended, and the UUID extraction fails.

For example, if the current code looks roughly like:

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

it should be changed to something like:

btrfs subvolume show / | awk -F' :' '/^[[:blank:]]*UUID/ { gsub(/^[[:blank:]]+/, "", $2); print $2}'

Expected / Correct Behavior

To be POSIX‑compliant and portable across different awk implementations (mawk, gawk, nawk, etc.), the script should use POSIX character classes within bracket expressions instead of \s. For matching whitespace, the portable options are:

[[:space:]] to match any whitespace character (space, tab, formfeed, newline, etc.).

[[:blank:]] to match horizontal whitespace only (space and tab).

[thoschworks]

Confirmed on Ubuntu 24.04 LTS (Noble Numbat) with grub-btrfs 4.13

Same issue on Ubuntu 24.04, which also uses mawk as default awk. The error "UUID of the root subvolume is not available" is purely caused by the mawk incompatibility with \s.

There are two affected lines in /etc/grub.d/41_snapshots-btrfs (line numbers may vary by version):

1. root_uuid_subvolume detection (no exit)
2. boot_uuid_subvolume detection (with exit)

The fix suggested by the original reporter works for both. Applied with:

sudo sed -i '125c\                       awk '"'"'$1=="UUID:" {print $2}'"'"')"' /etc/grub.d/41_snapshots-btrfs
sudo sed -i '141c\        awk '"'"'$1=="UUID:" {print $2; exit}'"'"'' /etc/grub.d/41_snapshots-btrfs

After the fix, sudo grub-mkconfig -o /boot/grub/grub.cfg correctly detected all 23 snapshots.

(Assisted by Claude Code.)