UUID detection fails on Debian 13 (mawk default, \s not supported)

[hjjanssen-ui]

This is what the Claude.ai told me to post, the fix worked for me (but I don't know shit).

Bug: \s regex not supported in mawk — UUID detection fails on Debian 13 (Trixie)

Environment:

• OS: Debian 13 "Trixie" (13.4)
• grub-btrfs: master (2026-05-10)
• btrfs-progs: v6.14
• awk: mawk 1.3.4 (Debian default)

Symptom:

After a clean install, update-grub always fails with:

Detecting snapshots ...
UUID of the root subvolume is not available

Snapshots exist and are valid. snapper -c root list shows them correctly.

Root cause:

In 41_snapshots-btrfs, line 124-125:

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

The regex \s is used to match whitespace, but mawk does not support \s. It is a gawk extension. Debian (and many Debian-based distros) ship mawk as the default awk (/usr/bin/awk → mawk via alternatives). As a result, the pattern never matches, root_uuid_subvolume is empty, and the script exits early.

This can be confirmed by running:

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

Fix:

Replace \s with the POSIX-compliant [[:space:]], which works correctly in mawk, gawk, and nawk:

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

After this change, the UUID is correctly extracted and snapshots are detected normally:

Detecting snapshots ...
Found snapshot: 2026-05-16 14:00:16 | @/.snapshots/2/snapshot | single | timeline
Found snapshot: 2026-05-16 13:34:17 | @/.snapshots/1/snapshot | single | fresh install baseline
Found 2 snapshot(s)

The same pattern may exist elsewhere in the script (e.g. the boot_uuid_subvolume detection) and should be audited for the same issue.

Note: There may be a second contributing factor — btrfs-progs v6.14 outputs btrfs subvolume show with tab indentation (\t) rather than spaces, which could affect other parsing logic in the script on older btrfs-progs versions too.

[dima-online]

I have the same problem on Ubuntu 26.04 LTS (GNU/Linux 7.0.0-15-generic x86_64)

Fix it

sudo sed -i 's|/^\\s\*UUID/|/^[ \\t]*UUID/|g' /etc/grub.d/41_snapshots-btrfs

[hendrikreuhl]

Just install gawk (sudo apt install gawk)
No need to change anything in the scripts itself

[mimferpo2]

Quick workaround without editing is:
sudo apt install gawk
Installing gawk replaces mawk as the default awk and the original script works immediately. This also survives grub-btrfs updates unlike script patches.

Confirm it works:
sudo update-grub

However— based on my experience on Linux Mint 22.3 there are required steps plus some config fixes to get Timeshift snapshots appearing in GRUB. Full guide here(check the solution in last message):

https://forums.linuxmint.com/viewtopic.php?p=2790327#top

Hope this helps.

[mimferpo2]

So the complete correct step order is(Sharing here as the post in mint forum might get auto-removed in 6 months)

Step 1 — Install grub-btrfs:

cd ~
git clone https://github.com/Antynea/grub-btrfs.git
cd grub-btrfs
sudo make install

Step 2 — Install required dependencies:
sudo apt install inotify-tools gawk
Step 3 — Fix the service:
sudo systemctl edit grub-btrfsd.service
Add between the ### blocks:

[Service]
ExecStart=
ExecStart=/usr/bin/grub-btrfsd --syslog --timeshift-auto

Save: Ctrl+O → Enter → Ctrl+X

The editor should look exactly like this when done:

### Anything between here and the comment below will become the contents of the drop-in file

[Service]
ExecStart=
ExecStart=/usr/bin/grub-btrfsd --syslog --timeshift-auto

### Edits below this comment will be discarded

Apply the changes:

sudo systemctl daemon-reload
sudo systemctl restart grub-btrfsd.service
sudo systemctl status grub-btrfsd.service

Confirm it shows Active: active (running)

Step 4 — Fix config:
sudo nano /etc/default/grub-btrfs/config
Change/uncomment these three lines:

GRUB_BTRFS_IGNORE_SPECIFIC_PATH=("")
GRUB_BTRFS_IGNORE_SNAPSHOT_TYPE=("")
GRUB_BTRFS_OVERRIDE_BOOT_PARTITION_DETECTION="true"

Save: Ctrl+O → Enter → Ctrl+X

Step 5 — Update GRUB:
sudo update-grub
Step 6 — Verify:
sudo grep -i snapshot /boot/grub/grub.cfg
Should return:
submenu 'Linux Mint snapshots' {

Step 7 — Reboot:
sudo reboot
Also make sure you don't have any leftover backup scripts in /etc/grub.d/:
ls /etc/grub.d/
If you have saved any backup like 41_snapshots-btrfs.backup delete it. Otherwise GRUB will run them as scripts and show the Linux Mint snapshots submenu twice at boot.
sudo rm /etc/grub.d/41_snapshots-btrfs.backup

No manual sudo update-grub after each tmeshift snapshot. gawk + the service handles everything automatically.