Using date in backup path possible?

[BEBU88]

I would like to take a backup from a zfs snapshot directory. The backup path is changing every day with the latest taken snapshot, e.g. /.zfs/snapshot/auto-2026-09-26_00-00. I tried .zfs/snapshot/auto-$(date +%Y-%m-%d)_00-00 but that didn't work. Is it possible to pass the actual year, month and day to the backup path?

[jmrplens]

Not in the path field itself: backrest passes the plan's paths to restic exactly as written, with no shell and no date expansion, so $(date ...) stays literal. You can get the same result with a hook and restic's --files-from, and backrest (1.13.0 and later) allows a plan with no paths when the backup flags supply them (its validation says "unless backup_flags supplies paths, e.g. --files-from").

1. Add a hook to the plan: type Command, condition CONDITION_SNAPSHOT_START (it runs before restic starts), error behavior ON_ERROR_CANCEL, with a script that writes the newest snapshot directory to a file and fails if there is none:

   latest=$(ls -d /tank/.zfs/snapshot/auto-*_00-00 | tail -n 1)
   [ -n "$latest" ] || exit 1
   echo "$latest" > /config/zfs-latest.txt

   Your names sort by date, so the last one is the newest, and it still works on a day the snapshot is late. Use your dataset's mountpoint instead of /tank, and any path backrest can write and read (/config only exists if your compose file mounts it).

2. Remove the path from the plan and add the backup flag --files-from /config/zfs-latest.txt.

Command hooks run through sh, so the script can use anything the shell can, $(date +%Y-%m-%d) included, if you prefer building the name from today's date.

One thing to expect: because the path is different every day, restic treats every file as new and reads all of them again, even with a parent snapshot. It only stores what actually changed, so the repository does not grow more than usual, and retention is unaffected since backrest groups the plan's snapshots by its tag rather than by path, but each run takes as long as a first backup in disk reads. I checked it with restic 0.19.1: backing up the same files under a new directory name reported every file as new and added only 18 KiB, while the same files under an unchanged path came back as unmodified.

If the reads matter, mount the day's snapshot at a fixed path in the same hook (mkdir -p /mnt/zfs-latest, then mount --bind "$latest" /mnt/zfs-latest, with a umount in a CONDITION_SNAPSHOT_END hook) and back up /mnt/zfs-latest instead. In Docker that needs --cap-add SYS_ADMIN --security-opt apparmor=unconfined, or --privileged.