From 1b00ad849a9e80a466759499ad582694f7af5e9f Mon Sep 17 00:00:00 2001
From: Andras Schmelczer <andras@schmelczer.dev>
Date: Mon, 21 Sep 2026 20:55:49 +0100
Subject: [PATCH] Add prefix

---
 README.md          | 6 ++++++
 docker-compose.yml | 1 +
 src/backup.sh      | 5 ++++-
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 7ffd082..bf38fe0 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,8 @@ Over the past 2 years, this backup setup has enabled me to successfully restore
 
 ### Multi-target backups
 
+Set the required `ARCHIVE_PREFIX` environment variable to a non-empty prefix shared by all backup targets. Include any separator in the value: `ARCHIVE_PREFIX=my-host-` creates archives named `my-host-{now:%Y-%m-%dT%H:%M:%S}`. Pruning uses the glob `${ARCHIVE_PREFIX}*`, so retention rules only apply to matching archives. To continue pruning existing archives, set the prefix to their previous hostname followed by `-`.
+
 To adhere to the [3-2-1 backup rule](https://en.wikipedia.org/wiki/Backup) without disk-level redundancy, you can configure backups to multiple destinations. For example, backups can be sent simultaneously to [rsync.net](rsync.net) and a local HDD.
 
 The [`docker-compose.yml`](docker-compose.yml) file demonstrates how to set up multiple backup targets using environment variables such as `BORG_REPO_0`, `BORG_REPO_1`, `BORG_PASSPHRASE_0`, `BORG_PASSPHRASE_1`, and so forth. The backup script sequentially handles each repository defined by the environment variables, ensuring your source volume is backed up across all specified targets.
@@ -32,6 +34,7 @@ The backup script first takes `BORG_REPO_0` and the corresponding env vars and s
 Thus, the following sets of environment variables are valid for multi-target backups:
 
 - ```sh
+    - ARCHIVE_PREFIX=my-host-
     - BORG_PASSPHRASE=$PASSWORD
     - BORG_REPO=/local-backup
   ```
@@ -39,6 +42,7 @@ Thus, the following sets of environment variables are valid for multi-target bac
   > This just backs up to a local repository
 
 - ```sh
+    - ARCHIVE_PREFIX=my-host-
     - BORG_PASSPHRASE_0=$PASSWORD
     - BORG_REPO_0=/local-backup
   ```
@@ -46,6 +50,7 @@ Thus, the following sets of environment variables are valid for multi-target bac
   > This just backs up to a local repository
 
 - ```sh
+    - ARCHIVE_PREFIX=my-host-
     - BORG_PASSPHRASE_0=$PASSWORD
     - BORG_REPO_0=/local-backup
 
@@ -56,6 +61,7 @@ Thus, the following sets of environment variables are valid for multi-target bac
   > This backs up to two different local repositories
 
 - ```sh
+    - ARCHIVE_PREFIX=my-host-
     - BORG_PASSPHRASE_0=$PASSWORD
     - BORG_REMOTE_PATH_0=borg1
     - BORG_REPO_0=my-username@my-username.rsync.net:~/backup
diff --git a/docker-compose.yml b/docker-compose.yml
index 7c56ea5..4fedfa7 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -6,6 +6,7 @@ services:
     environment:
       - TZ=Europe/London
       - SLEEP_TIME=1h # we will wait an hour between backups
+      - ARCHIVE_PREFIX=my-host- # required; include any separator before the timestamp
 
       # first backup goes to rsync.net
       - BORG_PASSPHRASE_0=MY_PASSWORD_CHANGE_THIS
diff --git a/src/backup.sh b/src/backup.sh
index 11a6611..a265356 100755
--- a/src/backup.sh
+++ b/src/backup.sh
@@ -1,5 +1,7 @@
 #!/bin/bash
 
+: "${ARCHIVE_PREFIX:?Set ARCHIVE_PREFIX to a non-empty archive prefix (e.g. my-host-)}"
+
 KEEP_DAILY=${KEEP_DAILY:-6}
 KEEP_WEEKLY=${KEEP_WEEKLY:-3}
 KEEP_MONTHLY=${KEEP_MONTHLY:-48}
@@ -71,11 +73,12 @@ borg create --stats \
     --filter=AMCE \
     --files-cache=ctime,size,inode \
     --compression=zstd,12 \
-    "${EXCLUDE_ARGS[@]}" ::"{hostname}-{now:%Y-%m-%dT%H:%M:%S}" .
+    "${EXCLUDE_ARGS[@]}" ::"${ARCHIVE_PREFIX}{now:%Y-%m-%dT%H:%M:%S}" .
 
 cd -
 
 borg prune --list --stats \
+    --glob-archives="${ARCHIVE_PREFIX}*" \
     --keep-daily="$KEEP_DAILY" \
     --keep-weekly="$KEEP_WEEKLY" \
     --keep-monthly="$KEEP_MONTHLY" \