From a826828d7649d7a49ad2f4edeaaa83f996b5adab Mon Sep 17 00:00:00 2001
From: Lennard Schwarz <lennard.schwarz@icloud.com>
Date: Thu, 1 Jan 2026 13:49:19 +0100
Subject: [PATCH] Support "BackupFilenameExpand" option

---
 cmd/backup/show_config.go           | 15 +++++++++++++++
 test/show-config/docker-compose.yml |  3 +++
 test/show-config/run.sh             |  5 +++++
 3 files changed, 23 insertions(+)

diff --git a/cmd/backup/show_config.go b/cmd/backup/show_config.go
index eb7e614..51720f1 100644
--- a/cmd/backup/show_config.go
+++ b/cmd/backup/show_config.go
@@ -5,6 +5,7 @@ package main
 
 import (
 	"fmt"
+	"os"
 	"regexp"
 
 	"github.com/offen/docker-volume-backup/internal/errwrap"
@@ -22,6 +23,20 @@ func runShowConfig() error {
 			fmt.Println("source=<nil>\n<nil>")
 			continue
 		}
+		if config.BackupFilenameExpand {
+			unset, err := config.applyEnv()
+			if err != nil {
+				fmt.Printf("error applying env: %v\n", err) // print error to stdout for debugging
+				return errwrap.Wrap(err, "error applying env")
+			}
+			config.BackupFilename = os.ExpandEnv(config.BackupFilename)
+			config.BackupLatestSymlink = os.ExpandEnv(config.BackupLatestSymlink)
+			config.BackupPruningPrefix = os.ExpandEnv(config.BackupPruningPrefix)
+			if err := unset(); err != nil {
+				fmt.Printf("error unsetting env: %v\n", err) // print error to stdout for debugging
+				return errwrap.Wrap(err, "error unsetting environment variables")
+			}
+		}
 		// insert line breaks before each field name, assuming field names start with uppercase letters
 		formatted := regexp.MustCompile(`\s([A-Z])`).ReplaceAllString(fmt.Sprintf("%+v", *config), "\n$1")
 		fmt.Printf("source=%s\n%s\n", config.source, formatted)
diff --git a/test/show-config/docker-compose.yml b/test/show-config/docker-compose.yml
index 1b28c96..6ceadca 100644
--- a/test/show-config/docker-compose.yml
+++ b/test/show-config/docker-compose.yml
@@ -2,6 +2,9 @@ services:
   backup:
     image: offen/docker-volume-backup:${TEST_VERSION:-canary}
     environment:
+      BACKUP_FILENAME: "backup-$${BACKUP_TAG}.tar"
+      BACKUP_FILENAME_EXPAND: true
+      BACKUP_TAG: expanded
       BACKUP_SOURCES: /backup
       NOTIFICATION_URLS: "stdout://"
       AWS_S3_BUCKET_NAME: example-bucket
diff --git a/test/show-config/run.sh b/test/show-config/run.sh
index 3e0586e..5979f7b 100755
--- a/test/show-config/run.sh
+++ b/test/show-config/run.sh
@@ -22,6 +22,11 @@ if ! echo "$logs" | grep -q "BackupSources:/backup"; then
 fi
 pass "BACKUP_SOURCES present."
 
+if ! echo "$logs" | grep -q "BackupFilename:backup-expanded.tar"; then
+  fail "Missing expanded BACKUP_FILENAME in output."
+fi
+pass "Expanded BACKUP_FILENAME present."
+
 if ! echo "$logs" | grep -q "NotificationURLs:\[stdout://\]"; then
   fail "Missing NOTIFICATION_URLS in output."
 fi