Add "print-config" subcommand (#698)

* Add "show-config" subcommand

* Add integration tests

* Handle wrong additional args

* Support "BackupFilenameExpand" option

* Rename to print-config

* Remove config nil check as it will never be nil

* Rework print config to work with config.resolve()

* Use defer unset()

* Add warning about feature not being stable yet

cmd/backup/main.go

@@ -11,8 +11,18 @@ func main() {
 	foreground := flag.Bool("foreground", false, "run the tool in the foreground")
 	profile := flag.String("profile", "", "collect runtime metrics and log them periodically on the given cron expression")
 	flag.Parse()
-
+	additionalArgs := flag.Args()
 	c := newCommand()
+
+	if len(additionalArgs) > 0 {
+		switch additionalArgs[0] {
+		case "print-config":
+			c.must(runPrintConfig())
+			return
+		default:
+			panic("unknown command: " + additionalArgs[0])
+		}
+	}
 	if *foreground {
 		opts := foregroundOpts{
 			profileCronExpression: *profile,

cmd/backup/print_config.go

@@ -0,0 +1,47 @@
+// Copyright 2025 - offen.software <hioffen@posteo.de>
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+
+	"github.com/offen/docker-volume-backup/internal/errwrap"
+)
+
+func runPrintConfig() error {
+	configurations, err := sourceConfiguration(configStrategyConfd)
+	if err != nil {
+		return errwrap.Wrap(err, "error sourcing configuration")
+	}
+
+	formatter := regexp.MustCompile(`\s([A-Z])`)
+	for _, config := range configurations {
+		if err := func() (err error) {
+			unset, warnings, err := config.resolve()
+			if err != nil {
+				return errwrap.Wrap(err, "error resolving configuration")
+			}
+			defer func() {
+				if derr := unset(); derr != nil {
+					err = errors.Join(err, errwrap.Wrap(derr, "error unsetting environment variables"))
+				}
+			}()
+
+			fmt.Printf("source=%s\n", config.source)
+			for _, warning := range warnings {
+				fmt.Printf("warning:%s\n", warning)
+			}
+			// insert line breaks before each field name, assuming field names start with uppercase letters
+			formatted := formatter.ReplaceAllString(fmt.Sprintf("%+v", *config), "\n$1")
+			fmt.Printf("%s\n", formatted)
+			return nil
+		}(); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}