cmd: Reject multiple configs for fmt command (#6717)

cmd/commandfuncs.go

@@ -560,10 +560,15 @@ func cmdValidateConfig(fl Flags) (int, error) {
 
 func cmdFmt(fl Flags) (int, error) {
 	configFile := fl.Arg(0)
-	if configFile == "" {
-		configFile = "Caddyfile"
+	configFlag := fl.String("config")
+	if (len(fl.Args()) > 1) || (configFlag != "" && configFile != "") {
+		return caddy.ExitCodeFailedStartup, fmt.Errorf("fmt does not support multiple files %s %s", configFlag, strings.Join(fl.Args(), " "))
+	}
+	if configFile == "" && configFlag == "" {
+		configFile = "Caddyfile"
+	} else if configFile == "" {
+		configFile = configFlag
 	}
-
 	// as a special case, read from stdin if the file name is "-"
 	if configFile == "-" {
 		input, err := io.ReadAll(os.Stdin)

cmd/commands.go

@@ -388,6 +388,7 @@ When reading from stdin, the --overwrite flag has no effect: the result
 is always printed to stdout.
 `,
 		CobraFunc: func(cmd *cobra.Command) {
+			cmd.Flags().StringP("config", "c", "", "Configuration file")
 			cmd.Flags().BoolP("overwrite", "w", false, "Overwrite the input file with the results")
 			cmd.Flags().BoolP("diff", "d", false, "Print the differences between the input file and the formatted output")
 			cmd.RunE = WrapCommandFuncForCobra(cmdFmt)