src/Pure/General/rsync.scala

         (if (prune_empty_dirs) " --prune-empty-dirs" else "") +
         (if (dry_run) " --dry-run" else "") +
         (if (clean) " --delete-excluded" else "") +
         (if (list) " --list-only --no-human-readable" else "") +
         filter.map(s => " --filter=" + Bash.string(s)).mkString +
-        (if (args.nonEmpty) " " + Bash.strings(args) else "")
+        if_proper(args, " " + Bash.strings(args))
     context.progress.bash(script, echo = true)
   }
 
   def init(context: Context, target: String,
     contents: List[File.Content] = Nil