public String ls() {
        File folder = new File(currentDirectory);
        File[] list = folder.listFiles();

        StringBuilder builder = new StringBuilder();
        for(File x : list) {
            builder.append(" ");
            builder.append(x.getName());
        }
        if (list.length != 0) {
            builder.deleteCharAt(0);
        }
        String result = builder.toString();
        if (result.equals("")) {
            result = null;
        }

        if (list.length == 0) {
            log.warn("no files in directory!");
        }

        return result;
    }