bootstrap怎么给input加图标

<div class="input-group">

    <div class="input-icon-group">

        <input type="text" class="form-control fc-clear" />

        <span id="scan" data-role="md" class="md md-center-focus-weak fa-lg input-icon input-icon-md" style="display: inline;">

        </span>

    </div>

    <span class="input-group-btn">

        <button class="btn btn-primary" type="button">

            批号        </button>

    </span>

</div>