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Bootstrap class: .progress-bar-striped

<div class="progress">
  <div class="progress-bar progress-bar-striped" role="progressbar" style="width: 10%" aria-valuenow="10" aria-valuemin="0" aria-valuemax="100"></div>
</div>

Preview

Sass source

/* _progress.scss:27 */
.progress-bar-striped {
  @include gradient-striped();
  background-size: $progress-height $progress-height;
}

Uses variables

/* _progress.scss:27 */
$progress-height
$progress-height