I tried to show a search icon in form but the only button is shown without the glyphs.
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit"><span class="glyphicon glyphicon-search"></span></button>
</form>
I use bootstrap version 4.1. I got the glyphicon code getbootstrap.com and form getbootstrap.com
Glyphicons are dropped from Bootstrap V4 and later.
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">
<form class="form-inline my-2 my-lg-0">
<div class="input-group">
<input class="form-control" type="search" placeholder="Search">
<div class="input-group-append">
<div class="input-group-text"><i class="fa fa-search"></i></div>
</div>
</div>
</form>
Here, Font Awesome is used. Refer this answer for more alternatives to Glyphicons and Font Awesome