Minimal shortcode sample.

shortcode-enclosed.php

<?php

// [caption]content[/caption]
function caption_shortcode( $atts, $content = null ) {
   extract( shortcode_atts( array(
      'class' => 'caption',
      ), $atts ) );
 
   return '<span class="' . esc_attr($class) . '">' . $content . '</span>';
}
add_shortcode( 'caption', 'caption_shortcode' );

shortcode.php

<?php

// [bartag foo="foo-value"]
function bartag_func( $atts ) {
  extract( shortcode_atts( array(
		'foo' => 'something',
		'bar' => 'something else',
	), $atts ) );

	return "foo = {$foo}";
}
add_shortcode( 'bartag', 'bartag_func' );