how to create shortcode

1// function that runs when shortcode is called
2function wpb_demo_shortcode() { 
3 
4// Things that you want to do. 
5$message = 'Hello world!'; 
6 
7// Output needs to be return
8return $message;
9} 
10// register shortcode
11add_shortcode('greeting', 'wpb_demo_shortcode'); 

1<?php
2function abc()
3{}
4////create shortcode////
5add_shortcode('page_name','abc');
6?>