# PHP, 56 bytes

    <? $r=$i=$argv[1];while($i>1){$i--;$r=$r*$i;} echo $r;?>

And heres my proof using Pi, Raspberry Pi. :D

    root@raspberrypi:~# php fact.php 5
    120root@raspberrypi:~#

But because it's a code golf here's a list of bugs:

 - there is no `\n`
 - throws error if no parameter
 - 0! seems to be 0