Using Db::getInstance() in a CLI script - prestashop

Question

I've been wanting to create a add-on cron script that utilises Prestashop's DB class instead of instantiating the database handle directly, but I can't seem to figure out where did the "Db" class commonly referenced by "Db::getInstance()" calls get defined.
classes/Db.php defines an abstract DbCore class. MySQLCore extends Db as you can see, however Db is never defined anywhere:
[/home/xxxx/www/shop/classes]# grep -r "extends Db" ../
../classes/MySQL.php:class MySQLCore extends Db
According to another thread on Prestashop forums, the abstract DbCore class is implemented in a class located in override/classes/db, however that directory does not exist.
[/home/xxxx/www/shop/override]# cd ../override/
[/home/xxxx/www/shop/override]# ls
./ ../ classes/ controllers/
[/home/xxxx/www/shop/override]# cd classes/
[/home/xxxx/www/shop/override/classes]# ls
./ ../ _FrontController.php* _Module.php* _MySQL.php*
Our shop is working, so obviously I am missing something. We are running Prestashop 1.4.1, so perhaps the docs are no longer applicable.
Quite clearly in many places in the code base functions from the Db class are being used, but this last grep through the code found nothing:
grep -rwI "Db" . | grep -v "::"
./modules/productcomments/productcommentscriterion.php:require_once(dirname(__FILE__).'/../../classes/Db.php');
./classes/MySQL.php:class MySQLCore extends Db
./classes/Db.php: * Get Db object instance (Singleton)
./classes/Db.php: * #return object Db instance
./classes/Db.php: * Build a Db object
Is there something I am missing? Where did this magical Db class come from?

Answer 1

To create a CLI script, the easiest way is to include the config file so you will have access to every classes. For example
<?php
require dirname(__FILE__).'/config/config.inc.php'; // assuming your script is in the root folder of your site
// you can then access to everything
$db = Db::getInstance();

Answer 2

You are confused a little bit with this. In PS 1.4.x. in the override directory, no files are palced. The documentation is following PS 1.5.x.
PS is using autoload feature to load classes. Lets take DB class as an example. The file name is Db.php , but class name is DbCore and when we want to get object of Db class, we do it like
Db::getInstance();
means not like
DbCore::getInstance();
How this works ? The thing is the php auto load function. Checkout the config/autoload.php (or a file with a name like that), and you will see that PS checking the class with and without Core at the end of the class name. Means that if the a name has Core or not, it will be loaded.
Starting with PS 1.5.x , PS placed over rided files in override/classes folder. Please note that all those classes are extending their respective classes. For example in override/classes/db the class Db.php is placed and this file has only the following code
<?php
abstract class Db extends DbCore
{
}
So it is clear if we want to use Db class, it will call the override/classes/db/Db.php which is extended from DbCore class.
Similarly for all other classes, the same criteria is used.
I hope those details will help.
Thank you