It’s inevitable that you’ll want to create a cron job for your Magento modules at some point. An obvious solution would be to make the user create a new crontab entry calling your script. There’s no need. Magento has cron functionality built in.

Simply edit your config.xml, adding the following:

<config>
    ...
    <crontab>
        <jobs>
            <shipwire_inventorysynch>
                <schedule><cron_expr>*/6 * * * *</cron_expr></schedule>
                <run><model>shipwire/api_inventorysynch::cron</model></run>
            </shipwire_inventorysynch>
        </jobs>
    </crontab>
</config>

When shipwire/api_inventorysynch::cron relates to the shipwire model namespace, api_inventorysynch relates to the model Meanbee_Shipwire_Model_Api_Invetorysynch and ::cron relates to the method cron() in that class.

Note: You need to make sure, of course, that the script ./cron.php, in the root of your Magento installation, has an entry in the crontab.