How to create a cronjob to update all nodes I have a content type that has a computed field that needs to be updated constantly. 
So I need to create a cronjob to update all nodes of a certain type (example 'page'), so that the code is executed from the computed field and its value updated.

thx
You need to create a custom module to do this.
Implement hook_cron to create a cron job in custom module
You can update a node programatically.. For more details see node_save