Display a CPT based on a metabox selection

I am trying to display multiple post based off a custom metabox selection. I am currently using CMB2 as my framework to display the CMB within the backend. The selection field which has an ID of work__assign is calling all post titles from another CPT called fleet. Below is my code for displaying the CMB2 select type which then displays the titles from a post-type fleet.

Execute php after post save/update

I have created a php script (generate.php) which is querying posts on set criteria and then writing result in custom xml file. I would like to execute the generate.php everytime the post Save/Update button is pressed. Do you have any clue on how to do this? May be with cron jobs every X minutes would be better?