Asset source file change detection in Unity Editor?

Situation:

• We have a text file X in Asset Folder.

• We have a Z.bat file that reads X and generates a Y file. Y is a .cs script file.

• This is cumbersome, because we need to check whether X is changed (e.g. after getting the latest version from non-UnityAssetServer source repository) and should run Z.bat.

Goal:

• We want UnityEditor to detect whether X is changed and automatically run Z.bat.

• We guess this can be done by making an Editor-classes-derived script file, but we have no idea where to start.

Question:

• Please let me know how to make an Editor plugin which detects Asset file change and do something.