Yahya Tawil posted a detailed how-to on writing ULP’s for Eagle CAD:
In this tutorial, you will learn how to write your first ULP in Eagle CAD to add a new capability to your CAD tool.
User Language Program (ULP) is a set of extensions for Eagle CAD users to either facilitate a routine job in an automated way or do a job that can’t be done without a ULP’s help. For example, the only way to import an image to your PCB design is by using the command import-bmp ULP. Auto-placement, exporting BOM, and renumbering parts in a schematic are all routine jobs with which ULP can help.
More details at allaboutcircuits.com.
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