Python script that launches a process that it needs to clean up after and kill on receiving SIGTERM/SIGINT
UPPER_CASE is reserved for constants, and your globals assuredly aren't constants. If you wish for a cleaner solution, you could possibly make a class and pass in instance methods to signal.signal.

Your final raise SystemExit seems unneeded.

You can use line.endswith('.ext') instead of slicing.

It's hard to give more detailed analysis without a better understanding of what the code is trying to do.