I am trying to get the amount of reputation that a user has
how to handle the response (or get one) from a script
You can do this with a Bash command that you can then embed in a script. Here's how:
1. Look up your user ID
Assuming you want to get the reputation on Stack Overflow, go to your account and look up your user ID in the URL:
Here's a quick'n'dirty userscript, tested on Greasemonkey on Firefox: https://github.com/BobVul/SEReopenReviewWarning/blob/master/SEReopenReviewWarning.user.js
Can be installed via the raw link: https://github.com/BobVul/SEReopenReviewWarning/raw/master/SEReopenReviewWarning.user.js
Currently only the pending/completed review warning is there. I might add ...