Linux (Retropie) - run interactive script immediately on boot

[Delta]

I have RetroPie installed on a Raspi 3, and running nicely.  I believe Retropie is built on Debian...

I have a simple script that will prompt for a password, and depending on input will hide the non-child friendly games. 

I am tearing my hair out over how to get it to run!  I want it to run before the GUI and Emulation Station start, and completely stall the boot until it exits.  (It does time out and exit, so you can play in "kid mode" without a keyboard).

I'm sure this is an obvious one for you Linux gurus, but I'm 

Thank you!

[Delta]

Well I've found a way to achieve what I want; via Emulation Station's autostart.sh script.  Not how I'd imagined doing it, but hey.

Is there a way to do such a thing purely through Linux - run a script before login, and boot will hang and wait until the script exits, before going continuing to login?