auto replace characters in highlighted text

I want to write a shell (bash) script that will replace all spaces with underscores within the currently-highlighted text, then assign that script to be executed when I press a certain key combination.
Is that a reasonable expectation?
Is that approach a logical choice?
How can I do it?
I'm running Mint 19.3 with a 5.0.0-32 kernel (all the 5.4.0 series I've tried freeze if I have Firefox running).