Surprisingly, https://learnxinyminutes.com/docs/bash/ is the best resource out there to learn shell scripting. This page basically turned me from a novice who could only write two commands with | or && between them to a competent shell programmer. My shell scripts have functions, descriptive variables, error handling, good formatting, etc.

I find shellcheck to be really great to go to the step beyound. It find a lot of things that make most shell script brittle and teach you some best practice.

Add shfmt on top and you are good to go.