This bash shell scripting series has been intended as a comprehensive introduction to shell scripting, but is lacking many details that are essential to shell script mastery.
Continuing on, you could research topics such as STDIN, command substitution, substitution operators, pattern matching, etc.
I also recommend Shell Check to help weed out any bugs (or unwanted features) in your shell scripts.
In the meantime, I threw together a little cheat sheet that includes most of the points that we have covered, as a quick-reference guide.
The cheat sheet is available exclusively to my mailing list subscribers.
Now, please excuse me while I go write a script to automate my backup process.