Since it seems like you don't know much about bash at all, I promise the book will help you.
You can be someone who actually knows what they're talking about instead of making embarrassing, snarky comments that expose your lack of education on the topic at hand.
Bash has had some nice minor features and syntax sugar added, but the fundamentals are entirely the same. All the examples in the book work just the same today as they did when it was written.
What was added in 4.X or 5.x that you can't live without? What do you think has changed that merits inclusion?
It's a 36 y/o language, mate. I still reference my copy all the time, and found it to be a great definitive resource when I was learning.
How many bash 4/5 features are you seriously using on a regular basis? What do you think is out-of-date?
I highly recommend O'Reilly's Learning the Bash Shell in paperback form: https://www.oreilly.com/library/view/learning-the-bash/0596009658/.
The other responses you've received so far don't offer much insight into the historical background and underlying mechanics of the shell, which are crucial to understanding the "Why?"s of command-line quirkiness.
Synology runs a proprietary OS OOTB that's had multiple sloppy vulns exposing full remote access to users' files. Putting your data in the hands of fuckups who have and will continue to leak it is the opposite of total control.
It's completely trivial to store any data you want to in a cloud provider 100% securely just by piping it through openssl before uploading.
You didn't start by asking a question. You needlessly trashed a helpful suggestion from a place of ignorance, then asked a naive question defensively to mask a lack of knowledge.
That is rude and trollish behavior.