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 What do you think is out-of-date?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.