The classic Advanced Bash-Scripting Guide:

http://tldp.org/guides.html#abs

Beginners might prefer the Bash Guide for Beginners:

http://tldp.org/guides.html#bbg

"The Bash manual page is concise because it is a Unix manual page. Unix manual pages are supposed to be concise, because they are meant to be reference documents, not tutorials. In the GNU project, this is what the Info documentation is for."

https://mywiki.wooledge.org/BashGuide

"This guide aims to aid people interested in learning to work with BASH. It aspires to teach good practice techniques for using BASH, and writing simple scripts.

This guide is targeted at beginning users. It assumes no advanced knowledge -- just the ability to login to a Unix-like system and open a command-line (terminal) interface. It will help if you know how to use a text editor; we will not be covering editors, nor do we endorse any particular editor choice. Familiarity with the fundamental Unix tool set, or with other programming languages or programming concepts, is not required, but those who have such knowledge may understand some of the examples more quickly."

https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/V3_chap02.html