Shell: Bourne, Bash and Zsh

If you spend any time programming a Unix system, you'll spend some time mucking around with sh (the Bourne shell), Bash (the popular GNU alternative) or Zsh (the default on macOS). Whether scripted or interactive, the shell is the glue that holds Unix systems together.

Manuals

Manual Page for sh, 2018
Shell Command Language, 2018
Bash Reference Manual, 2020-12-21
An Introduction to the Z Shell, 1995
A User's Guide to the Z-Shell, Peter Stephenson, 2003-03-23
The Z Shell Manual, Paul Falstad and Peter Stephenson, 2020-02-14

Tips and Tricks

Use the Unofficial Bash Strict Mode (Unless You Looove Debugging), Aaron Maxwell
Writing Safe Shell Scripts, MIT Student Information Processing Board, 2020-02-01
How to write idempotent Bash scripts, Fatih Arslan, 2019-07-03
How "Exit Traps" Can Make Your Bash Scripts Way More Robust And Reliable, Aaron Maxwell
What exactly was the point of [ “x$var” = “xval” ]?, Vidar Holen, 2021-04-12