Bash

Bash is easiest to learn by automating something you already do by hand. The Linux Command Line by William E. Shotts Jr. covers both interactive use and scripting, and Introduction to Bash Scripting by Bobby Iliev is a short, practical start. Explainshell breaks any command line into its parts, which helps when reading other people's scripts.

Shell scripts break in subtle ways around quoting, word splitting and exit codes. BashGuide by Maarten Billemont teaches safe habits, and the Google Shell Style Guide sets sensible conventions. The Advanced Bash-Scripting Guide and the GNU Bash manual are the deep references, and Sundeep Agarwal's books cover text processing with coreutils.