Linux
6 curated resources, easiest first.
A multi-part online tutorial covering navigation, viewing and manipulating files, redirection, expansion, permissions and job control. It opens with a short example of why command-line skills are worth learning.
The free PDF of William Shotts' book, hosted on SourceForge as linked from linuxcommand.org. Read it start to finish or dip into chapters.
A free 596-page book that teaches the shell and shell scripting from scratch and then moves on to common programs and advanced topics. The page links the PDF download.
A gentle introduction to the terminal: opening it, moving around the file system, creating and deleting files, pipes, man pages and safe use of sudo. The page says it has been replaced by a newer version and targets Ubuntu 18.04.
A broad post-install guide to the Debian system using shell examples. Chapters include Unix tutorials, package management, networking, authentication and data management.
Introduces shell scripting: writing and editing scripts, variables, functions, flow control, user input, positional parameters and error handling with traps.