Quick Answer
The Linux Foundation's Introduction to Linux (LFS101) on edX is the best free structured course for absolute beginners: it is the official course from the group that stewards the kernel, and you can audit it for free. For hands-on practice that sticks, Over The Wire's Bandit wargame is unmatched, because it forces you to actually use the command line to solve puzzles instead of watching someone else type. And freeCodeCamp's Linux crash course on YouTube is the fastest way to get comfortable at the terminal, roughly five to six hours end to end. Pick by how you learn: structured course, hands-on puzzles, or fast video.
Why learn Linux at all
Introduction to Linux, Linux Foundation on edX: best structured course
Over The Wire Bandit: best hands-on practice
freeCodeCamp Linux crash course and The Odin Project: best fast starts
Beginner path vs developer path, and how to practice
The honest limitation: free courses skip enterprise sysadmin
What to learn after Linux
FAQ