More resources to learn Linux

Special mention

Cisco's free Networking Academy, which includes Linux courses.

Linux Foundation's free, 60 hour Linux introductory course.

Complete newb level:

Tech Gee's free ITF+ courses

Early on, for beginners

LFS101x - Introduction to Linux - Linux Foundation fully free training TCMSec's Linux 100

LFD107X - Linux for developers - Linux Foundation fully free training

LFD108X - Git for distributed software development - Linux Foundation fully free training

LPI Linux Essentials

Linux Voyage (forked from Linux Journey)

OverTheWire's Bandit

KodeKloud's Linux & DevOps learning path (free after sign-up)

KodeKloud Engineer

Shawn Powers' free Linux+ YouTube course

Jadi's free LPIC1 book and YouTube course

Advanced

PluralSight's Bash scripting course

Exercism, has 80+ exercises for Bash scripting.

Exercises from the university of Cambridge

Exercises from TLDP

Cert Depot's RHCSA 7 exercises

Linux Upskill Challenge

Sander van Vugt's Linux+ training materials at O'Reilly.

Sander van Vugt's practice exercises for the RHCSA 8 exam.