GNU/Linux is an open-source operating system consisting of the Linux kernel plus GNU utilities. In my opinion, the best way to learn Linux is to use it as a daily driver (dual-booting Ubuntu is a great way to get started) and/or mess around with it.
Linux learningresources:
- Linux Journey
- List of Linux syscalls (with link to bootlin)
- Linux Kernel Newbies
- Linux is not secure
- Linux Hardening Guide
- Linux commands you should know
- OverTheWire Bandit
- some OSCP notes may also be helpful
- Debian security tips from klaver
sysctl.conffrom klaver- Linux hardening script
- linux-insides
- Linux From Scratch
- The Linux programming interface: a Linux and UNIX system programming handbook
- Linux kernel development
- Linux Device Drivers, 3rd Edition
- An old explanation on Notion that I made in high school