How Linux Boots, and Where It Goes Wrong
Firmware to bootloader to kernel to systemd, in order — what each stage does, how to tell which one failed, and how to recover from each.
The Linux Filesystem: What Every Directory Is For
A tour of the top-level directories on a Linux system — what belongs in each, why /usr and /var are split, and where to look for the file you are…
Users, Groups and Root: How Linux Decides Who Can Do What
UIDs and GIDs, why most accounts on your system are not people, what root actually is, and the group memberships that quietly hand out administrative power.