Linux

The system under everything.

Boot to hardening: the shell, processes, permissions, and systemd.

Firmware, kernel, init, and services

Everything is a file: paths, links, and filesystems

Pipes, redirection, and shell scripting

Execution, signals, and scheduling

Accounts, groups, and permissions

System calls and the kernel API

Device files, udev, and kernel modules

systemd, logging, and cron

Hardening, monitoring, and troubleshooting