Concepts

How core Linux fundamentals actually work.

Environment variables and PATH

Why a program you can see and execute is still not found, how the environment is copied into each new process, and where PATH is really set on Debian.
Exit codes and error handling

How $?, PIPESTATUS, set -e, and pipefail combine to tell a script what succeeded and what failed, and where each one falls short.
File permissions explained

The rwx/owner-group-other model, numeric vs symbolic chmod, umask, and why setuid/setgid on directories behave differently than you'd expect.
Pipes and redirection

How | connects commands together, and how >, >>, 2>&1, and <() route data to and from files, other commands, and each other.
Processes and signals

How processes are born, orphaned and reaped, what a signal actually does, why kill -9 is a last resort, and why Ctrl-C stops a whole pipeline at once.
Terminal, shell and tty

What a terminal, a tty, a shell and a console each actually are, why Ctrl-D is not a signal, and why Ctrl-S appears to freeze everything.