Exit codes and error handling
How $?, PIPESTATUS, set -e, and pipefail combine to tell a script what actually succeeded or failed, and where each one falls short.
Concepts
How core Linux fundamentals actually work.
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.