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.
sysadmin
System administration tasks
Find and kill whatever is using a port
Identify the process bound to a TCP port that's refusing to let a service start, then stop it.
Monitor a log file in real time
Watch a log file as new lines are written, filter it live, and keep watching across log rotation.
APT essentials
The apt, apt-get, and dpkg commands that cover almost everything you'll do to manage packages on Debian, and how remove differs from purge.