monitoring

Observing system state over time

journalctl

Tested journalctl examples: filtering by unit, priority and time, following logs live, output formats, and controlling journal disk usage.
ps

Tested ps examples: aux versus -ef, choosing columns with -o, selecting by user or name, sorting, the process tree, and pgrep instead of grep.
tail

Tested tail examples: -n and -c, the +N form that counts from the start, following a growing file with -f, and why log rotation needs -F instead.
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.