Linux

df reports free space per filesystem; du reports how much space a directory tree is actually using.

Sends a signal to a process — usually asking it to terminate, gracefully or not.

ps takes a snapshot of running processes; top (or htop) watches them live, sorted by resource usage.

Controls systemd services — start, stop, restart, and check whether one is set to run on boot.

Filtering the journal by unit, time and priority instead of scrolling.