This is a still growing collection of terminal commands that I find personally useful.
List all installed applications
dpkg -l
orapt list --installed
Show memory and swap usage
free
Power off
poweroff
(it powers of the system more or less immidiately, so be warned. I wondered if there is any potential harm to the system, but it seems to just the same thing as initiating a shutdown in the Gnome Desktop power options, for example (but there is a 60 seconds timer).
Check the content of a folder
ls --la(shows the folders content including hidden files and details like permissions and size)
Check the size of the current folder
du -hs .