Generalmore = view text file. Most Linux configurations are done in text files, so viewing these files is requisite to knowing your OS settings.
cat = also used to display files text files. cat /etc/issue displays Linux version
sudo = super user do... execute a command as root. This must be used in front of many of the commands which follow. Anything that changes outside of personal directory must be done as root.
su = change to super user... change session to super user
nano = simple text editor... often need to use: sudo nano [path/file]
man = manual for commands. use with command name after. Can often use [command] -h
info = more information about commands
apt-get = aptitude get used to get and install .deb software packages.
extensions: update; clean; autoclean; upgrade; install
ls = list folder contents; dir = also works as ls; cp = copy files; rm = remove; cd = change directory; mv = move files;
pico = another simple text editor
ufw = uncomplicated firewall.... used in Ubuntu server
extensions: allow; deny; delete; status; status verbose; disable; enable; reset; default
poweroff = kill systems and shutdown
reboot = obvious
uname -a = displays kernel version
SSH = secure shell to server/another linux machine ssh
SCP = secure shell copy from local to server or between servers