quick command list:

find file:
sudo find -name "example"

lists all users:
awk -F':' '{ print $1}' /etc/passwd

prints all administrator users:
grep '^sudo:.*$' /etc/group | cut -d: -f4

add user to group:
sudo usermod -a -G GROUP USERNAME

remove user from group:
sudo deluser USERNAME GROUP

sign into account of user:
sudo su - USERNAME

change password:
passwd

check if UFW is enabled:
sudo ufw status

enable ufw:
sudo ufw enable

find active services:
systemctl list-units --type=service --state=active

temporarily stop service:
sudo service SERVICE stop
sudo systemctl stop SERVICE

remove service:
update-rc.d -f SERVICE remove
sudo systemctl disable SERVICE

config ssh:
sudo gedit /etc/ssh/sshd_config

print out whole directory tree with perms, ignoring a few directories:
ls -Rl / -I{home,var,tmp}