9 bit model: drwxrwx---

1. create user/delete user:
   sudo useradd -m jack
sudo passwd jack
sudo userdel -r jack

2. add groups
   sudo groupadd students
sudo groupadd teacher
sudo usermod -a -G students jack

3. set permission

• chown, chmod
  sudo chown -R :students /home/assignments
sudo chmod -R g+rwx assignments/
• setfacl

sudo setfacl -m g:Teachers:rwx -R /home/exam
getfacl exam
sudo setfacl -m g:students:--- -R /home/exam

mask is the upper bound privileges. mask is a union of all users.

4. add user to sudoers (solve issues of "username is not in sudoers")
   add to sudo group
   sudo usermod -aG sudo adam
groups adam
visudo /etc/sudoers
   check if there exists a line of

# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL

3 Ways to Change a Users Default Shell in Linux

https://www.tecmint.com/change-a-users-default-shell-in-linux/