Locations of Access Control Files:

• /etc/passwd
• /etc/shadow
• /etc/group
• sudo adduser [username] = Create a new user
• sudo passwd [username] = Change password of a user
• su - [username] = Login as username ('su' = short for substitute or switch user)
• su - = Login as root
• sudo groupadd [groupname] = Create new group (System assigns next available GID)
• sudo adduser [username] = Switch to Insert Mode

Note 2 different User/Group commands:

adduser, addgroup, deluser, delgroup = interactive, more user friendly commands

useradd, groupadd, userdel, groupdel = low-level utilities, more infos need provided by yourself

• sudo usermod [OPTIONS] [username] = Modify a user account
• sudo usermod -g devops tom = Assign 'devops' as the primary group for 'tom' user
• sudo delgroup tom = Removes group 'tom'
• groups = Display groups the current logged in user belongs to
• groups [username] = Display groups of the given username
• sudo useradd -G devops nicole = Create 'nicole' user and add nicole to 'devops' group (-G = secondary group, not primary)
• sudo gpasswd -d nicole devops = Removes user 'nicole' from group 'devops'