useradd: Add new users commands:
-d specify home directory
-s shell
-p specify password
-g user’s primary group
-G user’s other group
-m create home directory

The following command creates a user “ali”, adds to groups users and admin and create his home directory:
useradd -g users -G admin -p <password> -d /home/ali -m ali -s /bin/shell

usermod: Modify existing users
Use above switches to modify a user

Add “ali” to another group:
usermod -G <groupname> ali

userdel: Delete a user
Removes the user “ali”. To delete home directories use -r switch:
userdel -r ali

passwd: user Password
Logged on users can just use passwd to change their password. If root is changing a user’s password:
passwd <username>

su: switch user
To switch from current user to another:
su <username>

You may switch from root to another user without entering user’s password, but not the other way around. This will not log off current user. To return to previous user shell type “exit”.

Users, passwords and group information are stored in these files:

Users: /etc/passwd
Groups: /etc/group
Passwords: /etc/shadow