etc/group

Rename group test to test3

groupmod test -n test3

Change gid of a group test to 2000

groupmod test -g 2000

Make two groups New,test equally the same

groupmod New -og 2000

etc/passwd

Structure of the file <user>:x:<uid>:<gid>:<description>:<home dir>:<shell> Add user

useradd <user>
    -m make user folder too
    -s specify the shell
    -g specify the group if any otherwise, primary group is <user>
    -G add user to any groups group1,group2,group3 ...
    -d specify user directory
    -c comment on a user properties, qoute if space any

Modify user

usermod <user>
    -l change to new user2
    -d change home directory
    -L Lock the user
    -U Unlock the user
    -aG append to user groups, not replace the groups

Delete User

userdel <user>
    -r delete home directory and mail content

Add, remove user from a group

gpasswd <group> -a <user>
gpasswd <group> -d <user>

Show User's group(s)

groups <user>

/etc/shadow

<user>:<password>:<1>:<2>:<3>:<4>:::

1. When was the last time this password changes
2. User wont be able to change the password 0 days after each change
3. After this many days, the user HAVE to change his password
4. ...and the user will be informed <4> days before the expiration to change his password

The password field can be:

• :: Without pass can be logged in
• :!: user is deactivated
• :*: user is deactivated
• :253%&^%576: user is activated
• :!253%&^%567: user is locked

Sudo

In debian based systems all the members of group sudo can run commands with root privileges without changing to root user. Also there is a config file to add users individually /etc/sudoers