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/etc/passwd File contains the configuration info for loacl users in the following format(world readable):

username : x: UID:GID:GECOS:homedir:shell

/etc/shadow File contains the username (which must match /etc/passwd) and the users MD5 encrypted password (only readable by root)

/etc/group file contains the configuration for local groups in the following format

groupname :x: GID : secondary users

/etc/gshadow file contains a list of groups on the server and their MD5 encrypted passwords.

useradd	used to add new users
usermod	used to modify the attributes of the user
groupadd	used to add new group to server
groupmod	used to change group attributes
passwd	used to create or change user passwords
chage	command used to create/edit password aging

Locating files with Special Permissions/attributes

find . / -perm +4000 |less

find root owned

find / -perm +2000  | less

get all GID/SUiD

find / -perm +1000 (sticky bit)

can be (used to find 777)

Setuid Suid Set UserID is used on Unix systems for tasks that require higher privileges. Has greatest impact on executable files.

chmod u+s <file>

setgid

chmod g+s  

or

chmod 2770 /some/directory

assigns group ownership to directories

example

Configure the users mike,chris and jamie

make these users part of the ru group

useradd -s /sbin/nologin ru
useradd chris -G ru -p '    ' 

(grub-md5-crypt can be used to generate system passwords)

configure the directory /home/ru so that each user can read,create, and modify files.

chmod 2770 /home/ru 

or

chmod u+s /home/ru