Where Linux User Group Information Stored

Where does linux stores user and group information?

User names and primary groups are stored in /etc/passwd. This file can be directly edited using the 'vi' editor, although this is not recommended. Format of the file is

* User name (normally all lower case)
* Password (encrypted - only contains the letter 'x')
* User ID (a unique number of each user)
* Primary Group ID
* Comment (Normally the person's full name)
* Home directory (normally /home/
* Default shell (normally /bin/bash)

Each field is separated by a colon.

for example:

# cat /etc/passwd
root:x:0:0:root:/root:/bin/sh
bin:x:1:1:bin:/bin:/bin/sh
daemon:x:2:2:daemon:/usr/sbin:/bin/sh
adm:x:3:4:adm:/adm:/bin/sh
lp:x:4:7:lp:/var/spool/lpd:/bin/sh
sync:x:5:0:sync:/bin:/bin/sync
shutdown:x:6:11:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
uucp:x:10:14:uucp:/var/spool/uucp:/bin/sh
operator:x:11:0:Operator:/var:/bin/sh
nobody:x:99:99:nobody:/home:/bin/sh
admin:x:1001:1000:admin:/home/admin:
guest:x:1002:1000:guest comment:/home/guest:

Says two users admin and guest on my linux box.
Also it is having cat /etc/passwd- file which contains same information as cat /etc/passwd

Passwords for each user are stored in /etc/shadow. This file should only be changed using the passwd command.

example:

# cat /etc/shadow
root:$1$yzGL7ddg$Eb9YHTiaMQ52qDUOkjHLM1:12963:0:99999:7:::
bin:*:12963:0:99999:7:::
daemon:*:12963:0:99999:7:::
adm:*:12963:0:99999:7:::
lp:*:12963:0:99999:7:::
sync:*:12963:0:99999:7:::
shutdown:*:12963:0:99999:7:::
halt:*:12963:0:99999:7:::
uucp:*:12963:0:99999:7:::
operator:*:12963:0:99999:7:::
nobody:*:12963:0:99999:7:::
admin:ddB035VORJOng:0:0:99999:7:::
guest:LkD5clThUzVjw:0:0:99999:7:::

It is having cat /etc/shadow- file which contains same information as cat /etc/shadow

Group information is stored in /etc/group. This file can be directly edited using the 'vi' editor. Format of the file is

* Group name
* Group password (hardly ever used)
* Group ID
* User names (separated by commas)

Each field is separated by a colon.
example

# cat /etc/group
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
lp:x:7:daemon,lp
uucp:x:14:uucp
ftp:x:50:
nobody:x:99:
users:x:100:
everybody:x:1000:admin,guest
administrators:x:1001:admin

It is having cat /etc/group- file which contains same information as cat /etc/shadow
There are two groups everybody and administartors with users are admin,guest and admin respectively.