[LINUX FILE] /etc/group

/etc/group is a text file which defines the groups to which users belong under Linux and UNIX operating system. Under Unix / Linux multiple users can be categorized into groups. Unix file system permissions are organized into three classes, user, group, and others.

Here is example of /etc/group file:

cat /etc/group

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:username
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:username
fax:x:21:
voice:x:22:
cdrom:x:24:username, username1 
floppy:x:25:
tape:x:26:
sudo:x:27:
audio:x:29:pulse
dip:x:30:
www-data:x:33:username
backup:x:34:
operator:x:37:
list:x:38:
irc:x:39:
src:x:40:
gnats:x:41:
shadow:x:42:
utmp:x:43:
video:x:44:
sasl:x:45:
plugdev:x:46:username
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
libuuid:x:101:
crontab:x:102:
syslog:x:103:

cdrom	:	x	:	24	:	username, username1
Group	:	Password	:	Group ID (GID)	:	Group List

Group: It is the name of group. If you run ls -l command, you will see this name printed in the group field.
Password: Generally password is not used, hence it is empty/blank. It can store encrypted password. This is useful to implement privileged groups.
Group ID (GID): Each user must be assigned a group ID. You can see this number in your / etc/ passwd file.
Group List: It is a list of user names of users who are members of the group. The user names, must be separated by commas.

To view in which group user belongs; type

groups [username]

Reade more about command groups.