Groups in Linux is as name suggest collection of user. Each group has its unique group ID (GID). Groups are of two types i.e. primary group and supplementary group, each system is user is member of 1 primary group and zero or more supplementary group. The group information is stored in /etc/group and the respective passwords are stored in the /etc/gshadow file. All information stored in /etc.As you must be aware a file is assigned separate read, write, and execute permissions for owner, group and everyone else.
Creating a group
groupadd command creates new group with default setting. To run this command you must have root privileges.
groupadd command creates group add its details to /etc/group file.
Creating group with specific group ID
groupadd command can group with specific group ID by using switch –g. If there is clash of group ID i.e. group ID already exists , the command will give error.
groupadd –g group_ID group_name
Set password for a group
To set or change password for a group, use command gpasswd followed by group name for which change is to be done.
Adding user to group
You can add user to a group by using command gpasswd with switch –a.
gpasswd –a username groupname
Removing user from group
Similar to adding user you can remove a user from a group by using command gpasswd with switch –d.
gpasswd –d username groupname
Removing a group
If you want to delete a group you can use groupdel command. This command deletes all entries for group from system files (/etc/group and /etc/gshadow).