Group Administration

Group Administration

Linux uses groups to define a set of related user accounts that can share access to a file or directory.

Important points:

A user can be participant to more than one group at the same time.

A user who is member of a group can change to that group without password but when a user  was not  member can only change to that group if the group password exist and the user gives it.

One or more users can become group administrators for specific groups.

Group Administrators can:

·         add/change/delete the password of the group

·        add/delete users to the group

·         reserve the group to members only

Types of groups

There are two types of groups. They are

Primary group: These groups are created automatically when user was created.

Secondary group: these groups are created by the admin manually.

The groups configuration files:

/etc/group: Where all the users for each groups are listed.

Format:

groupname: Password or x or ! : GID:  Memberlist

/etc/gshadow: Where the groups passwords are kept

Format:

groupname : Password or !  : AdminUsersList  : MemberUsersList

Creating a group

To create a new group, use the groupadd command:

groupadd group

where group specifies the name of the group to be added. Groups are stored in the file /etc/group, which can be read by any user but modified only by root.

Options:

-g gid for users

-o to override

Deleting a group

To delete a group, user the groupdel command.

groupdel group

where group specifies the name of the group to be deleted.

Group membership

To add user or delete users to group we use the gpasswd command

gpasswd [options] group

where group specifies the name of the group to be assigned to users

Options:

­a user Adds permanently a user to a group

­d user  Deletes permanently a user from a group.

-A adds a group administrator

-M adds multiple users to the group

Modifying  a  group

 groupmod command is used to modify the group contents

groupmod [options] group

where group specifies the name of the group to be modified

 options

-n to assign new name for group

-p to change password

-g to change gid

Examples:

 groupadd  -g  600 Student

groupmod –g 700 student

groupmod –n teachers student

gpasswd  -M raju koti Student

gpasswd –d raju Student

gpasswd –a raju Student

groupdel Student