31. Group Management

Question 1

Learning Objectives

By the end of this chapter, you should be able to:

Answer 1

• Explain why it is useful to have Linux users belong to one or more groups.
• Use utilities such as groupadd, groupdel, groupmod, and usermod to create, remove and manipulate groups and their membership.
• Describe User Private Groups.
• Explain the concept of group membership.

Question 2

Groups

Where is the groups config file location?

Answer 2

/etc/group

Question 3

Groups

Describe the components of a record in the /etc/group file.

Answer 3

Inside /etc/group file

groupname:password:GID:user1,user2,…

• groupname: is the name of the group
• password: is the password place holder. Group passwords may be set, but only if /etc/gshadow exists
• GID: is the group identifier. Values between 0 and 99 are for system groups. Values between 100 and GID_MIN (as defined in /etc/login.defs and usually the same as UID_MIN) are considered special. Values over GID_MIN are for UPG (User Private Groups)
• user1,user2,…: is a comma-separated list of users who are members of the group. The user need not be listed here if this group is the user’s principal group.

Question 4

Group Management

What is the command to add a new group?

Answer 4

groupadd

$ sudo groupadd -r -g 215 staff

🚩 Be very careful with the usermod -G command; the group list that follows is the complete list of groups, not just the changes. Any supplemental groups left out will be gone! Non-destructive use should utilize the -a option, which will preserve pre-existing group memberships when adding new ones.

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Question 5

Group Management

What is the command to remove a group?

Answer 5

groupdel

$ sudo groupdel newgroup

Question 6

Group Management

What is the command to modify a group and add new users?

Answer 6

groupmod

$ sudo groupmod -g 101 blah

Question 7

Group Management

What is the command to manage a user’s group memberships?

Answer 7

usermod

$ sudo usermod -G student,group1,group2 student

Question 8

Group Management

The group manipluation utilities modify which 2 group config files?

• groupadd
• groupdel
• groupmod
• usermod

Answer 8

/etc/group

/etc/gshadow (if it exists)

Question 9

User Private Groups

What is a User Private Group?

Answer 9

The idea behind UPGs is that each user will have his or her own group.

However, UPGs are not guaranteed to be private; additional members may be added to someone’s private group in /etc/group.

By default, users whose accounts are created with useradd have: primary GID = UID and the group name is also identical to the user name.

Question 10

Group Membership

A Linux user has ___ primary group; this is listed in ___ and will also be listed in ___.

Answer 10

• 1
• /etc/passwd
• /etc/group

Question 11

Group Membership

Group membership can be identified by running either of the following commands?

Answer 11

• $ groups [user1 user2 …]
• $ id -Gn [user1 user2 …]

With no arguments, either command reports on the current user. Note that the default groups can differ by distribution:

On CentOs:

[student@CentOS7 ~]$ groups
student

On Ubuntu:

student@ubuntu:~$ groups
student adm cdrom sudo dip plugdev lpadmin sambashare libvirt