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Linux - User Management: Group

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• Linux - User Management: Group
  • Group
  • Group Configuration Files
    • /etc/group
    • /etc/gshadow
  • Group Management
    • Adding a new group
    • Lab: create group
    • Changing a new group
    • Lab: Update and delete group
    • Group Administration
    • Deleting a new group
    • Display Group Info

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Group

• Group
  • a collection of users that share permissions to access files, directories, or other system resources.
• Groups are a fundamental part of Linux’s permission model, helping manage access efficiently.

• Every user is in at least one group - Primary Group

• Primary Group

  • the default group associated with a user account.
  • When a user creates a file, its group ownership is set to the user’s primary group.
  • Specified during user creation or modified using the usermod command.

• Secondary Groups

  • A user can belong to multiple secondary groups in addition to their primary group.
  • grant additional access to shared resources.

• Group ID (GID)

  • Each group is identified by a unique numeric Group ID (GID).
  • GIDs below 1000 are reserved for system groups.

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Group Configuration Files

/etc/group

• Lists all groups, their members, and group-related information.

group_name:x:group_id:member1,member2

• file permission:

-rw-r--r--. 1 root root 1054 Nov 19 16:37 /etc/group

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/etc/gshadow

• Contains secure group passwords and administrative information.
• If a group has a password, it is stored here in an encrypted format.

group_name:encrypted_password:group_admins:group_members

• group_name
  • The name of the group (must match the group in /etc/group).
• encrypted_password
  • The group password in encrypted form (if set).
  • ! or *:
    • Indicates that the group cannot be accessed with a password.
  • Empty field (::):
    • Group does not require a password.
• group_admins
  • A comma-separated list of users who can manage the group (add or remove members).
  • If empty, no specific administrators are assigned.

• group_members

  • A comma-separated list of users who belong to the group (same as /etc/group).
• file permission:

----------. 1 root root 850 Nov 19 16:37 /etc/gshadow

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Command	Desc
groups	show the groups the current user is in
id -Gn	show the groups the current user is in
groups user_name	show the groups a specific user is in
id user_name -Gn	show the groups a specific user is in

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Group Management

Adding a new group

CMD	DESC
groupadd g_name	create a new group
groupadd g_name -g GID	create a new group and specify a gid
groupadd g_name -g GID -o	create a new group and specify a duplicate gid
groupadd g_name -r	create a system group below 1000

• updates /etc/group

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Lab: create group

su -
groupadd -g 5000 linuxadmin
groupadd -o -g 5000 dba

# confirm
tail -2 /etc/group
# linuxadmin:x:5000:
# dba:x:5000:

# add user
usermod -aG dba user100

# confirm
id user100
# uid=1003(user100) gid=1003(user100) groups=1003(user100),5000(linuxadmin)

groups user100
# user100 : user100 linuxadmin

grep dba /etc/group
# dba:x:5000:user100

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Changing a new group

CMD	DESC
groupmod g_name -n new_name	Modify group’s name
groupmod g_name -g GID	Modify group’s GID
groupmod g_name -g GID -o	Modify group’s GID with a duplicate gid

• updates /etc/group

• Example

# Create a new user
useradd testuser

# Create a New Group
groupadd testgroup

# Change a User's Primary Group
usermod testuser -g testgroup

# Add a User to a Group
usermod testuser -aG testgroup,wheel

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Lab: Update and delete group

groupmod -n sysadm linuxadmin
groupmod -g 6000 sysadm

# confirm
grep sysadm /etc/group
# sysadm:x:6000:

# remove group
groupdel sysadm
grep sysadm /etc/group

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Group Administration

CMD	DESC
gpasswd group_name	Set or Change a Group Password
gpasswd group_name -r	Remove a Group Password
gpasswd group_name -A admin_user	Add a Group Administrator
gpasswd group_name -a username	Add a Member to a Group
gpasswd group_name -M user1,user2	Add a list of member to a Group
gpasswd group_name -d username	Remove a Member from a Group

• Prompts for a new group password and updates /etc/gshadow.

• Example

su -
# create users
useradd dev_admin
useradd dev1
useradd dev2

# set password for admin
passwd dev_admin

# create group
groupadd dev_group -g 3001

# Set group password
gpasswd dev_group

# add admin adn members
gpasswd dev_group -A dev_admin
gpasswd dev_group -a dev1
gpasswd dev_group -M root,rheladmin

# verify
tail /etc/group | grep dev_group
# dev_group:x:3001:root,rheladmin
tail /etc/gshadow | grep dev_group
# dev_group:$6$FRBtW/S9e$z8Xp7BKwwjBmv9pyIDfS.2cmwM9qNuO90sEfZe3nK9IeSOKZ2meY7mu9twHSRXcjrwlDvd7hkjlpfVUPHoIUZ/:dev_admin:root,rheladmin

su - dev_admin
# add user to group as admin
gpasswd dev_group -a dev2
# Adding user dev2 to group dev_group
gpasswd dev_group

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Deleting a new group

CMD	DESC
groupdel g_name	Delete a new group

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Display Group Info

CMD	DESC
groups	List Current logged-in user’s group
groups username	List group for a specific user

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