groupadd / groupdel / usermod (Group Management)

# -----------------------------------------------
#  Creating a group
# -----------------------------------------------

# groupadd {group-name}
#   → Creates a new group
#   → The group ID (GID) is assigned automatically
#   Example: sudo groupadd kiryu-clan

# groupadd -g {GID} {group-name}
#   → Creates a group with an explicit GID
#   Example: sudo groupadd -g 1200 kiryu-clan

# -----------------------------------------------
#  Deleting a group
# -----------------------------------------------

# groupdel {group-name}
#   → Deletes the specified group
#   → Fails if the group is the primary group of any user.
#     Change the user's primary group first before deleting
#   Example: sudo groupdel kiryu-clan

# -----------------------------------------------
#  Adding/removing members from a group (usermod)
# -----------------------------------------------

# usermod -aG {group-name} {username}
#   → Adds a user to a supplementary group
#   → The -a (append) option is required. Without it, existing group memberships are overwritten
#   Example: sudo usermod -aG kiryu-clan kazuma

# usermod -g {group-name} {username}
#   → Changes the user's primary group
#   Example: sudo usermod -g kiryu-clan kazuma

# gpasswd -d {username} {group-name}
#   → Removes a user from a group
#   Example: sudo gpasswd -d kazuma kiryu-clan

# -----------------------------------------------
#  Checking groups
# -----------------------------------------------

# groups [{username}]
#   → Lists the groups the specified user belongs to
#   → If the username is omitted, shows the current user's groups
#   Example: groups kazuma

# id [{username}]
#   → Displays UID, GID, and all group memberships
#   Example: id kazuma

# getent group {group-name}
#   → Retrieves group details from /etc/group
#   Example: getent group kiryu-clan

Operation	Command	Description
Create a group	groupadd {group-name}	Creates a new group. The GID is assigned automatically.
Create with a specific GID	groupadd -g {GID} {group-name}	Creates a group with an explicitly specified group ID.
Delete a group	groupdel {group-name}	Deletes the specified group. The group must not be the primary group of any user.
Add to a supplementary group	usermod -aG {group} {user}	Adds a user to a supplementary group. Always include -a; omitting it will remove the user from all existing groups.
Change primary group	usermod -g {group} {user}	Changes the user's primary group.
Remove from a group	gpasswd -d {user} {group}	Removes a user from the specified group.
Check group memberships	groups [{username}]	Lists the groups a user belongs to. If omitted, shows the current user's groups.
Check UID and GID	id [{username}]	Displays the UID, GID, and all group memberships at once.
Get group details	getent group {group-name}	Retrieves group details (GID and member list) from /etc/group.
List all groups	cat /etc/group	Displays the full contents of /etc/group. You can review group names, GIDs, and members at a glance.

Create a group and add users

# -----------------------------------------------
#  Create a group and add multiple users
# -----------------------------------------------

# Create the kiryu-clan group
sudo groupadd kiryu-clan

# Verify the group was created
getent group kiryu-clan

Run the following command:

$ getent group kiryu-clan
kiryu-clan:x:1200:

The following example demonstrates this:

# Add kazuma to the kiryu-clan group
# Using -a preserves existing group memberships when adding to the new group
sudo usermod -aG kiryu-clan kazuma

# Add majima to the same group
sudo usermod -aG kiryu-clan majima

# Check the group members
getent group kiryu-clan

Run the following command:

$ getent group kiryu-clan
kiryu-clan:x:1200:kazuma,majima

Check a user's group memberships

# -----------------------------------------------
#  Check group memberships with groups and id commands
# -----------------------------------------------

# List the groups kazuma belongs to
groups kazuma

Run the following command:

$ groups kazuma
kazuma : kazuma kiryu-clan sudo

The following example demonstrates this:

# Display UID, GID, and supplementary groups in detail with the id command
id kazuma

Run the following command:

$ id kazuma
uid=1001(kazuma) gid=1001(kazuma) groups=1001(kazuma),27(sudo),1200(kiryu-clan)

Remove a user from a group and delete the group

# -----------------------------------------------
#  Group management cleanup steps
# -----------------------------------------------

# Remove majima from the kiryu-clan group
sudo gpasswd -d majima kiryu-clan

Run the following command:

$ sudo gpasswd -d majima kiryu-clan
Removing user majima from group kiryu-clan

The following example demonstrates this:

# Remove all remaining users from kiryu-clan, then delete the group itself
# (Deletion is possible even with members remaining, but it will fail
# if the group is set as any user's primary group)
sudo gpasswd -d kazuma kiryu-clan
sudo groupdel kiryu-clan

# Confirm the group has been deleted (no output means deletion was successful)
getent group kiryu-clan

Run the following command:

$ getent group kiryu-clan
(no output means deletion was successful)

Inspect the structure of /etc/group

# -----------------------------------------------
#  Understanding the /etc/group format
#    Format: {group-name}:{password}:{GID}:{member-list}
#    The password field is usually x (shadowed) or empty
# -----------------------------------------------

# Check the sudo group entry
getent group sudo

Run the following command:

$ getent group sudo
sudo:x:27:kazuma

The following example demonstrates this:

# Extract only groups with a GID of 1000 or higher
# awk splits each line by colon and filters on the third field (GID)
awk -F: '$3 >= 1000' /etc/group

Run the following command:

$ awk -F: '$3 >= 1000' /etc/group
kazuma:x:1001:
majima:x:1002:
kiryu-clan:x:1200:kazuma,majima

Linux access permissions are managed in three layers — user, group, and others — and groups let you grant permissions to multiple users at once. The standard workflow is to create a group with groupadd and then add users with usermod -aG. Always include the -a (append) option with usermod -aG. Without it, all existing group memberships are overwritten, which can inadvertently remove the user from groups like sudo. Note that group changes are not reflected in an already-running shell session. Use newgrp {group-name} to switch groups temporarily, or log out and back in to apply the changes. Group information is stored in the /etc/group file, with one group per line in the format {group-name}:{password}:{GID}:{member-list}. For general user management, see the useradd / userdel / usermod (user management) page as well.