How do you manage groups in Linux?

groupadd creates, groupmod renames or re-numbers, groupdel removes; add members with usermod -aG group user (or gpasswd -a), and the user must re-login for it to take effect.

The re-login catch trips people up constantly: group membership is read into your session at login and baked into your running processes, so adding yourself to docker or wheel does nothing for your current shell until you log out and back in (or start a fresh session with newgrp).

Group management commands:

Create group:

sudo groupadd groupname
sudo groupadd -g 10000 groupname    # With specific GID
sudo groupadd -r systemgroup        # System group (low GID)

Modify group:

sudo groupmod -g 20000 groupname    # Change GID
sudo groupmod -n newname oldname    # Rename group

Delete group:

sudo groupdel groupname

View group membership:

groups username          # List user's groups
getent group groupname   # Show group details
cat /etc/group          # All groups

Add user to group:

# Recommended
sudo usermod -aG groupname username
# Alternative
sudo gpasswd -a username groupname

Note: Users must log out and back in for new group membership to take effect!