Textbook of Linux

/etc/group lists the groups on the system and their members. Each line has four colon-separated fields:

groupname:x:GID:member1,member2,member3
sudo:x:27:alice,bob
developers:x:1001:alice,bob,carol

Every user has one primary group (from their /etc/passwd entry) and zero or more supplementary groups (listed here). A user's primary group is the default for files they create; supplementary groups determine what additional resources they can access.

Groups matter because Linux permissions have an explicit "group" class—a file's group determines who, besides the owner, can access it through the group permission bits. Typical groups on a Linux system include sudo or wheel (administrators), adm (log readers), dialout (serial/modem access), video, audio, input, plugdev, docker, and libvirt.

Manage groups with groupadd, groupdel, gpasswd, or usermod -aG group user to add a user to a supplementary group. Like /etc/passwd, /etc/group is world-readable; the corresponding shadow file /etc/gshadow holds group passwords (almost never used in practice).