Frequently Asked Question

What does the /etc/group file actually do?

/etc/group defines every group on the system and lists which users belong to each one. A line like sudo:x:27:chris,alice says: there is a group called sudo, its GID is 27, the placeholder x means the (largely unused) group password lives in /etc/gshadow, and the members are chris and alice. The file is world-readable for the same reason /etc/passwd is: tools need to map GIDs to names.

A user's primary group comes from the GID field of /etc/passwd and does not need to be listed in /etc/group, that is the group new files inherit by default. All the other groups they belong to are supplementary and are listed in /etc/group. Running id shows both; running groups shows the names. Linux supports up to NGROUPS_MAX supplementary groups (65 536 on modern kernels), so there is no meaningful limit in practice.