Title: Is that redundant for groups=0(root) listed in id command output?
Tags: users;root;group
Question: sudo id
uid=0(root) gid=0(root) groups=0(root)
Maybe it is redundant  for groups=0(root) listed here, gid=0(root) can express the meaning of group root's id is 0?

Here is the accepted answer: What the id command lists is
The effective user ID and username of the user running id,
The effective group ID and group name of same user,
The list of all supplementary ("secondary") groups that this user belongs to.
In this case, the root user belongs to only one group, the root group, so this group is listed as the effective group as well as part of the supplementary groups.
On my OpenBSD system:
$ doas id
uid=0(root) gid=0(wheel) groups=0(wheel), 2(kmem), 3(sys), 4(tty), 5(operator), 20(staff), 31(guest)

Here is another answer: The result of id provides three values ugG (id and name together) by default.
From Linux man page,
-u, --user : Print only the effective user ID
-g, --group : Print only the effective group ID
-G, --groups : Print all group IDs
Here groups=0(root) represent that the user root belongs to a group called root. Since there are no secondary groups for root here, it looks like a redundant information.
For a user who belongs to multiple groups, the output would look like
uid=492(username) gid=492(primary groupname) groups=492(primary group), 493(secondary group)

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