groups(1)
Name
groups − show group memberships
Syntax
groups [user]
Description
The groups command shows the groups to which the specified user belongs determined by reading /etc/group. These are the groups that will be in effect the next time the usr logs in. If the groups command is used without the usr argument, it will report the groups currently in effect using the getgroups() system call. If the /etc/group file was changed after the login, this may not be the same.
Each user belongs to a group specified in the password file /etc/passwd and possibly to other groups as specified in the file /etc/group. If you do not own a file but you do belong to the group by which it is owned, you are granted group access to the file.
When a new file is created it is given the group specifications of the directory in which it resides.
Files
/etc/passwd
/etc/group