display current group names
Print group memberships for the current user
Print group memberships for a list of users
The groups command displays the current group names or ID values. If the value does not have a corresponding entry in /etc/group, the value will be displayed as the numerical group value. The optional user parameter will display the groups for the named user.
Systems which do not support concurrent group sets will have the information from /etc/group reported. The user must use newgrp or sg to change his current real and effective group ID.
Group account information.
newgrp(1), getgid(2), getgroups(2), getuid(2).