From LQWiki

The groups command tells what groups (and supplementary groups) a user or process belong to.

Process information

If groups is run without arguments, it writes the groups that the present process belongs to.

For example:

$ whoami
robot
$ groups
hive

This would be a process owned by user "robot", belonging to group "hive."

User information

If groups is run with a user as an argument, it writes the groups that the named user belongs to.

For example:

$ whoami
robot
$ groups robot
hive dockingbay command

This would be a user "robot", belonging to groups "hive," "dockingbay," and "command." The last two groups, "dockingbay," and "command," are likely suplementary groups, that can be activated with su or newgrp.

See also

• id - a more powerful command, providing much more information