— Test: -user uname
— Test: -group gname

True if the file is owned by user uname (belongs to group gname). A numeric ID is allowed.

— Test: -uid n
— Test: -gid n

True if the file's numeric user ID (group ID) is n. These tests support ranges (+n and -n), unlike -user and -group.

— Test: -nouser
— Test: -nogroup

True if no user corresponds to the file's numeric user ID (no group corresponds to the numeric group ID). These cases usually mean that the files belonged to users who have since been removed from the system. You probably should change the ownership of such files to an existing user or group, using the chown or chgrp program.