id

POSIX 7: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/id.html

Shows user and group ids and names.

Show all info for a given user:

u='root'
id "$u"

For current user:

id

Effective user ID:

id -u

Effective username:

id -un

Real user ID:

id -ur

Same but for groups:

id -g
id -gn
id -gr