The who command will give you information about who is logged on the system.
$ who
The whoami command tells you your username.
$ whoami
The w command shows you who is logged on and what they are doing.
$ w
The id command will give you your user id, primary group id, and a list of the groups that you belong to.
$ id
The su command allows a user to run a shell as another user.
$ su user_name
$ su jiya
$ su root