username different in Terminal and System Preferences I am new to Mac.

Recently I realized something for usernames.

When I type whoami in Terminal. It returns apple. However in System Preferences>Users and Groups" there is only one user and it says "sinan". (I am suspecting this could be the real name not the username)

Shouldn't be both the same?

Can I change the username in the terminal? Would that affect anything on the system (Lion)?
In Terminal the whoami displays your Short Name. This Short Name is also found as the name of your Home directory.

When you go into System Preferences and look at the user on the left it displays your Full Name.

Mac OS X does not let you easily change your Short Name, but you can change your Full Name from System Preferences.

Refer to this knowledge base from Apple if you wish to change your Short Name.
http://support.apple.com/kb/ht1428