Is it possible to query the current user's Apple ID from the command line?
The currently logged in Apple IDs for iCloud are stored in the MobileMeAccounts preferences. You can query the entire store with:

defaults read MobileMeAccounts Accounts


To return just the first account ID (an email address nowadays):

/usr/libexec/PlistBuddy -c "print :Accounts:0:AccountID" ~/Library/Preferences/MobileMeAccounts.plist