Is it possible to query the current user's Apple ID from the command line?
Solution 1:
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