user-init-file

user-init-file is a variable defined in `C source code'.
Its value is

nil

Documentation:
File name, including directory, of user's initialization file.
If the file loaded had extension `.elc', and the corresponding source file
exists, this variable contains the name of source file, suitable for use
by functions like `custom-save-all' which edit the init file.
While Emacs loads and evaluates the init file, value is the real name
of the file, regardless of whether or not it has the `.elc' extension.