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$Input is a global variable whose value is the name of the stream from which input to Mathematica is currently being sought.
During the execution of <<
file, $Input is set to "
During interactive input, $Input is "".
See the Mathematica book: Section 2.11.4, Section 2.13.1.
See also: Get, Streams, DirectoryName, ToFileName, $BatchInput, $ParentLink.