$Input
is a global variable whose value is the name of the stream from which input to the Wolfram Language is currently being sought.
Examples
Basic Examples (2)
Wolfram Research (1991), $Input, Wolfram Language function, https://reference.wolfram.com/language/ref/$Input.html.
Text
Wolfram Research (1991), $Input, Wolfram Language function, https://reference.wolfram.com/language/ref/$Input.html.
CMS
Wolfram Language. 1991. "$Input." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$Input.html.
APA
Wolfram Language. (1991). $Input. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$Input.html