FileReadLine from stdin

FileReadLine, OutputVar, Filename, LineNum

Parameters

Filename Standard Input (stdin): Specifying stdin for Filename causes Text to be read from standard input (stdin). Such text can be piped from a file
LineNum should be 1 with stdin

Example

FileReadLine, line1, stdin, 1
FileReadLine, line2, stdin, 1
listvars
msgbox
...
> type script.ahk | autohotkey.exe script.ahk