Simplified Common Lisp reference
read-line
Symbol class: Input and output
Syntax:
Symbol type: function
read-lineinput-stream(optional)eof-error-p(optional)eof-value(optional)recursive-p(optional) => line, missing-newline-p
Argument description:
input-stream an input stream, default is standard input
eof-error-p a boolean, true (default) is EOF should be signaled
eof-value an object that is returned as EOF value
recursive-p flag to note recursive processing

READ-LINE function reads a line from input stream into string. 

(let ((s (make-string-input-stream (format nil "line 1~%line 2~%line 3)")))) (read-line s))
=> "line 1", NIL
Function indexFull documentation for read-line (HyperSpec)