read

(read &optional stream)

Reads and parses a single S-expression. With no argument it reads from standard input; given a stream opened by open or with-open-file it reads from that stream. Blank and comment-only lines are skipped, the datum must fit on a single line, and EOF returns nil. Works in all three backends: the interpreter uses the full Lisp reader, the JVM emits a reader with full JDK parity, and the WASM reader is narrower (31-bit integers, decimal floats, no big integers or exponents).

(print (read))

Reading the line (+ 1 2) from standard input parses it into the list (+ 1 2), which print then echoes back. At end of input read returns nil.