Null program

From Esolang

The null program is the unique program of zero length. It is a valid program in many languages; in some of these languages, it is even a quine, in that it produces itself as output. (But if a zero-length program is used as a quine, it would be probably considered a cheating quine!)

In the language Homespring the message "In Homespring, the null program is not a quine." is printed and the program exited, in case one runs the null program/quine.

In languages where input is appended to the source code, the null program is a self-interpreter.