In logic, Quine's paradox shows that a sentence that is not directly self-referential can nonetheless be paradoxical. In particular, this sentence asserts that the sentence itself is false:
“yields falsehood when preceded by its quotation” yields falsehood when preceded by its quotation.
In computing, a quine is a program that prints its own source code, usually by using the same trick as Quine's sentence. For example, this Scheme program is a quine:
((lambda (x) `(,x ',x))
'(lambda (x) `(,x ',x)))
Ken Thompson's Turing Award lecture is all about quines and computer security.
This Git repository holds the source code to some other quines. Most are direct uses of the quotation pattern, like the two above, but a few are worth additional notes.
qshell takes advantage of the Unix kernel to arrange the quotation.
When the kernel is asked to run with a program beginning with #!,
it instead runs the program named by the remainder of that line,
with the name of the program file itself as an additional argument.
The file qshell/quine is a single line:
#!/bin/cat
To run this program, the kernel would execute:
/bin/cat qshell/quine
which will in turn print the source code.
The qzip directory contains compressed archives that manage to write a quine using the DEFLATE compression method's bytecode programs. See Zip Files All The Way Down for details about how they were constructed.