ROT-13 transform standard input The challenge:  To read an input of arbitrary length and produce the ROT13 of the input.  All characters besides A-Z should be copied to the output verbatim, and case should be preserved if possible.

Any language that can read and write standard streams is fair game.
Bash, 23 bytes

Canonical 23-character answer:

tr A-Za-z N-ZA-Mn-za-m