[jkn]

On Unix, an empty file is a working implementation of the "true" command (provided that is in the PATH and set as executable). This is because it is interpreted as a shell script, and an empty script of course exits successfully.

I'm pretty sure this was actually used in some Unix/Linux version, and they got bug reports due to the poor performance (executing a shell to do nothing), which makes for a lot of bugs per line of code. Unfortunately I can't find a reference. Instead I found that AT&T Unix implemented "true" as an empty file... with a copyright notice! See http://trillian.mit.edu/~jc/humor/ATT_Copyright_true.html