I was just poking around in /usr/bin and I found an ELF binary file called
/usr/bin/[. I have never heard of this file and my first thought was that it was a clever way of hiding a program, possibly a trojan. However it's present on all my CentOS servers and seems to have no manual entry. I can hazard a guess as to what it is but I was looking for a more authoritative answer...
It's an alternative form of the 'test' command. Mostly used in scripts.
if [ $VAR ] then echo $VAR exists! fi