What is /usr/bin/[?
Solution 1:
It's an alternative form of the 'test' command. Mostly used in scripts.
i.e.
if [ $VAR ]
then
echo $VAR exists!
fi
Solution 2:
It's what you call when you are using something like
if [ -e foo ]; then ...
in a shell script (but most shells have it as a buildin this days). man test should give you the docs.
Solution 3:
As others pointed out, [ is the shell's condition evaluation utility - test.
In fact, there is a manual page for that!
$ man [
should give you more details about the opening square bracket.
Btw, in OS X, [ is located in /bin/[ :)