Testfor

Testfor
Test for is a command added in 13w03a. If you know what it does place it in this treath.

The Testfor command, when used in a command block with a comparator next to it, will check whether or not a player is online, and if at least one is, it outputs a signal through the comparator. It can also be used to check whether a player (or more) is inside a radius, with: /testfor @p[100,64,100,r=5]. It outputs one extra strength per player inside the radius. Example

Known: You can only use /testfor in a Command Block.