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Counts entities (players, mobs, items, etc.) matching specified conditions.

testfor <victim: target>
victim: target
Specifies the targets to count. Must be a player name or a target selector (@e is permitted to target entities other than players).
Fails if the arguments are not specified correctly, or if player fails to resolve to one or more entities (named players must be online).
On success, produces a success count equal to the number of targets matching player, which can be measured by a redstone comparator facing away from the command block.
To test if Alice is online: testfor Alice
To count the number of players in survival mode within a 3-block radius of (0,64,0): testfor @a[x=0,y=64,z=0,r=3,m=0]
To count the number of zombies within a 20-block radius of (0,64,0): testfor @e[x=0,y=64,z=0,r=20,type=zombie]