Command Block

The Command Block is a block made with the intent to support Adventure mode and will allow mapmakers to expand the interaction with the player through server commands. The block can run specified server commands when activated by redstone current. Considering its functionality, they will not be able to be crafted and only administrators will be able to place them. Currently, it is not available in the creative mode inventory. To be able to use this block in multiplayer, you must replace  by   on line   in server.properties. If this line doesn't exist, you'll have to add it. Alternatively, you can use the 'give' command:  to spawn the command block entity to a certain player.

Usage
The Command Block will execute server commands like giving items or experience to a player.

To change the command executed by the block, simply right-click it, and a GUI will open.

In place of player names, it is possible to use multiple different shorthands:

When using commands, treat them as if entered from the console - you always need to specify the target, so it is not possible to use commands such as "kill" which do not take a player argument. For example, to change the mode to Survival for the nearest player type:. Note that you will have to be in Creative mode to change the command associated with the block.

Gamerules
As of Minecraft version 12w32a a new command has been added mainly was meant for custom maps using the command block.

Usage: /gamerule

Disabled commands
As of 12w33b, these commands have been disabled for the Command Block.


 * However "/me" command still works and allows to broadcast messages server-wide. Command block identifies itself as "@". Worth noting is the fact that "/me" command, is an example of command without player target that actually works with Command block.

Arguments
https://gist.github.com/3736487

History
On July 27th, 2012, Jeb began tweeting of a new block he had created to start expanding Adventure mode. He began posting pictures, ideas and hints of what the block will be able to do. During Notch's AMA he hinted the block may be in 1.4.

Bugs

 * If you use the command block to change either your own, or any one else's game mode to creative, any mobs which were aggressive towards a player in survival or adventure mode will stay aggressive; even though they don't inflict any damage.
 * Even if doFireTick command is set to false, lava will still start fires on nearby blocks, but they won't spread or damage flammable blocks.

Trivia

 * It is possible to make teleporters using the command block. (eg code: /tp @p[r=3] x y z )
 * You can make a kind of world guard using the command block. (eg code: /gamemode @a[r= ]
 * The commandBlockOutput gamerule does not affect the command block using the /say and /me commands
 * The commands  and   will cause the command block to say use name of the nearest player in the message, however   just causes it to say "Hello, @p"

Usage of the Command Block
Page detailing innovative, practical uses of the Command Block Tutorials/Command Block