Command blocks

Command blocks are a versatile addition to Minecraft which allows the player to control various aspects of the game (usually accessible via chat only) using redstone. Each command block stores exactly one command. This command is executed when the command block is powered.

While the chat console is limited and will truncate your input past a certain length, you can write more than 30000 characters in a command block and it will not complain.

This page will take you through the basics only, as the enormous variety of applications is impossible to cover here. Command blocks are used for entity manipulation, complex games, reward rooms, cellular automata, self-building structures or, in general, anything in need of a simple programming language in Minecraft.

Commands are considered the functions of command block programming. There are many, but the important thing is that almost every command has a series of arguments or parameters. When they are executed or called, they run a series of statements invisible to the player – thus, a give command will internally check if the player exists, check if the item exists, generate the item, find the first free space in the player’s inventory and place it there, returning errors if any of these steps fails (note this is merely an example and the real process is much more complicated if JSON data is added, see below).
Command blocks can be thought of as the rough equivalent of a multi-threaded language – that is, you can execute as many command blocks as you can reach with one redstone pulse in one tick at the same time. Barring lag, all of them will run in that same tick.
Control flow in programming refers to redirecting the execution of a program somewhere else, skipping a certain section or repeating it one or more times. In command block programming, this is done in what may seem a slightly unconventional manner.
- Commands can be chained together by sequentially powering them, be it with redstone repeaters or with the use of the chain command block, described below. You can create ever bigger functions by chaining many sections of your creation together and then taking the whole thing and executing it with just one pulse.
- Any of the logic gates mentioned previously can be used as a means of control flow. Conditional branches such as if-then-else with as many conditions as you want can be created by joining outputs representing certain inputs using Boolean algebra.
- If you want to emulate programming loops, you can use the repeat command block (more information below). While loops are easy as you can just unpower the command block when your desired condition (compound or not) is false. For loops are harder – a common way is using the scoreboard command to store and update a local database with a counter. We will not be covering scoreboard here as it is very hard for beginners, but there is a full reference on the official wiki.

Obtaining

If you search for command blocks in the creative menu, you will not find them. This is also the case for other reserved blocks such as structure blocks or barriers. To obtain a command block, you must type the following in your chat:

/give command_block

You can obtain an entire stack by typing the number 64 after that if you want, but you’ll only need one if you’re in creative. While on the topic, if you are in survival you won’t be able to break command blocks, as they are not vulnerable with any tool and have the same blast resistance as bedrock.

Types

It used to be the case that there was only one possible command block, the orange one. If you wanted functionality provided by the other two you had to use workarounds. These are the three currently possible types of command block:

The impulse command block is orange and functions exactly like the original command blocks. They execute once every time they are activated, that is, every time they receive redstone power from any source, regardless of whether it is weak or strong.
The chain command block is green. A command in one of these will run if and only if a command in a command block pointing to it has been processed and executed successfully. This is independent of how many command blocks are pointing towards it – one will suffice as a trigger.
The repeat command block is purple and will execute its command once per game tick (20 times per second). It is considered a redstone clock even though it does not involve any redstone. This, and other command blocks, are the only components capable of producing output and understanding input at a higher frequency than allowed by the redstone tick mechanic.

Options

When you right click a command block, not only do you get an input box for your command. you get 3 buttons below that allow you to cycle through a variety of options (left click to change). They are:

Impulse / Chain / Repeat, which deals with cycling between all 3 command block types described above.
Conditional / Unconditional:
- Making a command block conditional will restrict its activation. It will not execute until a command block directly behind it has produced a successful output (note the difference with chain blocks, which execute after anything pointing into them finishes.).
- Making it unconditional (default) will eliminate all dependencies and execute the command regardless.
Needs Redstone / Always Active:
- If a command block needs redstone, it will not execute unless it is somehow powered. This includes chained command blocks executing sequentially.
- If it is always active, the command block will not care if power is removed or added. With a repeat command block, for example, this means its command will keep executing 20 times a second forever unless broken (lag issues!).

Commands

There are about 60 commands in total, all of which can be found on the official wiki:

http://minecraft.gamepedia.com/Commands

However, they all follow approximately the same syntax. They will have a keyword followed by certain arguments, just like functions in programming. In the example above, give is the keyword and the rest are all parameters (the same as arguments).

When you type a command in the chat, you will need to prefix it with a slash (/).
When the command is included inside a command block, the slash is optional.

If you are unsure about the syntax of a certain command, the quickest way to check is by typing the command keyword in chat followed by no arguments. For example, typing /summon will bring up:

Usage: /summon [x] [y] [z] [dataTag]

Note the different brackets around the arguments.

Parameters surrounded by angle brackets are compulsory.
Parameters surrounded by square brackets are optional.

In this specific case, the summon command will not work without providing EntityName, but will assume a certain default behaviour for the other arguments if they are not provided.

In general, for commands, coordinates such as the x y and z above can be given in one of two formats:

Absolute coordinates will take the numbers provided literally as the coordinates you can see on your debug screen, accessible pressing F3 (Fn + F3 on Macs).
Relative coordinates are prefixed with a tilde (~) and add whatever X, Y and Z are provided to the position of the command block as an offset. No number after the tilde is equal to zero offset. Negative numbers work too, that is, to summon a spider just below the command block you could use /summon Spider ~ ~-2 ~.

dataTag is much harder to explain. They appear across many commands and they are nothing more than JSON data. JSON is a data serialization format consisting of an easily parse-able series of opening and closing curly or square brackets. For example, using a command which gives the player Paul a diamond sword with sharpness 100:

/give Paul diamond_sword 1 0 {ench:[{id:16, lvl:100}]}

The syntax of JSON never changes. Elements are delimited by curly brackets { and }, lists of elements are enclosed in square brackets [ and ]. However, there is a very important distinction between Minecraft JSON and normal JSON: Minecraft does not need double quotes around strings. You can re-write the above data tag in a neater format like this:

{

ench: [

{

id: 16,

lvl: 100

}

]

}

In fact, this is usually how a real JSON file will look, only with quotes around the strings.

Selectors

Selectors are used to target an entity or group of entities. The command which includes the selector will only apply to the entities specified. There are four possible options:

@p will target the nearest player.
@a will target all players.
@r will target a random player.
@e will target all entities, including the player. Commonly used in conjunction with the type modifier (see below).

Examples:

give @p 35:5 1 will give the nearest player 1 block of lime wool.
give @a cookie will give all players a cookie.
clear @r will clear a random player’s inventory.
kill @e[type=Spider] will kill all spiders around the player.

Specifiers

Specifiers are used to narrow down the group of entities we are targeting.

[r=radius] will target entities within radius
[rm=radius] will target entities outside radius
[m=gamemode] will target entities in gamemode
[l=xp] will target only entities whose XP level is below xp
[lm=xp] will target only entities whose XP level is above xp
[type=type] will target only entities of type type