Creating commands can allow a mod developer to add functionality that can be used through a command. This tutorial will teach you how to register commands and the general command structure of Brigadier.

INFO

Brigadier is a command parser and dispatcher written by Mojang for Minecraft. It is a tree-based command library where you build a tree of commands and arguments.

Brigadier is open-source: https://github.com/Mojang/brigadier

The Command Interface

com.mojang.brigadier.Command is a functional interface, which runs some specific code, and throws a CommandSyntaxException in certain cases. It has a generic type S, which defines the type of the command source. The command source provides some context in which a command was run. In Minecraft, the command source is typically a ServerCommandSource which can represent a server, a command block, a remote connection (RCON), a player or an entity.

The single method in Command, run(CommandContext<S>) takes a CommandContext<S> as the sole parameter and returns an integer. The command context holds your command source of S and allows you to obtain arguments, look at the parsed command nodes and see the input used in this command.

Like other functional interfaces, it is usually used as a lambda or a method reference:

Command<ServerCommandSource> command = context -> {
    return 0;
};

The integer can be considered the result of the command. Typically values less than or equal to zero mean a command has failed and will do nothing. Positive values mean the command was successful and did something. Brigadier provides a constant to indicate success; Command#SINGLE_SUCCESS.

What Can the ServerCommandSource Do?

A ServerCommandSource provides some additional implementation-specific context when a command is run. This includes the ability to get the entity that executed the command, the world the command was run in or the server the command was run on.

You can access the command source from a command context by calling getSource() on the CommandContext instance.

Command<ServerCommandSource> command = context -> {
    ServerCommandSource source = context.getSource();
    return 0;
};

Registering a Basic Command

Commands are registered within the CommandRegistrationCallback provided by the Fabric API.

INFO

For information on registering callbacks, please see the Events guide.

The event should be registered in your mod's initializer.

The callback has three parameters:

• CommandDispatcher<ServerCommandSource> dispatcher - Used to register, parse and execute commands. S is the type of command source the command dispatcher supports.
• CommandRegistryAccess registryAccess - Provides an abstraction to registries that may be passed to certain command argument methods
• CommandManager.RegistrationEnvironment environment - Identifies the type of server the commands are being registered on.

In the mod initializer, we just register a simple command:

CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) -> {
	dispatcher.register(CommandManager.literal("tater").executes(context -> {
		context.getSource().sendFeedback(() -> Text.literal("Called /tater with no arguments."), false);
		return 1;
	}));
});

In the sendFeedback() method, the first parameter is the text to be sent, which is a Supplier<Text> to avoid instantiating Text objects when not needed.

The second parameter determines whether to broadcast the feedback to other operators. Generally, if the command is to query something without actually affecting the world, such as query the current time or some player's score, it should be false. If the command does something, such as changing the time or modifying someone's score, it should be true.

If the command fails, instead of calling sendFeedback(), you may directly throw any exception and the server or client will handle it appropriately.

CommandSyntaxException is generally thrown to indicate syntax errors in commands or arguments. You can also implement your own exception.

To execute this command, you must type /foo, which is case-sensitive.

Registration Environment

If desired, you can also make sure a command is only registered under some specific circumstances, for example, only in the dedicated environment:

CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) -> {
	if (environment.dedicated) {
		dispatcher.register(CommandManager.literal("dedicatedtater").executes(context -> {
			context.getSource()
					.sendFeedback(() -> Text.literal("Called /dedicatedtater with no arguments."), false);
			return 1;
		}));
	}
});

Command Requirements

Let's say you have a command that you only want operators to be able to execute. This is where the requires() method comes into play. The requires() method has one argument of a Predicate<S> which will supply a ServerCommandSource to test with and determine if the CommandSource can execute the command.

CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) -> {
	dispatcher.register(CommandManager.literal("requiredtater")
			.requires(source -> source.hasPermissionLevel(1))
			.executes(context -> {
				context.getSource()
						.sendFeedback(() -> Text.literal("Called /requiredtater with no arguments."), false);
				return 1;
			}));
});

This command will only execute if the source of the command is a level 2 operator at a minimum, including command blocks. Otherwise, the command is not registered.

This has the side effect of not showing this command in tab completion to anyone who is not a level 2 operator. This is also why you cannot tab-complete most commands when you do not enable cheats.

Sub Commands

To add a sub command, you register the first literal node of the command normally. To have a sub command, you have to append the next literal node to the existing node.

CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) -> {
	dispatcher.register(CommandManager.literal("subtater1")
			.then(CommandManager.literal("subcommand").executes(context -> {
				context.getSource()
						.sendFeedback(
								() -> Text.literal("Called /subtater1 subcommand with no arguments."), false);
				return 1;
			})));
});

Similar to arguments, sub command nodes can also be set optional. In the following case, both /subtater and /subtater subcommand will be valid.

CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) -> {
	dispatcher.register(CommandManager.literal("subtater2")
			.executes(context -> {
				context.getSource()
						.sendFeedback(() -> Text.literal("Called /subtater2 with no arguments."), false);
				return 1;
			})
			.then(CommandManager.literal("subcommand").executes(context -> {
				context.getSource()
						.sendFeedback(
								() -> Text.literal("Called /subtater2 subcommand with no arguments."), false);
				return 1;
			})));
});

Client Commands

Fabric API has a ClientCommandManager in net.fabricmc.fabric.api.client.command.v2 package that can be used to register client-side commands. The code should exist only in client-side code.

ClientCommandRegistrationCallback.EVENT.register((dispatcher, registryAccess) -> {
	dispatcher.register(ClientCommandManager.literal("clienttater").executes(context -> {
		context.getSource().sendFeedback(Text.literal("Called /clienttater with no arguments."));
		return 1;
	}));
});

Command Redirects

Command redirects - also known as aliases - are a way to redirect the functionality of one command to another. This is useful for when you want to change the name of a command, but still want to support the old name.

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FAQ

Why Does My Code Not Compile?

• Catch or throw a CommandSyntaxException - CommandSyntaxException is not a RuntimeException. If you throw it, that should be in methods that throw CommandSyntaxException in method signatures, or it should be caught. Brigadier will handle the checked exceptions and forward the proper error message in the game for you.

• Issues with generics - You may have an issue with generics once in a while. If you are registering server commands (which are most of the case), make sure you are using CommandManager.literal or CommandManager.argument instead of LiteralArgumentBuilder.literal or RequiredArgumentBuilder.argument.

• Check sendFeedback() method - You may have forgotten to provide a boolean as the second argument. Also remember that, since Minecraft 1.20, the first parameter is Supplier<Text> instead of Text.

• A Command should return an integer - When registering commands, the executes() method accepts a Command object, which is usually a lambda. The lambda should return an integer, instead of other types.

Can I Register Commands at Runtime?

WARNING

You can do this, but it is not recommended. You would get the CommandManager from the server and add anything commands you wish to its CommandDispatcher.

After that, you need to send the command tree to every player again using CommandManager.sendCommandTree(ServerPlayerEntity).

This is required because the client locally caches the command tree it receives during login (or when operator packets are sent) for local completions-rich error messages.

Can I Unregister Commands at Runtime?

WARNING

You can also do this, however, it is much less stable than registering commands at runtime and could cause unwanted side effects.

To keep things simple, you need to use reflection on Brigadier and remove nodes. After this, you need to send the command tree to every player again using sendCommandTree(ServerPlayerEntity).

If you don't send the updated command tree, the client may think a command still exists, even though the server will fail execution.