Player NBT Modification
Why can't I modify a player's nbt?
The main thing to understand here is that NBT isn’t how the game remembers entities; it’s how they’re stored in files. While the game’s running, entities are stored as actual java objects. This means that accessing and modifying an entity’s nbt goes something like:
Command: “Hey, can you pretend you’re saving yourself?”
Entity: “Sure. Here’s my NBT.”
Command: “Thanks”
Command: Modifies NBT
Command: “Can you reload yourself in with this nbt?”
Entity: “Sure”
Entity: Is reloaded into the game
Thus, if we wanted to modify a player’s NBT, we’d need to kick the player out of the game and rejoin them. Which isn’t a very good idea.
Alternatives
Mojang is trying to allow us to do operations to the player, adding more commands every version. These commands act on the player object themselves and not the nbt; thus allowing the player to remain in the game.
There are some obvious ones:
- Effects: we can use
/effect - Advancements: we can use
/advancement - Tags: we can use
/tag - Recipes: we can use
/recipe
But generally people figure out those on their own. One thing people struggle with a lot is item modification.
Of course, to give an item you can use /give. In 1.16, there’s /replaceitem, which is slightly better but not useful if you want to modify an item. So you have some options (still 1.16):
- Use a loot table. This can work in some situations, using
loot replaceandcopy_nbtfunctions inside of it. - Use the shulker box loot trick.
In 1.17, we have the lovely /item command, where we can define item modifiers! Use it.