Minecraft – How would you /effect someone with chainmail boots with only 1 command block

You want to do this with scoreboards. My solution won't prevent getting diamond swords, but it will prevent crafting them and using them, i.e. a player can have a diamond sword in their inventory, as long as they never use it. How it gets there without crafting it, though, is pretty much impossible without cheats.

First, you need to set up 2 scoreboard objectives, one for crafting, and one for using:

/scoreboard objectives add craftedDiamondSword stat.craftItem.276
/scoreboard objectives add usedDiamondSword stat.useItem.276

Next, you need to set up two nearly identical chains of command blocks, one for each scoreboard objective. This will run on a clock like you already have. I'll only give the commands for using, so just replace that in the second chain with the crafting objective.

First, clear those players inventories:

clear @a[score_usedDiamondSword_min=1] 276

Next, tell the players that used a diamond sword to not use it, and reset their score:

tell @a[score_usedDiamondSword_min=1] Don't use diamond swords!
scoreboard players set @a[score_usedDiamondSword_min=1] usedDiamondSword 0

This last command block doesn't really need the [score_usedDiamondSword_min=1] but it's useful.

I got this to work in a single line of command blocks with redstone over top for the uses chain, but it wouldn't work for the crafting. The solution is to pipe the result from the clear command block into the tell and set scoreboard command blocks with comparators. Here's a pic of the two setups:

Use whatever clock you want.

Additionally, if you never, ever want players to have a diamond sword, even in their inventory, and even if they don't use it, you can still clear from @a, and then just say to not use diamond swords if one is found.

Update for added 1.8 functionality:

The 1.8 snapshots have added the option to match players against the Inventory NBT tag when setting or incrementing a scoreboard objective. This means we can set up a dummy scoreboard objective to track whenever someone has a Diamond Sword in their inventory, regardless if they've used it or crafted it. First, set up the scoreboard objective:

/scoreboard objectives add hasDiamondSword dummy

Next, we need to run two command blocks on the same clock; one to set players scores, and another to check those player's scores and clear the diamond swords:

scoreboard players add @a hasDiamondSword 1 {Inventory:[{id:"minecraft:diamond_sword"}]}
clear @a[score_hasDiamondSword_min=1] minecraft:diamond_sword

The second command is fairly similar to the old commands, except instead of using the old ID number, I'm using the name instead. The first is a bit more complicated, so let's walk through it. Basically it says add to all players on their hasDiamondSword objective the value 1 if that player has an inventory and that inventory contains a diamond sword. So, for anyone that has a diamond sword in their inventory, their hasDiamondSword objective will be incremented.

Finally, like in the old version, we need to tell the players to not use diamond swords, and to reset their hasDiamondSword objective:

tell @a[score_hasDiamondSword_min=1] Don't use diamond swords!
scoreboard players set @a[score_hasDiamondSword_min=1] hasDiamondSword 0

I stacked these two command blocks (with the tell on bottom) and fed them with a comparator from the clear command block. This is what it all looks like in the end:

It's very difficult to see whether someone has an object anywhere in their inventory. That would take 36 command blocks, one for each slot! You can, however, take the object, then see if taking the object worked.

The Commands

• Set up a fast clock (but not as fast as a fill clock)
• One command block does /clear @p iron_ingot 0 1 (clears 1 iron ingot from the nearest player. You can choose which player, but it must be exactly one. Also, you must use the same selector throughout all these commands.)
• Run a comparator from that command block to another command block running this command: /scoreboard players add @p ironIngots 1 (Increases the player's ironIngots score by 1. Make sure you create ironIngots first!)
• Create another command block that does whatever you want. In the case of giving them a diamond, you would do this: /give @p[score_ironIngots_min=10] diamond. Note the target selector; it gives the player the diamond only if their ironIngots score is 10 or greater.
• The last thing you want to do is take the ironIngots score away so they don't keep getting items. Just run /scoreboard players remove @p[score_ironIngots_min=10] ironIngots 10. This removes 10 iron ingots from the player if they have an ironIngots score of 10 or greater. Any player who matches this will have received a diamond in the previous step.

How to Set Them Up

The commands listed above must run in order. It is actually very simple to put them in order. Just open the debug menu and look at the stylish new crosshair. Align the command blocks along either the green or red line so that one of those lines points down the first to the last command block.

Notes

• You don't have to press a button to use this, you just have to be the nearest player. You can change the command to the nearest player within 10 blocks, nearest player on a team, etc.
• The commands will take a short amount of time to finish, but for 10 iron ingots, it will only take half a second. It's one tick (1/20 of a second) per item taken away.
• It will take away iron ingots even if you don't have ten. However, you can submit one, then later submit five, then submit four more later, and it will still give you the diamond. You can also submit 14 at a time and the extra 4 will be saved for later.