Minecraft – Place items in specific slots

As of 1.13, doing this no longer needs a scoreboard and can be done in one command block:

/execute as @a[nbt={Inventory:[{id:"blue_banner"}]}] run ...

This can be done rather easily by matching part of the Items data tag of the chest block entity in a testforblock command:

testforblock <x> <y> <z> minecraft:chest -1 {Items:[{id:minecraft:snowball,Slot:2s}]}

Replace <x> <y> <z> with the coordinates of the chest. The -1 allows all data values of the chest (i.e. all orientations) to match.

As usual, data tag matching is partial, meaning that everything not specified is not considered by the command, such as the Count of snowballs in the above command. I.e. any number of snowballs works, as but they need to be in the third slot from the top left, which is slot 2 because of 0-indexing. Any other items in the chest are also ignored.

You can add more item tags to the Items tag (comma separated, between the []) to match more items.