What command can I use to check a chest's contents?

Solution 1:

To check the NBT-data of a shulker box (or any other block) at the coordinates -42 23 23, you can use this command:

/data get block -42 23 23

To only get information about the contents of the shulker box, which is stored in the Items tag, you can use this command:

/data get block -42 23 23 Items

To check if the shulker box contains a disc 11, you would use one of these command, they fail if there is no item with the id "minecraft:music_disc_11" present within the Items tag, the first command also fails if there is more than one music disc 11 within the shulker box. The second command can imidiately run a command by adding run <command> at the end of it:

/data get block -42 23 23 Items[{id:"minecraft:music_disc_11"}]
/execute if data block -42 23 23 Items[{id:"minecraft:music_disc_11"}]

To change the id of all items with the id "minecraft:music_disc_11" to the id "minecraft:music_disc_stal" you can use this command:

/data modify block -42 23 23 Items[{id:"minecraft:music_disc_11"}].id set value "minecraft:music_disc_stal"