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

[jeanid]

Technically, I want to check a shulker box's contents for a map I'm making. I want the player to put a Disk 11 into the shulker box and then it will be replaced with Stal. What command would I use to do this, if there even is one?

I used /data get and got this:

{CustomName: '{         
    "text":"Disk Repairer"  
  }', 
  x: -42, 
  y: 23, 
  z: 23,    
  Items: [{         
    Slot: 13b, 
    id: "minecraft:music_disc_11", 
    Count:1b, 
    tag: {          
      display: {
        Lore: ['{
          "text":"Please return to",
          "color":"gray",
          "italic":false
        },], ['{
          "text":"a Disk Repairer.",
          "color":"gray",
          "italic":false
        }'], 
        Name: ['{
          "text":"Broken Music Disk",
          "italic":false
        }']             
      }         
    }   
  }],   
id: "minecraft:shulker_box" 
}

Would I use this for the path? Or what would I use?

[yajahira]

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 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"