Recently when attempting to kill an index operation with a customer we could not get the instructions here to work:
https://docs.mongodb.com/manual/reference/method/db.currentOp/#active-indexing-operations

The instructions say to run the following command to get active index operations:

db.currentOp(
    {
      $or: [
        { op: "query", "query.createIndexes": { $exists: true } },
        { op: "insert", ns: /\.system\.indexes\b/ }
      ]
    }
)

I had to change the op: "query" to op: "command" to get this to work. I am unsure about the second part of the $or and if that part still works.