simulate painless script

gistfile1.txt

POST _ingest/pipeline/_simulate
{
  "pipeline" :
  {
    "description": "_description",
    "processors": [
      {
        "script":
        {
          "lang": "painless",
          "inline": "if (ctx._id == '2') { ctx._id = params.foo; }",
          "params": 
          {
            "foo" : "bar"
          }
        }
      }
    ]
  },
  "docs": [
    {
      "_index": "shanetest",
      "_type": "shanetest",
      "_id": "1",
      "_source": {
        "field_a": 3
      }
    },
    {
      "_index": "shanetest",
      "_type": "shanetest",
      "_id": "2",
      "_source": {
        "field_a": 6
      }
    }
  ]
}