How can I view the contents of an ElasticSearch index?

You can view any existing index by using the below CURL. Please replace the index-name with your actual name before running and it will run as is.

View the index content

curl -H 'Content-Type: application/json' -X GET https://localhost:9200/index_name?pretty

And the output will include an index(see settings in output) and its mappings too and it will look like below output -

{
  "index_name": {
    "aliases": {},
    "mappings": {
      "collection_name": {
        "properties": {
          "test_field": {
            "type": "text",
            "fields": {
              "keyword": {
                "type": "keyword",
                "ignore_above": 256
              }
            }
          }
       }
    },
    "settings": {
      "index": {
        "creation_date": "1527377274366",
        "number_of_shards": "5",
        "number_of_replicas": "1",
        "uuid": "6QfKqbbVQ0Gbsqkq7WZJ2g",
        "version": {
          "created": "6020299"
        },
        "provided_name": "index_name"
      }
    }
  }
}

View ALL the data under this index

curl -H 'Content-Type: application/json' -X GET https://localhost:9200/index_name/_search?pretty

I can recommend Elasticvue, which is modern, free and open source. It allows accessing your ES instance via browser add-ons quite easily (supports Firefox, Chrome, Edge). But there are also further ways.

Just make sure you set cors values in elasticsearch.yml appropiate.

If you didn't index too much data into the index yet, you can use term facet query on the field that you would like to debug to see the tokens and their frequencies:

curl -XDELETE 'http://localhost:9200/test-idx'
echo
curl -XPUT 'http://localhost:9200/test-idx' -d '
{
    "settings": {
        "index.number_of_shards" : 1,
        "index.number_of_replicas": 0
    },
    "mappings": {            
        "doc": {
            "properties": {
                "message": {"type": "string", "analyzer": "snowball"}
            }
        }
    }

}'
echo
curl -XPUT 'http://localhost:9200/test-idx/doc/1' -d '
{
  "message": "How is this going to be indexed?"
}
'
echo
curl -XPOST 'http://localhost:9200/test-idx/_refresh'
echo
curl -XGET 'http://localhost:9200/test-idx/doc/_search?pretty=true&search_type=count' -d '{
    "query": {
        "match": {
            "_id": "1"
        }
    },
    "facets": {
        "tokens": {
            "terms": {
                "field": "message"
            }
        }
    }
}
'
echo