curl -XGET 'http://localhost:9200/_cluster/health?pretty=true'

curl http://localhost:9200/_cat/indices?v

curl -XGET 'http://localhost:9200/_cat/shards'

Try restarting

curl -XPOST -k https://admin:<password>@<hostname>:9200/_cluster/reroute?retry_failed=true --CApath /etc/elasticsearch/root-ca-key.pem

List indexes not assigned

curl -k --CApath /etc/elasticsearch/root-ca-key.pem -XGET https://admin:<password>@<hostname>:9200/_cat/shards?h=index,shard,prirep,state,unassigned.reason | grep -i UNASSIGNED

Detailed information on issue

curl -k --CApath /etc/elasticsearch/root-ca-key.pem -XGET https://admin:<password>@<hostname>:9200/_cluster/allocation/explain?pretty

curl -XGET 'http://localhost:9200/_cat/shards/filebeat?pretty=true'

curl -XDELETE 'http://localhost:9200/*.reindex'

When free disk space fall cluster fails.

curl -H 'Content-Type: application/json' -X PUT -d '{
  "transient": {
    "cluster.routing.allocation.disk.watermark.low": "89%",
    "cluster.routing.allocation.disk.watermark.high": "94%",
   "cluster.info.update.interval": "1m"
  }
}' http://localhost:9200/_cluster/settings

{
   "template": "filebeat-*",
   "version": 50001,
   "settings": {
      "index.refresh_interval": "5s",
      "number_of_replicas": 0
   },
   "mappings": {
      "_default_": {
         "dynamic_templates": [
            {
               "message_field": {
                  "path_match": "message",
                  "match_mapping_type": "string",
                  "mapping": {
                     "type": "text",
                     "norms": false
                  }
               }
            },
            {
               "string_fields": {
                  "match": "*",
                  "match_mapping_type": "string",
                  "mapping": {
                     "type": "text",
                     "norms": false,
                     "fields": {
                        "keyword": {
                           "type": "keyword",
                           "ignore_above": 256
                        }
                     }
                  }
               }
            }
         ],
         "properties": {
            "@timestamp": {
               "type": "date"
            },
            "@version": {
               "type": "keyword"
            },
            "beat.hostname": {
               "type": "string",
               "index": "not_analyzed",
               "ignore_above": 1024
            },
            "geoip": {
               "dynamic": true,
               "properties": {
                  "ip": {
                     "type": "ip"
                  },
                  "location": {
                     "type": "geo_point"
                  },
                  "latitude": {
                     "type": "half_float"
                  },
                  "longitude": {
                     "type": "half_float"
                  }
               }
            }
         }
      }
   }
}

{"error":"Content-Type header [application/x-www-form-urlencoded] is not supported","status":406}

In order to fix issue specify header with your curl XPUT.

Original Post

| exmple

curl -XPUT 'localhost:9200/_settings' -H 'Content-Type: application/json' -d '
{
    "index" : {
        "number_of_replicas" : 0
    }
}'

This should work on elastic.co but my steps are mainly taken from the Opendistro.

curl -k -XPOST https://admin:<password>@<hostname>:9200/_ml/set_upgrade_mode?enabled=true

curl -k -H 'Content-Type: application/json' -XPUT -d '{
"persistent": {
    "cluster.routing.allocation.enable": "primaries"
  }
}' https://admin:<password>@<hostname>:9200/_cluster/settings --CApath /etc/elasticsearch/root-ca-key.pem

curl -k -XPOST https://admin:<password>@<hostname>:9200/_flush/synced --CApath /etc/elasticsearch/root-ca-key.pem

systemctl stop kibana.service
systemctl stop elasticsearch.service
dnf install opendistroforelasticsearch-1.12.0 -y
dnf install opendistroforelasticsearch-kibana-1.12.0 -y
systemctl start elasticsearch.service
systemctl start kibana.service

curl -k -H 'Content-Type: application/json' -XPUT -d '{
"persistent": {
    "cluster.routing.allocation.enable": "all"
  }
}' https://admin:<password>@<hostname>:9200/_cluster/settings --CApath /etc/elasticsearch/root-ca-key.pem

Document Ref.

curl -XGET -k https://admin:<password>@<hostname>:9200/_cat/nodes?v=true\&h=id,ip,port,v,m,hp,l,r,j,fdc,fdm