原因：
为了安全不允许使用root用户启动
解决：
es5之后的都不能使用添加启动参数或者修改配置文件等方法启动了，必须要创建用户
问题：
es安装好之后，使用root启动会报错：can not run elasticsearch as root
[root@iZbp1bb2egi7w0ueys548pZ bin]# ./elasticsearch
[2019-01-21T09:50:59,387][WARN ][o.e.b.ElasticsearchUncaughtExceptionHandler] [] uncaught exception in thread [main]
org.elasticsearch.bootstrap.StartupException: java.lang.RuntimeException: can not run elasticsearch as root
at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:134) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Elasticsearch.execute(Elasticsearch.java:121) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:69) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:134) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.cli.Command.main(Command.java:90) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:92) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:85) ~[elasticsearch-6.0.0.jar:6.0.0]
Caused by: java.lang.RuntimeException: can not run elasticsearch as root
at org.elasticsearch.bootstrap.Bootstrap.initializeNatives(Bootstrap.java:104) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Bootstrap.setup(Bootstrap.java:171) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Bootstrap.init(Bootstrap.java:322) ~[elasticsearch-6.0.0.jar:6.0.0]
at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:130) ~[elasticsearch-6.0.0.jar:6.0.0]
... 6 more
1、创建用户：elasticsearch
[root@iZbp1bb2egi7w0ueys548pZ bin]# adduser elasticsearch
2、创建用户密码，需要输入两次
[root@iZbp1bb2egi7w0ueys548pZ bin]# passwd elasticsearch
3、将对应的文件夹权限赋给该用户
[root@iZbp1bb2egi7w0ueys548pZ local]# chown -R elasticsearch elasticsearch-6.0.0
4、切换至elasticsearch用户
[root@iZbp1bb2egi7w0ueys548pZ etc]# su elasticsearch
5、进入启动目录启动 /usr/local/elasticsearch-6.0.0/bin 使用后台启动方式：./elasticsearch -d
[elasticsearch@vmt10003 bin]$ ./elasticsearch -d
6、启动后测试
输入curl ip:9200,如果返回一个json数据说明启动成功