Hadoop3 启动，提示ERROR: Attempting to operate on hdfs namenode as root ***_starting namenodes on [xueai] error: attempting to

在CentOS-7 搭建伪分布式Hadoop3 测试环境，执行start-all.sh 提示如下错误信息：

Starting namenodes on [namenode]
ERROR: Attempting to operate on hdfs namenode as root
ERROR: but there is no HDFS_NAMENODE_USER defined. Aborting operation.
Starting datanodes
ERROR: Attempting to operate on hdfs datanode as root
ERROR: but there is no HDFS_DATANODE_USER defined. Aborting operation.
Starting secondary namenodes [datanode1]
ERROR: Attempting to operate on hdfs secondarynamenode as root
ERROR: but there is no HDFS_SECONDARYNAMENODE_USER defined. Aborting operation.
Starting resourcemanager
ERROR: Attempting to operate on yarn resourcemanager as root
ERROR: but there is no YARN_RESOURCEMANAGER_USER defined. Aborting operation.
Starting nodemanagers
ERROR: Attempting to operate on yarn nodemanager as root
ERROR: but there is no YARN_NODEMANAGER_USER defined. Aborting operation.

造成此问题的原因是：将Hadoop各进程(NameNode、DataNode、SECONDARYNAMENODE、ResourceManager、NodeManager)用户设置为root,而start-dfs.sh/stop-dfs.sh、start-yarn.sh/stop-yarn.sh 脚本没有添加root 账户声明。

解决办法：

在start-dfs.sh/stop-dfs.sh 开头处添加如下代码片段：

HDFS_DATANODE_USER=root
HADOOP_SECURE_DN_USER=hdfs
HDFS_NAMENODE_USER=root
HDFS_SECONDARYNAMENODE_USER=root 

在start-yarn.sh/stop-yarn.sh 开头处添加如下代码片段：

YARN_RESOURCEMANAGER_USER=root
HADOOP_SECURE_DN_USER=yarn
YARN_NODEMANAGER_USER=root