启动hadoop时发生如下错误：

[root@localhost ~]# cd /usr/local
[root@localhost local]# cd hadoop
[root@localhost hadoop]# ./sbin/start-dfs.sh
Starting namenodes on [localhost]
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 [localhost.localdomain]
ERROR: Attempting to operate on hdfs secondarynamenode as root
ERROR: but there is no HDFS_SECONDARYNAMENODE_USER defined. Aborting operation.

解决方法： 

1输入以下命令：

vi /etc/profile

按i进入编辑模式

2添加如下内容：

export HDFS_NAMENODE_USER=root
export HDFS_DATANODE_USER=root
export HDFS_SECONDARYNAMENODE_USER=root
export YARN_RESOURCEMANAGER_USER=root
export YARN_NODEMANAGER_USER=root

（按esc退出编辑模式，按shift+：后输入wq！点回车保存！）
3输入如下命令使编辑生效：

source /etc/profile

亲测有效哦！！