hadoop@litao-virtual-machine:/usr/local/hadoop$ ./sbin/start-dfs.sh

Starting namenodes on [localhost]

localhost: Error: JAVA_HOME is not set and could not be found.

localhost: Error: JAVA_HOME is not set and could not be found.

Starting secondary namenodes [0.0.0.0]

0.0.0.0: Error: JAVA_HOME is not set and could not be found.

  vim ./etc/hadoop/hadoop-env.sh  #打开你hadoop安装路径下的hadoop环境配置脚本 

 # The only required environment variable is JAVA_HOME.  All others are

 # optional.  When running a distributed configuration it is best to

 # set JAVA_HOME in this file, so that it is correctly defined on

 # remote nodes.

 # The java implementation to use.

 JAVA_HOME=/usr/local/java/jdk1..0_151 #你的java路径

 export JAVA_HOME   #添加这两行内容

 export JAVA_HOME=${JAVA_HOME}

 # The jsvc implementation to use. Jsvc is required to run secure datanodes

#由于我在写博客时已经运行过，所以会提示已经运行
1 hadoop@litao-virtual-machine:/usr/local/hadoop$ ./sbin/start-dfs.sh

 Starting namenodes on [localhost]

 localhost: namenode running as process . Stop it first.

 localhost: datanode running as process . Stop it first.

 Starting secondary namenodes [0.0.0.0]

 0.0.0.0: secondarynamenode running as process . Stop it first.