在伪分布式模式下安装hadoop时，我得到了以下错误

Starting namenodes on [localhost]
localhost: namenode running as process 20678. Stop it first.
localhost: datanode running as process 20831. Stop it first.
Starting secondary namenodes [0.0.0.0]
The authenticity of host '0.0.0.0 (0.0.0.0)' can't be established

我错误地执行了下面的命令

hduser@x7158796-dsmprojb:/usr/local/hadoop$ export HADOOP_CONF_DIR=$HADOOP_INSTALL:etc/hadoop/
hduser@x7158796-dsmprojb:/usr/local/hadoop$ sbin/start-dfs.sh
Error: JAVA_HOME is not set and could not be found

现在它显示了javahome没有设置
我是linux新手，有人能帮我吗。