1.想使用hadoop命令,发现无效
[root@hadoopnn-01 ~]# which hadoop
/usr/bin/which: no hadoop in (/usr/java/jdk1.7.0_67-cloudera/bin:/usr/java/jdk1.7.0_67-cloudera/bin:/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin)
2.进/usr/bin/目录查看hadoop
[root@hadoopnn-01 ~]# cd /usr/bin
[root@hadoopnn-01 bin]# ll hadoop
lrwxrwxrwx 1 root root 24 Nov 17 19:25 hadoop -> /etc/alternatives/hadoop
####这个 软连接 一直在跳动,说明这个 原路径/etc/alternatives/hadoop 不存在，如下图：
3.重新配置 hadoop的软连接，是由alternatives来动态管理的
[root@hadoopnn-01 bin]# alternatives --config hadoop
There are 3 programs which provide 'hadoop'.
Selection Command
-----------------------------------------------
*+ 1 /opt/cloudera/parcels/CDH-5.4.8-1.cdh5.4.8.p0.4/bin/hadoop
2 /opt/cloudera/parcels/CDH-4.7.1-1.cdh4.7.1.p0.47/bin/hadoop
3 /opt/cloudera/parcels/CDH-5.8.2-1.cdh5.8.2.p0.3/bin/hadoop
Enter to keep the current selection[+], or type selection number: 3 #输入当前CDH集群的版本
4.验证 hadoop
[root@hadoopnn-01 bin]# which hadoop
/usr/bin/hadoop
[root@hadoopnn-01 bin]#
