hadoop 伪分布式环境时运行wordcount程序，已经运行过一次，在次将input文件夹中的内容上传hafs文件系统时会出现重复，但是还是可以上传成功。

但是当再次运行wordcount程序时就会报以下的错误

hadoop@salve:~/programfile/hadoop$ bin/hadoop jar hadoop-examples-*.jar wordcount /input /output
14/05/16 08:50:00 INFO mapred.JobClient: Cleaning up the staging area hdfs://localhost:9000/tmp/hadoop-hadoop/mapred/staging/hadoop/.staging/job_201405160835_0004
14/05/16 08:50:00 ERROR security.UserGroupInformation: PriviledgedActionException as:hadoop cause:org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory /output already exists
org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory /output already exists
    at org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:137)
    at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:949)
    at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:912)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1136)
    at org.apache.hadoop.mapred.JobClient.submitJobInternal(JobClient.java:912)
    at org.apache.hadoop.mapreduce.Job.submit(Job.java:500)
    at org.apache.hadoop.mapreduce.Job.waitForCompletion(Job.java:530)
    at org.apache.hadoop.examples.WordCount.main(WordCount.java:67)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:68)
    at org.apache.hadoop.util.ProgramDriver.driver(ProgramDriver.java:139)
    at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:64)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
                                                                                 
at org.apache.hadoop.util.RunJar.main(RunJar.java:156)


从这行错误提示中

hadoop cause:org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory /output already exists

org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory /output already exists

可以看到导致错误的原因时hdfs文件系统中的output文件夹已经存在了，所以必须要删除。（因为outout文件夹是由hadoop自动生成的，所以会报错）


我使用的hadoop是 hadoop-1.1.0

查看hdfs文件系统的命令为

hadoop@salve:~/programfile/hadoop$ bin/hadoop fs -ls /


删除output文件夹和tmp文件夹

hadoop@salve:~/programfile/hadoop$ bin/hadoop fs -rmr /output
hadoop@salve:~/programfile/hadoop$ bin/hadoop fs -rmr /tmp

这样再试一下就可以了