Hadoop comes with a set of demonstration programs. They are located in here.
One of them is WordCount.java which will automatically compute the word frequency of all text files found in the HDFS directory you ask it to process. Follow the Hadoop Tutorial to run the example.
Creating a working directory for your data:
bin/hdfs dfs -mkdir /wordcount
Copy Data Files to HDFS:
bin/hdfs dfs -copyFromLocal /path/to/your/data /wordcount/input
bin/hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.5.1.jar wordcount /wordcount/input /wordcount/output
View the Results:
bin/hdfs dfs -cat /wordcount/output/part-r-00000
Download the Results:
bin/hdfs dfs -copyToLocal /wordcount/output/part-r-00000 .