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    <p>Try running without <code>wordcount</code>, like: <code>hadoop jar /usr/local/hadoop/hadoop-examples-1.1.2.jar input output</code></p> <p>You will receive something like follows:</p> <pre><code>Unknown program 'input' chosen. Valid program names are: aggregatewordcount: An Aggregate based map/reduce program that counts the words in the input files. aggregatewordhist: An Aggregate based map/reduce program that computes the histogram of the words in the input files. dbcount: An example job that count the pageview counts from a database. grep: A map/reduce program that counts the matches of a regex in the input. join: A job that effects a join over sorted, equally partitioned datasets multifilewc: A job that counts words from several files. pentomino: A map/reduce tile laying program to find solutions to pentomino problems. pi: A map/reduce program that estimates Pi using monte-carlo method. randomtextwriter: A map/reduce program that writes 10GB of random textual data per node. randomwriter: A map/reduce program that writes 10GB of random data per node. secondarysort: An example defining a secondary sort to the reduce. sleep: A job that sleeps at each map and reduce task. sort: A map/reduce program that sorts the data written by the random writer. sudoku: A sudoku solver. teragen: Generate data for the terasort terasort: Run the terasort teravalidate: Checking results of terasort wordcount: A map/reduce program that counts the words in the input files. </code></pre> <p>So basically the first argument <strong>is NOT the main-class name</strong>, rather it is the example program's name which you want to run. </p> <p>So, it shouldn't even accept <code>WordCount</code>, it isn't for me. The following command has the same result as shown above:</p> <p><code>bin/hadoop jar hadoop-examples-1.0.4.jar WordCount LICENSE.txt output</code> </p> <p><br> <strong>For your information:</strong> The main-class is already defined in the <code>META-INF/MANIFEST.MF</code> file contained in the jar : <code>Main-Class: org/apache/hadoop/examples/ExampleDriver</code></p>
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    1. POIs hadoop jar command class name case sensitive?
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    1. POIs hadoop jar command class name case sensitive?
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    1. COThe document [here](http://hadoop.apache.org/docs/r1.1.2/commands_manual.html#jar) says the usage is `Usage: hadoop jar <jar> [mainClass] args...`, so `wordcount` should be the `mainClass`. I checked again, and found `WordCount` does not work fine. But the document [here](http://hadoop.apache.org/docs/r1.1.2/mapred_tutorial.html#Usage) mislead me.
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    2. COWhat exactly in the above answer you did not understand?
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    3. COYou need to specify the mainclass only if you do not have it specified inside your jar's manifest. The square bracket in the document suggests that it is optional. And this is a different jar! In this case the first argument this jar expects is the example program's name (eg treasort, wordcount etc...). Simple.
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