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執行 MapReduce 基本運算
Running MapReduce by Examples

Sample 1: grep

• grep 這個命令是擷取文件裡面特定的字元，在Hadoop example中此指令可以擷取文件中有此指定文字的字串，並作計數統計
  grep is a command to extract specific characters in documents. In hadoop examples, you can use this command to extract strings match the regular expression and count for matched strings.

$ hadoop fs -put /etc/hadoop/conf lab5_input
$ hadoop fs -ls lab5_input
$ hadoop jar hadoop-examples.jar grep lab5_input lab5_out1 'dfs[a-z.]+' 
 

運作的畫面如下：
You should see procedure like this:

11/04/19 10:00:20 INFO mapred.FileInputFormat: Total input paths to process : 25
11/04/19 10:00:20 INFO mapred.JobClient: Running job: job_201104120101_0645
11/04/19 10:00:21 INFO mapred.JobClient:  map 0% reduce 0%
( ... skip ... )

• 接著查看結果
  Let's check the computed result of grep from HDFS :

$ hadoop fs -ls lab5_out1
Found 2 items
drwx------   - hXXXX supergroup          0 2011-04-19 10:00 /user/hXXXX/lab5_out1/_logs
-rw-r--r--   2 hXXXX supergroup       1146 2011-04-19 10:00 /user/hXXXX/lab5_out1/part-00000
$ hadoop fs -cat lab5_out1/part-00000 

結果如下
You should see results like this:

4	dfs.permissions
4	dfs.replication
4	dfs.name.dir
3	dfs.namenode.decommission.interval.
3	dfs.namenode.decommission.nodes.per.interval
3	dfs.
( ... skip ... )

Sample 2 : WordCount

• 如名稱，WordCount會對所有的字作字數統計，並且從a-z作排列
  WordCount example will count each word shown in documents and sorting from a to z.

~$ hadoop jar hadoop-examples.jar wordcount lab5_input lab5_out2

檢查輸出結果的方法同之前方法
Let's check the computed result of wordcount from HDFS :

$ hadoop fs -ls lab5_out2
$ hadoop fs -cat lab5_out2/part-r-00000 

More Examples

可執行的指令一覽表：
Here is a list of hadoop examples :

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.
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.
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.
wordcount	A map/reduce program that counts the words in the input files.

You could find more detail at ​org.apache.hadoop.examples