用Bash Script编写Hadoop MapReduce Streaming，hadoopmapreduce

用Bash Script编写Hadoop MapReduce Streaming

标签（空格分隔）： hadoop mapreduce bash

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MapReduce对外提供一个多语言编写MR的功能，就是Hadoop Streaming。我们可以通过自己喜欢的语言来编写Mapper和Reducer函数，运行MapReduce job。

根据Hadoop Streaming的定义，只要我们能够从标准输入（standard input）读入数据，然后从标准输出（standard output）读出数据就OK了。但是有一点需要记住，就是如果你要使用自己喜欢的的语言，如Python，就必须要事先在集群上安装该语言对应的版本和对应的lib等等。这里给出Shell Script的示例

输入事文本文件，功能是从特定的字符开始统计单词的平均长度。你可以在程序里实现做些检查来忽略一些字符，也可以少用Pipes和一些command来提升性能等。

#!/bin/bash
#This reducer script will take-in output from the mapper and emit starting letter of each word and average length
#Remember that the framework will sort the output from the mappers based on the Key
#Note that the input to a reducer will be of a form(Key,Value)and not (Key,
#This is unlike the input i.e.; usually passed to a reducer written in Java.
lastkey="";
count=0;
total=0;
iteration=1
while read line
 do
  newkey=`echo $line | awk '{print $1}'`
  value=`echo $line | awk '{print $2}'`
   if [ "$iteration" == "1" ] then
    lastkey=$newkey;c
    iteration=`expr $iteration + 1`;
   fi
   if [[ "$lastkey" != "$newkey" ]] then
    average=`echo "scale=5;$total / $count" | bc`;
    echo -e "$lastkey\t$average"
    count=0;
    lastkey=$newkey;
    total=0;
    average=0;
   fi
   total=`expr $total + $value`;
   lastkey=$newkey;
   count=`expr $count + 1`;
done
#The output would be Key,Value pairs(letter,average length of the words starting with this letter)

3 . run command

hadoop jar /usr/lib/hadoop-0.20-mapreduce/contrib/streaming/hadoop-streaming*.jar 
-input /input -output /avgwl 
-mapper mapper.sh 
-reducer reducer.sh 
-file /home/user/mr_streaming_bash/mapper.sh 
-file /home/user/mr_streaming_bash/reducer.sh 

还可以用其他语言鞋mapreduce来对比一下性能

翻译：Hadoop MapReduce Streaming Using Bash Script

Google 大牛的Github Project
mapreduce in bash