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From Robert Evans <ev...@yahoo-inc.com>
Subject Re: Question about invoking an executable from Hadoop mapper
Date Thu, 25 Aug 2011 16:32:23 GMT
I think this is a java issue.  I don't think that it is launching a shell to run your command.
 I think it is just splitting on white space and then passing all the args to hadoop.  What
you want to do is to run

sh -e 'hadoop dfs -cat file| myExec'

Or with streaming white a small shell script that has the command in it and then tall the
mapper/reducer to use that.  The key with streaming is that you have to make sure that you
read stdin before going on, or it will error out

#!/bin/sh
Hadoop fs -cat file | myExec;
#Read all of the input to this mapper.
cat > /dev/null;


--Bobby Evans

On 8/25/11 10:32 AM, "Zhixuan Zhu" <zzhu@calpont.com> wrote:

I think this should be a common use case. I'm trying to invoke an
executable from my mapper. Because my executable takes steaming input, I
used something like the following:

String lCmdStr = "hadoop dfs -cat file | myExec";

Process lChldProc = Runtime.getRuntime().exec(lCmdStr);

All executable and file names are full qualified name with path. I got
the following errors:

cat: File does not exist: |
cat: File does not exist: myExec

Looks like this kind of steaming/pipe method was not supported from a
mapper? I can take the exact command string and run it directly on a
Hadoop server and it works. Anybody have any experience with it?

I also tried to use Hadoop-steaming and it did not work either. It did
not give any error but nothing happened either. My program is supposed
to write a file on the local system and it's not there. I'm at the end
of my wit. Any help is most appreciated. Hadoop version 0.20.2.

String lCmdStr = "hadoop jar hadoop-0.20.2-streaming.jar -input
inputFile -output outputFile -mapper myExec";

Thanks very much,
Grace



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