Question

In Hadoop ‘grep’ example (that comes with the Hadoop package) what is the group parameter.Can you give me an example for that.

Answer 1

Disclaimer: I haven’t run this example and am pulling answering after just looking at http://wiki.apache.org/hadoop/Grep

The CLI call is: bin/hadoop org.apache.hadoop.examples.Grep <indir> <outdir> <regex> [<group>] and you want to know about <group>.

I suspect this is grouping in regex. (random link – http://www.exampledepot.com/egs/java.util.regex/Group.html)

As noted on the Hadoop Grep link

The command works different than the
Unix grep call: it doesn’t display the
complete matching line, but only the
matching string

What I take from this is that if you specify the <group> value (a number) it will output only the value for that group.

For an example (pulling from the Group link)

input: aba
regex: (a(b)*)+
group 0: aba
group 1: a
group 2: b

If the value for <group> is 1 then the result would be a. Group 0 is the full match, not the original string, In this case it just happens to be the same.

hth