Q:

What exactly does this Hadoop command performed in the shell?

I am absolutly new in Apache Hadoop and I am following a video course.
So I have correctly installed Hadoop 1.2.1 on a linux Ubuntu virtual machine.
After the installation the instructor perform this command in the shell:
bin/hadoop jar hadoop-examples-*.jar grep input output 'dfs[a-z.]+'

to see that Hadoop is working and that it is correctly installed.
But what exactly does this command?

A:

This command runs a grep job defined inside hadoop examples jar file (containing map, reduce and driver code) with input folder to search for specified in input folder in hdfs while output is the folder where output after searching for patter would be in that file while dfs[a-z.]+ is a regular expression which you are saying to grep for in input.

