Hadoop HDFS Basics

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Questions and Answers

What is the command used to move the input text file to HDFS?

hdfs dfs -transfer /home/cloudera/temp/wordcount.txt /user/cloudera/input

hdfs dfs -move /home/cloudera/temp/wordcount.txt /user/cloudera/input

hdfs dfs -put /home/cloudera/temp/wordcount.txt /user/cloudera/input (correct)

hdfs dfs -copy /home/cloudera/temp/wordcount.txt /user/cloudera/input

What should be checked before running the WordCount example?

Confirm that the input text file is in the local system.

Verify that the HDFS folder is empty.

Ensure that the local filesystem has sufficient space.

Make sure the YARN (MR2) service is running. (correct)

Which command will correctly list the content of a directory in HDFS?

hdfs dfs -ls /user/cloudera/input (correct)

hdfs dfs -list /user/cloudera/input

hdfs ls /user/cloudera/input

hdfs display -ls /user/cloudera/input

What will the output of the command 'hdfs dfs -cat /user/cloudera/input/wordcount.txt' display?

The content of the file 'wordcount.txt'. Signup and view all the answers

Which component is used to illustrate how MapReduce works?

WordCount Signup and view all the answers

What does the 'found' command output when checking the contents of the input directory?

1 item Signup and view all the answers

Where are the example programs located on the Cloudera Quickstart VM?

In the hadoop-mapreduce-examples.jar file Signup and view all the answers

What happens if the command 'ls /user' is executed in the local filesystem?

It shows an error saying no such file or directory. Signup and view all the answers

What is the primary purpose of the WordCount example in MapReduce?

To count the frequency of each word in a text file. Signup and view all the answers

What command is used to create a new directory in HDFS?

hdfs dfs -mkdir /user/cloudera/input Signup and view all the answers

Which command would correctly check the running status of the YARN service before executing the WordCount example?

service hadoop-yarn status Signup and view all the answers

What would be the result of attempting to access the directory '/user' in the local filesystem?

It will indicate no such file or directory exists. Signup and view all the answers

What is the command format to display the contents of a file in HDFS while paging through it?

hdfs dfs -cat /user/cloudera/input/wordcount.txt | more Signup and view all the answers

What should be included in the command to run the WordCount example from the Hadoop jar file?

hadoop jar hadoop-mapreduce-examples.jar WordCount /user/cloudera/input/wordcount.txt Signup and view all the answers

In the context of integrating HDFS and local filesystem, what unique characteristic does the 'ls' command have?

It operates independently in HDFS and local filesystem environments. Signup and view all the answers

Study Notes

Working with HDFS

To run the WordCount example, it is first necessary to create an input file in the local file system and then move it to HDFS.
The command echo “This is a hadoop tutorial test" > wordcount.txt creates a test file wordcount.txt in the local file system.
To move the file to HDFS, we can use the hdfs dfs command with the -put subcommand. The command will move the file to the specified location in HDFS.
Example command:hdfs dfs -put /home/cloudera/temp/wordcount.txt /user/cloudera/input
Use the 'ls' command to list the content in HDFS.
Example command: hdfs dfs -ls /user/cloudera/input
hdfs dfs -cat /user/cloudera/input/wordcount.txt is a command to view the content of a file in HDFS.
The hdfs dfs -cat wc-out/* | more command can be used to view the content of a large file by piping the output of the -cat subcommand through the local shell’s more command.

Running WordCount Example

The WordCount example is a common illustration of how MapReduce works.
It returns a list of words present in the input file along with their frequency.
The example programs can be found in the jar file hadoop-mapreduce-examples.jar on the Cloudera Quickstart VM.
Running the jar file without any arguments will show a list of available examples.
To run the WordCount example with the input file from HDFS:
Ensure that the YARN (MR2) service is running (check in Cloudera Manager).
Execute the WordCount example with the jar file and specify the input and output paths for the job.
The output will show each word found and its count, line by line.

Working in HDFS

To create input text files for Hadoop, first create a file on your local file system using the echo command and redirecting the output to a file.
Then, move the text file to HDFS using the hdfs dfs -put command.
The hdfs dfs -ls command lists files and directories in HDFS.
The hdfs dfs -cat command displays the content of a file on HDFS.
To view only the first or last parts of a large file in HDFS, use the more or tail commands, piped with hdfs dfs -cat.

Running the WordCount Example

The WordCount example counts the frequency of words in a text file.
This is a popular example to illustrate the MapReduce framework.
On the Cloudera Quickstart VM, the WordCount example is located in the hadoop-mapreduce-examples.jar file.
To run the WordCount example, use the following command: hadoop jar hadoop-mapreduce-examples.jar wordcount /user/cloudera/input /user/cloudera/output
Make sure the YARN (MR2) service is running before executing the command.

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Description

This quiz covers the essential commands for working with HDFS, specifically for running the WordCount example. You will learn how to create an input file, transfer it to HDFS, and explore the contents using various HDFS commands.