HDFS command line interface in Hadoop - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HDFS command?

Given the HDFS command hdfs dfs -ls /user/hadoop, what will it display?

Hadoop

hdfs dfs -ls /user/hadoop

A Lists all files and directories inside /user/hadoop with details like permissions, owner, size, and modification date.

B Deletes the directory /user/hadoop and all its contents.

C Creates a new directory named /user/hadoop.

D Displays the content of the file /user/hadoop.

💡 Hint

The command uses '-ls' which is similar to the Unix 'ls' command but for HDFS.

The 'hdfs dfs -ls' command lists files and directories in the specified HDFS path with detailed information.

❓ data_output

intermediate

What is the result of this HDFS command?

What will be the result of running hdfs dfs -mkdir /user/hadoop/newdir if the directory already exists?

Hadoop

hdfs dfs -mkdir /user/hadoop/newdir

A The command silently ignores and does nothing.

B The directory is overwritten with a new empty directory.

C The command creates a new directory inside /user/hadoop/newdir.

D An error message indicating the directory already exists.

💡 Hint

Think about what happens when you try to create a directory that already exists in HDFS.

The 'hdfs dfs -mkdir' command fails with an error if the directory already exists.

🔧 Debug

advanced

Identify the error in this HDFS command

What error will this command produce? hdfs dfs -put localfile.txt /user/hadoop/ when localfile.txt does not exist locally.

Hadoop

hdfs dfs -put localfile.txt /user/hadoop/

A Permission denied error on HDFS directory.

B FileNotFoundException: localfile.txt (No such file or directory)

C Command runs successfully and uploads an empty file.

D Syntax error due to missing arguments.

💡 Hint

The command tries to upload a file that does not exist locally.

The 'hdfs dfs -put' command fails with a FileNotFoundException if the local file does not exist.

🧠 Conceptual

advanced

What does the HDFS command hdfs dfs -du -h /user/hadoop show?

Choose the correct description of the output of hdfs dfs -du -h /user/hadoop.

A Displays the disk usage of files and directories in /user/hadoop in human-readable format.

B Displays the content of all files in /user/hadoop.

C Deletes all files in /user/hadoop and shows confirmation.

D Shows the replication factor of files in /user/hadoop.

💡 Hint

The '-du' option is similar to Unix 'du' command but for HDFS.

The 'hdfs dfs -du -h' command shows disk usage of files/directories with sizes in human-readable units.

🚀 Application

expert

Which command sequence correctly copies a local directory to HDFS and verifies the copy?

You want to copy a local directory data/ to HDFS path /user/hadoop/data/ and then list the contents to verify. Which sequence of commands is correct?

A hdfs dfs -put data /user/hadoop/data && hdfs dfs -ls /user/hadoop/data

B hdfs dfs -copyFromLocal data /user/hadoop/data && hdfs dfs -cat /user/hadoop/data

C hdfs dfs -mkdir /user/hadoop/data && hdfs dfs -put data /user/hadoop/data && hdfs dfs -ls /user/hadoop/data

D hdfs dfs -get data /user/hadoop/data && hdfs dfs -ls /user/hadoop/data

💡 Hint

Remember that the target directory must exist before copying if you want to keep the directory structure.

First create the target directory, then copy the local directory into it, then list contents to verify.

Practice

1. What does the HDFS command hdfs dfs -ls /user/hadoop do?

easy

A. Uploads local files to /user/hadoop in HDFS

B. Deletes the directory /user/hadoop from HDFS

C. Lists all files and directories in the HDFS path /user/hadoop

D. Displays the content of files in /user/hadoop

Solution

1. Step 1: Understand the command structure

   The command hdfs dfs -ls is used to list files and directories in HDFS.

2. Step 2: Analyze the given path

   The path /user/hadoop specifies the directory to list.

3. Final Answer:

   Lists all files and directories in the HDFS path /user/hadoop -> Option C

4. Quick Check:

   hdfs dfs -ls lists files [OK]

Hint: Remember: '-ls' always lists files in given HDFS path [OK]

Common Mistakes:

• Confusing '-ls' with '-rm' which deletes files
• Thinking '-ls' uploads files
• Assuming it shows file content

2. Which of the following is the correct syntax to upload a local file data.txt to HDFS directory /user/hadoop?

easy

A. hdfs dfs -put /user/hadoop data.txt

B. hdfs dfs -copyFromLocal /user/hadoop data.txt

C. hdfs dfs -get data.txt /user/hadoop

D. hdfs dfs -put data.txt /user/hadoop

Solution

1. Step 1: Identify the correct command for uploading

   -put uploads local files to HDFS, and the syntax is hdfs dfs -put <local_path> <hdfs_path>.

2. Step 2: Match the syntax with options

   hdfs dfs -put data.txt /user/hadoop correctly places data.txt as local file and /user/hadoop as HDFS destination.

3. Final Answer:

   hdfs dfs -put data.txt /user/hadoop -> Option D

4. Quick Check:

   -put local_file hdfs_path is correct [OK]

Hint: Remember: local file first, then HDFS path in '-put' [OK]

Common Mistakes:

• Swapping local and HDFS paths
• Using '-get' which downloads files
• Using '-copyFromLocal' with wrong argument order

3. What will be the output of the command hdfs dfs -cat /user/hadoop/sample.txt if the file contains:

Hello Hadoop
Welcome to HDFS

medium

A. Lists the file details of sample.txt

B. Prints the content of sample.txt to the console

C. Deletes sample.txt from HDFS

D. Uploads sample.txt to HDFS

Solution

1. Step 1: Understand the '-cat' command

   hdfs dfs -cat prints the content of a file in HDFS to the console.

2. Step 2: Analyze the file content

   The file sample.txt contains two lines: 'Hello Hadoop' and 'Welcome to HDFS'.

3. Final Answer:

   Prints the content of sample.txt to the console -> Option B

4. Quick Check:

   -cat shows file content [OK]

Hint: Use '-cat' to display file content in HDFS [OK]

Common Mistakes:

• Confusing '-cat' with '-ls' which lists files
• Thinking it deletes files
• Assuming it uploads files

4. You run the command hdfs dfs -rm /user/hadoop/data.csv but get an error saying file not found. What is the most likely cause?

medium

A. The file /user/hadoop/data.csv does not exist in HDFS

B. You used the wrong command; '-rm' cannot delete files

C. You need to use '-put' to delete files

D. The HDFS service is down

Solution

1. Step 1: Understand the error message

   The error 'file not found' means the specified file does not exist at the given path.

2. Step 2: Check command correctness

   -rm is the correct command to delete files in HDFS, so the error is not due to wrong command.

3. Final Answer:

   The file /user/hadoop/data.csv does not exist in HDFS -> Option A

4. Quick Check:

   File missing causes 'file not found' error [OK]

Hint: Check file existence before deleting with '-rm' [OK]

Common Mistakes:

• Assuming '-rm' is wrong command
• Trying to delete with '-put'
• Ignoring file path correctness

5. You want to copy all files from local directory /local/data/ to HDFS directory /user/hadoop/data/ and then verify the files are uploaded. Which sequence of commands is correct?

hard

A. hdfs dfs -put /local/data/* /user/hadoop/data/ then hdfs dfs -ls /user/hadoop/data/

B. hdfs dfs -get /local/data/* /user/hadoop/data/ then hdfs dfs -cat /user/hadoop/data/

C. hdfs dfs -rm /local/data/* then hdfs dfs -ls /user/hadoop/data/

D. hdfs dfs -copyToLocal /local/data/* /user/hadoop/data/ then hdfs dfs -rm /user/hadoop/data/

Solution

1. Step 1: Choose the correct upload command

   -put uploads local files to HDFS. Using /local/data/* uploads all files from local directory.

2. Step 2: Verify files in HDFS

   hdfs dfs -ls /user/hadoop/data/ lists files in the HDFS directory to confirm upload.

3. Final Answer:

   hdfs dfs -put /local/data/* /user/hadoop/data/ then hdfs dfs -ls /user/hadoop/data/ -> Option A

4. Quick Check:

   Upload with '-put' then list with '-ls' [OK]

Hint: Upload with '-put' then check with '-ls' [OK]

Common Mistakes:

• Using '-get' which downloads files
• Deleting local files before upload
• Using '-copyToLocal' which copies from HDFS to local