curl for HTTP requests in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this curl command?

You run this command in a terminal:

curl -s -o /dev/null -w "%{http_code}" https://httpbin.org/status/404

What will be the output?

Linux CLI

curl -s -o /dev/null -w "%{http_code}" https://httpbin.org/status/404

A No output

B 404

C Error: Not Found

D 200

💡 Hint

The -w option prints the HTTP status code. The URL returns a 404 status.

The command suppresses output (-s), discards body (-o /dev/null), and prints only the HTTP status code (-w "%{http_code}"). The URL returns 404, so output is '404'.

💻 Command Output

intermediate

What does this curl command output?

Consider this command:

curl -I https://example.com

What kind of output will it produce?

Linux CLI

curl -I https://example.com

A Only the HTTP status code

B Full HTML content of the page

C Only HTTP headers of the response

D An error message

💡 Hint

The -I option fetches headers only.

The -I option tells curl to fetch only the HTTP headers, not the body content.

📝 Syntax

advanced

Which curl command correctly sends JSON data with POST?

You want to send JSON data '{"name":"Alice"}' to https://api.example.com/users using POST. Which command is correct?

A curl -X POST -H "Content-Type: application/json" https://api.example.com/users

B curl -X POST -d '{"name":"Alice"}' -H "Content-Type: text/plain" https://api.example.com/users

C curl -X GET -H "Content-Type: application/json" -d '{"name":"Alice"}' https://api.example.com/users

D curl -X POST -H "Content-Type: application/json" -d '{"name":"Alice"}' https://api.example.com/users

💡 Hint

Use POST method, set content type to application/json, and provide data with -d.

Option D correctly uses POST, sets the header to application/json, and sends the JSON data with -d. Option D has wrong content type. Option D uses GET which ignores data. Option D sends no data.

🔧 Debug

advanced

Why does this curl command fail to send data?

This command is run:

curl -X POST https://api.example.com/data -d name=Bob&age=25

But the server reports only partial data received. Why?

Linux CLI

curl -X POST https://api.example.com/data -d name=Bob&age=25

A The -d option needs quotes around the data to be sent correctly

B POST method is not supported by curl

C The -X POST option must come after the URL

D The URL is missing a trailing slash

💡 Hint

Shell may split unquoted data into multiple arguments.

Without quotes, the shell splits name=Bob&age=25 into multiple parts due to the & operator, so curl sends incomplete data. Quoting the data keeps it as one argument.

🚀 Application

expert

How to save only HTTP response headers to a file using curl?

You want to save only the HTTP headers from https://example.com to a file named headers.txt, without saving the body. Which command does this?

A curl -s -D headers.txt -o /dev/null https://example.com

B curl -s -I https://example.com > headers.txt

C curl -s -o headers.txt -I https://example.com

D curl -s -o headers.txt https://example.com

💡 Hint

Use -D to save headers, -o /dev/null to discard body.

Option A saves headers to headers.txt (-D), discards body (-o /dev/null), and runs silently (-s). Option A saves headers to body file. Option A redirects headers to file but may include extra output. Option A saves full content.

Practice

1. What does the curl command primarily do in Linux?

easy

A. Monitor system processes

B. Send HTTP requests to websites

C. Manage file permissions

D. Compile source code

Solution

1. Step 1: Understand the purpose of curl

   The curl command is designed to send HTTP requests and interact with web servers.

2. Step 2: Compare with other options

   Compiling code, managing permissions, and monitoring processes are unrelated to curl's function.

3. Final Answer:

   Send HTTP requests to websites -> Option B

4. Quick Check:

   curl = HTTP requests [OK]

Hint: curl is for web requests, not system tasks [OK]

Common Mistakes:

• Confusing curl with system commands like chmod or ps
• Thinking curl compiles code
• Assuming curl manages files locally

2. Which of the following is the correct syntax to fetch the content of http://example.com using curl?

easy

A. curl http://example.com

B. curl --head http://example.com

C. curl -x http://example.com

D. curl -d http://example.com

Solution

1. Step 1: Identify basic curl usage

   To fetch a webpage, simply use curl URL. No extra flags are needed for a simple GET request.

2. Step 2: Analyze other options

   -x is for proxy, --head fetches headers only, -d sends data (POST), so they are incorrect for simple fetch.

3. Final Answer:

   curl http://example.com -> Option A

4. Quick Check:

   Basic fetch = curl URL [OK]

Hint: Use curl followed by URL to fetch page [OK]

Common Mistakes:

• Using -x which is for proxy
• Using -d which sends data instead of fetching
• Using --head which fetches headers only

3. What will be the output of this command?

curl -I https://example.com

medium

A. An error because -I is invalid

B. The full HTML content of the page

C. Only the HTTP headers of the response

D. The page content saved to a file

Solution

1. Step 1: Understand the -I option in curl

   The -I flag tells curl to fetch only the HTTP headers, not the full content.

2. Step 2: Check other options

   The full HTML content of the page is full content, which -I does not do. An error because -I is invalid is wrong because -I is valid. The page content saved to a file requires -o flag, not present here.

3. Final Answer:

   Only the HTTP headers of the response -> Option C

4. Quick Check:

   -I = headers only [OK]

Hint: Remember -I fetches headers only [OK]

Common Mistakes:

• Thinking -I fetches full page
• Confusing -I with -o for output file
• Assuming -I is invalid flag

4. You want to send JSON data {"name":"John"} to http://api.example.com/users using curl. Which command has an error?

medium

A. curl -d '{name:John}' -X POST -H "Content-Type: application/json" http://api.example.com/users

B. curl -X POST -H "Content-Type: application/json" -d '{"name":"John"}' http://api.example.com/users

C. curl -d '{"name":"John"}' http://api.example.com/users -H "Content-Type: application/json" -X POST

D. curl -X POST -d '{"name":"John"}' -H "Content-Type: application/json" http://api.example.com/users

Solution

1. Step 1: Check JSON data format

   curl -d '{name:John}' -X POST -H "Content-Type: application/json" http://api.example.com/users uses '{name:John}' which misses double quotes around keys and values, making it invalid JSON. Options A, C, D use correct JSON '{"name":"John"}'.

2. Step 2: Confirm curl flags order

   Order of flags does not matter in curl, so A, C, D are valid. Only data format in B is wrong.

3. Final Answer:

   curl -d '{name:John}' -X POST -H "Content-Type: application/json" http://api.example.com/users -> Option A

4. Quick Check:

   JSON must have quotes [OK]

Hint: JSON keys and values need quotes [OK]

Common Mistakes:

• Omitting quotes in JSON data
• Thinking flag order matters in curl
• Using wrong content-type header

5. You want to download a file from http://example.com/file.zip and save it as myfile.zip. Which curl command does this correctly?

hard

A. curl --save myfile.zip http://example.com/file.zip

B. curl -O http://example.com/file.zip > myfile.zip

C. curl -O myfile.zip http://example.com/file.zip

D. curl -o myfile.zip http://example.com/file.zip

Solution

1. Step 1: Understand curl file saving options

   -o filename saves the download to the specified filename. -O saves with original name from URL.

2. Step 2: Analyze each option

   B: curl -O http://example.com/file.zip > myfile.zip saves to 'file.zip' via -O and redirects empty stdout to myfile.zip. C: curl -O myfile.zip http://example.com/file.zip uses -O incorrectly, treating myfile.zip as URL. D: uses invalid flag --save.

3. Final Answer:

   curl -o myfile.zip http://example.com/file.zip -> Option D

4. Quick Check:

   -o sets output filename [OK]

Hint: Use -o to name downloaded file [OK]

Common Mistakes:

• Using > redirection for binary files
• Confusing -O and -o flags
• Using non-existent flags like --save