curl for HTTP requests in Linux CLI - Step-by-Step Execution

Concept Flow - curl for HTTP requests

Start curl command

↓

Parse URL and options

↓

Open network connection

↓

Send HTTP request

↓

Receive HTTP response

↓

Display or save response

↓

End

curl starts by reading your command, connects to the web address, sends the request, gets the response, and shows it to you.

Execution Sample

Linux CLI

curl https://example.com
curl -I https://example.com
curl -X POST https://example.com

These commands fetch a webpage, get headers only, and send a POST request respectively.

Execution Table

Step	Command Part	Action	Result/Output
1	curl https://example.com	Parse URL and default GET method	Ready to connect to example.com
2	Open network connection	Connect to example.com on port 443	Connection established
3	Send HTTP GET request	Send GET / HTTP/1.1	Request sent
4	Receive HTTP response	Get status and content	200 OK and HTML content received
5	Display response	Show HTML content in terminal	<!doctype html>...
6	curl -I https://example.com	Parse URL and HEAD method	Ready to connect to example.com
7	Open network connection	Connect to example.com on port 443	Connection established
8	Send HTTP HEAD request	Send HEAD / HTTP/1.1	Request sent
9	Receive HTTP headers	Get status and headers only	200 OK and headers received
10	Display headers	Show headers in terminal	HTTP/1.1 200 OK Content-Type: text/html...
11	curl -X POST https://example.com	Parse URL and POST method	Ready to connect to example.com
12	Open network connection	Connect to example.com on port 443	Connection established
13	Send HTTP POST request	Send POST / HTTP/1.1	Request sent
14	Receive HTTP response	Get status and content	200 OK or other response
15	Display response	Show response content in terminal	<response content>
16	End	No more commands	curl exits

💡 curl finishes after receiving and displaying the response or headers.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
URL	None	https://example.com	https://example.com	https://example.com	https://example.com
Method	GET (default)	GET	HEAD (with -I)	POST (with -X POST)	POST
Connection	Closed	Open	Open	Open	Closed
Response	None	HTML content	Headers only	Response content	Displayed

Key Moments - 3 Insights

Why does curl show HTML content for the first command but only headers for the second?

Because the first command uses the default GET method which fetches the full page (see execution_table rows 1-5), while the second uses -I which sends a HEAD request to get only headers (rows 6-10).

What does the -X option do in curl?

It changes the HTTP method curl uses to send the request, like POST instead of GET (see execution_table rows 11-15).

Why does curl need to open a network connection each time?

Because it must connect to the server to send the request and receive the response (see connection state in variable_tracker and steps 2,7,12).

Visual Quiz - 3 Questions

Look at the execution_table, what HTTP method does curl use by default when no option changes it?

A POST

B GET

C HEAD

D PUT

💡 Hint

Check the 'Method' variable in variable_tracker and steps 1-5 in execution_table.

At which step does curl send the HTTP HEAD request?

A Step 13

B Step 3

C Step 8

D Step 4

💡 Hint

Look at execution_table rows for the command with -I option.

If you add -X PUT to the curl command, what changes in the execution_table?

A The HTTP method changes to PUT in the 'Command Part' and 'Action' columns.

B The URL changes to a different website.

C The connection step is skipped.

D curl will not send any request.

💡 Hint

Refer to how -X POST changed the method in steps 11-15.

Concept Snapshot

curl sends HTTP requests from the command line.
Default method is GET.
Use -I for HEAD requests (headers only).
Use -X to specify other methods like POST.
curl connects, sends request, receives response, then shows it.

Full Transcript

curl is a command line tool to send HTTP requests. When you run curl with a URL, it connects to that address and sends a GET request by default. It then waits for the server to respond and shows you the response content. If you add -I, curl sends a HEAD request instead, which only fetches headers, not the full page. Using -X lets you change the HTTP method, for example to POST. Each time, curl opens a network connection, sends the request, gets the response, and displays it. This process ends when the response is fully received and shown.

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