How to Send POST Requests With cURL

cURL is a versatile command-line utility designed for seamless data transfers across different network protocols, including HTTP, HTTPS, and FTP. Available on all major operating systems, cURL has become the standard tool for sending HTTP requests from the command line. Its cross-platform utility supports protocols like HTTP, HTTPS, FTP, and IMAP, making it easy to send requests to APIs and scrape websites. Due to its wide availability and protocol support, cURL is commonly referenced in REST API documentation as a quick way to test API calls directly from the command line.

This tutorial delves into executing POST requests using cURL, enabling efficient communication with servers through simple command-line instructions.

What Is a POST Request?

A POST request is an HTTP method used to send data to a server and is one of the most common HTTP methods. Unlike GET requests, where data is included in the URL, POST requests transmit data in the request body. This allows for discreet data transfer and avoids the URL length limitations imposed by browsers on GET requests.

POST requests are frequently used for form submissions, file uploads, and sending JSON data to APIs. Since the data is stored in the request body, it is not visible in browser history, and unlike GET requests, POST requests are not typically cached by browsers.

Practical Use Cases for cURL POST Requests

cURL POST requests are widely used in various scenarios, including:

Testing REST APIs – Developers use cURL to quickly test API endpoints before implementing them in applications.
Automating Form Submissions – By using cURL, you can automate interactions with web forms, submitting data without manual input.
Uploading Files – Many APIs allow file uploads using POST requests, making cURL an effective tool for automating this process.
Interacting with Authentication Systems – Developers can use cURL to authenticate users and access protected resources with API tokens or credentials.
Data Scraping and Integration – Extracting and integrating data from different online sources is simplified using cURL’s POST request capabilities.

Executing a POST Request with cURL

To start using cURL for POST requests, ensure it is installed on your system. If you haven’t installed it yet, refer to our guide on setting up cURL. Below is a reference table summarizing essential cURL options:

Short Flag	Long Flag	Argument(s)	Description
-X	--request	(use "POST" for POST requests)	Defines the HTTP method for the request
-b	--cookie		Sends cookies in the request
-c	--cookie-jar		Saves received cookies to a file
-d	--data		Transmits data in a POST request
-f	--fail		Ensures silent failure on server errors
-F	--form	<name=content>	Submits form data (application/x-www-form-urlencoded)
-H	--header	header/@file	Includes custom HTTP headers
-i	--include		Displays HTTP headers in the response
-l	--head		Retrieves only headers
-k	--insecure		Bypasses SSL verification
-L	--location		Follows redirections (3XX)
-o	--output		Saves output to a specified file
-O	--remote-name		Saves output with the remote filename
-s	--silent		Suppresses output messages
-v	--verbose		Enables detailed output (debugging)
-w	--write-out		Outputs response details

To view the full list of available options, use:

curl --help

Common Errors and How to Fix Them

Even though cURL is powerful, users often encounter errors when sending POST requests. Here are some common issues and solutions:

curl: (6) Could not resolve host – This means the domain name is incorrect or there is a DNS resolution issue. Double-check the URL.
curl: (7) Failed to connect to host – Indicates that the server is not reachable. Ensure the server is online and that you have an active internet connection.
curl: (35) SSL connect error – If the request fails due to SSL issues, try using -k to bypass SSL verification.
HTTP 400 Bad Request – This means the server rejected the request, often due to incorrect data format. Check the content-type and data payload.
HTTP 401 Unauthorized – Occurs when authentication is required but missing or incorrect. Ensure API keys or credentials are correctly passed with -u or -H "Authorization: Bearer <token>".

Basic POST Request Syntax

curl -X POST -d "Hello" https://example.com/api

In this command:

-X POST specifies the request type.
-d "Hello" defines the data being sent.
https://example.com/api is the target URL.

Setting Content-Type Headers

curl -X POST -H "Content-Type: text/plain" -d "Hello" https://example.com/api

Adding -H "Content-Type: text/plain" informs the server that the data format is plain text.

Sending JSON Data

curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://example.com/api

Using application/json as the content type ensures the data is correctly interpreted as JSON.

Sending XML Data

curl -X POST -H "Content-Type: application/xml" -d '<?xml version="1.0" encoding="UTF-8"?><root><name>John Doe</name><age>30</age></root>' https://example.com/api

Uploading Files with POST

curl -X POST -F "file=@/path/to/img.png" https://example.com/api/upload

For multiple file uploads:

curl -X POST -F "file=@/path/to/img1.png" -F "file=@/path/to/img2.png" https://example.com/api/upload

Authentication with cURL

curl -u username:password https://example.com/login

Replace username and password with actual credentials.

Conclusion

cURL simplifies HTTP interactions with its powerful and flexible command-line options. By mastering POST requests, developers can efficiently manage API communications, data transmissions, and file uploads. Whether you need to test API endpoints, automate form submissions, or scrape data from websites, cURL provides a lightweight and effective solution. Keep experimenting with cURL to optimize your workflow!

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