Basics of Curl

Posted on September 4, 2023

In the vast landscape of command-line tools, one gem stands out for its versatility and power: the curl command. Whether you’re a seasoned sysadmin, a developer, or just someone curious about the command line, curl is a tool you’ll want to get to know. In this blog post, we’ll explore what curl is, how it works, and some practical use cases to demonstrate its capabilities.

What is curl?

curl, short for “Client URL,” is a command-line tool for transferring data with URLs. It’s available on most Unix-like operating systems, including Linux and macOS, and even on Windows through various packages or WSL (Windows Subsystem for Linux). curl is a versatile Swiss Army knife for making HTTP, HTTPS, FTP, SCP, and more requests to servers.

Basic Usage

To get started with curl, open your terminal and type:

curl [options] [URL]

Here’s a breakdown of this command:

  • [options]: Optional flags that modify how curl behaves.
  • [URL]: The URL you want to fetch or interact with.

For instance, to fetch the contents of a web page:

Bash
$ curl https://example.com

Key Features and Options

1. HTTP Methods

curl supports various HTTP methods like GET, POST, PUT, and DELETE. To specify a method, use the -X flag:

Bash
# Send a POST request
$ curl -X POST https://example.com/api/v1/resource

2. Headers

You can set custom headers with the -H flag:

Bash
# Add a custom user-agent header
$ curl -H "User-Agent: MyCoolApp" https://example.com

3. Data Transfer

curl can send data with requests. For example, to send a POST request with JSON data:

Bash
# Send a JSON POST request
$ curl -X POST -H "Content-Type: application/json" -d '{"key": "value"}' https://example.com/api/resource

4. File Uploads

Uploading files is a breeze:

Bash
# Upload a file via POST
$ curl -F "file=@/path/to/file.jpg" https://example.com/upload

5. Authentication

You can pass credentials with the -u flag:

Bash
# Authenticate with username and password
$ curl -u username:password https://example.com/private

6. Follow Redirects

curl follows redirects by default. Use -L to follow them:

Bash
# Follow redirects
$ curl -L https://example.com/redirect

Practical Use Cases

  1. API Testing: curl is an excellent tool for testing REST APIs. You can craft requests with specific data, headers, and authentication.
  2. Downloading Files: Downloading files from the web is a breeze. For instance, to download a file: curl -O https://example.com/file.zip
  3. Debugging: When troubleshooting network issues, you can use curl to diagnose problems by inspecting headers and responses.
  4. Automating Tasks: Incorporate curl into scripts to automate various tasks, like periodically checking a website’s status or fetching data.
  5. Web Scraping: While not as powerful as dedicated tools like wget or web scraping libraries, curl can help fetch web content for basic scraping.

Conclusion

The curl command-line tool is a must-have for anyone working with web services, APIs, or simply looking to harness the power of the command line for networking tasks. Its flexibility, ease of use, and extensive feature set make it an invaluable tool in your toolbox. So, next time you need to make HTTP requests or interact with web services from the command line, remember to reach for curl and unleash its capabilities.