Curl

curl is a versatile command-line tool that allows you to transfer data across networks using various protocols, such as HTTP, HTTPS, FTP, SFTP, and more. It’s widely used by developers, sysadmins, and network engineers for tasks such as testing APIs, downloading files, and debugging network issues.

Main usages

  1. Transfer Data Over the Network: curl can download or upload files, fetch web pages, and perform network-related operations.
  2. API Testing: You can use curl to interact with RESTful APIs by sending requests (GET, POST, PUT, DELETE, etc.) and receiving raw responses.
  3. Automating Requests: It’s commonly used in scripts to automate repetitive tasks like monitoring web services or downloading files.
  4. Debugging Network Issues: curl provides detailed output about server responses (e.g., HTTP headers, status codes), helping to troubleshoot connectivity or server-side problems.
  5. Customizing HTTP Requests: You can send custom headers, manipulate cookies, include authentication, and much more.

Command line examples

This section will present some basic curl usages.

Retrieve a webpage or download a file

curl https://example.com

This fetches the content of the webpage https://example.com and displays it in the terminal. To save it to a file:

curl -o page.html https://example.com

Get a webpage from a server using port 8000:

curl also permits to specify the port used by the web server:

curl http://www.example.com:8000/

Perform an HTTP GET request

Fetch data from an API using a GET request:

curl -X GET https://api.example.com/items

Send two fields with two field names

This permits to specify multipart data in the request body:

curl -F "docpicture=@dog.gif" -F "catpicture=@cat.gif" https://api.example.com/

Perform an HTTP POST request with JSON Data

This command will send data to an API:

curl -X POST -H "Content-Type: application/json" -d '{"name":"John","age":30}' https://api.example.com/users

Download a file with authentication

To download a file from a server that requires a username and password:

curl -u username:password https://example.com/file.zip -O

Or, if an API requires a Bearer Token for authentication:

curl -H "Authorization: Bearer YOUR_TOKEN" https://api.example.com/data

Display HTTP headers only

To view only the HTTP response headers:

curl -I https://example.com

Upload a file via a POST request

You can upload a file to a server:

curl -X POST -F "file=@myfile.txt" https://example.com/upload

Verbose Mode for Debugging

Enable verbose output to see details about the request and response:

curl -v https://example.com

Useful options

This section presents some useful curl options:

-o <filename>: Save the output to a file with a custom name.
-O: Save the output using the original filename from the server.
-H <header>: Add a custom HTTP header.
-X <method>: Specify the HTTP method (e.g., GET, POST).
-d <data>: Send data in the request body (e.g., JSON for POST requests).
-F: Send data as multipart/form-data (useful for file uploads).
-u <user:pass>: Provide username and password for authentication.
-k: Ignore SSL/TLS certificate errors.
--limit-rate: Limit the transfer speed.
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