CURL - CMD

Overview

curl is a command-line tool used to transfer data to or from a server using various protocols, including HTTP, HTTPS, FTP, FTPS, and more. It is widely utilized for testing, submitting forms, network diagnostics, downloading files, and interacting with APIs. curl is valuable for developers, system administrators, and automation scripts due to its versatility and wide support across platforms.

Syntax

The basic syntax for curl in CMD is:

curl [options] [URL...]
  • options: This denotes the various flags and settings that you can apply to modify the behavior of curl.
  • URL: This specifies the server URL that curl will target.

Options/Flags

Below are several commonly used options in curl:

  • -V, --version: Displays the curl version information.
  • -o, --output <file>: Writes output to <file> instead of stdout.
  • -I, --head: Fetches the headers only.
  • -L, --location: Follows HTTP redirects.
  • -u, --user <user:password>: Pass credentials for server authentication.
  • -x, --proxy [protocol://]host[:port]: Use the specified proxy.
  • -H, --header <header>: Pass custom header(s) to server.
  • -X, --request <command>: Specify a custom request method (e.g., GET, POST).
  • -d, --data <data>: Sends the specified data in a POST request.
  • --data-urlencode <data>: URL encodes and sends data in a POST request.

Default values are used for parameters not explicitly specified by the user, such as Content-Type for POST data, which typically defaults to application/x-www-form-urlencoded.

Examples

Example 1: Downloading a file

curl -o example.zip http://example.com/download.zip

Example 2: Using HTTP POST with data

curl -X POST -d "username=user&password=pass" http://example.com/login

Example 3: Sending JSON data

curl -H "Content-Type: application/json" -d "{\"user\":\"user\", \"pass\":\"pass\"}" http://example.com/api/login

Example 4: Saving response headers to a file

curl -D headers.txt http://example.com

Common Issues

Problem: Encountering SSL certificate verification failures.

Solution: Temporarily bypass SSL verification with -k or --insecure, although it’s a security risk. For a proper fix, ensure your system’s CA certificates are updated.

Problem: Data format issues with APIs.

Solution: Confirm that the Content-Type is correctly set and that the data is properly structured (e.g., JSON).

Integration

curl can be combined with scripts to monitor the health of web applications or to automate download and upload tasks. Here’s an example using a batch script:

@echo off
For /F "tokens=*" %%A in ('curl -s https://api.example.com/health') do (set response=%%A)
echo %response%

This script fetches the health status of an application and prints it.

  • wget: Another command-line utility for downloading files.
  • httpie: A more user-friendly HTTP client with colored output and simpler syntax for forms and JSON.

For additional resources, consult the official curl documentation.