curl.png

cURL Explained: A Practical Guide for Developers

Created At: 1/23/2026, 1:43:54 PM

#backend#rest-api#curl#api

If you’ve worked with APIs, debugged network issues, or automated system tasks, chances are you’ve encountered cURL. It’s one of those tools that quietly powers a huge amount of developer workflows—simple on the surface, incredibly powerful underneath.

This guide breaks down what cURL is, how it works, and how to use it effectively, with real-world examples you can apply immediately.

What Is cURL (and Why It Matters)?

cURL (Client URL) is a command-line tool and library for transferring data between systems using URLs. It supports a wide range of network protocols and is available on almost every operating system.

Why developers love cURL

  • Works everywhere (Linux, macOS, Windows)
  • Supports many protocols (HTTP, HTTPS, FTP, SFTP, SMTP, and more)
  • Ideal for testing and debugging APIs
  • Perfect for automation and scripting
  • No GUI required—fast and scriptable

If you can talk to a server over the network, cURL can probably do it.

A Brief History of cURL

  • Created in 1997 by Daniel Stenberg
  • Originally designed to fetch currency exchange rates
  • Evolved into a full-featured data transfer tool
  • Now maintained as an open-source project with millions of users

Today, cURL is embedded in CI/CD pipelines, backend services, cloud infrastructure, and developer toolchains worldwide.

Common Use Cases

cURL is often used for:

  • Calling REST and GraphQL APIs
  • Testing HTTP endpoints
  • Debugging request/response issues
  • Downloading or uploading files
  • Automating system and DevOps tasks
  • Verifying authentication and headers

How cURL Works

At its core, cURL:

  1. Takes a URL
  2. Applies options (method, headers, auth, body, etc.)
  3. Sends a request over a supported protocol
  4. Outputs the server’s response

Supported Protocols (Partial List)

  • HTTP / HTTPS
  • FTP / FTPS
  • SCP / SFTP
  • SMTP / IMAP / POP3
  • LDAP
  • WebSockets (via HTTP)

Basic Command Structure

curl [options] <URL>

Example

curl https://api.example.com/status

Basic cURL Examples

Simple GET Request

curl https://api.example.com/users

Pretty-Print JSON (with jq)

curl https://api.example.com/users | jq

Include Response Headers

curl -i https://api.example.com/users

Advanced cURL Examples

POST Request with JSON Body

curl -X POST https://api.example.com/users \\
 -H \"Content-Type: application/json\" \\
 -d '{
\"name\": \"Alice\",
\"email\": \"alice@example.com\"
}'

Custom Headers

curl https://api.example.com/data \\
 -H \"Authorization: Bearer YOUR_TOKEN\" \\
 -H \"Accept: application/json\"

Basic Authentication

curl -u username:password https://api.example.com/secure

Bearer Token Authentication

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

File Uploads and Downloads

Download a File

curl -O https://example.com/file.zip

Download and Rename

curl -o report.pdf https://example.com/latest-report

Upload a File (Multipart Form)

curl -X POST https://api.example.com/upload \\
 -F \"file=@image.png\"

Real-World Scenarios

API Testing

Quickly verify endpoints without Postman or a browser:

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

Debugging Issues

See exactly what’s being sent and received:

curl -v https://api.example.com/users

Automation & Scripting

Use cURL in shell scripts and CI pipelines:

curl -f https://api.example.com/status || exit 1

Performance Checks

Measure request timing:

curl -w \"@curl-format.txt\" -o /dev/null -s https://example.com

Common Mistakes (and How to Avoid Them)

Forgetting the HTTP method

  • cURL defaults to GET
  • Use -X POST, -X PUT, etc. when needed

Missing Content-Type headers

-H \"Content-Type: application/json\"

Not escaping JSON properly

  • Use single quotes in bash
  • Watch out for shell interpolation

Exposing secrets

  • Never commit tokens or passwords in scripts
  • Use environment variables instead

Best Practices

  • Use -v or --trace for debugging
  • Prefer --fail in scripts to catch errors
  • Store tokens in environment variables
  • Combine cURL with tools like jq, grep, and awk
  • Use --compressed when supported

Alternatives to cURL (and When to Use Them)

Tool When to Use
HTTPie More human-friendly CLI syntax
Postman Exploratory API testing with UI
Insomnia Lightweight API client
wget Simple file downloads
Language SDKs Application-level integrations

Use cURL when you want speed, portability, automation, or low-level control.

Conclusion

cURL is one of those tools every developer should know—not because it’s flashy, but because it’s reliable, universal, and incredibly versatile. Whether you’re testing APIs, debugging production issues, or automating workflows, cURL gives you full control over network requests.

Mastering cURL pays dividends across backend, DevOps, and cloud development.