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Md Abu Musa
Md Abu Musa

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cURL: The Swiss Army Knife of Data Transfer

#api #cli #networking #tutorial

Introduction

curl (Client URL) is one of the most powerful and widely used command-line tools in software engineering. It enables data transfer between a client and a server using various network protocols. From testing REST APIs to downloading files, debugging HTTP requests, and automating workflows, curl is an essential tool for developers, system administrators, and DevOps engineers.

This blog explores what curl is, how it works, its most common use cases, and practical examples that you can apply in real-world scenarios.

What Is cURL?

curl is an open-source command-line tool and library (libcurl) used to transfer data to or from a server using URLs. It supports a wide range of protocols, including:

  • HTTP / HTTPS
  • FTP / FTPS
  • SCP / SFTP
  • SMTP / POP3 / IMAP
  • LDAP
  • MQTT
  • FILE

The tool is available by default on most Linux and macOS systems and can be installed easily on Windows.

Why cURL Is Important

curl is popular because it is:

  • Lightweight and fast
  • Scriptable (perfect for automation)
  • Protocol-agnostic
  • Widely supported
  • Ideal for API testing and debugging

In backend development and DevOps, curl is often the first tool used to verify whether a service or endpoint is working correctly.

Basic cURL Syntax 

curl [options] URL
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Example:

curl https://example.com
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This sends a basic HTTP GET request and prints the response body to the terminal.

Common HTTP Operations with cURL

1. GET Request 

curl https://api.example.com/users
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Used to retrieve data from a server.

2. POST Request 

curl -X POST https://api.example.com/users \
     -H "Content-Type: application/json" \
     -d '{"name":"Musa","email":"musa@example.com"}'
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Used to send data to the server, commonly for creating resources.

3. PUT Request 

curl -X PUT https://api.example.com/users/1 \
     -H "Content-Type: application/json" \
     -d '{"name":"Updated Name"}'
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Used to update existing resources.

4. DELETE Request 

curl -X DELETE https://api.example.com/users/1
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Used to delete a resource.

Working with Headers

Send Custom Headers 

curl -H "Authorization: Bearer TOKEN" \
     -H "Accept: application/json" \
     https://api.example.com/profile
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View Response Headers 

curl -i https://example.com
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Authentication with cURL

Basic Authentication 

curl -u username:password https://api.example.com
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Bearer Token Authentication 

curl -H "Authorization: Bearer YOUR_TOKEN" https://api.example.com
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Handling JSON Responses

To pretty-print JSON responses, combine curl with jq:

curl https://api.example.com/users | jq
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This is extremely useful when debugging APIs.

File Upload and Download

Download a File 

curl -O https://example.com/file.zip
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Upload a File 

curl -F "file=@image.png" https://api.example.com/upload
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Debugging and Verbose Mode

Verbose Output 

curl -v https://example.com
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This shows:

  • Request headers
  • Response headers
  • TLS handshake details

Very useful for debugging network issues.

Timeouts and Retries

Set Timeout 

curl --max-time 10 https://example.com
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Retry on Failure 

curl --retry 3 https://example.com
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Following Redirects 

curl -L https://example.com
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By default, curl does not follow redirects unless explicitly told.

Using cURL in Automation and Scripts

curl is commonly used in:

  • CI/CD pipelines
  • Health checks
  • Cron jobs
  • Shell scripts
  • Server monitoring

Example health check:

curl -f https://api.example.com/health || exit 1
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Security Best Practices

  • Avoid exposing tokens in terminal history
  • Use environment variables for credentials
  • Prefer HTTPS over HTTP
  • Validate SSL certificates (avoid -k in production)

cURL vs Postman

Feature cURL Postman
Command-line ✅ ❌
Automation ✅ Limited
GUI ❌ ✅
Lightweight ✅ ❌
CI/CD friendly ✅ ❌

Best practice: Use Postman for exploration and curl for automation and production debugging.

Conclusion

curl is an indispensable tool for modern software engineering. Its simplicity, flexibility, and power make it ideal for API testing, system administration, and automation tasks. Mastering curl significantly improves your ability to debug, test, and interact with network services efficiently.

Whether you are a backend engineer, DevOps professional, or software architect, curl deserves a permanent place in your toolkit.

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