Linux `curl` Command — Transferring Data and Testing APIs¶

curl (Client URL) is one of the most powerful command-line tools used to transfer data between a client and a server using various network protocols. It is widely used for testing REST APIs, downloading data, uploading files, debugging HTTP/HTTPS requests, interacting with cloud services, Kubernetes APIs, and automating web-based workflows. curl supports more than 20 protocols, including HTTP, HTTPS, FTP, SFTP, SCP, SMTP, LDAP, and more. Every Linux administrator, DevOps engineer, Cloud Architect, Platform Engineer, Site Reliability Engineer (SRE), Network Engineer, and Security Engineer should master the curl command.

Learning Path¶

Networking Mastery → Module 9: Linux Networking → Lesson 8

Difficulty: Beginner

Reading Time: 150 Minutes

Course Progress

Course: Networking Mastery

Module: Linux Networking

Lesson: 8 of 10

What You'll Learn¶

After completing this lesson, you'll be able to:

Understand the curl command
Send HTTP and HTTPS requests
Test REST APIs
Work with request headers
Authenticate API requests
Upload and download files
Troubleshoot web services

Prerequisites¶

Complete:

Basic understanding of:

HTTP
HTTPS
REST APIs
JSON

Why Learn `curl`?¶

Suppose users report:

API Not Responding
Website Returns Error
Authentication Failure
Kubernetes API Issues
Cloud Service Errors

Instead of opening a browser, Linux engineers use:

curl

to verify:

Is the server reachable?
What HTTP status code is returned?
What headers are sent?
What response body is received?
Does authentication work?

What is `curl`?¶

curl stands for:

Client URL

It sends requests to servers and displays responses.

Supported protocols include:

HTTP
HTTPS
FTP
FTPS
SFTP
SCP
SMTP
LDAP
MQTT
Many others

Basic Syntax¶

curl [options] URL

Example:

curl https://example.com

Simple HTTP Request¶

curl http://example.com

Returns:

HTML Response

HTTPS Request¶

curl https://example.com

Uses Transport Layer Security (TLS) automatically.

Display Response Headers¶

curl -I https://example.com

Example output:

HTTP/1.1 200 OK

Content-Type: text/html

Content-Length: 1234

Display Headers and Body¶

curl -i https://example.com

Verbose Mode¶

curl -v https://example.com

Displays:

Domain Name System (DNS) Resolution
Transmission Control Protocol (TCP) Connection
TLS Handshake
HTTP Request
HTTP Response

Useful for troubleshooting.

Follow Redirects¶

curl -L https://example.com

Automatically follows HTTP redirects.

Save Output to File¶

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

Save Using Remote Filename¶

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

Uses the filename from the URL.

Download Multiple Files¶

curl -O https://example.com/file1.txt \
     -O https://example.com/file2.txt

Specify HTTP Method¶

GET (default):

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

POST:

curl -X POST https://api.example.com/users

PUT:

curl -X PUT https://api.example.com/users/1

DELETE:

curl -X DELETE https://api.example.com/users/1

Send JSON Data¶

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

Send Custom Headers¶

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

Basic Authentication¶

curl -u username:password \
https://example.com

Bearer Token Authentication¶

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

Common for REST APIs and cloud platforms.

Upload a File¶

curl -F "file=@report.pdf" \
https://example.com/upload

Upload JSON File¶

curl -X POST \
-H "Content-Type: application/json" \
-d @data.json \
https://api.example.com

Display Only HTTP Status Code¶

curl -o /dev/null -s -w "%{http_code}\n" \
https://example.com

Useful in monitoring scripts.

Ignore TLS Certificate Validation¶

curl -k https://example.com

Warning: Use -k only for testing or trusted internal environments. Do not disable certificate validation in production.

Set Request Timeout¶

curl --max-time 10 https://example.com

Stops the request after ten seconds.

Test REST API¶

curl https://jsonplaceholder.typicode.com/posts/1

Returns:

{
  "id": 1,
  "title": "...",
  "body": "..."
}

Enterprise Example¶

Test a production API.

curl -H "Authorization: Bearer TOKEN" \
https://api.company.com/v1/users

Verify:

Authentication
Response Time
Status Code
Response Body

Cloud Perspective¶

Cloud engineers use curl to:

Test Cloud APIs
Verify Load Balancers
Check Health Endpoints
Retrieve Metadata
Validate Service Availability

Kubernetes Perspective¶

Test Kubernetes API.

curl https://KUBE_API_SERVER:6443

Health endpoint.

curl http://SERVICE_IP/health

Ingress endpoint.

curl https://app.example.com

curl is widely used to verify:

Services
Ingress
API Gateways
Health Checks

Linux Perspective¶

Basic request.

curl https://example.com

Verbose mode.

curl -v https://example.com

Download file.

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

POST request.

curl -X POST \
-H "Content-Type: application/json" \
-d '{"test":"value"}' \
https://api.example.com

Common HTTP Status Codes¶

Status Code	Meaning
200	OK
201	Created
204	No Content
301	Permanent Redirect
302	Temporary Redirect
400	Bad Request
401	Unauthorized
403	Forbidden
404	Not Found
500	Internal Server Error
503	Service Unavailable

Common `curl` Commands¶

Command	Purpose
`curl URL`	GET request
`curl -I URL`	Headers only
`curl -i URL`	Headers and body
`curl -v URL`	Verbose output
`curl -O URL`	Download using remote filename
`curl -o file URL`	Save with custom filename
`curl -X POST`	POST request
`curl -H`	Add headers
`curl -d`	Send request body
`curl -u`	Basic authentication

Hands-on Lab¶

Task 1¶

Retrieve a web page.

curl https://example.com

Task 2¶

Display response headers.

curl -I https://example.com

Task 3¶

Use verbose mode.

curl -v https://example.com

Task 4¶

Download a file.

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

Task 5¶

Send a POST request.

curl -X POST \
-H "Content-Type: application/json" \
-d '{"message":"Hello"}' \
https://httpbin.org/post

Task 6¶

Retrieve only the HTTP status code.

curl -o /dev/null -s -w "%{http_code}\n" \
https://example.com

Task 7¶

Test a REST API endpoint with a custom Authorization header.

Task 8¶

Use curl to test a Kubernetes Ingress or application health endpoint and verify the HTTP response.

Production Troubleshooting¶

Problem:

API

Returns

500 Error

Check:

curl -v \
https://api.company.com

↓

Inspect:

DNS Resolution
TLS Handshake
HTTP Status
Response Headers
Response Body

↓

Identify:

Authentication Failure
Server Error
Timeout
Redirect
Certificate Issue

curl vs wget¶

curl	wget
API Testing	File Download
Supports Many Protocols	Optimized for Downloads
Sends Custom Requests	Recursive Downloads
REST API Automation	Website Mirroring
Preferred for HTTP Debugging	Preferred for Bulk Downloads

Common Mistakes¶

❌ Forgetting quotes around JSON.

✅ Always quote JSON payloads.

❌ Using the wrong HTTP method.

✅ Verify the API documentation.

❌ Ignoring response headers.

✅ Inspect headers during troubleshooting.

❌ Disabling TLS validation in production.

✅ Use valid certificates instead of -k.

❌ Forgetting authentication headers.

✅ Include required credentials or tokens.

Best Practices¶

Use verbose mode when troubleshooting.
Check HTTP status codes before parsing responses.
Protect API tokens and credentials.
Validate TLS certificates in production.
Use -L when redirects are expected.
Format JSON responses using tools like jq when available.
Automate health checks with curl.

Interview Questions¶

Beginner¶

What is curl?
How do you display HTTP headers?
How do you download a file?
What does curl -v do?

Intermediate¶

How do you send a POST request?
How do you authenticate using a Bearer token?
Explain common HTTP status codes.
How do you troubleshoot an API using curl?

Architect Level¶

Design an API health-check workflow using curl.
Explain how you would debug intermittent API failures.
How would you automate production endpoint monitoring with curl?

Summary¶

In this lesson, you learned:

The curl command
HTTP and HTTPS Requests
REST API Testing
Authentication
Request Headers
File Downloads
HTTP Status Codes
Enterprise API Troubleshooting

curl is one of the most versatile networking tools available on Linux. It enables engineers to interact with web servers, APIs, cloud services, and Kubernetes applications directly from the command line. Mastering curl is essential for testing APIs, automating HTTP requests, validating service health, and troubleshooting modern web-based infrastructure.

Key Takeaways¶

curl is the standard command-line tool for HTTP and API communication.
It supports GET, POST, PUT, DELETE, and many other request methods.
Use -H to send custom headers and -d to send request data.
Use -v for detailed troubleshooting and -I to inspect response headers.
curl is widely used for REST APIs, Kubernetes, cloud services, automation, and monitoring.
Understanding HTTP status codes is essential for diagnosing application issues.

What's Next?¶

wget

In the next lesson, you'll learn about wget.

You'll explore:

What wget is
Downloading Files
Recursive Downloads
Resume Interrupted Downloads
Background Downloads
Website Mirroring
Automation with wget

By the end of the lesson, you'll understand how to efficiently download files, mirror websites, automate file retrieval, and use wget for system administration, DevOps workflows, and software distribution.

Linux curl Command — Transferring Data and Testing APIs¶