Comprehensive Guide to Using the curl Command-Line Tool

Introduction

curl is a widely used command-line utility for making HTTP requests, acting as a client URL tool with dozens of options that can replace graphical tools like Postman.

Common Options

2.1 -A and -H (User-Agent and custom headers)

Use # curl https://www.example.com for a simple GET request. The -A flag sets the User-Agent header, while -H allows arbitrary header specification.

# -A parameter specifies the client User-Agent header
curl -A 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.100 Safari/537.36' https://google.com

# Remove User-Agent header
curl -A '' https://google.com

# Set a custom User-Agent header
curl -H 'User-Agent: php/1.0' https://google.com

2.2 -b (Send cookies)

The -b option sends cookies to the server, either directly or from a file.

# Send a single cookie
curl -b 'foo=bar' https://google.com

# Send multiple cookies
curl -b 'foo1=bar;foo2=bar2' https://google.com

# Read cookies from a file
curl -b cookies.txt https://www.google.com

2.3 -c (Save cookies)

Store server‑set cookies into a file for later reuse.

# Save cookies to a file
curl -c cookies.txt https://www.google.com

2.4 -d and --data-urlencode (POST data)

Use -d to send POST data; --data-urlencode automatically URL‑encodes the payload.

# Simple POST with form data
curl -d 'login=emma&password=123' -X POST https://google.com/login

# Read data from a file
curl -d '@data.txt' https://google.com/login

# URL‑encode data
curl --data-urlencode 'comment=hello world' https://google.com/login

2.5 -H (Add arbitrary HTTP headers)

Specify additional request headers such as Accept-Language or Content-Type .

# Add Accept-Language header
curl -H 'Accept-Language: en-US' https://google.com

# Add multiple headers
curl -H 'Accept-Language: en-US' -H 'Secret-Message: xyzzy' https://google.com

# Send JSON with appropriate Content-Type
curl -d '{"login": "emma", "pass": "123"}' -H 'Content-Type: application/json' https://google.com/login

2.6 -F (Upload files)

Upload binary files using multipart/form‑data.

# Upload a file
curl -F '[email protected]' https://google.com/profile

# Specify MIME type
curl -F '[email protected];type=image/png' https://google.com/profile

# Rename the uploaded file
curl -F '[email protected];filename=me.png' https://google.com/profile

2.7 -k (Skip SSL verification)

Disable certificate validation for HTTPS requests.

# Ignore SSL certificate errors
curl -k https://www.example.com

2.8 -L (Follow redirects)

Make curl follow HTTP 3xx redirects automatically.

# Follow redirects
curl -L http://codebelief.com

2.9–2.11 (Saving output)

Various ways to save responses: shell redirection, -o (lowercase) for a specific filename, -O (uppercase) to keep the remote filename, and patterns for batch downloads.

# Shell redirection
curl http://www.linux.com >> linux.html

# Save to a specific file
curl -o linux.html http://www.linux.com

# Save using remote filename
curl -O http://www.linux.com/hello.sh

# Batch download with brace expansion
curl -O http://www.linux.com/{hello,bb}/dodo[1-5].JPG

2.12 Testing HTTP response codes

Use -w %{http_code} to output the status code, useful in scripts.

# Print HTTP status code only
curl -o /dev/null -s -w "%{http_code}" www.linux.com

2.13 Proxy support

Set a proxy server with -x .

# Use proxy at 192.168.100.100:1080
curl -x 192.168.100.100:1080 http://www.linux.com

2.14 Save response headers

Store response headers in a file using -D .

# Save headers to cookied.txt
curl -D cookied.txt http://www.linux.com

2.15 Fake Referer (anti‑hotlinking)

Set the Referer header with -e to bypass hotlink protection.

# Set custom referer
curl -e "www.linux.com" http://mail.linux.com

2.16 File upload

Upload files via FTP using -T .

# Upload via FTP
curl -T dodo1.JPG -u username:password ftp://www.linux.com/img/

2.17 Show errors

Fail silently on HTTP errors with -f and customize output with -w .

# Fail on HTTP errors
curl -f http://www.linux.com/error

2.18 Show HTTP headers

Display only headers with -I or both headers and body with -i .

# Show headers only
curl -I http://www.codebelief.com

# Show headers and body
curl -i http://www.codebelief.com

Conclusion

The guide covers a broad set of curl functionalities, from basic GET requests to complex multipart uploads, proxy configuration, header manipulation, and response testing, providing ready‑to‑use command snippets for developers and system administrators.