curl - HTTP Client

Command-line tool for making HTTP requests and transferring data.

Basic Requests

GET requests

# Simple GET request
curl https://api.example.com

# Save output to file
curl https://example.com -o output.html
curl https://example.com/file.zip -O  # Use remote filename

# Follow redirects
curl -L https://example.com

# Show response headers
curl -i https://example.com

# Show only headers
curl -I https://example.com

# Verbose output (debugging)
curl -v https://example.com

POST requests

# POST with data
curl -X POST https://api.example.com/users \
  -d "name=John&email=john@example.com"

# POST JSON data
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name":"John","email":"john@example.com"}'

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

# Form upload
curl -X POST https://api.example.com/upload \
  -F "file=@document.pdf" \
  -F "description=My document"

Other HTTP methods

# PUT request
curl -X PUT https://api.example.com/users/1 \
  -H "Content-Type: application/json" \
  -d '{"name":"Jane"}'

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

# PATCH request
curl -X PATCH https://api.example.com/users/1 \
  -H "Content-Type: application/json" \
  -d '{"email":"newemail@example.com"}'

Headers & Authentication

Custom headers

# Add custom header
curl -H "User-Agent: MyApp/1.0" https://example.com

# Multiple headers
curl -H "Accept: application/json" \
     -H "Authorization: Bearer token123" \
     https://api.example.com

Authentication

# Basic auth
curl -u username:password https://api.example.com

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

# API key in header
curl -H "X-API-Key: your_api_key" \
  https://api.example.com

# API key in URL
curl "https://api.example.com?api_key=your_key"

Advanced Features

Timeouts & retries

# Connection timeout (seconds)
curl --connect-timeout 10 https://example.com

# Max time for entire operation
curl --max-time 30 https://example.com

# Retry on failure
curl --retry 3 https://example.com

# Retry delay
curl --retry 3 --retry-delay 5 https://example.com

Cookies

# Send cookies
curl -b "session=abc123" https://example.com

# Save cookies to file
curl -c cookies.txt https://example.com

# Load cookies from file
curl -b cookies.txt https://example.com

# Both save and load
curl -b cookies.txt -c cookies.txt https://example.com

Proxy

# Use HTTP proxy
curl -x http://proxy.example.com:8080 https://api.example.com

# With proxy authentication
curl -x http://proxy:8080 -U user:pass https://api.example.com

# SOCKS proxy
curl --socks5 127.0.0.1:1080 https://api.example.com

SSL/TLS

# Ignore SSL certificate errors (not recommended for production)
curl -k https://self-signed.example.com

# Use specific SSL version
curl --tlsv1.2 https://example.com

# Use client certificate
curl --cert client.crt --key client.key https://example.com

# Show SSL handshake details
curl -v https://example.com 2>&1 | grep -i ssl

Response Handling

Output formatting

# Silent mode (no progress bar)
curl -s https://api.example.com

# Show only HTTP status code
curl -s -o /dev/null -w "%{http_code}" https://example.com

# Custom output format
curl -w "\nTime: %{time_total}s\nStatus: %{http_code}\n" \
  https://example.com

# Pretty print JSON (with jq)
curl -s https://api.example.com | jq '.'

Range requests

# Download specific byte range
curl -r 0-1000 https://example.com/large-file.zip

# Resume download
curl -C - -O https://example.com/large-file.zip

File Operations

Downloading files

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

# Download with custom name
curl -o myfile.zip https://example.com/file.zip

# Download multiple files
curl -O https://example.com/file1.zip \
     -O https://example.com/file2.zip

# Resume interrupted download
curl -C - -O https://example.com/large-file.zip

Uploading files

# FTP upload
curl -T file.txt ftp://ftp.example.com/upload/

# HTTP PUT upload
curl -T file.txt https://example.com/upload

# Form file upload
curl -F "file=@document.pdf" https://example.com/upload

Testing & Debugging

API testing

# Test REST API
curl -X GET https://api.example.com/users
curl -X GET https://api.example.com/users/1
curl -X POST https://api.example.com/users -d @user.json
curl -X PUT https://api.example.com/users/1 -d @updated.json
curl -X DELETE https://api.example.com/users/1

# Test with verbose output
curl -v -X POST https://api.example.com/login \
  -H "Content-Type: application/json" \
  -d '{"username":"test","password":"pass"}'

Performance testing

# Measure request time
curl -w "Total time: %{time_total}s\n" https://example.com

# Detailed timing
curl -w "\nDNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTLS: %{time_appconnect}s\nTransfer: %{time_starttransfer}s\nTotal: %{time_total}s\n" \
  -o /dev/null -s https://example.com

Common debugging

# Show request and response headers
curl -v https://api.example.com

# Trace request
curl --trace-ascii trace.txt https://api.example.com

# Include response headers in output
curl -i https://api.example.com

Common Patterns

Quick JSON API test:

curl -s https://api.github.com/users/octocat | jq '{name, bio, followers}'

Download with progress bar:

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

POST JSON and extract field:

curl -s -X POST https://api.example.com/login \
  -H "Content-Type: application/json" \
  -d '{"user":"test","pass":"secret"}' | jq -r '.token'

Check if URL is accessible:

if curl -s --head --fail https://example.com > /dev/null; then
  echo "Site is up"
else
  echo "Site is down"
fi

Parallel downloads:

for i in {1..10}; do
  curl -O https://example.com/file$i.jpg &
done
wait

Useful Flags

-X: HTTP method (GET, POST, PUT, DELETE, etc.)
-d: Data to send (POST/PUT)
-H: Custom header
-o: Output file
-O: Save with remote filename
-L: Follow redirects
-i: Include headers in output
-I: Headers only
-v: Verbose output
-s: Silent mode
-S: Show errors even in silent mode
-f: Fail silently on HTTP errors
-k: Insecure (ignore SSL)
-u: Basic authentication
-F: Multipart form data
-b: Send cookies
-c: Save cookies
-w: Custom output format

Tips

Use -s in scripts to suppress progress bar
Combine -sS for silent but show errors
Use -L for redirects (e.g., shortened URLs)
Add -v for debugging
Use jq to process JSON responses
Save common requests as shell aliases or scripts
Use --config for complex reusable requests

Documentation

Official docs: https://curl.se/docs/ Manual: man curl HTTP methods: https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods

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