🌐 Linux curl Command

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

# Use saved cookies
curl -b cookies.txt https://example.com/dashboard

# Create curl-format.txt with:
time_namelookup:  %{time_namelookup}s\n
time_connect:     %{time_connect}s\n
time_appconnect:  %{time_appconnect}s\n
time_pretransfer: %{time_pretransfer}s\n
time_redirect:    %{time_redirect}s\n
time_starttransfer: %{time_starttransfer}s\n
time_total:       %{time_total}s\n

📚 Additional Information

Common curl Options

HTTP Methods (REST API)

Response Status Codes

# Get just the HTTP status code
curl -o /dev/null -s -w "%{http_code}\n" https://example.com

# Common status codes:
# 200 OK - Success
# 201 Created - Resource created
# 204 No Content - Success, no body
# 301 Moved Permanently - Redirect
# 302 Found - Temporary redirect
# 400 Bad Request - Invalid request
# 401 Unauthorized - Authentication required
# 403 Forbidden - No permission
# 404 Not Found - Resource doesn't exist
# 500 Internal Server Error - Server error
# 503 Service Unavailable - Server down

Advanced curl Techniques

Rate Limiting

# Limit download speed to 100KB/s
curl --limit-rate 100k -O https://example.com/largefile.zip

Timeouts

# Set connection timeout to 10 seconds
curl --connect-timeout 10 https://example.com

# Set maximum time for entire operation to 60 seconds
curl --max-time 60 https://example.com

# Both combined
curl --connect-timeout 10 --max-time 60 https://example.com

Retries

# Retry failed requests up to 5 times
curl --retry 5 https://api.example.com

# Add delay between retries (5 seconds)
curl --retry 5 --retry-delay 5 https://api.example.com

# Retry only on specific errors
curl --retry 3 --retry-connrefused https://api.example.com

Parallel Downloads

# Download multiple files in parallel (curl 7.66+)
curl -Z --parallel --parallel-max 10 \
  -O https://example.com/file1.pdf \
  -O https://example.com/file2.pdf \
  -O https://example.com/file3.pdf

GraphQL Queries

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

Testing Different HTTP Versions

# Force HTTP/1.1
curl --http1.1 https://example.com

# Force HTTP/2
curl --http2 https://example.com

# Try HTTP/2 but fallback to HTTP/1.1
curl --http2-prior-knowledge https://example.com

Proxy Usage

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

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

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

Real-World Sysadmin Use Cases

Health Check Monitoring

# Check if service is up
curl -f -s -o /dev/null https://myapp.com/health || echo "Service down!"

# Check multiple endpoints
for url in https://api1.com/health https://api2.com/health; do
  status=$(curl -o /dev/null -s -w "%{http_code}" "$url")
  echo "$url: $status"
done

API Testing in Scripts

#!/bin/bash
# Test API endpoint and parse response
response=$(curl -s -w "\n%{http_code}" https://api.example.com/status)
body=$(echo "$response" | head -n -1)
status=$(echo "$response" | tail -n 1)

if [ "$status" -eq 200 ]; then
  echo "API is healthy: $body"
else
  echo "API error: Status $status"
  exit 1
fi

Download and Verify Checksum

# Download file and its checksum
curl -O https://example.com/file.tar.gz
curl -O https://example.com/file.tar.gz.sha256

# Verify
sha256sum -c file.tar.gz.sha256

Send Alerts to Slack/Discord

# Slack webhook
curl -X POST -H 'Content-type: application/json' \
  --data '{"text":"Server disk space critical!"}' \
  https://hooks.slack.com/services/YOUR/WEBHOOK/URL

# Discord webhook
curl -X POST -H "Content-Type: application/json" \
  -d '{"content":"Backup completed successfully"}' \
  https://discord.com/api/webhooks/YOUR/WEBHOOK

Automated Login and Session

# Login and save session cookie
curl -c cookies.txt -X POST \
  -d "username=admin&password=secret" \
  https://example.com/login

# Use session for authenticated requests
curl -b cookies.txt https://example.com/dashboard

# Logout
curl -b cookies.txt -X POST https://example.com/logout
rm cookies.txt

Check SSL Certificate Expiration

# Get certificate expiration date
curl -vI https://example.com 2>&1 | grep "expire date"

# More detailed certificate info
curl -vI https://example.com 2>&1 | awk '/expire date/{print}'

Download Latest Release from GitHub

# Get latest release URL
latest_url=$(curl -s https://api.github.com/repos/user/repo/releases/latest \
  | grep "browser_download_url.*\.tar\.gz" \
  | cut -d : -f 2,3 \
  | tr -d \")

# Download it
curl -L -O $latest_url

Security Best Practices

• Avoid plaintext passwords: Use -u username (prompts for password) or environment variables
• Verify SSL certificates: Don't use -k/--insecure in production
• Use API tokens: Prefer token authentication over username/password
• Be careful with -v: Verbose output may expose sensitive data in logs
• Clear bash history: Use space before command or history -d to remove sensitive commands
• Store credentials securely: Use .netrc file with proper permissions (600)

Using .netrc for Authentication

# Create ~/.netrc with permissions 600
cat > ~/.netrc << EOF
machine api.example.com
login myusername
password mypassword
EOF

chmod 600 ~/.netrc

# curl will automatically use these credentials
curl https://api.example.com/secure

Debugging Tips

• Use -v for verbose: See full request/response cycle
• Use --trace or --trace-ascii: Even more detailed debugging
• Check DNS resolution: curl -v shows IP address connected to
• Test with curl -I first: Check headers before downloading large files
• Use --trace-time: Add timestamps to trace output

curl vs wget

Performance Optimization

# Use HTTP/2 for better performance
curl --http2 https://example.com

# Enable compression
curl --compressed https://example.com

# Keep-alive (reuse connection for multiple requests)
curl --keepalive-time 60 https://example.com

# Use DNS cache
curl --dns-servers 8.8.8.8 https://example.com

Common Pitfalls and Solutions

Quick Reference Commands

# Simple GET
curl https://example.com

# GET with headers shown
curl -i https://example.com

# POST with JSON
curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' URL

# POST with form data
curl -X POST -d "key=value" URL

# Upload file
curl -F "file=@path/to/file" URL

# Download with progress bar
curl -# -O URL

# Follow redirects
curl -L URL

# Save to file
curl -o filename URL

# Silent but show errors
curl -sS URL

# With authentication
curl -u user:pass URL

# With custom header
curl -H "Authorization: Bearer TOKEN" URL

# Timing information
curl -w "%{time_total}\n" -o /dev/null -s URL

# Just status code
curl -o /dev/null -s -w "%{http_code}\n" URL

Useful Shell Functions

# Check HTTP status
http_status() {
  curl -o /dev/null -s -w "%{http_code}\n" "$1"
}

# Pretty print JSON response
curl_json() {
  curl -s "$1" | jq .
}

# Download with retry and resume
curl_reliable() {
  curl -C - --retry 5 --retry-delay 3 -# -O "$1"
}

# Usage:
# http_status https://google.com
# curl_json https://api.github.com/users/octocat
# curl_reliable https://example.com/largefile.zip

Testing APIs: Complete Workflow

# 1. Check if API is reachable
curl -I https://api.example.com

# 2. Test authentication
curl -H "Authorization: Bearer YOUR_TOKEN" https://api.example.com/auth/test

# 3. GET request
curl -H "Authorization: Bearer YOUR_TOKEN" https://api.example.com/users

# 4. POST request (create)
curl -X POST \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name":"John","email":"john@example.com"}' \
  https://api.example.com/users

# 5. PUT request (update)
curl -X PUT \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name":"John Updated"}' \
  https://api.example.com/users/123

# 6. DELETE request
curl -X DELETE \
  -H "Authorization: Bearer YOUR_TOKEN" \
  https://api.example.com/users/123

Fun Facts

• curl was first released in 1997 as httpget, then renamed to curl
• The creator, Daniel Stenberg, still maintains curl after 25+ years
• curl is used in virtually every connected device - cars, TVs, phones, spacecraft
• NASA used curl on the Mars helicopter Ingenuity
• curl supports over 25 different protocols
• The official curl website uses curl in its logo: curl.se
• There's a "curl up" conference dedicated to curl and HTTP