What is the difference between --head and --request HEAD in Curl?

When working with HTTP requests in cURL, developers often need to retrieve only the headers of a response without downloading the entire body content. This is particularly useful for checking response codes, content types, server information, or file metadata without the overhead of downloading large files. cURL provides two seemingly similar options: --head and --request HEAD. While both can be used to make HEAD requests, they have important differences that can affect your request behavior and results.

Understanding HTTP HEAD Requests

Before diving into the differences, it's important to understand what HTTP HEAD requests are designed for. A HEAD request is identical to a GET request except that the server must not return a message body in the response. The server should return the same headers that would be returned for a GET request, making HEAD requests perfect for:

• Checking if a resource exists
• Retrieving metadata about a resource
• Validating cache entries
• Testing server availability
• Getting file size information without downloading the file

The --head Option

The --head option (short form: -I) is cURL's dedicated flag for making HEAD requests. When you use this option, cURL automatically:

1. Changes the HTTP method to HEAD
2. Stops after receiving the headers
3. Ignores any response body
4. Follows the HTTP specification for HEAD requests

Basic Usage

curl --head https://example.com
# or the short form
curl -I https://example.com

This command will output something like:

HTTP/2 200 
date: Fri, 19 Jul 2024 10:30:45 GMT
server: nginx/1.18.0
content-type: text/html; charset=UTF-8
content-length: 1256
last-modified: Thu, 18 Jul 2024 14:22:33 GMT
etag: "64b2f8a9-4e8"
accept-ranges: bytes

Advanced --head Examples

# Check multiple URLs for their status
curl -I https://api.example.com/users/1
curl -I https://api.example.com/users/2

# Follow redirects while making HEAD requests
curl -I -L https://short.url/abc123

# Add custom headers to HEAD request
curl -I -H "Authorization: Bearer token123" https://api.example.com/protected

# Check file size without downloading
curl -I https://example.com/largefile.zip | grep -i content-length

The --request HEAD Option

The --request HEAD option (short form: -X HEAD) is part of cURL's general request method specification. This option explicitly sets the HTTP method to HEAD but doesn't automatically configure other HEAD-specific behaviors.

Basic Usage

curl --request HEAD https://example.com
# or the short form
curl -X HEAD https://example.com

Key Differences from --head

While --request HEAD sets the HTTP method to HEAD, it doesn't automatically:

1. Stop processing after headers (though it typically will due to HTTP specification)
2. Configure optimal HEAD request handling
3. Provide the same level of HEAD-specific optimizations

Practical Differences and Use Cases

When to Use --head

The --head option is generally preferred for standard HEAD requests because:

# Recommended: Using --head for simple HEAD requests
curl -I https://httpbin.org/status/200

# Check if a file exists and get its metadata
curl -I https://example.com/documents/report.pdf

# Verify API endpoint availability
curl -I https://api.service.com/health

When to Use --request HEAD

The --request HEAD option is useful when you need more control or are combining it with other options:

# When combining with other request options
curl -X HEAD --data "param=value" https://api.example.com/endpoint

# When building complex request chains
curl -X HEAD -H "Content-Type: application/json" https://api.example.com

# When you need explicit method specification for clarity
curl --request HEAD --include https://example.com

Combining with Other cURL Options

Following Redirects

Both options work with redirect following, but --head is more straightforward:

# Follow redirects with HEAD requests
curl -I -L https://bit.ly/shortened-url

# Equivalent with --request HEAD
curl -X HEAD -L https://bit.ly/shortened-url

Including Response Headers in Output

# Show only headers (default behavior with -I)
curl -I https://example.com

# Show headers and any body content (though HEAD shouldn't return body)
curl -X HEAD --include https://example.com

Authentication and Custom Headers

Both options work seamlessly with authentication:

# Basic authentication with HEAD request
curl -I -u username:password https://secure.example.com

# API key authentication
curl -I -H "X-API-Key: your-api-key" https://api.example.com/data

# OAuth token with custom request
curl -X HEAD -H "Authorization: Bearer your-token" https://api.example.com/protected

Performance Considerations

Network Efficiency

HEAD requests are inherently more efficient than GET requests for metadata retrieval:

# Efficient: Only retrieve headers (typical size: 500-2000 bytes)
curl -I https://example.com/large-video.mp4

# Inefficient: Download entire file to check metadata
curl https://example.com/large-video.mp4 | head -1

Batch Operations

When checking multiple resources, HEAD requests can significantly reduce bandwidth:

#!/bin/bash
# Check status of multiple API endpoints efficiently
endpoints=(
    "https://api.service.com/users"
    "https://api.service.com/orders"
    "https://api.service.com/products"
)

for endpoint in "${endpoints[@]}"; do
    echo "Checking $endpoint:"
    curl -I "$endpoint" 2>/dev/null | head -1
    echo
done

Error Handling and Debugging

Status Code Checking

# Extract just the status code
status=$(curl -I -s https://example.com 2>/dev/null | head -1 | cut -d' ' -f2)
echo "Status: $status"

# Check if resource exists
if curl -I --fail -s https://example.com >/dev/null 2>&1; then
    echo "Resource exists"
else
    echo "Resource not found or error occurred"
fi

Verbose Output for Debugging

# See detailed request/response information
curl -I -v https://example.com

# Save headers to file for analysis
curl -I https://example.com > headers.txt

Integration with Web Scraping Workflows

When building web scraping applications, HEAD requests are valuable for preliminary checks before committing to full downloads. While tools like Puppeteer handle browser-based interactions, cURL's HEAD requests excel at quick HTTP-level validations.

For API-based scraping workflows, you might combine HEAD requests with subsequent data retrieval:

# First, check if the API endpoint is available
if curl -I --fail -s https://api.example.com/data >/dev/null 2>&1; then
    # Then proceed with actual data retrieval
    curl -H "Accept: application/json" https://api.example.com/data
else
    echo "API endpoint unavailable"
    exit 1
fi

Advanced Scenarios and Edge Cases

Handling Non-Standard Servers

Some servers may not properly implement HEAD requests. In such cases, understanding the differences becomes crucial:

# Standard HEAD request (preferred)
curl -I https://well-behaved-server.com/api

# If server has issues with -I, try explicit method
curl -X HEAD https://problematic-server.com/api

# For debugging server behavior
curl -X HEAD -v https://server.com/api 2>&1 | grep -E "(< |> )"

Testing API Endpoints

When developing APIs, HEAD requests help verify endpoint behavior:

# Test if API endpoint supports HEAD method
curl -I https://api.example.com/v1/users

# Verify CORS headers are present
curl -I -H "Origin: https://frontend.com" https://api.example.com/v1/data

# Check rate limiting headers
curl -I -H "X-API-Key: test-key" https://api.example.com/v1/limited-endpoint

Content Validation Workflows

HEAD requests are perfect for content validation in CI/CD pipelines:

#!/bin/bash
# Validate deployed content without downloading
validate_deployment() {
    local base_url="$1"
    local endpoints=("/" "/api/health" "/static/app.js" "/static/app.css")

    for endpoint in "${endpoints[@]}"; do
        echo "Checking ${base_url}${endpoint}..."
        if curl -I --fail -s "${base_url}${endpoint}" >/dev/null 2>&1; then
            echo "✓ ${endpoint} is accessible"
        else
            echo "✗ ${endpoint} failed"
            return 1
        fi
    done
    return 0
}

validate_deployment "https://myapp.com"

Best Practices and Recommendations

Choose --head for Standard Cases

For most HEAD request scenarios, use the --head or -I option:

# Preferred for standard HEAD requests
curl -I https://example.com

# Good for checking redirects
curl -I -L https://shortened-url.com

# Optimal for file metadata checks
curl -I https://cdn.example.com/assets/large-file.zip

Use --request HEAD for Complex Scenarios

Reserve --request HEAD for cases requiring explicit method specification or complex option combinations:

# When you need explicit control over request building
curl -X HEAD --max-time 5 --retry 3 https://unreliable-service.com

# When combining with data for unusual server requirements
curl -X HEAD --data "@query.json" -H "Content-Type: application/json" https://api.example.com

# When scripting requires explicit method clarity
curl --request HEAD --silent --show-error https://example.com

Error Handling Best Practices

Always implement proper error handling when using HEAD requests:

# Robust error handling with detailed feedback
check_resource() {
    local url="$1"
    local response

    if response=$(curl -I -s --fail "$url" 2>/dev/null); then
        local status=$(echo "$response" | head -1 | cut -d' ' -f2)
        local content_type=$(echo "$response" | grep -i "content-type:" | cut -d' ' -f2-)
        echo "✓ $url - Status: $status, Type: $content_type"
        return 0
    else
        local exit_code=$?
        echo "✗ $url - Failed (exit code: $exit_code)"
        return $exit_code
    fi
}

# Usage
check_resource "https://example.com/api/endpoint"

Performance Optimization

When making multiple HEAD requests, consider parallel execution:

# Parallel HEAD requests for faster batch checking
urls=(
    "https://api.service1.com/health"
    "https://api.service2.com/health"
    "https://api.service3.com/health"
)

for url in "${urls[@]}"; do
    {
        if curl -I --fail -s --max-time 10 "$url" >/dev/null 2>&1; then
            echo "$url: UP"
        else
            echo "$url: DOWN"
        fi
    } &
done
wait # Wait for all background processes to complete

Troubleshooting Common Issues

Server Doesn't Support HEAD

Some servers may return errors for HEAD requests:

# If HEAD fails, fall back to GET with range header
check_with_fallback() {
    local url="$1"

    if curl -I --fail -s "$url" >/dev/null 2>&1; then
        echo "HEAD supported"
        curl -I "$url"
    else
        echo "HEAD not supported, using GET with range"
        curl -s -r 0-0 "$url" | head -1
    fi
}

Redirect Handling

When dealing with redirects, understand the behavior differences:

# HEAD with redirects - both options work similarly
curl -I -L https://short.url/redirect

# But explicit method gives more control
curl -X HEAD -L --max-redirs 5 https://multiple.redirects.com

Conclusion

While both --head and --request HEAD can achieve similar results for HTTP HEAD requests, --head (or -I) is generally the better choice for standard HEAD operations due to its optimized behavior and cleaner syntax. Use --request HEAD when you need explicit control over the request method or when combining with complex option sets.

The key takeaways are:

• Use --head (-I) for standard HEAD requests - it's optimized and follows HTTP specifications perfectly
• Use --request HEAD (-X HEAD) when you need explicit method control or complex option combinations
  
• HEAD requests are essential for efficient metadata retrieval, server health checks, and content validation
• Always implement error handling when working with HEAD requests in scripts
• Consider parallel execution for batch operations to improve performance

Understanding these differences helps you write more efficient and reliable scripts for web scraping, API testing, and HTTP debugging tasks. Whether you're monitoring network requests in automated browser sessions or performing quick HTTP validations, choosing the right HEAD request option ensures optimal performance and predictable behavior in your applications.