When working with HTTP requests in command line, these two commands appear similar but behave differently:

# Standard HEAD request
curl -I http://example.com

# Manual HEAD method specification
curl -X HEAD http://example.com

At the protocol level, both commands send identical HEAD requests as seen in tshark output:

HEAD / HTTP/1.1
Host: example.com
User-Agent: curl/7.68.0
Accept: */*

• Connection Handling: -I automatically closes connection after receiving headers
• Output Behavior: -X HEAD waits for potential body data (despite HEAD method)
• Protocol Compliance: -I properly implements HTTP spec for HEAD requests

The difference stems from how curl internally handles different request types:

// Simplified curl internal logic
if (option_I) {
  setup_as_HEAD();
  disable_body_processing();
  auto_close_connection();
} else if (custom_method) {
  send_exact_method_provided();
  // No special handling for HEAD
}

For production use cases, prefer:

# Recommended for header inspection
curl -I https://api.example.com/resources

# Alternative with proper HEAD handling
curl --head https://api.example.com/resources

When investigating similar issues:

# Verbose output shows internal handling
curl -vI http://example.com
curl -vX HEAD http://example.com

# Network-level comparison
tcpdump -i any -nn -v port 80

According to RFC 7231 Section 4.3.2:

"The HEAD method is identical to GET except that the server MUST NOT send a message body in the response."

This explains why -X HEAD may appear to "hang" - it's waiting for a body that should never arrive.

For more consistent HEAD behavior consider:

# Using httpie
http HEAD example.com

# Using wget
wget --server-response --spider http://example.com

While both commands appear similar at first glance, they handle HTTP HEAD requests differently:

# Standard HEAD request with -I
curl -I https://example.com

# Manual HEAD request specification
curl -X HEAD https://example.com

The key behavioral differences observed in network traffic:

• curl -I properly closes the connection after receiving headers
• curl -X HEAD keeps the connection alive waiting for potential body data

The divergence stems from how curl implements these options:

// Simplified internal logic
if (option == -I) {
    set_header_only(true);
    set_automatic_behavior(true);
} else if (option == -X HEAD) {
    set_method("HEAD");
    // Missing automatic header handling
}

For web service debugging, curl -I is generally preferred because:

1. It automatically handles connection closing
2. Properly displays response headers
3. Follows HTTP protocol expectations

If you need to force a HEAD request with different behavior:

# Combine methods for better control
curl -X HEAD -I https://example.com

# Alternative using --head
curl --head https://example.com

Some web servers handle these differently due to:

• Protocol compliance variations
• Connection keep-alive settings
• HTTP/1.1 vs HTTP/2 implementations