Difference between `curl -I` and `curl -X HEAD`

It seems the difference has to do with the Content-Length header and how it is treated by both commands.

But before going into that, curl -X HEAD does not give any output because, by default, curl does not print headers if switch -i is not provided (not needed on -I though).

In any case, curl -I is the proper way to fetch the headers. It just ask for the header and close the connection.

On the other hand curl -X HEAD -i will wait for the transmission of the number of bytes stated by Content-Length. In the case no Content-Length is not specified, I guess it will wait for some data or for that particular header.

Some examples that shows this behaviour:

$ curl -X HEAD -i http://www.elpais.es
HTTP/1.1 301 Moved Permanently
Server: AkamaiGHost
Content-Length: 0
Location: http://www.elpais.com/
Date: Wed, 12 May 2010 06:35:57 GMT
Connection: keep-alive

Because Content-Length is 0, in this case both commands behave the same. And the connection is closed afterwards.

$ curl -X HEAD -i http://slashdot.org
HTTP/1.1 200 OK
Server: Apache/1.3.41 (Unix) mod_perl/1.31-rc4
SLASH_LOG_DATA: shtml
X-Powered-By: Slash 2.005001296
X-Bender: Since I love you all so much, I'd like to give everyone hugs.
X-XRDS-Location: http://slashdot.org/slashdot.xrds
Cache-Control: no-cache
Pragma: no-cache
Content-Type: text/html; charset=iso-8859-1
Content-Length: 115224
Date: Wed, 12 May 2010 06:37:20 GMT
X-Varnish: 1649060825 1649060810
Age: 1
Connection: keep-alive

curl: (18) transfer closed with 115224 bytes remaining to read

In this case, there seems to be a timeout (probably by Varnish), so curl protests that the connection was closed before having received the Content-Length number of bytes.

By the way, look at the funny X-Bender (shown in the example) and X-Fry (try it for yourself) headers :).

I think this is a bug in curl. If I specify a method with -X, curl should handle the response according to the RFC. Unfortunately, the maintainer of curl does not agree. Someone filed a bug and even submitted a patch:

http://sourceforge.net/tracker/?func=detail&atid=100976&aid=1810273&group_id=976

but the curl maintainer rejected it. Apparently a broken "-X HEAD" option is "working as designed".

--Jamshid