How can I see the request headers made by curl when sending a request to the server?

Question

I think curl --verbose/-v is the easiest. It will spit out the request headers (lines prefixed with '>') without having to write to a file:

$ curl -v -I -H "Testing: Test header so you see this works" http://stackoverflow.com/
* About to connect() to stackoverflow.com port 80 (#0)
*   Trying 69.59.196.211... connected
* Connected to stackoverflow.com (69.59.196.211) port 80 (#0)
> HEAD / HTTP/1.1
> User-Agent: curl/7.16.3 (i686-pc-cygwin) libcurl/7.16.3 OpenSSL/0.9.8h zlib/1.2.3 libssh2/0.15-CVS
> Host: stackoverflow.com
> Accept: */*
> Testing: Test header so you see this works
>
< HTTP/1.0 200 OK
...

The question did not specify if command line command named curl was meant or the whole cURL library.

The following PHP code using cURL library uses first parameter as HTTP method (e.g. "GET", "POST", "OPTIONS") and second parameter as URL.

<?php
$ch = curl_init();
$f = tmpfile(); # will be automatically removed after fclose()
curl_setopt_array($ch, array(
    CURLOPT_CUSTOMREQUEST  => $argv[1],
    CURLOPT_URL            => $argv[2], 
    CURLOPT_RETURNTRANSFER => 1,
    CURLOPT_FOLLOWLOCATION => 0,
    CURLOPT_VERBOSE        => 1,
    CURLOPT_HEADER         => 0,
    CURLOPT_CONNECTTIMEOUT => 5,
    CURLOPT_TIMEOUT        => 30,
    CURLOPT_STDERR         => $f,
));
$response = curl_exec($ch);
fseek($f, 0);
echo fread($f, 32*1024); # output up to 32 KB cURL verbose log
fclose($f);
curl_close($ch);
echo $response;

Example usage:

php curl-test.php OPTIONS https://google.com

Note that the results are nearly identical to following command line

curl -v -s -o - -X OPTIONS https://google.com

The --trace-ascii option to curl will show the request headers, as well as the response headers and response body.

For example, the command

curl --trace-ascii curl.trace http://www.google.com/ 

produces a file curl.trace that starts as follows:

== Info: About to connect() to www.google.com port 80 (#0)
== Info:   Trying 209.85.229.104... == Info: connected
== Info: Connected to www.google.com (209.85.229.104) port 80 (#0)
=> Send header, 145 bytes (0x91)
0000: GET / HTTP/1.1
0010: User-Agent: curl/7.16.3 (powerpc-apple-darwin9.0) libcurl/7.16.3
0050:  OpenSSL/0.9.7l zlib/1.2.3
006c: Host: www.google.com
0082: Accept: */*
008f: 

It also got a response (a 302 response, to be precise but irrelevant) which was logged.

---

If you only want to save the response headers, use the --dump-header option:

curl -D file url
curl --dump-header file url

If you need more information about the options available, use curl --help | less (it produces a couple hundred lines of output but mentions a lot of options). Or find the manual page where there is more explanation of what the options mean.

The only way I managed to see my outgoing headers (curl with php) was using the following options:

curl_setopt($ch, CURLOPT_HEADER, 1);
curl_setopt($ch, CURLINFO_HEADER_OUT, true);

Getting your debug info:

$data = curl_exec($ch);
var_dump($data);
var_dump(curl_getinfo($ch));

curl --trace-ascii {filename} or use a single dash instead of file name to get it sent to stdout:

curl --trace-ascii - {URL}

CURLOPT_DEBUGFUNCTION if you're using libcurl

This shows you everything curl sends and receives, with some extra info thrown in.

I tried the answers here and found that the most useful and easiest one is not listed as an answer yet, but it is:

curl -v https://example.com/path

This prints out the REQUEST headers as well as the RESPONSE headers plus other useful such as the SSL cert and whether an existing TCP connection was reused. the -v flag can be combined with other flags, of course, such as to follow redirects and prompt for HTTP authentication:

curl -vL --user my_username https://example.com/path

Hope this helps.