Verbose
If your curl command does not execute or return what you expected it to, yourfirst gut reaction should always be to run the command with the-v / --verbose option to get more information.
When verbose mode is enabled, curl gets more talkative and explains and showsa lot more of its doings. It adds informational tests and prefix them with'*'. For example, let's see what curl might say when trying a simple HTTPexample (saving the downloaded data in the file called 'saved'):
$ curl -v http://example.com -o saved* Rebuilt URL to: http://example.com/Ok so we invoked curl with a URL that it considers incomplete so it helps usand it adds a trailing slash before it moves on.
* Trying 93.184.216.34...This tells us curl now tries to connect to this IP address. It means the name'example.com' has been resolved to one or more addresses and this is the first(and possibly only) address curl tries to connect to.
* Connected to example.com (93.184.216.34) port 80 (#0)It worked. curl connected to the site and here it explains how the name mapsto the IP address and on which port it has connected to. The '(#0)' part iswhich internal number curl has given this connection. If you try multiple URLsin the same command line you can see it use more connections or reuseconnections, so the connection counter may increase or not increase dependingon what curl decides it needs to do.
If we use anHTTPS:// URL instead of an HTTP one, there are also a wholebunch of lines explaining how curl uses CA certs to verify the server'scertificate and some details from the server's certificate, etc. Includingwhich ciphers were selected and more TLS details.
In addition to the added information given from curl internals, the-vverbose mode also makes curl show all headers it sends and receives. Forprotocols without headers (like FTP, SMTP, POP3 and so on), we can considercommands and responses as headers and they thus also are shown with-v.
If we then continue the output seen from the command above (but ignore theactual HTML response), curl shows:
> GET / HTTP/1.1> Host: example.com> User-Agent: curl/7.45.0> Accept: */*>This is the full HTTP request to the site. This request is how it looks in adefault curl 7.45.0 installation and it may, of course, differ slightlybetween different releases and in particular it changes if you add commandline options.
The last line of the HTTP request headers looks empty, and it is. Itsignals the separation between the headers and the body, and in this requestthere is no "body" to send.
Moving on and assuming everything goes according to plan, the sent requestgets a corresponding response from the server and that HTTP response startswith a set of headers before the response body:
< HTTP/1.1 200 OK< Accept-Ranges: bytes< Cache-Control: max-age=604800< Content-Type: text/html< Date: Sat, 19 Dec 2015 22:01:03 GMT< Etag: "359670651"< Expires: Sat, 26 Dec 2015 22:01:03 GMT< Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT< Server: ECS (ewr/15BD)< Vary: Accept-Encoding< X-Cache: HIT< x-ec-custom-error: 1< Content-Length: 1270<This may look mostly like mumbo jumbo to you, but this is a normal set of HTTPheaders—metadata—about the response. The first line's "200" might be themost important piece of information in there and means "everything is fine".
The last line of the received headers is, as you can see, empty, and that is themarker used for the HTTP protocol to signal the end of the headers.
After the headers comes the actual response body, the data payload. Theregular -v verbose mode does not show that data but only displays
{ [1270 bytes data]That 1270 bytes should then be in the 'saved' file. You can also see thatthere was a header named Content-Length: in the response that contained theexact file length (though it may not always be present in responses).
HTTP/2 and HTTP/3
When doing file transfers using version two or three of the HTTP protocol,curl sends and receivescompressed headers. To display outgoing andincoming HTTP/2 and HTTP/3 headers in a readable and understandable way, curlshows the uncompressed versions in a style similar to how they appear withHTTP/1.1.
Silence
The opposite of verbose is, of course, to make curl more silent. With the-s(or--silent) option you make curl switch off the progress meter and notoutput any error messages for when errors occur. It gets mute. It stilloutputs the downloaded data you ask it to.
With silence activated, you can ask for it to still output the error message onfailures by adding-S or--show-error.