Regardless of what the CGI program does with its input, it’s responsible for giving the browser something to display when it’s done. It must either create a new document to be served to the browser or point to an existing document. On Unix, programs send their output to standard output (STDOUT) as a data stream that consists of two parts. The first part is either a full or partial HTTP header that (at minimum) describes the format of the returned data (e.g., HTML, ASCII text, GIF, etc.). A blank line signifies the end of the header section. The second part is the body of the output, which contains the data conforming to the format type reflected in the header. For example:
Content-type: text/html
<HTML>
<HEAD><TITLE>Thanks!</TITLE></HEAD>
<BODY><H1>Thanks for signing my guest book!</H1>
...
</BODY></HTML>In this case, the only header line generated is
Content-type, which gives the media format of the output as HTML
(text/html). This line is
essential for every CGI program, since it tells the browser what
kind of format to expect. The blank line separates the header from
the body text (which, in this case, is in HTML format as
advertised).
The server transfers the results of the CGI program back to the browser. The body text is not modified or interpreted by the server in any way, but the server generally supplies additional headers with information such as the date, the name and version of the server, etc.
CGI programs can also supply a complete HTTP header itself, in which case the server does not add any additional headers but instead transfers the response verbatim as returned by the CGI program. The server needs to be configured to allow this behavior; see your server documentation on NPH (no-parsed headers) scripts for more information.
Here is the sample output of a program generating an HTML virtual document, with a complete HTTP header:
HTTP/1.0 200 OK
Date: Thursday, 28-June-96 11:12:21 GMT
Server: NCSA/1.4.2
Content-type: text/html
Content-length: 2041
<HTML>
<HEAD><TITLE>Thanks!</TITLE></HEAD>
<BODY>
<H1>Thanks for signing my guestbook!</H1>
...
</BODY>
</HTML>The header contains the communication protocol, the date and
time of the response, and the server name and version. (200 OK is a status code generated by the
HTTP protocol to communicate the status of a request, in this case
successful.) Most importantly, the header also contains the content
type and the number of characters (equivalent to the number of
bytes) of the enclosed data.
The result is that after users click the “submit” button, they see the message contained in the HTML section of the response thanking them for signing the guestbook.