Apache: The Definitive Guide, 3rd Edition

HTTP Header

One of the most crucial elements of a script is also hard to see: the HTTP header that goes ahead of everything else and tells the browser what is coming. If it isn’t right, nothing happens at the far end.

A CGI script produces headers and a body. Everything up to the first blank line (strictly speaking, CRLF CRLF, but Apache will tolerate LF LF and convert it to the correct form before sending to the browser) is header, and everything else is body. The lines of the header are separated by LF or CRLF.

The CGI module (if you are using it) and Apache will send all the necessary headers except the one you need to control. This is normally:

print "Content-Type: text/html\n\n";

If you don’t want to send HTML — but ordinary text — as if to your own screen, use the following:

print "Content-Type: text/plain\n\n";

Notice the second \n (C and Perl for newline), which terminates the headers (there can be more than one; each on its own line), which is always essential to make the HTTP header work. If you find yourself looking at a blank browser screen, suspect the HTTP header.

If you want to force your visitor’s browser to go to another URL, include the following line:

print "Location: http://URL\n\n"

CGIs can emit almost any legal HTTP header (note that although “Location” is an HTTP header, using it causes Apache to return a redirect response code as well as the location specified — this is a special case for redirects). A complete list of HTTP headers can be found in section 14 ...