Detour: Redirection
May 10, 1999
Not that it's rude or anything, but sometimes you just need
to send the user to some other page. It's for their
own good. Perhaps their form input contained an error and
you have a standard error page you'd like to send them
to. Or, perhaps they submitted some data which requires
you to send them off to one of several possible pages;
for instance, a submitted ZIP code might return one of
three possible pages depending on whether your CGI script
determines their location in the East, Central, or
Western time zones.
The CGI object provides a method named redirect(),
which accepts a URL string. As you probably infer,
this method causes the user's browser to be redirected
to the named URL.
print $cgiobject->redirect('http://www.somesite.com/page2.html');
Notice that the redirect() call is part of a
print statement -- this is because this method produces
a redirection header which is sent to the browser as output.
Also note that the destination URL should be a fully
qualified URL, as seen in the example, not simply a relative
path such as "page2.html".
The main cause of confusion in using this redirect concerns
the fact that it produces a header. Remember earlier
that our Perl scripts often contained the line:
print $cgiobject->header;
But if you output a normal header to the page,
you cannot then output a redirect! This
means that you cannot output some HTML content to the
page and then redirect it to another URL. The redirect
has to be the first and only output sent to the page.
So, put very briefly, our ZIP code-based redirect might look
like:
#read submitted ZIP code and redirect to appropriate page
use CGI;
$cgiobject=new CGI;
$userZIP=$cgiobject->param("ZIP");
$redirURL="http://www.mysite.com/timezone-";
$redirURL.=timeZone(ZIP);
#Note: timeZone(ZIP) is a fictional function which returns a string
#representing the timezone given the ZIP, such as "EST" or "PST".
print $cgiobject->redirect($redirURL);
Again notice that we did not output a normal header
($cgiobject->header) in this page, or the redirect
would have failed to work properly.
The natural follow-up question: "How do I redirect
the page after having output some content?"
The answer is that you don't, not using CGI itself. What
you can do, if the browser supports JavaScript,
is output some JavaScript to the page which in turn
redirects the user's browser. In this case, you would output
the normal header, since the JavaScript output is exactly
like outputting any HTML content to the page. In this
example, we output some text using the traditional
CGI means and then output some JavaScript which, once it hits
the browser, will cause the browser to redirect to a given URL.
#read submitted ZIP code and redirect to appropriate page
use CGI;
$cgiobject=new CGI;
print $cgiobject->header;
print "<html><body>".
"Thanks for your submission ... ".
"I will now send you to the main page.".
"If your browser doesn't go anywhere, click ".
"<a href=\"http://www.mysite.com/main.html\">here".
"</a>";
print "<script language='JavaScript'>".
"window.location='http://www.mysite.com/main.html'".
"</script></body></html>";
The neat thing is, to create the above style of redirect,
we've used one programming language -- Perl -- to
write code in another programming language -- JavaScript!
A Peek at Environment Variables
The Perl You Need to Know
Conclusion
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