Learnthat.com - Beginning CGI Programming with Perl

Exercise 2.1. Reading and decoding the `User-Agent` field

The CGI program to determine which browser is calling your Web page has two basic steps. First, it must figure out which browser is accessing it. Then, it must return the correct Location headers based on the information figured out in step 1.

Because Netscape is the offending browser by going off on its own and implementing all those cool extensions that are so much fun to use, let's just deal with the Netscape browser. If Netscape were the only browser that could handle tables, this program would be complete. In practice, this code should deal with all the browsers that can and can't handle the HTML+ extensions.

The format of HTTP_USER_AGENT is illustrated by how these two popular browsers define their User-Agent request header:

Mozilla/1.1N (Windows; I; 16bit)
AIR_Mosaic (16bit)/v1.00.198.07

You can find out what types of browsers are looking at your Web page by looking in the server log files. These log files are discussed in further detail in Tutorial 10, "Keeping Track of Your Web Page Visitors."

The easiest thing to do is to split HTTP_USER_AGENT into fields and then compare them against browsers you know will work for your enhanced Web page. Listing 2.3 contains the Perl code to do this. As with all the code in these tutorials, I step through the new and relevant Perl code. You are not expected to know Perl. However, I hope you will feel comfortable enough with Perl by the time you complete these tutorials to write CGI programs of your own.

Listing 2.3. Perl code to return a Web page based on a browser.

01: #!/usr/local/bin/perl
02
03: @user_agent = split(/\//,$ENV{'HTTP_USER_AGENT'});
04:
05: if ($user_agent[0] eq "Mozilla"){
06:     @version = split(/ /,$user_agent[1]);
07:     $version_number = substr($version[0], 0, 3);
08:     if ($version_number < 1.1){
09:        print "Location: http://www.county-line-bbq/clbbq-plus.html.com\n\n";
10:     }
11:     else{
12:        print "Location: http:// www.county-line-bbq/clbbq-minus.html.com  \n\n";
13:     }
14: }
15: else{
16:     print "Location: http:// www.county-line-bbq/clbbq-minus.html.com \n\n";

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Exercise 2.1. Reading and decoding the User-Agent field

Exercise 2.1. Reading and decoding the `User-Agent` field