Beginning Web Development With Perl : From Novice to ... - Nabo

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CHAPTER 2 ■ POPULAR CGI MODULES 45Interaction Based on Environment VariablesAs noted in Chapter 1, environment variables, which are available to your programs from theserver itself, can be useful for CGI programming. Using variables like the system time or thebrowser used by the client, you can change the behavior of your program. As an example ofusing environment variables, let’s look at displaying a page to the browser based on thatbrowser and browser version.Some sites misuse the HTTP_USER_AGENT and attempt to block visitors based on the value ofthis variable. Such sites frequently claim to work only in certain browsers. These sites are theresult of poor design by someone who doesn’t understand the Internet. Since HTTP_USER_AGENTis based on a client-side setting, it can be forged or made to look like an acceptable browser.The user agent is covered in Chapter 5, where you’ll see how to change this value. You shouldnot rely on any values coming from the client side when designing a web application, and youcertainly don’t want to design a site that works with only certain browsers.With that in mind, since different browsers and different versions of a browser render webpages differently, in some cases, it’s helpful to customize the page to suit the browser. But beaware that the customization may not work in all cases. In the end, it’s much better to designa site that adheres to well-defined and widely adopted standards, or at least default to thesesettings when the user agent cannot be determined.Performing an action based on the HTTP_USER_AGENT variable is more of an exercise in Perlprogramming than an exercise in anything specific to CGI programming. Programming a setof conditional statements on the value of the variable is all that’s necessary, as shown in the examplein Listing 2-6.Listing 2-6. Determining the User Agent and Printing the Appropriate Result#!/usr/bin/perl -Tuse strict;use CGI qw/:standard/;my $useragent = $ENV{'HTTP_USER_AGENT'};print header,start_html('User Agent Example');if ($useragent =~ /Firefox/) {print p("You are visiting with a Firefox browser");} elsif ($useragent =~ /MSIE/) {print p("You are visiting with an Internet Explorer browser");} else {print p("Could not determine browser: $useragent");}print end_html;exit;The relevant bits of code within this program include the line to set the user agent intoa local variable:
46CHAPTER 2 ■ POPULAR CGI MODULESmy $useragent = $ENV{'HTTP_USER_AGENT'};Next is the Perl conditional if-elsif-else to take an action based on a word found withinthe $useragent variable that is unique to the browser:if ($useragent =~ /Firefox/) {print p("You are visiting with a Firefox browser");} elsif ($useragent =~ /MSIE/) {print p("You are visiting with an Internet Explorer browser");} else {print p("Could not determine browser: $useragent");}Notice that if the browser cannot determine the browser, the default action is to dosomething sane to handle the problem—as opposed to producing an error or, worse yet,crashing, when this variable isn’t present or is something unexpected.The result when viewing this program through a web browser is shown in Figure 2-5.Figure 2-5. Using the user agent to determine the browser
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Beginning Web Development with Perl
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PART 5 ■ ■ ■ Creating Web Tem
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■CONTENTSvii■CHAPTER 4 System I
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■CONTENTSixCreating a SOAP Listen
Page 12 and 13: ■CONTENTSxiTemplate Toolkit Synta
Page 14 and 15: About the Author■STEVE SUEHRING i
Page 16 and 17: AcknowledgmentsThanks to James Lee
Page 18 and 19: xx■INTRODUCTIONThis book will use
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Page 66 and 67: 50CHAPTER 3 ■ DATABASES AND PERLT
Page 68 and 69: 52CHAPTER 3 ■ DATABASES AND PERLm
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Page 74 and 75: 58CHAPTER 3 ■ DATABASES AND PERLR
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Page 104 and 105: CHAPTER 5■ ■ ■LWP ModulesLWP
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Page 108 and 109: CHAPTER 5 ■ LWP MODULES 93Get Fun
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CHAPTER 5 ■ LWP MODULES 99Recall
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CHAPTER 5 ■ LWP MODULES 101which
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108CHAPTER 6 ■ NET:: TOOLSCreatin
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110CHAPTER 6 ■ NET:: TOOLSThe out
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112CHAPTER 6 ■ NET:: TOOLSmy $pop
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114CHAPTER 6 ■ NET:: TOOLSthe POP
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116CHAPTER 6 ■ NET:: TOOLS($messa
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118CHAPTER 6 ■ NET:: TOOLSListing
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120CHAPTER 6 ■ NET:: TOOLSFor the
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122CHAPTER 6 ■ NET:: TOOLSNet::SM
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124CHAPTER 6 ■ NET:: TOOLS$smtpco
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126CHAPTER 6 ■ NET:: TOOLSuse Net
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128CHAPTER 6 ■ NET:: TOOLSListing
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130CHAPTER 6 ■ NET:: TOOLSFor exa
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132CHAPTER 6 ■ NET:: TOOLSSending
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PART 3■ ■ ■XML and RSS
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138CHAPTER 7 ■ SOAP-BASED WEB SER
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154CHAPTER 8 ■ PERL AND RSSFigure
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156CHAPTER 8 ■ PERL AND RSShttp:/
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158CHAPTER 8 ■ PERL AND RSSDebugg
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160CHAPTER 8 ■ PERL AND RSS$rsswr
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162CHAPTER 8 ■ PERL AND RSSListin
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CHAPTER 9■ ■ ■XML Parsing wit
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CHAPTER 9 ■ XML PARSING WITH PERL
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PART 4■ ■ ■PerformanceEnhance
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184CHAPTER 10 ■ APACHE AND MOD_PE
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202CHAPTER 11 ■ DEVELOPMENT WITH
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CHAPTER 12■ ■ ■The Template T
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CHAPTER 12 ■ THE TEMPLATE TOOLKIT
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CHAPTER 13■ ■ ■Perl Web Sites
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CHAPTER 13 ■ PERL WEB SITES WITH
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284APPENDIX ■ PERL BASICSKeywords
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286APPENDIX ■ PERL BASICS$ perl n
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288APPENDIX ■ PERL BASICSAnd line
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290APPENDIX ■ PERL BASICSCan you
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292APPENDIX ■ PERL BASICSBy defau
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294APPENDIX ■ PERL BASICS#!/usr/b
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296APPENDIX ■ PERL BASICSThe last
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298APPENDIX ■ PERL BASICSThe NOT
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300APPENDIX ■ PERL BASICSprint "S
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302APPENDIX ■ PERL BASICSthe syst
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304APPENDIX ■ PERL BASICS$ perl s
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306APPENDIX ■ PERL BASICSThis cod
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308APPENDIX ■ PERL BASICS$a = 6 *
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310APPENDIX ■ PERL BASICS$a = "A9
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312APPENDIX ■ PERL BASICS#!/usr/b
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314APPENDIX ■ PERL BASICSmy $name
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316APPENDIX ■ PERL BASICSis short
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318APPENDIX ■ PERL BASICSLet’s
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320APPENDIX ■ PERL BASICSOther Te
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322APPENDIX ■ PERL BASICS}}}actio
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324APPENDIX ■ PERL BASICSNotice t
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326APPENDIX ■ PERL BASICSLooping
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328APPENDIX ■ PERL BASICSuse stri
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330APPENDIX ■ PERL BASICSuntil ($
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332APPENDIX ■ PERL BASICSuse stri
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334APPENDIX ■ PERL BASICSdoneAll
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336APPENDIX ■ PERL BASICS$ perl r
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338APPENDIX ■ PERL BASICSSummaryT
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340■INDEXmethods for retrieving r
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342■INDEXdo() method for, 62dumpi
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344■INDEXhello option for Net::SM
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346■INDEXNet::DNS module, 125-128
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348■INDEXrecord with Net::DNS mod
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350■INDEXstoring mail server in e
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352■INDEXlogging based on, 229pri
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