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16 Using the CGI Module to Write HTML CGI Built-in www.perl.com Most people are aware of the CGI module as a way of parsing HTML form information and making it available within Perl eit

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SYBEX Sample Chapter

Martin C Brown

Part 3: CGI Tricks

ISBN: 0-7821-2862-9

The author and publisher have made their best efforts to prepare this book, and the content is based upon final release software whenever possible Portions of the manuscript may be based upon pre-release versions supplied by software manufacturer(s) The author and the publisher make no representation or warranties of any kind with regard to the completeness or accuracy of the contents herein and accept no liability of any kind including but not limited to performance, merchantability, fitness for any particular purpose, or any losses or damages of any kind caused or alleged to be caused directly or indirectly from this book Copyright ©2000 SYBEX Inc., 1151 Marina Village Parkway, Alameda, CA 94501 World rights reserved No part of this publica-tion may be stored in a retrieval system, transmitted, or reproduced in any way, including but not limited to photocopy, photo-graph, magnetic or other record, without the prior agreement and written permission of the publisher.

This sample chapter may contain images, text, trademarks, logos, and/or other material owned by third parties All rights reserved Such material may not be copied, distributed, transmitted, or stored without the express, prior, written consent of the owner.

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CGI Tricks

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Did you know that Perl is one of the most commonly used languages for Web development? The reason is quite simple: it’s superb at processing the sort of text information that comes from Web browsers Perl’s also great

at talking to databases, too So not only can it get information from the browser, but it can also search, format, and display it back with ease And you don’t have to compile between each revision!

16 Using the CGI Module

to Write HTML

CGI Built-in www.perl.com Most people are aware of the CGI module as a way of parsing HTML form information and making it available within Perl either through an object interface or a simpler functional interface What many people don’t real-ize, however, is that the same module can also be used to generate the HTML in the first place

So at a base level, simple operations, such as outputting headers and the formatting of individual sections of text, become much easier At a more complex level, you can use the module to generate clean HTML that works

on a particular vendor’s browser

One issue the CGI module addresses is that of complexity Normally, when you want to produce HTML, you use a simple print statement and some suitable text But it is prone to problems because you only have to mistype

a tag name or the angle brackets, and the HTML produced is useless By using a set of functions to generate the HTML, like those supplied by the CGI module, you eliminate most of those problems before you’ve started The module provides two methods for generating HTML: an object-based method that works with the object-based browser interface, and a simpler functional interface Both methods use the same functions, and each func-tion is named after the HTML tag it represents For example, the following creates a “Hello World!” page using the object method:

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U S I N G T H E C G I M O D U L E T O W R I T E H T M L 61

use CGI;

$page = new CGI;

print $page->header, $page->start_html('Hello World!'), $page->h1('Hello World!'),

$page->end_html;

You can achieve the same result with the functional interface as follows:

use CGI qw/:standard/;

print header, start_html('Hello World!'), h1('Hello World!'),

end_html;

The latter code imports the standard set of functions from the CGI module and outputs the header, consisting of the content-type information that must be output to the browser; the HTML preamble; and the piece of text formatted using the <H1> HTML tag It then closes the page The text gen-erated by the script follows:

Content-Type: text/html; charset=ISO-8859-1

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

"http://www.w3.org/TR/html4/loose.dtd">

<HTML LANG="en-US"><HEAD><TITLE>Hello World!</TITLE>

</HEAD><BODY><H1>Hello World!</H1></BODY></HTML>

You can see from the example that the H1 function accepts an argument—

the text to be formatted—and that the resultant HTML includes the neces-sary start and end tags The start_html function does the same basic oper-ation, except the accepted argument is used as the <title>-tag text for the page title

Other tags work in a similar fashion, and tags that accept multiple argu-ments are populated using a hash For example, when creating a form, you can supply the method and action arguments to the tag by supplying a suit-able hash, as shown:

print startform(-method => 'PUT', -action => '/cgi/mycgi.cgi'), textfield('roman');

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C G I T R I C K S

6 2

Note that the action defaults to the name of the CGI script You probably only need to explicitly define the script name if it’s a different script from the one generating the form I’ve also added a text field in the foregoing example

The sample script on the CD provides a quick way of converting a sequence

of Roman numerals, like those seen at the end of TV programs, to a deci-mal The script uses the CGI module to generate HTML and pass the infor-mation returned by the form

Export Sets

The CGI module doesn’t export all of its symbols by default Instead, it uses a series of import sets than define a set of functions to be exported These are divided into sets of supported HTML tags, such as HTML 2.0 and HTML 3.0, and the core functions used to parse and supply parame-ters and other information In the table you can see the import sets and the list of functions (and therefore HTML tags) exported

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U S I N G T H E C G I M O D U L E T O W R I T E H T M L 6 3

Import Set Exported Symbols/Symbol Sets

html2 h1 h2 h3 h4 h5 h6 p br hr ol ul li dl dt dd menu code

var strong em tt u i b blockquote pre img a address cite samp dfn html head base body link nextid title meta kbd start_html end_html input select option comment

html3 div table caption th td TR Tr sup Sub strike applet

Param embed basefont style span layer ilayer font frameset frame script small big

netscape blink fontsize center

form textfield textarea filefield password_field hidden

checkbox checkbox_group submit reset defaults radio_group popup_menu button autoescape scrolling_list image_button start_form end_form startform endform start_multipart_form isindex tmpfilename uploadinfo url_encoded multipart

cgi param path_info path_translated url self_url

script_name cookie dump raw_cookie request_method query_string accept user_agent remote_host

remote_addr referer server_name server_software server_port server_protocol virtual_host

remote_ident auth_type http use_named_parameters save_parameters restore_parameters param_fetch remote_user user_name header redirect import_names put delete delete_all url_param

cgi-lib readparse printheader htmltop htmlbot splitparam

html html2 html3 netscape

standard html2 html3 form cgi

push multipart_init multipart_start multipart_end

all html2 html3 netscape form cgi internal

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17 Better Table Handling

CGI Built-in www.perl.com One of my own personal bugbears when writing HTML is how HTML tables work The basis is right—tables are split into table rows (the <tr> tag) and table cells (the <td> tag) But problems can arise as the table gets more complex and you add more tags and other elements Missing a tag

is generally OK, until you start to use tables embedded in tables, at which point the formatting fails Forgetting to add a closing </table> tag will also cause the entire table to be ignored by the browser

The solution is straightforward—use a function that puts the tags around

a piece of text, as follows:

print td("Name"),td("Martin");

The CGI module provides this functionality for you The real advantage of the CGI module, though, is that the function calls for creating table cells and rows and other multitag components, like lists, can be nested and therefore called more simply

Basic Components

When you want to introduce a list into your HTML, you can create a multi-item list just by supplying a reference to an array of list multi-items to the li func-tion, as follows:

print li(['Martin','Sharon','Richard','Julie']);

The code correctly produces the following HTML:

<LI>Sneezy</LI>

<LI>Sleepy</LI>

<LI>Happy</LI>

All you have to do is supply the <ul> or other list tags to define the type of list

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B E T T E R T A B L E H A N D L I N G 6 5

For table components the same rule applies The following call generates

three table cells with the correct start and end tags:

td(['Foxtrot', 'no', 'no' ]);

You can also supply attributes to the td tag by supplying a hash of the

attributes and values as the first argument To center the text in each of

those cells, write the following code:

td({-align => CENTER}, ['Foxtrot', 'no', 'no' ]);

It generates the HTML shown:

<TD ALIGN="CENTER">Foxtrot</TD>

Obviously, the method is much quicker than either manually generating

the HTML or making individual calls to the td function to generate each

cell It also ensures that the HTML is valid—the HTML tags are started and

completed, and the attributes are properly quoted

Nesting Components

Going back to the list example, to produce a bulleted list you can embed the

call to li in a call to the ul function:

print ul(li(['Martin','Sharon','Richard','Julie']));

It correctly produces the HTML shown:

<UL>

<LI>Sneezy</LI>

<LI>Sleepy</LI>

<LI>Happy</LI>

</UL>

Again, the format applies to table generation You can embed a call to the

td function in a call to Tr to build up the rows of a table, and the Tr function

call can be embedded into a table function call to produce an entire table

For example, consider the following code:

print table({-border => 1},

caption('Cartoons'),

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6 6

Tr({-align => CENTER,-valign => TOP}, [th(['Toon', 'Download','Archive']), td(['Dilbert', 'no', 'yes' ]), td(['Foxtrot', 'no', 'no' ]), td(['Grand Avenue', 'yes',' yes']) ]

));

It generates the following HTML:

<CAPTION>Cartoons</CAPTION>

<TH>Toon</TH> <TH>Download</TH> <TH>Archive</TH>

</TR>

<TD>Dilbert</TD> <TD>no</TD> <TD>yes</TD>

</TR>

<TD>Foxtrot</TD> <TD>no</TD> <TD>no</TD>

</TR>

<TD>Grand Avenue</TD> <TD>yes</TD> <TD> yes</TD>

</TR>

</TABLE>

The script on the CD uses these features to provide a restricted directory-browsing service through a Web site Just adjust the base directory speci-fication for the download directory you want to use

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S E T T I N G U P A C O O K I E 67

18 Setting Up a Cookie

CGI::Cookie Lincoln Stein www.perl.com Unfortunately, Perl can’t make chocolate-chip cookies for you, but it can make the sort of cookies used to store small pieces of site information within a browser Cookies store Web-site login or greeting information, and even simple preferences Your browser will only return the cookie to the host or domain configured by the cookie when created Cookies are thus more secure than some people realize Despite what you may have heard, it’s impossible for a cookie to be obtained by anything other than the site that asks for it

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Amazon stores your login information in a cookie, so that it knows who you are; you may have noticed that whenever you visit the site it greets you by name and even customizes the layout Cookies don’t have to be related to login information, though; they can store any data you like The Ananova news site (part of the Press Association) records your TV region so that it can show you the correct TV listings, for example

The exchange of cookie data happens at the browser’s request—for example, when it sends the GET command to the server to obtain a page and when the server sends the response Although the cookie process sounds like a security nightmare, each cookie is tagged with and sent to specific hostnames, pages,

or scripts As standard, the cookie information is not encrypted, so you’ll need

to think about how to encrypt the data if it’s sensitive

Although the principles of the cookie are simple, setting up and using it manually can be quite complex It won’t surprise you to know, though, that

a Perl module makes it all easier That module is CGI::Cookie

Cookie Components

Cookies consist of six pieces of information: the cookie name, value, expiry date, domain, path, and a simple security field

The cookie name, of course, identifies the cookie A server or domain can sup-ply multiple cookies—you might want to store login and preference informa-tion separately, for example—so the name acts as a unique identifier

The value is the information you want to store Don’t store too much infor-mation, as limited storage for cookies is available You can supply a simple string, an array, or a hash reference You can leave the formatting of the information to the CGI::Cookie module, but I’ll look at that later

Be careful when storing data in a cookie; although cookies are relatively secure, they are still open to abuse Many browsers allow users to see the cookies stored there, and a Perl module will even allow you to access the information

As a general rule, don’t store passwords without some form of encryption Unless absolutely vital, don’t record the login and password within the same cookie; two cookies make it more difficult to collate the two pieces of information If you want to use cookies for authentication purposes, con-sider using a system that separates the login authentication (see number

20for information)

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