Teach Yourself CGI Programming with PERL 5 in a Week, 2E

by Eric Herrmann

C  O  N  T  E  N  T  S

Introduction

Chapter 1   An Introduction to CGI and Its Environment

• The Common Gateway Interface (CGI)
• HTML, HTTP, and Your CGI Program
  • The Role of HTML
  • The HTTP Headers
  • Your CGI Program
• The Directories on Your Server
  • The Server Root
  • The Document Root
• File Privileges, Permissions, and Protection
• WWW Servers
  • MS-Based Servers
  • The CERN Server
  • The ncSA Server
  • The Netscape Server
• The CGI Programming Paradigm
  • CGI Programs and Security
  • The Basic Data-Passing Methods of CGI
  • CGI's Stateless Environment
• Preventing the Most Common CGI Bugs
  • Tell the Server Your File Is Executable
  • Finding Things on Your System
  • Make Your Program Executable
• Learning Perl
  • Hello World
  • Exercise 1.1. Working with Perl variables
  • Exercise 1.2. Using the scalar variable
• Summary
• Q&A

Chapter 2   Understanding How the Server and Browser Communicate

• Using the Uniform Resource Identifier
  • The Protocol
  • The Domain Name
  • The Directory, File, or CGI Program
• Requesting Your Web Page with the Browser
• Using the Internet Connection
  • TCP/IP, the Public Socket, and the Port
  • One More Time, Using the Switchboard Analogy
• Using the HTTP Headers
  • Status Codes in Response Headers
  • The Method Request Header
  • The Full Method Request Header
  • The Accept Request Header
  • The HTTP Response Header
• Changing the Returned Web Page Based on the User-Agent Header
• Learning Perl
  • Exercise 2.2. Using the Perl regular array
  • Exercise 2.3. Using Perl's associative array
• Summary
• Q&A

Chapter 3   Using Server Side Include Commands

• Using SSI Negatives
• Understanding How SSIs Work
  • Deciding Whether to Enable SSIs
  • Using the Options Directive
  • Using the AddType Command for SSIs
  • Using the srm.conf File
• Automatically Adding the Last Modification Date to Your Page
• Examining the Full Syntax of SSI Commands
• Using the SSI config Command
• Using the include Command
  • Analyzing the include Command
  • Using the virtual Command Argument
  • Using the file Command Argument
• Examining the flastmod Command
  • The Syntax of the SSI echo Command
  • The exec Command and CGI Scripts
• Looking At Security Issues with SSIs
• Learning Perl
  • Exercise 3.2. The Perl conditional statement if(){} else{}
• Summary
• Q&A

Chapter 4   Using Forms to Gather and Send Data

• Understanding HTML Form Tags
• Using the HTML Form Method Attribute
  • The Get and Post Methods
  • The Get Method
  • The Post Method
• Generating Your First Web Page On-the-Fly
  • Comparing CGI Web Pages to HTML Files
  • Analyzing first.cgi
  • Sending Variables in Your CGI Program
• Using the HTML Input Tag
  • Sending Data to Your CGI Program with the Text Field
  • Using the Submit Button to Send Data to Your CGI Program
• Making Your Text-Entry Form Fast and Professional Looking
• NPH-CGI Scripts
  • NPH-CGI Scripts Are Faster
  • URI-Encoded Data Ends Up in the Location Window
• Seeing What Happens to the Data Entered on Your Form
  • Name/Value Pairs
  • Path Information
• Using URI Encoding
  • Reserved Characters
  • The Encoding Steps
• Learning Perl
  • The Perl for Statement
  • The Perl foreach Statement
• Summary
• Q&A

Chapter 5   Decoding Data Sent to Your CGI Program

• Using the Post Method
• Using Radio Buttons in Your Web Page Forms and Scripts
  • The HTML Radio Button Format
  • The Name Attribute
  • The Value Attribute
  • The Checked Attribute
  • Radio Button Rules
• Reading and Decoding Data in Your CGI Program
  • Using the ReadParse Function
  • Creating Name/Value Pairs from the Query String
  • Decoding the Name/Value Pairs
  • Using the Post Method
  • Using the Perl read Function
  • Including Other Files and Functions in Your CGI Programs
  • Using the Data Passed with Radio Buttons
  • Using Perl's If Elsif Block
  • Using the HTML Checkbox
• Using a Database with Your CGI Program
• Using Pull-Down Menus in Your Web Page Forms and Scripts
  • Using the HTML Form Select Tag
  • Using the Option Field
• Using File Data in Your CGI Program
  • Opening a File
  • Reading Formatted Data
  • Using Formatted File Data
  • Using Data to Make Your CGI Programming Easier
• Learning Perl
  • Exercise 5.2. Using ARGV
  • Printing with Perl
• Summary
• Q&A

Chapter 6   Using Environment Variables in Your Programs

• Understanding Environment Variables
  • Program Scope
  • The Path Environment Variable
• Printing Your Environment Variables
• Sending Environment Variables to Your E-Mail Address
  • Perl Subroutines
  • The Unescape Subroutine
  • The cgi_encode Subroutine
  • The Main Mail Program
• Using the Two Types of Environment Variables
  • Environment Variables Based on the Server
  • Environment Variables Based on the Request Headers
• Finding Out Who Is Calling at YourWeb Page
• Getting the Username of Your Web Site Visitor
• Using the Cookie
• Returning the Cookie
• Learning Perl
  • Exercise 6.1. Using files with Perl
  • Understanding the Filename and Pathname
  • Understanding the Filehandle
  • Opening and Closing the File
  • Exercise 6.2. Using filehandles
  • Using Perl's Special Variables
• Summary
• Q&A

Chapter 7   Building an Online Catalog

• Using Forms, Headers, and Status Codes
• Registering Your Customer
• Setting Up Password Protection
  • Using the Password File
  • Using the Authentication Scheme
• Dealing with Multiple Forms
• Learning Perl
  • Perl's write Statement
  • The format Statement
  • Field Formatting
• Summary
• Q&A

Chapter 8   Using Existing CGI Libraries

• Using the cgi-lib.pl Library
  • Determining the Requesting Method
  • Decoding Incoming CGI Data
  • Printing the Magic HTTP Content Header
  • Printing the Variables Passed to Your CGI Program
  • Printing the Variables Passed to Your CGI Program in a Compact Format
• Using CGI.pm for Creating and Reading Web Forms
  • Installing CGI.pm
  • Reading Input Data
  • Saving Your Incoming Data
  • Saving the Current State of a Form
  • Creating the HTTP Headers
  • Creating an HTML Header
  • Ending an HTML Document
  • Creating Forms
  • Creating a Submit Button
  • Creating a Reset Button
  • Creating a Defaults Button
  • Creating a Hidden Field
  • Creating a Clickable Image Button
  • Controlling HTML Autoescaping
• Using the CGI Library for C Programmers: cgic
  • Writing a cgic Application
    
  • Using String Functions
  • Using Numeric Functions
  • Using Header Output Functions
  • A cgic Variable Reference
• Summary
• Q&A

Chapter 9   Using Imagemaps on Your Web Page

• Defining an Imagemap
• Sending the x,y Coordinates of a Mouse Click to the Server
  • Using the Ismap Attribute and the Img Tag
  • Using the Ismap Attribute with <INPUT TYPE=IMAGE>
• Creating the Link to the Imagemap Program
• Using the imagemap.c Program
• Using the Mapfile
  • Looking At the Syntax of the Imagemap File
  • Deciding Where to Store the Imagemap File
  • Increasing the Efficiency of Imagemap Processing
  • Using the Default URI
  • Ordering Your Mapfile Entries
• Using Client-Side Imagemaps
  • The Usemap Attribute
  • The HTML Map Tag
  • The Area Tag and Its Attributes
• Summary
• Q&A

Chapter 10   Keeping Track of Your Web Page Visitors

• Defining an Access Counter
• Using the Existing Access Log File
• Using page-stats.pl to Build Log Statistics
• Getting Access Counts for Your Entire Server from wusage 3.2
  • Configuring wusage
  • Charting Access by Domain
  • Running wusage
  • Purging the access_log File (How and Why)
• Examining Access Counter Graphics and Textual Basics
• Working with DBM Files
  • Locking a File
  • Creating Your Own File Lock
  • Using the flock() Command
• Excluding Unwanted Domains from Your Counts
• Printing the Counter
• Turning Your Counter into an Inline Image
  • Generating Counters from a Bitmap
  • Using the WWW Homepage Access Counter
  • Using the gd 1.2 Library to Generate Counter Images On-the-Fly
  • Using the gd 1.2 Library to Produce Images On-the-Fly
  • Global Types
  • Create, Destroy, and File Functions
  • Drawing Functions
  • Query Functions
  • Font and Text-Handling Functions
  • Color-Handling Functions
  • Copying and Resizing Functions
• Summary
• Q&A

Chapter 11   Using Internet Mail with Your Web Page

• Looking At Existing Mail Programs
  • The UNIX Mail Program
  • The UNIX sendmail Program
• Using Existing CGI E-Mail Programs
  • The WWW Mail Gateway Program
  • Using a Multilingual E-Mail Tool
• Building Your Own E-Mail Tool
  • Making Your Own E-Mail Form
  • Sending the Blank Form
  • Restricting Who Mail Can Be Sent To
• Implementing E-Mail Security
• Defining a Regular Expression
  • Positioning Your Regular Expression Match
  • Specifying the Number of Times a Pattern Must Occur
  • Using Regular Expression Special Characters
• Summary
• Q&A

Chapter 12   Guarding Your Server Against Unwanted Guests

• Protecting Your CGI Program from User Input
• Protecting Your Directories with Access-Control Files
  • The Directory Directive
  • The AllowOverride Directive
  • The Options Directive
  • The Limit Directive
  • The allow from Directive
  • The deny from Directive
  • The order Directive
  • The require Directive
• Setting Up Password Protection
  • The htpasswd Command
  • The Groupname File
• Using the Authorization Directives
  • The AuthType Directive
  • The AuthName Directive
  • The AuthUserFile Directive
  • The AuthGroupFile Directive
• Examining Security Odds and Ends
  • The emacs Files
  • The Path Variable
  • The Perl Taint Mode
• Cleaning Up Cookie Crumbs
• Summary
• Q&A

Chapter 13   Debugging CGI Programs

• Determining Which Program Has a Problem
• Determining Whether the Program Is Being Executed
• Checking the Program's Syntax
  • Checking Syntax at the Command Line
  • Interpreting Perl Error Messages
  • Looking At the Causes of Common Syntax Errors
• Viewing HTML Sources of Output
  • Using MIME Headers
  • Examining Problems in the HTML Output
• Viewing the CGI Program's Environment
  • Displaying the Raw Environment
  • Displaying Name/Value Pairs
• Debugging at the Command Line
  • Testing without the HTTP Server
  • Simulating a Get Request
  • Using Perl's Debug Mode
• Reading the Server Error Log
• Debugging with the Print Command
• Looking At Useful Code for Debugging
  • Show Environment
  • Show Get Values
  • Show Post Values
  • Display Debugging Data
• A Final Word about Debugging
• Summary
• Q&A

Chapter 14   Tips, Tricks, and Future Directions

• Making Browser-Sensitive Pages
• Simplifying Perl Code
• Looking At the Future of Perl
• Examining Python: A New Language for CGI
  • Comparing Python and Perl
  • Understanding the Python Language
  • Implementing Python
• Examining Java and JavaScript: Bringing Life to HTML
  • Understanding How Java Works
  • Understanding How a Java Program Is Executed
  • Looking At the Java Language
  • Implementing Java in Your System
  • Looking At JavaScript: Scripting for the Web
• Finding Useful Internet Sites for CGI Programmers
  • CGI Information
  • Perl Information
  • Specific Product Information
• Summary
• Q&A

Appendix A   MIME Types and File Extensions

Appendix B   HTML Forms

• Form Attributes
  • Action
  • Enctype
  • Method
  • Script
• Input Fields
  • Checkbox Fields
  • File Attachments
  • Hidden Fields
  • Image Fields
  • Password Fields
  • Radio Buttons
  • Range Fields
  • Reset Buttons
  • Scribble on Image Fields
  • Single-Line Text Fields
  • Submit Buttons
• Permitted Attributes for the Input Element
  • Accept
  • Align
  • Checked
  • Class
  • Disabled
  • Error
  • ID
  • Lang
  • Max
  • Maxlength
  • MD
  • Min
  • Name
  • Size
  • SRC (Source)
  • Type
  • Value
• Textarea
  • Cols
  • Rows
• Select Elements
  • Height
  • Multiple
  • SRC (Source)
  • Units
  • Width
• The Option Element

Appendix C   Status Codes and Reason Phrases

Appendix D   The ncSA imagemap.c Program

Appendix E   The Perl Quick Reference Manual

• Perl Operators and Their Precedence
• Perl Operators and Their Meanings
• Special Variables
• Perl Commands
• Miscellaneous Perl Rules
• GNU License Information

Credits

Copyright 1997 by Sams.net Publishing

SECOND EDITION

All rights reserved. No part of this book shall be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission from the publisher. No patent liability is assumed with respect to the use of the information contained herein. Although every precaution has been taken in the preparation of this book, the publisher and author assume no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use of the information contained herein. For information, address Sams.net Publishing, 201 W. 103rd St., Indianapolis, IN 46290.

International Standard Book Number: 1-57521-196-3

Acknowledgments

It's not possible to write a book without a lot of help from all kinds of places:

• Dad definitely hasn't been around very much in the last year, and hardly at all in the last 90 days. My oldest son, Scott, took over a lot of the work that Dad normally does, with very little complaint. Thanks, Scott.
• This book probably would not have happened without the initial encouragement to get into the Internet business, provided by my friend and mentor Mario V. Boykin. Thanks, Mario, for your business and personal support.
• Lorraine Bier is a dear friend who had the guts to tell me how awful the first couple of chapters were. Without Lori's honest early appraisal, I think my editor would have shot me. Thanks, Lori, for your editing help.
• James Martin, one of my partners and friends in this high-tech world, gave me the freedom and encouragement to spend the hours required to write a book. Thanks, James.
• A book on any subject on the Internet is always a collaborative effort, with lots of cyberspace help. The newsgroup
  comp.infosystems.www.authoring.cgi
  was a big research tool for me. Thanks to everyone who answered all the myriad questions about CGI programming. Especially Thomas Boutell, Tom Christianson, Mark Hedlund, and Lincoln Stein.
• Michael Moncur was a great help in getting this book done in a timely manner. When I was tired and didn't think I could write another word, Michael stepped in and wrote Chapters 13 and 14. Thanks, Mike, for the Great Work.
• It is amazing how much effort it is to write a book. My production editor, Fran Blauw, kept her sense of humor throughout the process of fixing my poor grammar and geeky English. Thanks a lot, Fran, for the hard work and keeping me smiling during the editing process.

About the Author

Eric Herrmann is the owner of Practical Internet, an online catalog and Web-page development company, and partner in Application Software Solutions Inc., a software development company focused on building intranets. Eric has a master's degree in Computer Science, 10 years of application programming experience in various asynchronous parallel processing environments, and is fluent in most of today's buzzwords: OOP, C++, UNIX, TCP/IP, Perl, and Java. Eric is happily settled on 10 acres of lovely Texas hill country in Dripping Springs, Texas, with his wife, Sherry, a riding instructor who speaks fluent horse; his three children, Scott, Jessica, and Steve; and 10 horses, 3 dogs, 4 cats, and 8 pet chickens :). When not playing at his computer, Eric helps with the horses, takes the kids fishing, or plays with model trains in the garage.

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Introduction

Teach Yourself CGI Programming with Perl 5 in a Week, 2E collects all the information you need to do Internet programming in one place.

In the first chapter, you will learn:

• The requirements needed to run CGI programs on your HTTP server
• How to set up the directories and configuration files on your server
• The common mistakes that keep your CGI programs from working

From there, you will learn about the basic client/server architecture of the server, and you will get a detailed description of the HTTP request/response headers. You will learn the client/server model in straightforward and simple terms, and throughout the book, you will learn about several methods for keeping track of the state of your client.

A full explanation of the unique environment of CGI programming is included in the chapters covering environment variables and server communications with the browser. The heart of CGI programming-understanding how data is managed between the client and the server-gets full coverage. Each step in data management-sending, receiving, and decoding data-is fully covered in its own chapter.

Each chapter of Teach Yourself CGI Programming with Perl 5 in a Week, 2E includes lots of programming and HTML examples. This book is an excellent resource for the novice Perl programmer; a detailed explanation of Perl is included with most programming examples. There is no assumption of the programming skills of the reader. Every programming example includes a detailed explanation of how the code works.

After teaching you the foundations of CGI programming, this book explores and explains the hottest topics of CGI programming. Make your Web page come alive with a clickable imagemap. Learn how to define the hotspots, where the existing tools are, and how to configure your server for imagemaps. Count the number of visitors to your Web page and learn about the pitfalls of getting their names. Learn how to create customizable mailing applications using the Internet sendmail format. And learn how to protect yourself from hackers, in a full chapter on Internet and CGI security.

You will find that this book is a great introduction and resource to the CGI programming environment on the Internet. Read on to begin understanding this fantastic programming environment, and good luck in all your programming endeavors. Have fun! It's more fun than not having fun.

What's New in This Edition

Teach Yourself CGI Programming with Perl 5 in a Week, 2E is a practical, hands-on guide to the world of CGI and Perl. This edition offers many new and revised examples and explanations. Furthermore, several readers suggested that more Perl foundation information would be helpful. The second edition includes new "Learning Perl" sections that focus exclusively on Perl concepts. This will give you, the reader, a much better understanding of both CGI and Perl as you work through each lesson in this book.

What This Book Is About

This book starts where most CGI tutorials leave off-just before you get into the really cool stuff! Fear not. If you are looking to take your Internet knowledge to the next level, you've made the right purchase. This book provides useful tips and hands-on examples for developing your own applications within the CGI programming environment using the Perl language. You get a complete understanding of the important CGI concepts, such as HTTP request/response headers, status codes, CGI/URI data encoding and decoding, and Server Side Include commands. You learn application development through examples in every chapter and with a complete application when you design an online catalog.

Specific features you'll see throughout the book follow:

What You Will Gain from This Book

Anyone who wants to know about programming on the Internet and in the CGI environment will benefit by reading this book. You spend several days covering advanced topics, yet a majority of this book is dedicated to helping you understand the CGI environment and Perl and then applying that knowledge to real applications. It is this hands-on approach to the CGI environment and the Perl language that sets this book apart from others. In addition to helping you develop an application, you learn the concepts involved in development.