Chapter 9:

Perl and CGI Programming

Guide To UNIX Using Linux Third Edition

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Objectives

• Understand the basics of the Perl language

• Identify and use data types in Perl scripts

• Understand differences between the Awk program and Perl programming

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Objectives (continued)

• Access disk files in Perl

• Use Perl to sort information

• Set up a simple HTML Web page

• Understand how Perl and CGI are used for creating Web pages

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Introduction to Perl

Perl contains features found in other languages – it is very similar to C and also contains features found in Awk and shell programs

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Introduction to Perl (continued)

Perl can be directed to read its input from the keyboard

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Introduction to Perl (continued)

Perl uses decision structures such as if statements to control program flow

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Introduction to Perl (continued)

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Introduction to Perl (continued)

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Identifying Data Types

• Data may be represented in Perl in a variety of ways:– Variables and constants– Scalars– Numbers– Strings– Arrays– Hashes

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Variables and Constants

• Variables and constants are symbolic names that represent values stored in memory

• The value of a variable can change while the program runs

• The value of a constant does not change while the program runs

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Scalars

• Scalars are simple variables that hold a number or a string

• A string is any nonnumeric sequence of characters (including numbers treated as characters)

• Scalar variable names begin with a dollar sign ($)

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Numbers

• Numbers are stored as either signed integers, or as double-precision floating-point values

– Numeric literals can be either integers or floating-point values

– Perl uses an added convention with numeric literals to improve legibility: the underscore character (_)

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Strings

• Sequences of any types of characters

– Often used for logical analysis, sorts, or searches

– String literals are usually delimited by either single (‘) or double quotes (“)

– To put control and escape characters into strings, need to use \ notation, e.g., \n is a newline character

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Strings (continued)

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Strings (continued)

The use of special codes determined the output of this Perl script

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Arrays

• Variables that store an ordered list of scalar values accessed with numeric subscripts

– An at sign (@) precedes the name of an array when assigning it values

– Use the dollar sign ($) when processing the individual elements of an array

– Subscripts are zero-based

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Hashes

• Variables that represent key/value pairs

– A percent sign (%) precedes the name of a hash variable when assigning it a value

– Use the dollar sign ($) to refer to a single element of a hash

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Perl versus the Awk Program

This Awk program counts comment lines in a file. Awk doesn’t use while-type statements for looping.

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How Perl Accesses Disk Files

• Perl uses filehandles to reference files

• A filehandle is the name for an I/O connection between Perl and the OS

• Used to open, read, write, and close a file

• 3 standard filehandles: STDIN, STDOUT, STDERR

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How Perl Accesses Disk Files (continued)

• Can open a file

– With an explicit open statement

– By providing the file name at the command line (storing it in ARGV[0])

• Should always check for failure or EOF when opening a file

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How Perl Accesses Disk Files (continued)

Perl can access a file by passing the filename on the command line

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Using Perl to Sort

• Perl has a powerful sort operator

• Can sort strings or numbers

• Can sort in ascending or descending order

• Advanced sorting operations allow you to define your own sorting routine

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Using Perl to Sort Alphanumeric Fields

You can sort words in a Perl program into alphabetical order using the sort function

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Using Perl to Sort Numeric Fields

You can sort numeric fields in a Perl program by using a sort subroutine

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Setting Up a Web Page

• Web pages can be created using HTML (Hypertext markup Language)

• HTML is a format for creating documents with embedded tags

• Tags give the document special properties when it is viewed in a Web browser

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Setting Up a Web Page (continued)

• Hyperlinks load another document into the browser when clicked

• Web pages are published on a web server

– Apache is a common Web server software

• Linux has a loopback networking feature

– Lets you access your own system as if it were an external network

– Useful for testing Web pages before publishing

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Creating a Simple Web Page

• Two ways to create HTML documents:

– Typing the text and desired embedded tags

– Using a visual HTML editor

• Two main parts to HTML code

– Head contains the title, which appears on the top bar of the browser window

– Body defines what appears in the browser window

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Creating a Simple Web Page (continued)

An HTML document viewed in Mozilla

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CGI Overview

• CGI (Common Gateway Interface) is a protocol, or set of rules, governing how browsers and servers communicate

• Scripts that send or receive information from a server need to follow the CGI protocol

• Perl is the most commonly used language for CGI programming

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CGI Overview (continued)

• Perl scripts are written to get, process, and return information through Web pages (dynamic pages)

• Main objects in dynamic Web pages are forms that allow you to collect input data from a Web page and send it to a server

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CGI Overview (continued)

This Web page contains a form that collects information from a user to submit to a server via CGI

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Chapter Summary• Perl is a interpreted scripting language that can be

combined with CGI to create interactive Web pages

• Perl blends features found in C, Awk, and shell programs

• Perl includes

– An if-else statement as a decision structure

– Numeric and string relational operators

– Arithmetic operators

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Chapter Summary

• Perl’s data types include numbers, strings, arrays, and hashes

• Perl and Awk are both good for applications requiring pattern matching

• Unlike Awk, Perl includes an explicit while looping structure

• Perl includes a powerful sort feature

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Chapter Summary

• Web pages are created using Hypertext Markup Language (HTML)

• An HTML document contains embedded tags that specify document properties and links to other pages

• CGI is a protocol or set of rules governing how browsers and servers communicate