Perl and CGI Programming

1
Guide To UNIX Using Linux Third Edition

• Chapter 9
• Perl and CGI Programming

2
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

3
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

4
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
5
Introduction to Perl (continued)
Perl can be directed to read its input from the
keyboard
6
Introduction to Perl (continued)
Perl uses decision structures such as if
statements to control program flow
7
Introduction to Perl (continued)
8
Introduction to Perl (continued)
9
Identifying Data Types

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

10
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

11
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
  ()

12
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 (_)

13
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

14
Strings (continued)
15
Strings (continued)
The use of special codes determined the output of
this Perl script
16
Arrays

• Variables that store an ordered list of scalar
  values accessed with numeric subscripts
• An at sign (_at_) 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

17
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

18
Perl versus the Awk Program
This Awk program counts comment lines in a file.
Awk doesnt use while-type statements for looping.
19
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

20
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 ARGV0)
• Should always check for failure or EOF when
  opening a file

21
How Perl Accesses Disk Files (continued)
Perl can access a file by passing the filename on
the command line
22
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

23
Using Perl to Sort Alphanumeric Fields
You can sort words in a Perl program into
alphabetical order using the sort function
24
Using Perl to Sort Numeric Fields
You can sort numeric fields in a Perl program by
using a sort subroutine
25
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

26
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

27
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

28
Creating a Simple Web Page (continued)
An HTML document viewed in Mozilla
29
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

30
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

31
CGI Overview (continued)
This Web page contains a form that collects
information from a user to submit to a server via
CGI
32
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

33
Chapter Summary

• Perls 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

34
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