Lecture 7: Introduction to CGI and Perl

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Lecture 7 Introduction to CGI and Perl

• Web Architecture Web server
• CGI Common Gateway Interface
• examples of server-side programming
• Perl
• Data types
• String processing
• Client-server environment variables
• Form Processing Business Logic
• SSI

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1. Web Architecture

• Three-tier architecture
• Presentation clients contains both the
  presentation and application logic components.
• Content web server provides interactive view of
  information from a data store.
• Data and service level provides data for the web
  server.

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1. Web Architecture

• Multi-tier architecture
• Application-level or middleware has an
  application server, which is used to find
  requested data and services, makes them available
  for viewing, and carries out transactions.
• Data and service level has a variety of data and
  services accessible by the application server.

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1. Web Server

• A web server is a network server that manages
  access to files, folders and other resources over
  Internet or local Intranet via HTTP
• Handle permissions
• Execute programs
• Keep track of directories and files
• Communicate with client computers
• A web server consists of a computer with
• a suitable operating system
• a suitable file system
• a network connection
• web server software which accepts requests from
  browsers such as IE and Netscape

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1. Examples of Web Server Software

• Microsoft Personal Web Server
• Microsoft Internet Information Server
• W3C Jigsaw Web Server
• http//jigsaw.w3.org/Distrib/
• Apache
• http//Apache-Server.Com/tutorials/

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2. CGI

• Server-side technologies such as CGI(Common
  Gateway Interface) , SSI(Server Side Include) and
  ASPs (Active Server pages)
• CGI.
• is a standard for interfacing external
  applications with web servers.
• extends Web server's functionality through
  outside scripts or compiled programs.
• Data path of a typical CGI-based application

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2. CGI

• Web browser
• Take info from user
• typically http//www.mydomain.com/cgi-bin/ name
• Using HTTP, sends info to a Web server
• Server-side CGI program executed
• Standard output from server-side applications or
  scripts redirected or piped to CGI
• Output sent from CGI over the Internet to the
  client browser for rendering
• CGI is an interface
• Cannot be directly programmed
• Script or executable program must be used to
  interact with it

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2. CGI

• CGI programs can be written in any language, e.g.
  C/C, Fortran, Perl, TCL, any Unix shell, Visual
  Basic
• CGI programs need to reside in a special
  directory, so that the Web server knows to
  execute the program rather than just display it
  to the browser.
• typically resides in /cgi-bin directory.
• Within Web server
• Permission needs to be granted by Web master to
  allow specific programs to be executed on the
  server

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3. Perl

• Perl Practical Extraction and Report Language
• High-level programming language
• Developed by Larry Wall in 1987
• Rich, easy-to-use text-processing capabilities
• Alternative to the terse and strict C programming
  language
• Powerful alternative to UNIX shell scripts
• To check current version type perl v (in
  UNIX)
• Logical choice for programming the server side of
  interactive Web-based applications
• Most popular language for doing so today
• Is continuously corrected and evolved by the
  online Perl community
• Stays competitive with newer server-side
  technologies

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3. Perl

• Perl initially developed for UNIX platform
• Always intended to be a cross-platform computer
  language
• ActivePerl
• Version of Perl for Windows
• Free download at http//www.activestate.com
• Includes the core Perl package
• Predefined functionality expected to behave the
  same across all platforms
• Perl Interpreter - perl.exe placed in bin
  directory
• Loaded into memory each time Perl program invoked
• Extension of Perl programs is .pl
• Associated with Perl interpreter by default
• Perl program execution
• Type perl followed by filename of Perl source
  code at command line (DOS prompt)

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3. Perl

• Comment Character -
• Goes at beginning of every line with comment
• Function print
• Outputs text indicated by quotation marks ()
• Escape sequence \n
• When inside a string, moves cursor to next line
• Statements terminated with semicolons ()
• Exception where braces () used to denote block
  of code

Welcome to Perl!
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3.1. Perl Data Types

• Perl contains set of data types
• Represent different kinds of information
• Each variable name has special character
  preceding it
• - variable contains scalar value
• Strings, integer numbers and floating-point
  numbers
• _at_ - indexed array
• Uses an integer (called an index) to reference
  array elements
• An ordered list of scalar variables that can be
  accessed using integer indices
• - hash
• Uses keys that are strings to reference
  individual array elements
• An unordered set of scalar variables whose values
  are accessed using unique scalar values (ie,
  strings) called keys.
• Variables do not have to be initialized before
  being used
• Variable names in strings
• Serve as place-holders for values they represent
• If have no declared value set to null (empty)
  value

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3.1. Perl Data Types

• Example to demonstrate scalar variables

Program to illustrate the use of scalar
variables. number 5 print( "The value of
variable \number is number\n\n" ) number
5 print( "Variable \number after adding 5 is
number\n" ) number 2 print( "Variable
\number after multiplying by 2 is " ) print(
"number\n\n\n" )
The value of variable number is 5 Variable
number after adding 5 is 10 Variable number
after multiplying by 2 is 20
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3.1. Perl Data Types

• Example to demonstrate uninitialized variables

using an uninitialized variable in the context
of a string print( "Using a variable before
initializing variable\n\n" ) using an
uninitialized variable in a numeric context test
undefined 5 print( "Adding uninitialized
variable \undefined " ) print( "to 5 yields
test\n" )
Using a variable before initializing Adding
uninitialized variable undefined to 5 yields 5

• uninitialized variables have the value undef
• Undef is evaluated to different values depending
  on variables context
• Numeric context, undef evaluates to 0
• String context, undef evaluates to

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3.1. Perl Data Types

• Example adding strings and numeric variables

using strings in numeric context string "A
string value" number 1 number
string print("Adding a string to an integer
\n") print(" \number \number \string
number \n") string2 "15charactersand1" num
ber2 number string2 print("Adding a string
with numbers in the beginning of the string\n
") print("\number2 \number \string2
number2\n\n") print("evaluating a string in
numeric context does not") print(" the value of
the string, \n \"string2\" yields
") print("string2\n")
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3.1. Perl Data Types

• Adding a string to an integer
• number number string 1
• Adding a string with numbers in the beginning of
  the string
• number2 number string2 16
• evaluating a string in numeric context does not
  the value of the string,
• "15charactersand1" yields 15charactersand1

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3.1. Perl Data Types

• Perl can store arrays
• Array definition
• _at_arrayName (element1, element2, ,
  elementN)
• Elements are referenced as scalar values with
  element number in square brackets ()
• _at_ refers to array as a whole, refers to
  elements
• First array element is 0
• e.g. array2refers to the third element in
  _at_array
• Range Operator ..
• Used to store all values between given arguments
• e.g. _at_array2 (A..Z)
• Creates array _at_array2 containing all capital
  letters in alphabet (all letters between A and Z)

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3.1. Perl Data Types

• Example to demonstrate arrays

Program to demonstrate arrays in Perl _at_array
("Bill", "Bobby", "Sue", "Michelle") print "The
array contains\n\n" print "_at_array \n\n" print
"Third element array2\n\n" _at_array2
(A..Z) print "The range operator is used to
store all\n" print "letters from capital A to
Z\n\n" print "_at_array2 \n"
The array containsBill Bobby Sue
MichelleThird element SueThe range operator
is used to store allletters from capital A to
ZA B C D E F G H I J K L M N O P Q R S T U V W
X Y Z
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3.1. Perl Data Types

• array3 3 "4th"
• print( "Array with just one element initialized
  _at_array3 \n\n" )
• print( 'Printing literal using single quotes '
  )
• print( '_at_array and \n', "\n" )
• print( "Printing literal using backslashes " )
• print( "\_at_array and \\n\n" )

• Array with just one element initialized 4th
• Printing literal using single quotes _at_array and
  \n
• Printing literal using backslashes _at_array and \n

• uninitialized elements of an array take the
  value undef
• Eg, array30 array31 array32
  (empty string)
• 3 spaces printed before printing 4th
• Perl interpreter interprets strings inside
  single quotes literally, eg _at_array
• use back slash to escape special characters, eg
  \_at_

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3.2. Perl String Processing

• Processing textual data easily and efficiently
• One of Perls most powerful capabilities
• Usually done through use of regular expressions
• Patterns of characters used to search through
  text files and databases
• Allows large amounts of text to be searched using
  relatively simple expressions
• eq equality operator
• Tests whether two strings are equivalent
• eg. if (hello eq Good Morning)
• Keyword my
• Indicates designated variable only valid for
  block of code in which it is declared

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3.2. Perl String Processing

• Example to demonstrate string processing

my stringa "Test" my stringb
"Testing" if (stringa eq "Test") print
"stringa matches Test.\n" else print
"stringa does not match Test.\n" if
(stringb eq "Test") print "stringb matches
Test.\n" else print "stringb does not
match Test.\n"
Test matches Test.Testing does not match Test.
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3.2 Perl String Processing
Program to demonstrate the eq, ne, lt, gt
operators. _at_fruits qw( apple orange banana )
qw means quote word foreach item ( _at_fruits )
if ( item eq "banana" ) print(
"String 'item' matches string 'banana'\n" )
if ( item ne "banana" ) print( "String
'item' does not match string 'banana'\n" )
if ( item lt "banana" ) print( "String
'item' is less than string 'banana'\n" ) if
( item gt "banana" ) print( "String
'item' is greater than string 'banana'\n" )

• String 'apple' does not match string 'banana'
• String 'apple' is less than string 'banana'
• String 'orange' does not match string 'banana'
• String 'orange' is greater than string 'banana'
• String 'banana' matches string 'banana'

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3.2 Perl String Processing

• Searches using the matching operator and
  regular expressions.search "Now is is the
  time"print( "Test string is 'search'\n\n"
  )if ( search /Now/ ) print( "String
  'Now' was found.\n" )if ( search /Now/ )
  print( "String 'Now' was found at the
  beginning of the line." ) print( "\n" )if
  ( search /Now/ ) print( "String 'Now'
  was found at the end of the line.\n" )if (
  search /\b ( \w ow ) \b/x ) print(
  "Word found ending in 'ow' 1 \n" )if (
  search /\b ( \w ) \s ( \1 ) \b/x )
  print( "Repeated words found 1 2\n"
  )_at_matches ( search / \b ( t \w ) \b
  /gx )print( "Words beginning with 't' found
  _at_matches\n" )

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3.2 Perl String Processing

• Test string is 'Now is is the time'
• String 'Now' was found.
• String 'Now' was found at the beginning of the
  line.
• Word found ending in 'ow' Now
• Repeated words found is is
• Words beginning with 't' found the time

• \w ow - indicates searching for pattern ending
  with ow
• \b \b - word boudary
• match result(s) stored in special Perl
  variables, eg, 1, 2, 3
• x indicates that whitespace in the regular
  expression are to be ignored
• eg, If the expression search /\b ( \w ow
  ) \b/
• then the script will search for
• a word boundary,
• 2 spaces,
• one or more alphanumeric characters,
• one space, the characters ow,
• 2 spaces
• word boundary.

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3.3. Perl Packages

• In addition to core Perl package
• Add-ons called packages provide additional
  functionality
• The packages can be including using use command
• Packages
• Often provide platform specific features
• Are available free of charge at
  http//www.activestate.com/packages

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3.3. Perl Samples Viewing Client/Server
Environment Variables

• Knowing info about client very useful to system
  administrators
• CGI environment variables
• Contains info about client
• Version of CGI server running
• HTTP host, HTTP connection
• Much more
• Note In the following Perl example for
  Client/server environment variables, the purpose
  is to demonstrate how one can access the
  environment variables. The intention is not to
  study each and every of the environment
  variables.
• Use statement
• Allows inclusion of predefined library packages
  in programs

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3.3 Perl Samples Viewing Client/Server
Environment Variables

• CGI Library
• Included to provide functionality that makes it
  easier to write HTML sent to Web browser
• Contains keywords that represent HTML tags
• foreach loop
• Iterates through keys in given hashtable,
  performs indicated actions
• foreach key (sort keys ENV)
• Iterates through ENV hashtable
• Built-in table in Perl that contains names and
  values of all CGI environment variables
• sort function
• returns list in alphabetical order
• Assigns current key to key and performs
  indicated actions

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3.3 Perl Samples Viewing Client/Server
Environment Variables

• Line 2
• use instruct Perl to include the contents of
  predefined functions
• standard import a standard set of pre-defined
  functions
• Line 4-7
• Change the default document type definition from
  HTMLs DTD to the value in dtd
• Line 9
• Instruct Perl to print a valid HTTP header using
  the header() function

1 !/usr/local/bin/perl 2 use CGI qw
(standard) 3 4 dtd 5 "-//W3C//DTD XHTML
1.0 Transitional//EN\" 6 \"http//www.w3.org/T
R/xhtml1/DTD/xhtml1- 7 transitional.dtd" 8 9
print( header() )
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3.3 Perl Samples Viewing Client/Server
Environment Variables

• Line 10
• start_html() is a CGI function that prints the
  document type definition, and the opening tags
  such as , ,
• Line 16
• Tr() and th() are CGI functions that place
  arguments between Table row and table header tags
  (ref, p 923, Deitel)
• Line 19
• td() is a CGI function that prints tags
• hr() is a CGI function that prints a horizontal
  line and a line break above and below the
  horizontal line

10 print( start_html( dtd dtd, 11
title "Environment Variables..." ) ) 12 13
print( "2 14 font-weight bold\"" ) 15 16
print( Tr( th( "Variable Name" ), 17
th( "Value" ) ) ) 18 19 print( Tr( td( hr() ),
td( hr() ) ) )
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3.3 Perl Samples Viewing Client/Server
Environment Variables

• Line 20
• ENV hash is a built-in table in Perl that
  contains the names and values of all the
  environment variables
• each element in a hash is accessed using a unique
  string key that is associated with that elements
  value.
• keys() is a function that returns an unordered
  array containing all keys in ENV
• foreach iterates sequentially through the array
  returns by sort
• Line 24
• hash values are accessed using the syntax
  hashNamekeyName, eg, ENV variable

20 foreach variable ( sort( keys( ENV ) ) )
21 22 print( Tr( td( style
"background-color 11bbff" , 23
variable ), 24 td( style
"font-size 12pt" , 25
ENV variable ) ) ) 26 27 print( Tr( td(
hr() ), td( hr() ) ) ) 28 29 30 print(
"" ) 31 print( end_html() )
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3.3 Perl Samples Viewing Client/Server
Environment Variables

• Script output

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3.4 Perl Samples Form Processing and Business
Logic

• Two parts
• client side HTML forms
• server side CGI programs, e.g. Perl script
• HTML FORMs
• 1. Allow users to enter data
• 2. Data sent to Web server for processing
• 3. Program processes data
• Allows users to interact with server
• Vital to electronic commerce
• FORM element
• Indicates what action should occur when user
  submits form
• Attribute ACTION cgi-bin/form.pl
• Directs server to execute form.pl Perl script

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• Line 17
• Open FORM
• Define FORM attributes
• Specify the action
• IMG SRC specify the path to display the image
• Line 23-30
• Define form INPUT elements
• TYPE text inserts a one-line text box
• NAME provides a unique identification for INPUT
  element
• Line 33
• Specify correct input format

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• Line 55-64
• Radio buttons
• similar in function and usage to checkboxes
• Only one radio button in a group can be selected

35
3.4 Perl Samples Form Processing and Business
Logic

• Script Output

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3.4 Perl Samples Form Processing and Business
Logic

• CGI Scripts written in Perl
• Retrieving data from form output
• Assign to variables
• Example Assign data from form INPUT OS to
  variable OS
• os param(OS)
• Testing for correct form input
• Example Make sure phone number in format
  [phone]
• if ( phone / \( \d3 \) \d3 - \d4 /x)
  actions
• dn tests for n digits
• \ is escape character
• Close-bracket ()) character is used in Perl
  statements, needs escape character \ to appear
  as part of search test string

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• Line 2
• Use (include) standard CGI library
• Line 3-8
• Assign form field values to variables
• param() is part of the Perl CGI module that
  retrieves values from a form fields value

1 !/usr/local/bin/perl 2 use CGI qw
(standard) 3 os param( "os" ) 4
firstName param( "fname" ) 5 lastName
param( "lname" ) 6 email param( "email" ) 7
phone param( "phone" ) 8 book param(
"book" ) 9 dtd 10 "-//W3C//DTD XHTML 1.0
Transitional//EN\" 11 \"http//www.w3.org/TR/xht
ml1/DTD/xhtml1-transitional.dtd" 12 print(
header() ) 13 print( start_html( dtd
dtd, 14 title "Form
Results" ) )
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• Line 15
• Test for correct phone number input form using
  if structure
• Indicate actions to be performed if test returns
  TRUE result
• check if phone no. has the format (555)-555-5555
• \d3 means 3 digits
• ensure that there are no extra characters
  in front or at the end
• Line 22
• br() function adds a break (ie ,
  )
• Line 24
• span() function adds a tag

15 if ( phone / \( \d3 \) \d3 - \d4
/x ) 16 print( "Hi " ) 17 print( span(
style "color blue font-weight bold" , 18
firstName ) ) 19 print( "!"
) 20 21 print( "\nThank you for completing the
survey." ) 22 print( br(), "You have been added
to the " ) 23 24 print( span( style
"color blue font-weight bold" , 25
book ) ) 26 print( " mailing list.",
br(), br() ) 27 28 print( span( style
"font-weight bold" , 29 "The
following information has 30
been saved in our database " ), br() )
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31 print( table( 32 Tr( th( style
"background-color ee82ee" , 33
"Name" ), 34 th( style
"background-color 9370db" , 35
"E-mail" ), 36 th( style
"background-color 4169e1" , 37
"Phone" ), 38 th( style
"background-color 40e0d0" , 39
"OS" ) ), 40 41 Tr( style
"background-color c0c0c0" , 42
td( "firstName lastName" ), 43 td(
email ), 44 td( phone ), 45
td( os ) ) ) )

• Line 32
• Tr() and th() are CGI functions that place
  arguments between Table row and table header tags
• (ref, p 923, Deitel)
• Line 42
• td() is a CGI function that prints tags

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46 print( br() ) 47 print( div( style
"font-size x-small" , 48 "This is
only a sample form. You have not been 49
added to a mailing list." ) ) 50 51 else
52 print( div( style "color red
font-size x-large" , 53 "INVALID
PHONE NUMBER" ), br() ) 54 print( "A valid
phone number must be in the form " ) 55 print(
span( style "font-weight bold" , 56
"[phone]." ) ) 57 print( div(
style "color blue" , 58 "Click
the Back button, and enter a 59
valid phone number and resubmit." ) ) 60
print( br(), br() ) 61 print( "Thank you."
) 62 print( end_html() )

• Line 47
• div() is a CGI function that generates a
  tag
• Line 51
• Set actions to be performed if the if structure
  returns a FALSE value
• Line 51
• end_html() returns the closing tags for the page
  ( and )

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3.4 Perl Samples Form Processing and Business
Logic

• Script Output 1 If phone number is valid

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3.4 Perl Samples Form Processing and Business
Logic

• Script Output 2 If phone number is invalid

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4. SSI

• SSI Server-Side Include
• Commands embedded in HTML documents
• Provide for content creation
• Allow inclusion of current time, date or even
  contents of different html document
• Cf. Original CGI is executed by client commands.
• http//www.mydomain.com/cgi-bin/
• With SSI, the commands to execute CGI programs
  are embedded in HTML scripts.

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4. SSI

• SSI commands
• Execute CGI scripts on a server
• Are capable of connecting to an ODBC data source
• Use to create customized Web pages depending for
  certain conditions
• Document containing SSI commands has .shtml file
  extension
• EXEC CGI command
• Issued to execute a Perl script before document
  sent to client
• Example
• Executes the Perl script counter.pl, located in
  the cgi-bin directory

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4. SSI

• ECHO command
• Used to display variable information
• Is followed by the keyword VAR and variables
  constant name
• Example
• Returns the current local time
• Other variables
• DATE_GMT
• Contains current Greenwich Mean Time
• DOCUMENT_NAME
• Contains name of current document
• Many more

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4. SSI Samples

• Common usage
• For tracking clients
• Where client coming from
• What client views on your site
• Where client goes after your site
• Tracking Web data important, allows Web masters
  to
• Know which sites visited most frequently
• Know how effective advertisements and products are

47

• Line 14
• Execute Perl script counter.pl using EXEC CGI
  statement
• Line 18
• Use ECHO VAR statements to display environmental
  variables

48

• Continue printing environmental variables using
  ECHO VAR statements

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4. SSI Samples

• The Perl Script counter.pl

• Line 5-8
• Open counter.dat, assign to filehandle COUNTREAD
• read 1 line from file referred by filehandler
  COUNTEREAD
• Increment data in COUNTREAD
• Close COUNTREAD
• Line 9-11
• means write mode, open counter.dat for
  writing, referred to by filehandler COUNTERWRITE
• print indicates the filehandler COUNTERWRITE
  where data is written

1 !/usr/local/bin/perl 2 Program to track the
number of times a Web page has been accessed. 3
counter.cgi 4 use CGI qw( standard ) 5 open(
COUNTREAD, "counter.dat" ) 6 data
7 data 8 close( COUNTREAD ) 9
open( COUNTWRITE, "counter.dat" ) 10 print(
COUNTWRITE data ) 11 close( COUNTWRITE ) 12
print( header(), "center font-weight bold\"" ) 13 print( "You
are visitor number", br() ) 14 for ( count
0 count number substr( data, count, 1 ) 17 print(
img( src "number.gif" ), "\n" ) 18 19
print( "" )
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4. SSI Samples

• Perl script uses built-in functions
• open() and close() function
• to open and close a file
• print() function
• to redirect output to a file
• substr( x, y, z ) function
• Similar to JavaScripts substr function
• First argument (x)
• Specifies string from which to take a substring
• Second argument (y)
• Specifies offset in characters from beginning of
  the string
• Third argument (z)
• Specifies length of substring to return

51
4. SSI Samples

• Script output

52
Further Readings

• Note This topic is designed with the objective
  of providing an introduction to CGI, Perl and
  SSI.
• Advanced features of CGI, Perl and SSI are beyond
  the scope of this course and will NOT be taught
  or discussed. Students who wish to invest more
  time on studying advanced features and topics of
  CGI, Perl and SSI are referred to the following
  resources
• Deitel Chapter 27
• http//hoohoo.ncsa.uiuc.edu/cgi/overview.html -
  CGI info
• http//hoohoo.ncsa.uiuc.edu/docs/tutorials/include
  s.html - SSI info

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Further Readings

• Students who wish to invest more time on studying
  Web servers are referred to the following
  resources
• Deitel Chapter 24
• Apache
• http//Apache-Server.Com/tutorials/