1. A Guide to Unix Using Linux Fourth 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
Access disk files in Perl
A Guide to Unix Using Linux, Fourth Edition

3. Objectives (continued)
Use Perl to sort information
Set up a simple HTML Web page
Understand how Perl and CGI are used for creating Web pages
A Guide to Unix Using Linux, Fourth Edition

4. Introduction to Perl Perl: Practical Extraction and Report Language
Free script language
Runs on many operating systems
Examples: UNIX, Linux, Windows, Mac OS X
Manipulates text, displays output, handles mathematical processes, and works with files
Generating reports
Used for Web programming
Released by Larry Wall in 1987
Interpreted language
Interpreter: /usr/bin/perl
A Guide to Unix Using Linux, Fourth Edition

5. Introduction to Perl (continued)
#!/usr/bin/perl
# Program name: example1.pl
print("This is a simple\n");
print("Perl program.\n");
A Guide to Unix Using Linux, Fourth Edition

6. Introduction to Perl (continued)
The following script uses a variable:
Output:
#!/usr/bin/perl
# Program name: example2.pl
$name = "Charlie";
print ("Greetings $name\n");
~]$ ./example2.pl
Greetings Charlie
A Guide to Unix Using Linux, Fourth Edition

7. Introduction to Perl (continued)
#!/usr/bin/perl
# Program name: example3.pl
print ("Enter a number: ");
$number = <STDIN>;
print ("You entered $number\n");
A Guide to Unix Using Linux, Fourth Edition

8. Introduction to Perl (continued)
#!/usr/bin/perl
# Program name: example4.pl
print ("Enter a number: ");
$number = <STDIN>;
if ($number == 10) {
print ("That is the number I was thinking of.\n"); }
else
print ("You entered $number\n");
A Guide to Unix Using Linux, Fourth Edition

9. Introduction to Perl (continued)
A Guide to Unix Using Linux, Fourth Edition

10. Introduction to Perl (continued)
A Guide to Unix Using Linux, Fourth Edition

11. Introduction to Perl (continued)
Perl offers the if-else statement:
Output:
#!/usr/bin/perl
# Program name: example5.pl
$my_name = "Ellen";
$your_name = "Charlie";
if ($my_name eq $your_name) {
print ("Your name is the same as mine.\n"); }
else
print ("Hello. My name is $my_name\n");
~]$ ./example5.pl
Hello. My name is Ellen
A Guide to Unix Using Linux, Fourth Edition

12. Introduction to Perl (continued)
Perl provides standard arithmetic operators:
Output:
#!/usr/bin/perl
# Program name: example6.pl
$num1 = 10;
$num2 = 50;
$num3 = 12;
$average = ($num1 + $num2 + $num3) / 3;
print ("The average is $average\n");
~]$ ./example6.pl
The average is 24
A Guide to Unix Using Linux, Fourth Edition

13. Identifying Data Types
In Perl, data can be represented in a variety of ways
Variables and constants
Scalars
Numbers
Strings
Arrays
Hashes
A Guide to Unix Using Linux, Fourth Edition

14. Variables and Constants
Variables: symbolic names that represent values stored in memory
Examples:
$x might hold the value 100
$name might hold the sequence of characters Charlie
Value can change while program runs
Constants do not change value as the program runs
Written into the program code itself
Example: $num =
A Guide to Unix Using Linux, Fourth Edition

15. Scalars Scalar: simple variable that holds a number or string
Name begins with a $
Examples:
$x = 12;
$name = "Jill";
$pay = ;
A Guide to Unix Using Linux, Fourth Edition

16. Numbers Numbers are stored inside the computer as:
Signed integers (e.g., 14321)
Double-precision, floating-point values (e.g., 56.85)
Numeric literals are integers or floating-point values
Perl uses underscore (_) to improve legibility:
Example: 5_456_678_901
Only works within literal numbers specified in a program
0x used to express hexadecimal constants
A Guide to Unix Using Linux, Fourth Edition

17. Strings Strings are sequences of any types of characters
Delimited by ‘’ or “”
Single-quoted strings are not subject to interpolation
Except for \' and \\
In double-quoted strings, variables are interpolated
Backslash (\) used to ensure variable or control character is not interpolated
A Guide to Unix Using Linux, Fourth Edition

18. Strings (continued)
A Guide to Unix Using Linux, Fourth Edition

19. A Guide to Unix Using Linux, Fourth Edition
#!/usr/bin/perl
# Program name: example7.pl
print ("\\words\\separated\\by\\slashes\n");
print ("This is a \"quote\"\n");
print ("\Uupper case\n");
print ("\LLOWER CASE\n");
A Guide to Unix Using Linux, Fourth Edition

20. Arrays
Arrays are variables that store an ordered list of scalar values
Elements are accessed with numeric subscripts
Starting at zero
Elements are usually of the same data type
An “at” sign precedes the name of an array when assigning it values
$ character used when processing the individual elements
A Guide to Unix Using Linux, Fourth Edition

21. A Guide to Unix Using Linux, Fourth Edition
#!/usr/bin/perl
# Program name: example8.pl
@pets = ("dog", "cat", "parrot", "hamster" );
print ("My pets are:\n");
print ("$pets[0]\n");
print ("$pets[1]\n");
print ("$pets[2]\n");
print ("$pets[3]\n");
A Guide to Unix Using Linux, Fourth Edition

22. Hashes Hash: variable that represents a set of key/value pairs
Hash variables are preceded by % when they are assigned values
%animals = (’Tigers’, 10, ’Lions’, 20, ’Bears’, 30);
%animals = (Tigers ==> 10, Lions ==> 20, Bears ==> 30);
To refer to an element, use $ before the variable name followed by the key in {}
$animals{’Bears’}
A Guide to Unix Using Linux, Fourth Edition

23. Hashes (continued) Example: Output:
#!/usr/bin/perl
# Program name: example9.pl
%animals = (’Tigers’, 10, ’Lions’, 20, ’Bears’, 30);
print ("The animal values are:\n");
print ("$animals{’Tigers’}\n");
print ("$animals{’Lions’}\n");
print ("$animals{’Bears’}\n");
~]$ ./example9.pl
The animal values are: 10 20 30
A Guide to Unix Using Linux, Fourth Edition

24. Perl Versus the Awk Program
Unlike Perl, Awk does not require programmer to explicitly set up looping structures
Uses fewer lines of code to resolve pattern-matching extractions than Perl
Similarities:
Perl and Awk use # to specify a comment line
Pattern-matching code is the same in both programs
Both are portable across many UNIX/Linux systems
A Guide to Unix Using Linux, Fourth Edition

25. A Guide to Unix Using Linux, Fourth Edition
#!/usr/bin/awk -f
# Program name: awkcom.a
# Purpose: Count the comment lines in a file.
# Enter the file name on the command line.
END {
print "The file has ", line_count, " comment lines." }
/^#/ && !/^#!/ { ++line_count } # This occurs for
every line.
A Guide to Unix Using Linux, Fourth Edition

26. <> is the diamond operator
#!/usr/bin/perl
# Program name: perlcom.pl
# Purpose: Count the source file’s comment lines
# ==============================================
$filein = $ARGV[0];
while (<>) {
if (/^#/ && !/^#!/)
++$line_count }
print ("File \"$filein\" has $line_count comment lines. \n");
<> is the diamond operator
A Guide to Unix Using Linux, Fourth Edition

27. How Perl Accesses Disk Files
Perl uses filehandles to reference files
Filehandle: name for an I/O connection between Perl program and OS
Used to open, read, write, and close the file
Convention: use all uppercase letters for filehandles
Must issue an open before you can access file
Exception: when file name passed through ARGV[0]
Three standard filehandles in Perl:
STDIN, STDOUT, and STDERR
A Guide to Unix Using Linux, Fourth Edition

28. A Guide to Unix Using Linux, Fourth Edition
#!/usr/bin/perl
# Program name: perlread1.pl
# Purpose: Display records in a file and count lines
$filein = $ARGV[0];
while (<>) {
print "$_";
++$line_count; }
print ("File \"$filein\" has $line_count lines. \n");
#!/usr/bin/perl
# Program name: perlread2.pl
# Purpose: Open disk file. Read and display the records
# in the file. Count the number of records in
# the file.
open (FILEIN, "students") || warn "Could not open
students file\n";
while (<FILEIN>) {
print "$_";
++$line_count; }
print ("File \"students\" has $line_count lines. \n");
close (FILEIN);
A Guide to Unix Using Linux, Fourth Edition

29. Using Perl to Sort Perl provides a powerful and flexible sort operator
Can sort string or numeric data
In ascending or descending order
Allows advanced sorting
Define your own sorting routine
A Guide to Unix Using Linux, Fourth Edition

30. Using Perl to Sort Alphanumeric Fields
Perl can be used to sort information
Example:
#!/usr/bin/perl
# Program name: perlsort1.pl
# Purpose: Sort a list of names contained inside an array
# Syntax: perlsort1.pl <Enter>
#========================================================
@somelist = ("Oranges", "Apples", "Tangerines", "Pears",
"Bananas", "Pineapples");
@sortedlist =
print
print"\n";
A Guide to Unix Using Linux, Fourth Edition

31. Using Perl to Sort Alphanumeric Fields (continued)
Another example:
#!/usr/bin/perl
# Program name: perlsort2.pl
# Purpose: Sorts a text file alphabetically. File name is
# entered on the command line.
# Syntax: perlsort2.pl file name <Enter>
#========================================================
$x = 0;
while (<>) {
$somelist[$x] = $_;
$x++; }
A Guide to Unix Using Linux, Fourth Edition

32. Using Perl to Sort Numeric Fields
For numeric fields, you can define a subroutine with the comparison conditions
sub numbers {
if ($a < $b) { -1; }
elsif ($a == $b) { 0; }
else { +1; } }
$sortednumbers = sort numbers 101, 87, 34, 12, 1, 76;
A Guide to Unix Using Linux, Fourth Edition

33. Using Perl to Sort Numeric Fields (continued)
Perl has a special operator for numeric sorts: <=>
Spaceship operator
The inline sort block is even more compact
sub numbers {
$a <=> $b; }
$sortednumbers = sort numbers 101, 87, 34, 12, 1, 76;
@sortednumbers = sort { $a <=> $b;
A Guide to Unix Using Linux, Fourth Edition

34. A Guide to Unix Using Linux, Fourth Edition
#!/usr/bin/perl
# Program name: perlsort3.pl
# Purpose: Sorts numerically using a subroutine. File name
# is entered on the command line.
# Syntax: perlsort3.pl file name <Enter>
#========================================================
$x = 0;
while (<>) {
$somelist[$x] = $_;
$x++; }
@sortedlist = sort
sub numbers
if ($a < $b)
{ -1; }
elsif ($a == $b)
{ 0; }
else
{ +1; }
A Guide to Unix Using Linux, Fourth Edition

35. Using Perl to Sort Numeric Fields (continued)
A Guide to Unix Using Linux, Fourth Edition

36. Using Perl to Sort Numeric Fields (continued)
#!/usr/bin/perl
# Program name: perlsort4.pl
# Purpose: Sort numerically using the spaceship operator
# (<=>)
# syntax: perlsort4.pl file name <Enter>
#========================================================
$x = 0;
while (<>) {
$somelist[$x] = $_;
$x++; }
@sortedlist = sort
sub numbers
$a <=> $b;
A Guide to Unix Using Linux, Fourth Edition

37. Setting Up a Web Page Create a Web page using HTML
HTML: Hypertext Markup Language
Format for creating documents with embedded tags
Tags give the document special properties and let you place hyperlinks in a document
Publish a Web page on a Web server
Experiment with and test HTML documents using your system’s loopback address
or localhost
A Guide to Unix Using Linux, Fourth Edition

38. Creating a Simple Web Page
To create Web pages:
Use a visual HTML editor
Examples: Adobe Dreamweaver, Microsoft Expression Web
“What you see is what you get”
Use a text editor
Examples: vi, Emacs
Just type text and the desired embedded tags
A Guide to Unix Using Linux, Fourth Edition

39. A Guide to Unix Using Linux, Fourth Edition
<HTML>
<HEAD><TITLE>My Simple Web Page</TITLE></HEAD>
<BODY>
<H1>Just a Simple Web Page</H1>
This is a Web page with no frills!
</BODY>
</HTML>
A Guide to Unix Using Linux, Fourth Edition

40. Creating a Simple Web Page (continued)
<HTML>
<HEAD><TITLE>UNIX/Linux Programming Tools</TITLE></HEAD>
<BODY>
<H1><CENTER>My UNIX/Linux Programming Tools</CENTER></H1>
<H2>Languages</H2>
<P>Perl</P>
<P>Shell Scripts</P>
<P>C and C++</P>
<H2>Editors</H2>
<P>vi</P>
<P>Emacs</P>
<H2>Other Tools</H2>
<P>awk</P>
<P>sed</P>
</BODY>
</HTML>
A Guide to Unix Using Linux, Fourth Edition

41. CGI Overview
Perl is the most commonly used language for Common Gateway Interface (CGI) programming
CGI is a protocol governing how browsers and servers communicate
Exchanging and processing a form containing information typically involves:
Using CGI for communication between the client’s Web browser and the Web server
A program that can be executed
Often a Perl script or a program written in C
Often stored in cgi-bin subdirectory on Web server
A Guide to Unix Using Linux, Fourth Edition

42. CGI Overview (continued)
Programs in cgi-bin are set up to have executable permissions
Also typically have r permissions so client can view associated Web page
To allow HTML document to accept input, precede input area with a description:
Total Cost? <INPUT TYPE=text NAME=cost SIZE=10>
Some links of interest:
A Guide to Unix Using Linux, Fourth Edition

43. Summary Perl is a scripting language
Data types: variables, constants, scalars, strings, arrays, and hashes
Perl and Awk are both powerful processing languages that function in different ways
Perl uses filehandles for the I/O connection between a file and Perl
In Perl, use <=> operator for numeric sorts
HTML is used to format text in a Web page
CGI is a protocol that governs how browsers and servers communicate
A Guide to Unix Using Linux, Fourth Edition

44. Command Summary
A Guide to Unix Using Linux, Fourth Edition