Transcript Perl and CGI Programming

A Guide to Unix Using Linux
Fourth Edition
Chapter 9
Perl and CGI Programming
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
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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
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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
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Introduction to Perl (continued)
#!/usr/bin/perl
# Program name: example1.pl
print("This is a simple\n");
print("Perl program.\n");
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Introduction to Perl (continued)
• The following script uses a variable:
#!/usr/bin/perl
# Program name: example2.pl
$name = "Charlie";
print ("Greetings $name\n");
• Output:
[ellen@localhost ~]$ ./example2.pl
Greetings Charlie
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Introduction to Perl (continued)
#!/usr/bin/perl
# Program name: example3.pl
print ("Enter a number: ");
$number = <STDIN>;
print ("You entered $number\n");
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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");
}
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Introduction to Perl (continued)
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Introduction to Perl (continued)
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Introduction to Perl (continued)
• Perl offers the if-else statement:
#!/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 $my_name\n");
}
• Output:
[ellen@localhost ~]$ ./example5.pl
Hello. My name is Ellen
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Introduction to Perl (continued)
• Perl provides standard arithmetic operators:
#!/usr/bin/perl
# Program name: example6.pl
$num1 = 10;
$num2 = 50;
$num3 = 12;
$average = ($num1 + $num2 + $num3) / 3;
print ("The average is $average\n");
• Output:
[ellen@localhost ~]$ ./example6.pl
The average is 24
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Identifying Data Types
• In Perl, data can be represented in a variety of
ways
–
–
–
–
–
–
Variables and constants
Scalars
Numbers
Strings
Arrays
Hashes
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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 = 127.89
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Scalars
• Scalar: simple variable that holds a number or
string
– Name begins with a $
• Examples:
– $x = 12;
– $name = "Jill";
– $pay = 12456.89;
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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
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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
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Strings (continued)
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#!/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");
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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
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#!/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");
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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 {}
$animals{’Bears’}
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Hashes (continued)
• Example:
#!/usr/bin/perl
# Program name: example9.pl
%animals = (’Tigers’, 10, ’Lions’, 20, ’Bears’,
30);
print ("The animal values are:\n");
print ("$animals{’Tigers’}\n");
print ("$animals{’Lions’}\n");
print ("$animals{’Bears’}\n");
• Output:
[ellen@localhost ~]$ ./example9.pl
The animal values are:
10
20
30
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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
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#!/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.
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#!/usr/bin/perl
# Program name: perlcom.pl
# Purpose: Count the source file’s comment lines
# ==============================================
$filein = $ARGV[0];
while (<>)
<> is the diamond operator
{
if (/^#/ && !/^#!/)
{
++$line_count
}
}
print ("File \"$filein\" has $line_count comment lines. \n");
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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
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#!/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);
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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
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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 = sort @somelist;
print "@sortedlist";
print"\n";
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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] = $_;
$x++;
}
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Using Perl to Sort Numeric Fields
• For numeric fields, you can define a subroutine
with the comparison conditions
sub numbers
{
if ($a < $b) { -1; }
elsif ($a == $b) { 0; }
else { +1; }
}
$sortednumbers = sort numbers 101, 87, 34, 12, 1, 76;
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Using Perl to Sort Numeric Fields
(continued)
• Perl has a special operator for numeric sorts: <=>
– Spaceship operator
sub numbers
{
$a <=> $b;
}
$sortednumbers = sort numbers 101, 87, 34, 12, 1, 76;
• The inline sort block is even more compact
@sortednumbers = sort { $a <=> $b; } @numberlist;
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#!/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] = $_;
$x++;
}
@sortedlist = sort numbers @somelist;
print @sortedlist;
sub numbers
{
if ($a < $b)
{ -1; }
elsif ($a == $b)
{ 0; }
else
{ +1; }
}
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Using Perl to Sort Numeric Fields
(continued)
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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 numbers @somelist;
print @sortedlist;
sub numbers
{
$a <=> $b;
}
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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
– 127.0.0.1 or localhost
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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
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<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>
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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>
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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
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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:
– www.scriptarchive.com
– www.perl.com
– www.perlaccess.com
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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
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Command Summary
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