Perl, CGI and DBI

Ashley George

ageorge@cs.dal.ca

March 7th, 2005

Overview

• Each of the following will be presented with explanations and brief exercises:– Perl usage, syntax, semantics, subroutines– CGI and Apache– DBI and Oracle

Perl In Brief

• Using the Perl Interpreter

• A Perl Program

• Elements of Perl– Syntax, Data Structures, Subroutines and Refs

• Perl Library and Builtin Functions

• perldoc, perl manpages

Tutorial Files

• All the programs seen in this tutorial are available on the web

http://www.cs.dal.ca/~ageorge/4173/

• and in the following directory on torch

/users/grad/ageorge/4173/

• Three subdirectories containing perl scripts:– "perllang", "CGI", "DBI"

Log into Torch…

$ mkdir ~/4173tutorial

$ cd 4173tutorial

$ cp ~ageorge/4173tut.tar.gz .

$ gtar zxvf 4173tut.tar.gz

$ cd perllang; ls (and see: )

helloworld.pl insertsort.pl …

• (use more/less/pico/vim/emacs to view/edit!)

Using the Perl Interpreter

• The interpreter is invoked from the command line on torch.cs.dal.ca.

• Use the syntax

perl -w filename

where filename is the name of a text file containing your Perl program.

• (Typically filename.pl or filename.cgi )

A Very Short Perl Program

print("Hello world!\n");

http://www.cs.dal.ca/~ageorge/4173/perllang/helloworld.pl

A Slightly Longer Perl Program@nums = (5, 2, 7, 4, 8, 9);

for ($j = 1; $j < @nums; $j++) {

$key = $nums[$j];

$i = $j - 1;

while($i >= 0 && $nums[$i] > $key) {

$nums[$i+1] = $nums[$i];

$i = $i - 1;

}

$nums[$i+1] = $key;

}

print("@nums\n"); http://www.cs.dal.ca/~ageorge/4173/perllang/insertsort.pl

Almost the Same, But Different

@list = (5, 2, 7, 4, 8, 9);

@sorted = shift(@list);

for (@list) {

$i = 0;

while ( $i < @sorted &&

$sorted[$i] < $_ ) { $i++; }

splice(@sorted, $i, 0, $_);

}

print("@sorted\n");http://www.cs.dal.ca/~ageorge/4173/perllang/insertsort2.pl

What are those things?

• Scalar variables (scalar: storing a single value)$identifier

$arrayidentifier[$index]

$assocarrayidentifier{$key}

• Arrays and Associative Arrays@arrayidentifier

@arrayidentifier[$slice0, $slice1, …$sliceN]

%assocarrayidentifier

How do I use 'em?

• Declare arrays:@array = ($scalar1, $scalar2, $scalar3, … $scalarN);

• Access array elements:$array[$index] = $scalarvalue;

• Declare associative arrays:%hash = (key1, val1, key2, val2, key3, val3);

• Access associative array elements:$hash{$key} = $scalarvalue;

How do I use 'em? (cont'd)

• There are many operators– +, -, /, *, &&, ||, ==, !=, <, >

• Many of those are familiar to you..– also see perlop man page

•eq, lt, gt, ne, ?:, ., x, **

• You can also pass scalar variables, arrays and associative arrays into subroutines.

Literals and Scalar Variables

$i = "abcdefg"; # strings

$j = 0.78; # real numbers

$k = 255; # decimal integers

$l = 0377; # octal integers

$m = 0x00ff; # hexadecimal integers

$n = 2.55E+2; # also

# interpreted quotes!

print("$i $j $k $l $m $n\n");

$o = $l + $m;

print("$o\n");

http://www.cs.dal.ca/~ageorge/4173/perllang/variables.pl

Arrays and Array Elements@names =

("Zhiyong", "Jamie", 52, "Ashley", "Helmut");

# if you have @array, use $array[$index]

print("$names[2]\n\n"); # prints "52\n\n"

# sort them lexically

@sortednames = sort(@names);

# for each element in sorted names, print...

for $element (@sortednames) {

print("$element\n");

}

http://www.cs.dal.ca/~ageorge/4173/perllang/lists.pl

Associative Arrays%employees = ( "Bob" => 7.50,

"Martin" => 8.50,

"Stevie" => 6.75

);

# Cost for Bob and Martin per hour

print( $employees{"Bob"} +

$employees{"Martin"} .

"\n"); # prints "16\n"

@names = keys(%employees);

print("@names\n");

http://www.cs.dal.ca/~ageorge/4173/perllang/associativearrays.pl

Control flow statements

• for, while, if/elsif/else, until, unless, etc… see perlsyn for details. (man perlsyn or see the webpage…)

• Note that blocks are required -- you cannot omit braces { } anywhere.

Control Flow (cont'd)

• An interesting feature:

for (@array) {

# now $_ has each element

...

}• At each iteration of the loop, $_ will contain

the next item in the array.

Subroutines

• Syntax:

sub identifier {

statements…

}

• Usage (for the above subroutine):

identifier()

Subroutines and Parameters

• At the beginning of any subroutine, a special array called @_ ("at underscore") contains all the parameters passed to a subroutine.

• Example on following slide– subroutine declarations can follow code which

invokes them.

Subroutinessortandprintnames("Zhiyong", "Ashley",

"Jamie", "Helmut");

print(factorial(5) . "\n"); # prints 120

sub sortandprintnames { for (sort(@_)){ print("$_\n"); }

}

sub factorial { my $n = shift(@_); # get one parameter if ($n == 0) { return 1; } return $n * factorial($n-1);}

http://www.cs.dal.ca/~ageorge/4173/perllang/subroutines.pl

Naming Your Parameters

• You can assign @_ to an array of named scalars:

sub namedparams {

my ($p1, $p2, $p3) = @_;

etc etc etc

}

• Of course, use whatever names you please.

Returning

• You can also return scalars, arrays and associative arrays from a subroutine.return $value;

return @list;

return %hash;

References

• What about …– passing more than one data structure?– returning more than one data structure?

• You might initially trysub myroutine {

my (@array1, %array2) = @_;

… # doesn't work.

}

References (cont'd)

• You can take a reference to any data structure by prefixing it with a backslash ("\") (ie, $ref = \@array; )

• You can dereference a reference by prefixing it with the appropriate symbol: $, @ or %.– (ie, @array = @$ref; )

Incorrect attempt@names = ("Zhiyong", "Jamie", 52, "Ashley", "Helmut");

%classes = ( "4173" => "Thursday", "3171" => "Tuesday", "1101" => "Friday" );

printstuff(@names, %classes); # no good..

sub printstuff { my (@names2, %classes2) = @_; print("@names2\n");

my @keys = keys(%classes2); print("@keys\n"); # empty! }

http://www.cs.dal.ca/~ageorge/4173/perllang/subproblems.pl

Fixed with References.@names =("Zhiyong", "Jamie", 52, "Ashley", "Helmut");

%classes = ( "4173" => "Thursday", "3171" => "Tuesday", "1101" => "Friday" );

printstuff(\@names, \%classes);

sub printstuff { my ($names2, $classes2) = @_; print("@$names2\n"); my @keys = keys(%$classes2); print("@keys\n");}

http://www.cs.dal.ca/~ageorge/4173/perllang/subproblemsfixed.pl

Library Functions

• Perl has many builtin functions that are useful for manipulating scalars and arrays.

• The next few slides list some of the commonly used functions… you can look them up as necessary usingperldoc -f functionname

• Or, you can look at – http://www.perl.com/doc/manual/html/

Scalar String Functions

chomp: remove a newline from a string

lc (uc): get the lowercase (uppercase) version of a string

length: get the length of a string

substr: get substrings of a string

split: split a string into substrings ("tokens")

Useful Array Functions

• pop: removes and returns last value

• push: adds a new last value

• shift: removes and returns first value

• unshift: adds a new first value

• splice: inserts (and possibly overwrites) a list of elements into an array

• sort: mutable sort function - flexible!

I/O Functions• open, close

open FILE, "file.txt";

• read, write (low-level I/O)

• Use the < > operator to read lines.while (<FILE>) {

# now $_ contains the next line

# (might want to chomp, split…)

…

}

Review Exercise

• Merge two sorted arrays– Write a subroutine that takes two sorted arrays

as its parameters by reference.– Merge the two arrays, maintaining sorted order

and return the result.

• Try using push, shift in a loop.

• You can append arrays like this:@newarray = (@array1, @array2);

Merging Two Sorted Arrayssub merge {

my ($ref1, $ref2) = @_;

my @ar1 = @$ref1; my @ar2 = @$ref2;

my @ar3 = ();

while ( @ar1 > 0 && @ar2 > 0 ) {

if ($ar1[0] <= $ar2[0]) {

push @ar3, shift @ar1;}

elsif ($ar1[0] > $ar2[0]) {

push @ar3, shift @ar2;}

} @ar3 = (@ar3, @ar1); @ar3 = (@ar3, @ar2);

return @ar3;

} http://www.cs.dal.ca/~ageorge/4173/perllang/merge.pl

CGI, Perl, Apache

• CGI: Common Gateway Interface

• A standard interface for programs running on a webserver to receive data from a user.

• Specifies a set of environment variables

• A CGI program retrieves these values, parses them and acts on the input, typically a set of key-value pairs.

Preparing cgi-bin on Torch

$ cd ~/public_html

$ mkdir cgi-bin

$ chmod a+rwx,og-rw cgi-bin

Using Perl with CGI

• In order to use Perl scripts with CGI, you must place the script in ~/public_html/cgi-bin

• Note that:– At the top of your perl script, the very first line

MUST be:#!/usr/bin/perl -w

– Your script must be called filename.cgi– also, do chmod u+x filename.cgi

Invoking your CGI Program• If you called your Perl script

test.cgi

• You would run it by entering into a web browser the following URL (with your username)http://www.cs.dal.ca/~username/cgi-bin/test.cgi

• The web server on torch will process your script and report failure or print your script's output.

More CGI Caveats

• Any calls to Perl's print function will be printed to the user's browser.

• Furthermore, you must callprint "Content-type: text/html\n\n";

before you make any other calls to print.

CGI Exercise

• Can you make a simple CGI script that printsHello world!

in the user's browser?• Remember to place the script in ~/public_html/cgi-bin.

• It should be named filename.cgi• chmod u+x filename.cgi

Hello World!

#!/usr/bin/perl

print "Content-type: text/html\n\n";

print "Hello world!\n";

Printing Here Docs

• Sometimes you want to print something fairly long… like an HTML document.

• You can use Here docs for this purpose.print << EOF

this is a lot of text that will be printed out

blah blah blah… text… lots of it.

even more text..

EOF

Here Docs

• One neat thing about here docs is using them to print out HTML incorporating Perl variables.

• Just as in a regular print statement, Perl variables can be embedded in here docs.

Example#!/usr/bin/perl

$sum = 2 + 2;

print <<EOFContent-type: text/html

<html><head><title>The value of 2 and 2...</title></head><body><p>The value of 2 + 2 is: $sum</p></body></html>EOF

CGI Input

• So now you could– generate web pages dynamically– perform any calculation or operation

• But how do you handle user input?

• You need to parse CGI variables.

• These are passed to you in environment variables accessible through %ENV.

• It's a nuisance.

CGI.pm

• A feature-filled Perl module that handles the details of CGI for you.

• Parses CGI environment variables

• Handles POST and GET methods of HTML form submission

• More...

An HTML Form<html>

<head><title>Name Collection</title>

</head>

<body>

<form method="GET" action="http://www.cs.dal.ca/~ageorge/cgi-bin/test.cgi">

<p>What is your name?</p>

<p><input name="name" size="30"></p>

<input type=submit>

</form>

</body>

</html>

http://www.cs.dal.ca/~ageorge/4173/CGI/input.html

A Script Processing That Form's Input

#!/opt/bin/perl

use CGI qw(:standard);

print "Content-type: text/html\n\n";

print "Hello world!\n";

$name = param('name');

print "Your name is $name\n";