Revision as of 00:20, 19 July 2012

Practical Extraction and Report Language is the oldest of the interpreted languages, python being its 3 years younger sibling. The perl interpreter is written in C, a compiled language.

Perl is flexible and can be used to write web applications, command line applications, or services.

Basics

Development Environment

To develop in perl you will need only a perl interpreter and a text editor. For those of you who find un highlighted perl, there are a variety of windows & linux text editors with syntax highlighting support.

Windows:

• notepad++
• cygwin's vim implementation
• gvim for windows

Linux:

• vim
• nano
• emacs
• geany
• gedit

Linux & Unix

On most distributions, perl and cpan come bundled by default. In the case it is not, a simple apt-get, emerge, yum install, pacman, or any other package manager should install it quickly. You can determine if perl is installed by typing `which perl' at the bash command line. If a filename is returned, you're good to go.

Windows

You can do everything we're going over by installing perl on cygwin. CYGWIN is available at http://www.cygwin.com/install.html

 For compilation to .exe, we recommend "pp", you can install this by `typing cpan -i pp' from your cygwin shell.

There is also a perl implementation for Windows written by activestate, searching for "activestate perl" in any search engine will find it.

CPAN

CPAN is the module and package installer for perl. It can be accessed on most distributions simply by typing `cpan'. On windows, you can access it by typing `cpan' in your CYGWIN shell. Note: If `cpan' does not work, try `perl -e 'shell' -mCPAN'. If this does not work, your installation may be broken.

Your first program

Code

To run this code, you'll only need to put it in a text file. Save it as "hello.pl", and then you can execute the following to run it from either cygwin or bash:

• chmod +x hello.pl
• ./hello.pl

Alternatively you can simply type:

• perl hello.pl

Analysis

The shebang declares the location of the code's interpreter. I.e. if you're writing bash, you'll need to put:

 #!/bin/bash

at the top of your file. In perl, it's typically:

 #!/usr/bin/perl

This should be the first line in any perl you write. You can also use:

 #!env perl

If you are unsure of the path and you have it in your environment variables. If for some reason `#!env perl' and `#!/usr/bin/perl' do not work, running `which perl' from the bash command line will return the proper path.

This is only required if you want to directly execute your script (i.e. ./script.pl). If you get permissions errors when attempting this, you can execute it via `perl script.pl' or running `chmod +x script.pl' before running `./script.pl'.

With perl in particular, its real easy for ugliness to occur. To counter this, the next lines are:

Strict perl forces you to maintain some semblence of syntax. Without the strict usage, you can basically run amok with code, perl will not care.

The print "Hello world!\n" line simply prints "Hello world!" with a newline character on the end. On windows you may need to change "\n" to "\r\n", depending on which interpreter you've installed.

You can also reference the hex code for this via a \x character, "\x0a\x0d".

Variables & Data Types

In strict perl, variables must be declared using the "my" or "our" operators. "my" is used implicitly in non-shared memory, whereas "our" is used explicitly for shared memory to pass data between threads.

Scalars

Scalars in perl are prefixed with a $. A scalar may contain any string, integer, or floating point value. It may also contain a reference pointer. An example declaration:

Arrays

Arrays (or lists) have elements. Typically an array in perl can contain anything - each element can be something different. An array element may be a hash, hash reference, scalar, or another array.

Arrays are prefixed by the @ character:

You can access and modify array elements directly:

Helper Functions

join()

Join will compile an array into a scalar. Using the array example above, @messages, the following code will generate the string "Hello world!\n, I like perl!\n" as a scalar:

split()

Split takes a scalar and converts it to an array using a delimiter. Using our string from earlier:

push()

The push() function is used to append an element or elements to the end of an array, similar to the push instruction in assembly and treats the array like a stack.

 
my @array;
push(@array,'element one');
push(@array,('element two','element three'));
 

$array[$#array] = "new element";

pop()

The pop() function is similar to the pop instruction in assembly and treats the array like a stack.

my @array;
$array[$#array] = 1;
$popped = pop(@array);
 

The same affect can be acheived with:

$popped = $array[$#array--];

Executing pop() on an array will delete the highest order array element.

unshift()

The unshift() function is like the inverse of the push() function and treats the array like a stack. In stead of pushing to the top of the stack, this function operates against the bottom of the stack.

my @array;
$array[0] = 1;
unshift(@array,0); # $array[0] now contains "0" and $array[1] now contains [1].
 

shift()

The shift() function is like the inverse of the pop() function and treats the array like a stack. In stead of popping from the top of the stack, this function operates against the bottom of the stack.

my @array = (0,1);
my $first_element = shift(@array); # $array[0] now contains one, and @array only contains one element
 

Hashes

A hash is very similar to a struct in C.

Introduction

Hashes are prefixed by the % character. Hash element values are prefixed by $. A hash element may contain another hash, an array, or a scalar.

• You can directly modify the key inside of a hash

• You can also create a key => value pair on declaration

• Example:

Helper Functions

each()

"while my each" can be used to isolate $key => $value pairs from a hash as follows with our %user hash:

keys

This uses a foreach() loop and casting. We can isolate $key=>$value pairs the same as above using keys in stead of each:

References

A reference is very similar to a pointer in C.

Hash References

A hash reference is a scalar created using the \ operator as follows:

 
my %user;
$user{'name'}    = "hatter";
$user{'network'} = "irc.blackhatacademy.org";
 
my $hashref = \%user;
 

Once you've created a hashref (hash reference) you must use pointers to access a key:

 
print $user->{'name'} . "\n";
print $user->{'network'} . "\n";
 

Callback References

This involves user-defined functions. User-defined functions are covered later in this article. A callback reference is a scalar that points to a function. To create a callback reference:

my $callback = \&function_name;
 

To execute the callback function and pass it arguments:

$callback->($arg1, $arg2);

Casting

Casting is the process of transitioning from one data type to another. This is typically done using curly brackets {} preceeded by a data type designator ($,%, or @).

• To cast a hash reference back to a hash:

 
my %hash;
my $hashref = \%hash; #create the hash reference
 
my %casted  = %{$hashref}; #Cast back to a hash.
 

• To cast a list of keys in a hash into an array:

 
my @casted = @{keys %hash};
 

• To cast a scalar value to an integer:

 
my $integer = int($scalar);
 

Scalars

Scalars in perl are prefixed with a $. A scalar may contain any string, integer, or floating point value. It may also contain a reference pointer. An example declaration:

Arrays

Arrays (or lists) have elements. Typically an array in perl can contain anything - each element can be something different. An array element may be a hash, hash reference, scalar, or another array.

Arrays are prefixed by the @ character:

You can access and modify array elements directly:

Helper Functions

join()

Join will compile an array into a scalar. Using the array example above, @messages, the following code will generate the string "Hello world!\n, I like perl!\n" as a scalar:

split()

Split takes a scalar and converts it to an array using a delimiter. Using our string from earlier:

push()

The push() function is used to append an element or elements to the end of an array, similar to the push instruction in assembly and treats the array like a stack.

 
my @array;
push(@array,'element one');
push(@array,('element two','element three'));
 

$array[$#array] = "new element";

pop()

The pop() function is similar to the pop instruction in assembly and treats the array like a stack.

my @array;
$array[$#array] = 1;
$popped = pop(@array);
 

The same affect can be acheived with:

$popped = $array[$#array--];

Executing pop() on an array will delete the highest order array element.

unshift()

The unshift() function is like the inverse of the push() function and treats the array like a stack. In stead of pushing to the top of the stack, this function operates against the bottom of the stack.

my @array;
$array[0] = 1;
unshift(@array,0); # $array[0] now contains "0" and $array[1] now contains [1].
 

shift()

The shift() function is like the inverse of the pop() function and treats the array like a stack. In stead of popping from the top of the stack, this function operates against the bottom of the stack.

my @array = (0,1);
my $first_element = shift(@array); # $array[0] now contains one, and @array only contains one element
 

Helper Functions

join()

Join will compile an array into a scalar. Using the array example above, @messages, the following code will generate the string "Hello world!\n, I like perl!\n" as a scalar:

split()

Split takes a scalar and converts it to an array using a delimiter. Using our string from earlier:

push()

The push() function is used to append an element or elements to the end of an array, similar to the push instruction in assembly and treats the array like a stack.

 
my @array;
push(@array,'element one');
push(@array,('element two','element three'));
 

$array[$#array] = "new element";

pop()

The pop() function is similar to the pop instruction in assembly and treats the array like a stack.

my @array;
$array[$#array] = 1;
$popped = pop(@array);
 

The same affect can be acheived with:

$popped = $array[$#array--];

Executing pop() on an array will delete the highest order array element.

unshift()

The unshift() function is like the inverse of the push() function and treats the array like a stack. In stead of pushing to the top of the stack, this function operates against the bottom of the stack.

my @array;
$array[0] = 1;
unshift(@array,0); # $array[0] now contains "0" and $array[1] now contains [1].
 

shift()

The shift() function is like the inverse of the pop() function and treats the array like a stack. In stead of popping from the top of the stack, this function operates against the bottom of the stack.

my @array = (0,1);
my $first_element = shift(@array); # $array[0] now contains one, and @array only contains one element
 

Hashes

Perl/Basics/Hashes

Helper Functions

each()

Perl/Basics/Hashes/Each

keys

Perl/Basics/Hashes/Keys

References

Perl/Basics/References

Hash References

Perl/Basics/References/Hash

Callback References

Perl/Basics/References/Callback

Casting

Perl/Basics/Casting

Boolean Logic

Operators

Mathematical

Perl/Basics/Operators/Mathematical

Regular Expression

Perl/Basics/Operators/Regular Expressions

Statements

if

Perl/Basics/Statements/If

unless

Perl/Basics/Statements/Unless

AND and OR

Perl/Basics/Statements/And and Or

switch

Perl/Basics/Statements/Switch

Golfing

Perl/Basics/Statements/Golfing

Helper Natives

Perl/Basics/Helper Natives

exists

Perl/Basics/Helper Natives/Exists

defined

Perl/Basics/Helper Natives/Defined

undef

Perl/Basics/Helper Natives/Undef

Bitwise Manipulations

Perl/Basics/Bitwise Manipulations

AND

Perl/Basics/Bitwise Manipulations/AND

NOT

Perl/Basics/Bitwise Manipulations/NOT

OR

Perl/Basics/Bitwise Manipulations/OR

XOR

Perl/Basics/Bitwise Manipulations/XOR

Bit Shifting

Perl/Basics/Bitwise Manipulations/Bit Shifting

Bit Rotation

Perl/Basics/Bitwise Manipulations/Bit Rotation

Loops

A loop is a block of code that continues to execute until a condition is met.

While

• A while loop executes while a condition is true.

my $switch;
my $counter;
while (undef $switch) {
    print $counter;
    $counter++;
    $switch = 1 if ($counter > 100);
}

 The above code will execute until $switch is defined.

It is possible to create an infinite loop using while (1) { ... }.

Until

• An until loop executes until a condition is true.

my $switch;
my $counter;
until (defined $switch) {
    print $counter;
    $counter++;
    $switch = 1 if ($counter > 100);
}

 The above code will execute until $switch is defined.

For

• A for loop has a built-in counter and stops at a pre-defined number.

my @messages = ("Hello world!\n","I like perl!\n");
for (my $counter = 0; $counter < $#array; ++$counter) {
   print $messages[$counter];
}

 The above code will iterate through every element in an array.

It is possible to create an infinite loop using for (;;) {...}.

Foreach

• A foreach loop is built specifically for array handling and iterates through all of the elements in an array.

my @messages = ("Hello world!\n","I like perl!\n");
foreach my $message (@messages) {
   print $message;
}

 The above code will iterate through every element in an array.

While

• A while loop executes while a condition is true.

my $switch;
my $counter;
while (undef $switch) {
    print $counter;
    $counter++;
    $switch = 1 if ($counter > 100);
}

 The above code will execute until $switch is defined.

It is possible to create an infinite loop using while (1) { ... }.

Until

• An until loop executes until a condition is true.

my $switch;
my $counter;
until (defined $switch) {
    print $counter;
    $counter++;
    $switch = 1 if ($counter > 100);
}

 The above code will execute until $switch is defined.

For

• A for loop has a built-in counter and stops at a pre-defined number.

my @messages = ("Hello world!\n","I like perl!\n");
for (my $counter = 0; $counter < $#array; ++$counter) {
   print $messages[$counter];
}

 The above code will iterate through every element in an array.

It is possible to create an infinite loop using for (;;) {...}.

Foreach

• A foreach loop is built specifically for array handling and iterates through all of the elements in an array.

my @messages = ("Hello world!\n","I like perl!\n");
foreach my $message (@messages) {
   print $message;
}

 The above code will iterate through every element in an array.

User Input

Command Line Arguments

Command line arguments are passed at execution time; e.g.

 perl script.pl -a arg1 -b arg2 ...

Getopt::Std

This requires Getopt::Std. The perldoc is here.

Code

 
use strict;
use warnings;
use Getopt::Std;
 
my %opts;
getopts('m:b',\%opts);
 
print $opts{m} . "\n";
print "The boolean -b option was set!\n" if defined $opts{b};
print "The boolean -b option was not set!\n" if undef $opts{b};
 

Analysis

The getopts() function takes a string of flags to parse as well as a hash reference. You can execute the script as follows:

 perl script.pl -m "hello" -b
 perl script.pl -m "hello"

In the above example, we see the line:

 getopts('m:b',\%opts);

The 'm:b', the first argument to the function, designates what command line arguments to parse. The colon after the 'm' specifies that it takes an additional parameter, in this case, the message to say. The -b does not have a colon; we are using it to demonstrate a flag that does not require an additional parameter.

The second argument is a hash reference to designate where the return data is stored; in this case, $opts{m} contains "hello" and opts{b} is either defined or undefined based on whether or not it was present in the flags when the script was executed.

Getopt::Long

This requires Getopt::Long. The perldoc is here.

Code

 
use strict;
use warnings;
use Getopt::Long;
 
my $message, $boolean;
GetOptions('message=s' => \$message, 'boolean' => \$boolean);
 
print $message . "\n";
print "The boolean -b option was set!\n" if defined $boolean;
print "The boolean -b option was not set!\n" if undef $boolean;
 

Analysis

The GetOptions() function receives message formats and references for variable assignment. You can execute the script as follows:

 perl script.pl --message "hello" --boolean
 perl script.pl --message "hello"

In the above example, we see the line:

 GetOptions('message=s' => \$message, 'boolean' => \$boolean);

You can see from the execution pattern above that the GetOptions() function provides an interface for the "double-dash" style command line arguments. The GetOptions() function receives a hash. The =s after message designates that the --message parameter receives a string data type. An =i will change it to integer. Simple no = will set the flag to a boolean; similar to an argument without a colon in Getopt::Std. Notice each variable is passed as a reference.

Getopt::Std

This requires Getopt::Std. The perldoc is here.

Code

 
use strict;
use warnings;
use Getopt::Std;
 
my %opts;
getopts('m:b',\%opts);
 
print $opts{m} . "\n";
print "The boolean -b option was set!\n" if defined $opts{b};
print "The boolean -b option was not set!\n" if undef $opts{b};
 

Analysis

The getopts() function takes a string of flags to parse as well as a hash reference. You can execute the script as follows:

 perl script.pl -m "hello" -b
 perl script.pl -m "hello"

In the above example, we see the line:

 getopts('m:b',\%opts);

The 'm:b', the first argument to the function, designates what command line arguments to parse. The colon after the 'm' specifies that it takes an additional parameter, in this case, the message to say. The -b does not have a colon; we are using it to demonstrate a flag that does not require an additional parameter.

The second argument is a hash reference to designate where the return data is stored; in this case, $opts{m} contains "hello" and opts{b} is either defined or undefined based on whether or not it was present in the flags when the script was executed.

Code

 
use strict;
use warnings;
use Getopt::Std;
 
my %opts;
getopts('m:b',\%opts);
 
print $opts{m} . "\n";
print "The boolean -b option was set!\n" if defined $opts{b};
print "The boolean -b option was not set!\n" if undef $opts{b};
 

Analysis

The getopts() function takes a string of flags to parse as well as a hash reference. You can execute the script as follows:

 perl script.pl -m "hello" -b
 perl script.pl -m "hello"

In the above example, we see the line:

 getopts('m:b',\%opts);

The 'm:b', the first argument to the function, designates what command line arguments to parse. The colon after the 'm' specifies that it takes an additional parameter, in this case, the message to say. The -b does not have a colon; we are using it to demonstrate a flag that does not require an additional parameter.

The second argument is a hash reference to designate where the return data is stored; in this case, $opts{m} contains "hello" and opts{b} is either defined or undefined based on whether or not it was present in the flags when the script was executed.

Getopt::Long

This requires Getopt::Long. The perldoc is here.

Code

 
use strict;
use warnings;
use Getopt::Long;
 
my $message, $boolean;
GetOptions('message=s' => \$message, 'boolean' => \$boolean);
 
print $message . "\n";
print "The boolean -b option was set!\n" if defined $boolean;
print "The boolean -b option was not set!\n" if undef $boolean;
 

Analysis

The GetOptions() function receives message formats and references for variable assignment. You can execute the script as follows:

 perl script.pl --message "hello" --boolean
 perl script.pl --message "hello"

In the above example, we see the line:

 GetOptions('message=s' => \$message, 'boolean' => \$boolean);

You can see from the execution pattern above that the GetOptions() function provides an interface for the "double-dash" style command line arguments. The GetOptions() function receives a hash. The =s after message designates that the --message parameter receives a string data type. An =i will change it to integer. Simple no = will set the flag to a boolean; similar to an argument without a colon in Getopt::Std. Notice each variable is passed as a reference.

Code

 
use strict;
use warnings;
use Getopt::Long;
 
my $message, $boolean;
GetOptions('message=s' => \$message, 'boolean' => \$boolean);
 
print $message . "\n";
print "The boolean -b option was set!\n" if defined $boolean;
print "The boolean -b option was not set!\n" if undef $boolean;
 

Analysis

The GetOptions() function receives message formats and references for variable assignment. You can execute the script as follows:

 perl script.pl --message "hello" --boolean
 perl script.pl --message "hello"

In the above example, we see the line:

 GetOptions('message=s' => \$message, 'boolean' => \$boolean);

You can see from the execution pattern above that the GetOptions() function provides an interface for the "double-dash" style command line arguments. The GetOptions() function receives a hash. The =s after message designates that the --message parameter receives a string data type. An =i will change it to integer. Simple no = will set the flag to a boolean; similar to an argument without a colon in Getopt::Std. Notice each variable is passed as a reference.

STDIN (Standard Input)

Reading from standard input in perl is very simple.

 
print "Enter your name :";
my $name = <>;
print "Your name is $name\n";
 

User-Defined Functions

A function is defined by the programmer to create re-usable code. In our example, we will make an is_integer function that returns either 1 or undef depending on whether the scalar passed is an integer or not.

sub is_integer {
    my $scalar = shift;
    return 1 if (int($scalar) == $scalar);
    return undef;
}

Usage:

 
print "This scalar is an integer.\n" if (defined is_integer($scalar)) else print "This is not an integer.\n";
 

return($scalar,@array);

my ($scalar,@array) = function();

Helpful Libraries

Throughput

Perl/Throughput

Download

Perl/Throughput/Download

Usage

Perl/Throughput/Usage

Config.pm

Perl/Throughput/Usage/Config

Log.pm

Perl/Throughput/Usage/Log

Server.pm

Perl/Throughput/Usage/Server