Latest revision as of 03:28, 20 September 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.

Special thanks to hatter for his contributions to this article.

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);
 

Boolean Logic

Operators

Mathematical

• =

The = operator assigns a value to a variable.

• ==

The == operator is used to test if a variable is equal to a value or another variable.

• !

The ! operator means "not". It applies to = and any variable. For example, "if (!$scalar)" is used to determine if the $scalar variable is null, zero, or undefined. When used before a = operator, this becomes "not equal to".

• eq

Sometimes because variables may not be a string or integer, the eq operator is used to determine if the two are equal.

The following operators are used for greater than, less than, and greater than or equal to, less than or equal to, etc; similar to other languages:

• >
• gt
• <
• lt
• >=
• gte
• <=
• lte

Regular Expressions

The ~ operator is used with regular expressions, which are covered later in this article. The ~ operator can be used in a variety of ways:

• =~
• !~

Regular expressions can also be very useful when using perl as a shell glue language. As an example:

"find | perl -nwl -e "m:zs: and print" 

will pipe the output of the find command into perl, which will then apply the regular expression m:zs: which is an expression which only looks for those two characters in those orders. The use of "and" makes perl only apply the print operation to the line if the first match returns a "true." The use of -nwl means that newlines are stripped, warnings are enabled, and that the input which doesn't match isn't printed as well.

Statements

if

An if statement may have 3 types of clauses: if,elsif, and else. For the below example, assume that the $age scalar is passed as a command line argument:

 
if (int($age) == $age) {  #Making sure it's an integer.
    if ($age < 18) {      #If the age is less than 18:
        print "You must be at least 18 to view this.\n";
    } elsif ($age < 21) { # If the age is more than 18, but less than 21:
        print "Because you are under 21, some features may be restricted.\n";
    } else {              # If none of the conditions have been met:
        display_content();
    }
}
 

unless

An unless statement may only have the unless clause and an else clause.

unless ($age >= 21) {
    print "All content is restricted for users under the age of 21.\n";
} else {
    print "Welcome to our sample age gate!\n";
    display_content();
}
 

AND and OR

"And" and "or" are used to apply multiple conditions to a boolean statement.

• && is the way perl represents "and"
• || is the way perl represents "or"

Example:

 
  if ($age < 21 && $age >= 18) {
      print "Some content will be restricted because you are not older than 21.\n";
  }
 

switch

To use perl's switch() routine you must have use Switch; before your switch() statement. A switch statement allows a programmer to avoid long chains of "elsif" statements. It condenses the amount of required lines of code. Perl's switch statement is very similar to the switch() statement in C and C++, though the syntax is a little different. A perl switch() statement may contain case and else clauses. Perl switch cases can also be used to determine if a value is in a list, an array element, hash key, or matches a regular expression or string. In this example, suppose $option was a numeric value for an integer based menu with 3 options.

 
use Switch;
switch(int($option)) {
    case 1 {  # Essentially the same as if ($option == 1)
        print "You picked option 1!\n";
    }
    case 2 { # Essentially the same as elsif ($option == 2)
        print "You picked option 2!\n";
    }
    case 3 { # Essentially the same as elsif ($option == 3)
        print "You picked option 3!\n";
    }
    else { 
        print "invalid menu option!\n";
    }
}
 

For more information, see perldoc switch, or: here.

Golfing

The term golfing applies to condensing a boolean statement into one line. Golfing is typically used when you only need to execute one line of code for a boolean statement.

 
print "You are not 18 or older!\n" unless ($age >= 18);
 

• Is essentially the same as:

 
print "You are not 18 or older!\n" if ($age < 18);
 

• Is essentially the same as this un-golfed statement:

 
unless ($age >= 18) {
    print "You are not 18 or older!\n";
}
 

Helper Natives

These helper natives are boolean statements that assist with the determination of the existence of or the defining of a variable.

exists

The exists native applies specifically to hashes and hash references.

print "This user has an age.\n" if exists $user->{'age'};
 

defined

The defined native determines if a scalar value is defined:

 
print "We received a response from the server.\n" if defined $response;
 

undef

The undef native determines if a scalar value is un-defined:

 
print "We received a response from the server.\n" unless undef $response;
 

Bitwise Manipulations

Perl's bitwise manipulations cover the syntax for performing bitwise math on variables.

AND

• & - The AND operator.

 
my $num = 10;
$num = $num & 25;
print $num . "\n";
 

NOT

• ~ - The NOT operator

 
my $num = 10;
$num = ~$num;
print $num . "\n";
 

OR

• | - The OR operator

 
my $num = 10;
$num = $num | 25;
print $num . "\n";
 

XOR

• ^ - The xor (exclusive or) operator

 
my $num = 10;
$num = $num ^ 25;
print $num . "\n";
 

Bit Shifting

• << - The shift left operator
• >> - The shift right operator

Bit Rotation

Perl bit rotation requires the Bit::ShiftReg package from CPAN. More information available there.

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.

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.

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

Throughput is a library that provides abstraction for sockets, logging, and simple configuration.

Download

Download Throughput

Usage

To set up your development environment for Throughput, you'll need to put the contents of Throughput.tgz in your perl include directory or put the Throughput directory in the same directory as the application you are developing.

Config.pm

• A simple config parser

use Throughput::Config qw(parse);
my %config = Throughput::parse('config.conf');

• The config file should be formatted as:

 variable=value # comment

• The %config hash will return as :

$config{variable}

Log.pm

• A simple logger

my $logger = new Log();
$logger->error("an error has occured");

• Output defaults to STDERR but can be set to files in the constructor or using accessors.

$logger->error_log($filehandle);

• Log also supports info, warn and digest:

 
$logger->info("info message");
$logger->warn("warning!");
 
$logger->info_log($filehandle);
$logger->warn_log($filehandle);

$logger->digest_log($filehandle);

Server.pm