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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


Some of the highlights from perlref:

In addition to consulting the obvious documents such as the Perl man pages, look at the Perl source code for standard modules more information. Also the 't/op' directory in the Perl source tree has regression tests that should definitely get you thinking.

Some useful documents and informative posts are available at the Web sites and PerlMonks

Some typical question with answers:

  1. Q: How do I know what type of address a pointer is pointing to?

    A: Function ref provides the type of reference. Also the address printed out with the print statement on a reference has a qualifier word in front of it. For example, a reference to a hash has the word HASH followed by an address value, an array has the word ARRAY, and so on.

  2. Q: How are multidimensional arrays possible using Perl?

    A: References in Perl can point to arrays. Elements of arrays can be references to other arrays, hashes, and so on. 

  3. Q: What's the best way to pass more than one array into a subroutine?

    A: Pass references to the arrays, using the \@arrayname for each array passed-as in the following call:
    mysub(\@one, \@two);
    Within the subroutine, take each reference off one at a time.
    my ($a, $b) = @_;
    Now use @$a and @$b to get to the arrays passed into the subroutines.

  4. Q: Why is *moo more efficient to use than $_main{'moo'}? Is there a difference in usage?

    A: Both *moo and $_main{'moo'} mean the same variable (as long as you aren't using a package). *moo is more efficient because the reference is looked up at compile time, whereas $_main{'moo'} is evaluated at runtime each time the statement with this expression is executed.


In Perl, you can pass only one kind of argument to a subroutine: a scalar. To pass any other kind of argument, you need to convert it to a scalar. You do that by passing a reference to it. A reference to anything is a scalar. If you're a C programmer you can think of a reference as a pointer (sort of).

The following table discusses the referencing and de-referencing of variables. Note that in the case of lists and hashes, you reference and dereference the list or hash as a whole, not individual elements (at least not for the purposes of this discussion).

Variable Instantiating
the scalar
Instantiating a
reference to it
Referencing it Dereferencing it Accessing an element
$scalar $scalar = "steve";
$ref = \"steve";
$ref = \$scalar $$ref or
@list @list = ("steve", "fred");
$ref = ["steve", "fred"];
$ref = \@list @{$ref} ${$ref}[3]
%hash %hash = ("name" => "steve",
   "job" => "Troubleshooter");
$hash = {"name" => "steve",
   "job" => "Troubleshooter"};
$ref = \%hash %{$ref} ${$ref}{"president"}

$ref = \*FILE {$ref} or scalar <$ref>

The two types of references in Perl 5 are hard and symbolic.

You can have references to scalars, arrays, hashes, subroutines, and even other references. References themselves are scalars and have to be de-referenced to the context before being used. Use @$pointer for an array, %$pointer for a hash, &$pointer for a subroutine, and so on for dereferencing.

Multidimensional arrays are possible using references in arrays and hashes.

Parameters are passed into a subroutine through references. The @_ array is really all the passed parameters concatenated in one long array. To send separate arrays, use the references to the individual items.

Tomorrow's lesson covers Perl objects and references to objects. We have deliberately not covered Perl objects in this chapter because it requires some knowledge of references. References are used to create and refer to objects, constructors, and packages.

  1. Scalar references:

    $ra  = \$a;              # reference to scalar
    $$ra = 2;                # dereference scalar-ref
    $ra  = \1.6;             # reference to constant scalar
  2. Array references:

    $rl  = \@l;              # reference to existing
    $rl  = [1,2,3];          # reference to anonymous array
    push (@$rl, "a");        # Dereference
    print $rl->[3]           # 4th element of array pointed to by $rl
  3. Hash references:

    $rh = \%h;               # reference to hash
    $rh = {"laurel" => "hardy", "romeo" => "juliet"}; # ref to anon-hash
    print keys (%$rh);       # Dereference
    $x = $rh->{"laurel"};    # Arrow notation to extract single element
    @slice = @$rh{"laurel","romeo"}; # Hash slice
  4. Code references:

    $rs = \&foo;             # reference to existing subroutine foo
    $rs = sub {print "foo"}; # reference to anonymous subroutine 
                             # (remember the semicolon at the end)
    &$rs();                  # dereference: call the subroutine
  5. Generalized dereferences. Any code inside a block yielding a reference can be dereferenced:

    @a = @{foo()};           # dereference the array reference 
                             # returned by foo()
  6. References gotchas. All the examples below are wrong. Always use -w in developing and testing.

    @foo = [1,3,4];          # Assigning an array-ref to an array
                             # Use parentheses instead.
    %foo = {"foo" => "bar"}; # Assigning a hash-ref to a hash.
                             # Use parentheses instead.
    $foo = \($a, @b);        # Identical to $foo = (\$a, \@b)
                             # Assiging an enumerated list to a 
                             # scalar yields the last element (so, 
                             # $foo gets \@b). Use [ ] if you need 
                             # an array reference



Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy


War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotesSomerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose BierceBernard Shaw : Mark Twain Quotes


Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law


Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOSProgramming Languages History : PL/1 : Simula 67 : C : History of GCC developmentScripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-MonthHow to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Haterís Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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