Larry Wall

[ Pobierz całość w formacie PDF ]
By Larry Wall, Tom Christiansen, & Randal Schwartz; 1-56592-149-6, 646 pages.
2nd Edition, September 1996
Table of Contents
Examples-
Warning:
this directory includes long filenames which may confuse some older
operating systems (notably Windows 3.1).
Searchthe text of
Programming Perl
.
Copyright© 1996, 1997 O'Reilly & Associates. All Rights Reserved.
      Preface
Preface
Contents:
Perl in a Nutshell
Perl in a Nutshell
Perl is a language for getting your job done.
Of course, if your job is programming, you can get your job done with any "complete" computer
language, theoretically speaking. But we know from experience that computer languages differ not so
much in what they make
possible
, but in what they make
easy
. At one extreme, the so-called "fourth
generation languages" make it easy to do some things, but nearly impossible to do other things. At the
other extreme, certain well known, "industrial-strength" languages make it equally difficult to do
almost everything.
Perl is different. In a nutshell, Perl is designed to make the easy jobs easy, without making the hard
jobs impossible.
And what are these "easy jobs" that ought to be easy? The ones you do every day, of course. You
want a language that makes it easy to manipulate numbers and text, files and directories, computers
and networks, and especially programs. It should be easy to run external programs and scan their
output for interesting tidbits. It should be easy to send those same tidbits off to other programs that
can do special things with them. It should be easy to develop, modify, and debug your own programs
too. And, of course, it should be easy to compile and run your programs, and do it portably, on any
modern operating system.
Perl does all that, and a whole lot more.
Initially designed as a glue language for the UNIX operating system (or any of its myriad variants),
Perl also runs on numerous other systems, including MS-DOS, VMS, OS/2, Plan 9, Macintosh, and
any variety of Windows you care to mention. It is one of the most portable programming languages
 available today. To program C portably, you have to put in all those strange
#ifdef
markings for
different operating systems. And to program a shell portably, you have to remember the syntax for
each operating system's version of each command, and somehow find the least common denominator
that (you hope) works everywhere. Perl happily avoids both of these problems, while retaining many
of the benefits of both C and shell programming, with some additional magic of its own. Much of the
explosive growth of Perl has been fueled by the hankerings of
former
UNIX programmers who
wanted to take along with them as much of the "old country" as they could. For them, Perl is the
portable distillation of UNIX culture, an oasis in the wilderness of "can't get there from here". On the
other hand, it works in the other direction, too: Web programmers are often delighted to discover that
they can take their scripts from a Windows machine and run them unchanged on their UNIX servers.
Although Perl is especially popular with systems programmers and Web developers, it also appeals to
a much broader audience. The hitherto well-kept secret is now out: Perl is no longer just for text
processing. It has grown into a sophisticated, general-purpose programming language with a rich
software development environment complete with debuggers, profilers, cross-referencers, compilers,
interpreters, libraries, syntax-directed editors, and all the rest of the trappings of a "real" programming
language. (But don't let that scare you: nothing requires you to go tinkering under the hood.) Perl is
being used daily in every imaginable field, from aerospace engineering to molecular biology, from
computer-assisted design/computer-assisted manufacturing (CAD/CAM) to document processing,
from database manipulation to client-server network management. Perl is used by people who are
desperate to analyze or convert lots of data quickly, whether you're talking DNA sequences, Web
pages, or pork belly futures. Indeed, one of the jokes in the Perl community is that the next big stock
market crash will probably be triggered by a bug in a Perl script. (On the brighter side, any
unemployed stock analysts will still have a marketable skill, so to speak.)
There are many reasons for the success of Perl. It certainly helps that Perl is freely available, and
freely redistributable. But that's not enough to explain the Perl phenomenon, since many freeware
packages fail to thrive. Perl is not just free; it's also fun. People feel like they can be creative in Perl,
because they have freedom of expression: they get to choose what to optimize for, whether that's
computer speed or programmer speed, verbosity or conciseness, readability or maintainability or
reusability or portability or learnability or teachability. You can even optimize for obscurity, if you're
entering an Obfuscated Perl contest.
Perl can give you all these degrees of freedom because it's essentially a language with a split
personality. It's both a very simple language and a very rich language. It has taken good ideas from
nearly everywhere, and installed them into an easy-to-use mental framework. To those who merely
like it, Perl is the
Practical Extraction and Report Language
. To those who love it, Perl is the
Pathologically Eclectic Rubbish Lister
. And to the minimalists in the crowd, Perl seems like a
pointless exercise in redundancy. But that's okay. The world needs a few reductionists (mainly as
physicists). Reductionists like to take things apart. The rest of us are just trying to get it together.
Perl is in many ways a simple language. You don't have to know many special incantations to compile
a Perl program--you can just execute it like a shell script. The types and structures used by Perl are
easy to use and understand. Perl doesn't impose arbitrary limitations on your data--your strings and
arrays can grow as large as they like (so long as you have memory), and they're designed to scale well
as they grow. Instead of forcing you to learn new syntax and semantics, Perl borrows heavily from
other languages you may already be familiar with (such as C, and
sed
, and
awk
, and English, and
Greek). In fact, just about any programmer can read a well-written piece of Perl code and have some
idea of what it does.
Most important, you don't have to know everything there is to know about Perl before you can write
useful programs. You can learn Perl "small end first". You can program in Perl Baby-Talk, and we
promise not to laugh. Or more precisely, we promise not to laugh any more than we'd giggle at a
child's creative way of putting things. Many of the ideas in Perl are borrowed from natural language,
and one of the best ideas is that it's okay to use a subset of the language as long as you get your point
across. Any level of language proficiency is acceptable in Perl culture. We won't send the language
police after you. A Perl script is "correct" if it gets the job done before your boss fires you.
Though simple in many ways, Perl is also a rich language, and there is much to be learned about it.
That's the price of making hard things possible. Although it will take some time for you to absorb all
that Perl can do, you will be glad that you have access to the extensive capabilities of Perl when the
time comes that you need them. We noted above that Perl borrows many capabilities from the shells
and C, but Perl also possesses a strict superset of
sed
and
awk
capabilities. There are, in fact,
translators supplied with Perl to turn your old
sed
and
awk
scripts into Perl scripts, so you can see how
the features you may already be familiar with correspond to those of Perl.
Because of that heritage, Perl was a rich language even when it was "just" a data-reduction language,
designed for navigating files, scanning large amounts of text, creating and obtaining dynamic data,
and printing easily formatted reports based on that data. But somewhere along the line, Perl started to
blossom. It also became a language for filesystem manipulation, process management, database
administration, client-server programming, secure programming, Web-based information
management, and even for object-oriented and functional programming. These capabilities were not
just slapped onto the side of Perl--each new capability works synergistically with the others, because
Perl was designed to be a glue language from the start.
But Perl can glue together more than its own features. Perl is designed to be modularly extensible.
Perl allows you to rapidly design, program, debug, and deploy applications, but it also allows you to
easily extend the functionality of these applications as the need arises. You can embed Perl in other
languages, and you can embed other languages in Perl. Through the module importation mechanism,
you can use these external definitions as if they were built-in features of Perl. Object-oriented external
libraries retain their object-orientedness in Perl.
Perl helps you in other ways too. Unlike a strictly interpreted language such as the shell, which
compiles and executes a script one command at a time, Perl first compiles your whole program
quickly into an intermediate format. Like any other compiler, it performs various optimizations, and
gives you instant feedback on everything from syntax and semantic errors to library binding mishaps.
Once Perl's compiler frontend is happy with your program, it passes off the intermediate code to the
interpreter to execute (or optionally to any of several modular back ends that can emit C or bytecode.)
This all sounds complicated, but the compiler and interpreter are quite efficient, and most of us find
that the typical compile-run-fix cycle is measured in mere seconds. Together with Perl's many fail-soft
characteristics, this quick turnaround capability makes Perl a language in which you really can do
rapid prototyping. Then later, as your program matures, you can tighten the screws on yourself, and
make yourself program with less flair but more discipline. Perl helps you with that too, if you ask
nicely.
Perl also helps you to write programs more securely. While running in privileged mode, you can
temporarily switch your identity to something innocuous before accessing system resources. Perl also
guards against accidental security errors through a data tracing mechanism that automatically
determines which data was derived from insecure sources and prevents dangerous operations before
they can happen. Finally, Perl lets you set up specially protected compartments in which you can
safely execute Perl code of dubious lineage, masking out dangerous operations. System administrators
and CGI programmers will particularly welcome these features.
But, paradoxically, the way in which Perl helps you the most has almost nothing to do with Perl, and
everything to do with the people who use Perl. Perl folks are, frankly, some of the most helpful folks
on earth. If there's a religious quality to the Perl movement, then this is at the heart of it. Larry wanted
the Perl community to function like a little bit of heaven, and he seems to have gotten his wish, so far.
Please do your part to keep it that way.
Whether you are learning Perl because you want to save the world, or just because you are curious, or
because your boss told you to, this handbook will lead you through both the basics and the intricacies.
And although we don't intend to teach you how to program, the perceptive reader will pick up some of
the art, and a little of the science, of programming. We will encourage you to develop the three great
virtues of a programmer:
laziness
,
impatience
, and
hubris
. Along the way, we hope you find the book
mildly amusing in some spots (and wildly amusing in others). And if none of this is enough to keep
you awake, just keep reminding yourself that learning Perl will increase the value of your resume. So
keep reading.
The Rest of This Book
     [ Pobierz całość w formacie PDF ]