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130 | .\" | |
131 | .IX Title "PERLFAQ1 1" | |
132 | .TH PERLFAQ1 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | perlfaq1 \- General Questions About Perl ($Revision: 1.19 $, $Date: 2005/12/31 00:54:37 $) | |
135 | .SH "DESCRIPTION" | |
136 | .IX Header "DESCRIPTION" | |
137 | This section of the \s-1FAQ\s0 answers very general, high-level questions | |
138 | about Perl. | |
139 | .Sh "What is Perl?" | |
140 | .IX Subsection "What is Perl?" | |
141 | Perl is a high-level programming language with an eclectic heritage | |
142 | written by Larry Wall and a cast of thousands. It derives from the | |
143 | ubiquitous C programming language and to a lesser extent from sed, | |
144 | awk, the Unix shell, and at least a dozen other tools and languages. | |
145 | Perl's process, file, and text manipulation facilities make it | |
146 | particularly well-suited for tasks involving quick prototyping, system | |
147 | utilities, software tools, system management tasks, database access, | |
148 | graphical programming, networking, and world wide web programming. | |
149 | These strengths make it especially popular with system administrators | |
150 | and \s-1CGI\s0 script authors, but mathematicians, geneticists, journalists, | |
151 | and even managers also use Perl. Maybe you should, too. | |
152 | .Sh "Who supports Perl? Who develops it? Why is it free?" | |
153 | .IX Subsection "Who supports Perl? Who develops it? Why is it free?" | |
154 | The original culture of the pre-populist Internet and the deeply-held | |
155 | beliefs of Perl's author, Larry Wall, gave rise to the free and open | |
156 | distribution policy of perl. Perl is supported by its users. The | |
157 | core, the standard Perl library, the optional modules, and the | |
158 | documentation you're reading now were all written by volunteers. See | |
159 | the personal note at the end of the \s-1README\s0 file in the perl source | |
160 | distribution for more details. See perlhist (new as of 5.005) | |
161 | for Perl's milestone releases. | |
162 | .PP | |
163 | In particular, the core development team (known as the Perl Porters) | |
164 | are a rag-tag band of highly altruistic individuals committed to | |
165 | producing better software for free than you could hope to purchase for | |
166 | money. You may snoop on pending developments via the archives at | |
167 | http://www.xray.mpe.mpg.de/mailing\-lists/perl5\-porters/ | |
168 | and http://archive.develooper.com/perl5\-porters@perl.org/ | |
169 | or the news gateway nntp://nntp.perl.org/perl.perl5.porters or | |
170 | its web interface at http://nntp.perl.org/group/perl.perl5.porters , | |
171 | or read the faq at http://simon\-cozens.org/writings/p5p\-faq , | |
172 | or you can subscribe to the mailing list by sending | |
173 | perl5\-porters\-request@perl.org a subscription request | |
174 | (an empty message with no subject is fine). | |
175 | .PP | |
176 | While the \s-1GNU\s0 project includes Perl in its distributions, there's no | |
177 | such thing as \*(L"\s-1GNU\s0 Perl\*(R". Perl is not produced nor maintained by the | |
178 | Free Software Foundation. Perl's licensing terms are also more open | |
179 | than \s-1GNU\s0 software's tend to be. | |
180 | .PP | |
181 | You can get commercial support of Perl if you wish, although for most | |
182 | users the informal support will more than suffice. See the answer to | |
183 | \&\*(L"Where can I buy a commercial version of perl?\*(R" for more information. | |
184 | .Sh "Which version of Perl should I use?" | |
185 | .IX Subsection "Which version of Perl should I use?" | |
186 | (contributed by brian d foy) | |
187 | .PP | |
188 | There is often a matter of opinion and taste, and there isn't any | |
189 | one answer that fits anyone. In general, you want to use either | |
190 | the current stable release, or the stable release immediately prior | |
191 | to that one. Currently, those are perl5.8.x and perl5.6.x, respectively. | |
192 | .PP | |
193 | Beyond that, you have to consider several things and decide which | |
194 | is best for you. | |
195 | .IP "\(bu" 4 | |
196 | If things aren't broken, upgrading perl may break | |
197 | them (or at least issue new warnings). | |
198 | .IP "\(bu" 4 | |
199 | The latest versions of perl have more bug fixes. | |
200 | .IP "\(bu" 4 | |
201 | The Perl community is geared toward supporting the most | |
202 | recent releases, so you'll have an easier time finding help for | |
203 | those. | |
204 | .IP "\(bu" 4 | |
205 | Versions prior to perl5.004 had serious security problems with | |
206 | buffer overflows, and in some cases have \s-1CERT\s0 advisories (for | |
207 | instance, http://www.cert.org/advisories/CA\-1997\-17.html ). | |
208 | .IP "\(bu" 4 | |
209 | The latest versions are probably the least deployed and | |
210 | widely tested, so you may want to wait a few months after their | |
211 | release and see what problems others have if you are risk averse. | |
212 | .IP "\(bu" 4 | |
213 | The immediate, previous releases (i.e. perl5.6.x ) are usually | |
214 | maintained for a while, although not at the same level as the | |
215 | current releases. | |
216 | .IP "\(bu" 4 | |
217 | No one is actively supporting perl4.x. Five years ago it was | |
218 | a dead camel carcass (according to this document). Now it's barely | |
219 | a skeleton as its whitewashed bones have fractured or eroded. | |
220 | .IP "\(bu" 4 | |
221 | There is no perl6.x for the next couple of years. Stay tuned, | |
222 | but don't worry that you'll have to change major versions of Perl | |
223 | soon (i.e. before 2006). | |
224 | .IP "\(bu" 4 | |
225 | There are really two tracks of perl development: a | |
226 | maintenance version and an experimental version. The | |
227 | maintenance versions are stable, and have an even number | |
228 | as the minor release (i.e. perl5.8.x, where 8 is the minor | |
229 | release). The experimental versions may include features that | |
230 | don't make it into the stable versions, and have an odd number | |
231 | as the minor release (i.e. perl5.9.x, where 9 is the minor release). | |
232 | .Sh "What are perl4, perl5, or perl6?" | |
233 | .IX Subsection "What are perl4, perl5, or perl6?" | |
234 | (contributed by brian d foy) | |
235 | .PP | |
236 | In short, perl4 is the past, perl5 is the present, and perl6 is the | |
237 | future. | |
238 | .PP | |
239 | The number after perl (i.e. the 5 after perl5) is the major release | |
240 | of the perl interpreter as well as the version of the language. Each | |
241 | major version has significant differences that earlier versions cannot | |
242 | support. | |
243 | .PP | |
244 | The current major release of Perl is perl5, and was released in 1994. | |
245 | It can run scripts from the previous major release, perl4 (March 1991), | |
246 | but has significant differences. It introduced the concept of references, | |
247 | complex data structures, and modules. The perl5 interpreter was a | |
248 | complete re-write of the previous perl sources. | |
249 | .PP | |
250 | Perl6 is the next major version of Perl, but it's still in development | |
251 | in both its syntax and design. The work started in 2002 and is still | |
252 | ongoing. Many of the most interesting features have shown up in the | |
253 | latest versions of perl5, and some perl5 modules allow you to use some | |
254 | perl6 syntax in your programs. You can learn more about perl6 at | |
255 | http://dev.perl.org/perl6/ . | |
256 | .PP | |
257 | See perlhist for a history of Perl revisions. | |
258 | .Sh "What is Ponie?" | |
259 | .IX Subsection "What is Ponie?" | |
260 | At The O'Reilly Open Source Software Convention in 2003, Artur | |
261 | Bergman, Fotango, and The Perl Foundation announced a project to | |
262 | run perl5 on the Parrot virtual machine named Ponie. Ponie stands for | |
263 | Perl On New Internal Engine. The Perl 5.10 language implementation | |
264 | will be used for Ponie, and there will be no language level | |
265 | differences between perl5 and ponie. Ponie is not a complete rewrite | |
266 | of perl5. | |
267 | .PP | |
268 | For more details, see http://www.poniecode.org/ | |
269 | .Sh "What is perl6?" | |
270 | .IX Subsection "What is perl6?" | |
271 | At The Second O'Reilly Open Source Software Convention, Larry Wall | |
272 | announced Perl6 development would begin in earnest. Perl6 was an oft | |
273 | used term for Chip Salzenberg's project to rewrite Perl in \*(C+ named | |
274 | Topaz. However, Topaz provided valuable insights to the next version | |
275 | of Perl and its implementation, but was ultimately abandoned. | |
276 | .PP | |
277 | If you want to learn more about Perl6, or have a desire to help in | |
278 | the crusade to make Perl a better place then peruse the Perl6 developers | |
279 | page at http://dev.perl.org/perl6/ and get involved. | |
280 | .PP | |
281 | Perl6 is not scheduled for release yet, and Perl5 will still be supported | |
282 | for quite awhile after its release. Do not wait for Perl6 to do whatever | |
283 | you need to do. | |
284 | .PP | |
285 | \&\*(L"We're really serious about reinventing everything that needs reinventing.\*(R" | |
286 | \&\-\-Larry Wall | |
287 | .Sh "How stable is Perl?" | |
288 | .IX Subsection "How stable is Perl?" | |
289 | Production releases, which incorporate bug fixes and new functionality, | |
290 | are widely tested before release. Since the 5.000 release, we have | |
291 | averaged only about one production release per year. | |
292 | .PP | |
293 | Larry and the Perl development team occasionally make changes to the | |
294 | internal core of the language, but all possible efforts are made toward | |
295 | backward compatibility. While not quite all perl4 scripts run flawlessly | |
296 | under perl5, an update to perl should nearly never invalidate a program | |
297 | written for an earlier version of perl (barring accidental bug fixes | |
298 | and the rare new keyword). | |
299 | .Sh "Is Perl difficult to learn?" | |
300 | .IX Subsection "Is Perl difficult to learn?" | |
301 | No, Perl is easy to start learning\*(--and easy to keep learning. It looks | |
302 | like most programming languages you're likely to have experience | |
303 | with, so if you've ever written a C program, an awk script, a shell | |
304 | script, or even a \s-1BASIC\s0 program, you're already partway there. | |
305 | .PP | |
306 | Most tasks only require a small subset of the Perl language. One of | |
307 | the guiding mottos for Perl development is \*(L"there's more than one way | |
308 | to do it\*(R" (\s-1TMTOWTDI\s0, sometimes pronounced \*(L"tim toady\*(R"). Perl's | |
309 | learning curve is therefore shallow (easy to learn) and long (there's | |
310 | a whole lot you can do if you really want). | |
311 | .PP | |
312 | Finally, because Perl is frequently (but not always, and certainly not by | |
313 | definition) an interpreted language, you can write your programs and test | |
314 | them without an intermediate compilation step, allowing you to experiment | |
315 | and test/debug quickly and easily. This ease of experimentation flattens | |
316 | the learning curve even more. | |
317 | .PP | |
318 | Things that make Perl easier to learn: Unix experience, almost any kind | |
319 | of programming experience, an understanding of regular expressions, and | |
320 | the ability to understand other people's code. If there's something you | |
321 | need to do, then it's probably already been done, and a working example is | |
322 | usually available for free. Don't forget the new perl modules, either. | |
323 | They're discussed in Part 3 of this \s-1FAQ\s0, along with \s-1CPAN\s0, which is | |
324 | discussed in Part 2. | |
325 | .Sh "How does Perl compare with other languages like Java, Python, \s-1REXX\s0, Scheme, or Tcl?" | |
326 | .IX Subsection "How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?" | |
327 | Favorably in some areas, unfavorably in others. Precisely which areas | |
328 | are good and bad is often a personal choice, so asking this question | |
329 | on Usenet runs a strong risk of starting an unproductive Holy War. | |
330 | .PP | |
331 | Probably the best thing to do is try to write equivalent code to do a | |
332 | set of tasks. These languages have their own newsgroups in which you | |
333 | can learn about (but hopefully not argue about) them. | |
334 | .PP | |
335 | Some comparison documents can be found at http://www.perl.com/doc/FMTEYEWTK/versus/ | |
336 | if you really can't stop yourself. | |
337 | .Sh "Can I do [task] in Perl?" | |
338 | .IX Subsection "Can I do [task] in Perl?" | |
339 | Perl is flexible and extensible enough for you to use on virtually any | |
340 | task, from one-line file-processing tasks to large, elaborate systems. | |
341 | For many people, Perl serves as a great replacement for shell scripting. | |
342 | For others, it serves as a convenient, high-level replacement for most of | |
343 | what they'd program in low-level languages like C or \*(C+. It's ultimately | |
344 | up to you (and possibly your management) which tasks you'll use Perl | |
345 | for and which you won't. | |
346 | .PP | |
347 | If you have a library that provides an \s-1API\s0, you can make any component | |
348 | of it available as just another Perl function or variable using a Perl | |
349 | extension written in C or \*(C+ and dynamically linked into your main | |
350 | perl interpreter. You can also go the other direction, and write your | |
351 | main program in C or \*(C+, and then link in some Perl code on the fly, | |
352 | to create a powerful application. See perlembed. | |
353 | .PP | |
354 | That said, there will always be small, focused, special-purpose | |
355 | languages dedicated to a specific problem domain that are simply more | |
356 | convenient for certain kinds of problems. Perl tries to be all things | |
357 | to all people, but nothing special to anyone. Examples of specialized | |
358 | languages that come to mind include prolog and matlab. | |
359 | .Sh "When shouldn't I program in Perl?" | |
360 | .IX Subsection "When shouldn't I program in Perl?" | |
361 | When your manager forbids it\*(--but do consider replacing them :\-). | |
362 | .PP | |
363 | Actually, one good reason is when you already have an existing | |
364 | application written in another language that's all done (and done | |
365 | well), or you have an application language specifically designed for a | |
366 | certain task (e.g. prolog, make). | |
367 | .PP | |
368 | For various reasons, Perl is probably not well-suited for real-time | |
369 | embedded systems, low-level operating systems development work like | |
370 | device drivers or context-switching code, complex multi-threaded | |
371 | shared-memory applications, or extremely large applications. You'll | |
372 | notice that perl is not itself written in Perl. | |
373 | .PP | |
374 | The new, native-code compiler for Perl may eventually reduce the | |
375 | limitations given in the previous statement to some degree, but understand | |
376 | that Perl remains fundamentally a dynamically typed language, not | |
377 | a statically typed one. You certainly won't be chastised if you don't | |
378 | trust nuclear-plant or brain-surgery monitoring code to it. And Larry | |
379 | will sleep easier, too\*(--Wall Street programs not withstanding. :\-) | |
380 | .ie n .Sh "What's the difference between ""perl"" and ""Perl""?" | |
381 | .el .Sh "What's the difference between ``perl'' and ``Perl''?" | |
382 | .IX Subsection "What's the difference between perl and Perl?" | |
383 | One bit. Oh, you weren't talking \s-1ASCII\s0? :\-) Larry now uses \*(L"Perl\*(R" to | |
384 | signify the language proper and \*(L"perl\*(R" the implementation of it, | |
385 | i.e. the current interpreter. Hence Tom's quip that \*(L"Nothing but perl | |
386 | can parse Perl.\*(R" You may or may not choose to follow this usage. For | |
387 | example, parallelism means \*(L"awk and perl\*(R" and \*(L"Python and Perl\*(R" look | |
388 | \&\s-1OK\s0, while \*(L"awk and Perl\*(R" and \*(L"Python and perl\*(R" do not. But never | |
389 | write \*(L"\s-1PERL\s0\*(R", because perl is not an acronym, apocryphal | |
390 | folklore and post-facto expansions notwithstanding. | |
391 | .Sh "Is it a Perl program or a Perl script?" | |
392 | .IX Subsection "Is it a Perl program or a Perl script?" | |
393 | Larry doesn't really care. He says (half in jest) that \*(L"a script is | |
394 | what you give the actors. A program is what you give the audience.\*(R" | |
395 | .PP | |
396 | Originally, a script was a canned sequence of normally interactive | |
397 | commands\*(--that is, a chat script. Something like a \s-1UUCP\s0 or \s-1PPP\s0 chat | |
398 | script or an expect script fits the bill nicely, as do configuration | |
399 | scripts run by a program at its start up, such \fI.cshrc\fR or \fI.ircrc\fR, | |
400 | for example. Chat scripts were just drivers for existing programs, | |
401 | not stand-alone programs in their own right. | |
402 | .PP | |
403 | A computer scientist will correctly explain that all programs are | |
404 | interpreted and that the only question is at what level. But if you | |
405 | ask this question of someone who isn't a computer scientist, they might | |
406 | tell you that a \fIprogram\fR has been compiled to physical machine code | |
407 | once and can then be run multiple times, whereas a \fIscript\fR must be | |
408 | translated by a program each time it's used. | |
409 | .PP | |
410 | Perl programs are (usually) neither strictly compiled nor strictly | |
411 | interpreted. They can be compiled to a byte-code form (something of a | |
412 | Perl virtual machine) or to completely different languages, like C or | |
413 | assembly language. You can't tell just by looking at it whether the | |
414 | source is destined for a pure interpreter, a parse-tree interpreter, | |
415 | a byte-code interpreter, or a native-code compiler, so it's hard to give | |
416 | a definitive answer here. | |
417 | .PP | |
418 | Now that \*(L"script\*(R" and \*(L"scripting\*(R" are terms that have been seized by | |
419 | unscrupulous or unknowing marketeers for their own nefarious purposes, | |
420 | they have begun to take on strange and often pejorative meanings, | |
421 | like \*(L"non serious\*(R" or \*(L"not real programming\*(R". Consequently, some Perl | |
422 | programmers prefer to avoid them altogether. | |
423 | .Sh "What is a \s-1JAPH\s0?" | |
424 | .IX Subsection "What is a JAPH?" | |
425 | These are the \*(L"just another perl hacker\*(R" signatures that some people | |
426 | sign their postings with. Randal Schwartz made these famous. About | |
427 | 100 of the earlier ones are available from | |
428 | http://www.cpan.org/misc/japh . | |
429 | .Sh "Where can I get a list of Larry Wall witticisms?" | |
430 | .IX Subsection "Where can I get a list of Larry Wall witticisms?" | |
431 | Over a hundred quips by Larry, from postings of his or source code, | |
432 | can be found at http://www.cpan.org/misc/lwall\-quotes.txt.gz . | |
433 | .Sh "How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?" | |
434 | .IX Subsection "How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?" | |
435 | If your manager or employees are wary of unsupported software, or | |
436 | software which doesn't officially ship with your operating system, you | |
437 | might try to appeal to their self\-interest. If programmers can be | |
438 | more productive using and utilizing Perl constructs, functionality, | |
439 | simplicity, and power, then the typical manager/supervisor/employee | |
440 | may be persuaded. Regarding using Perl in general, it's also | |
441 | sometimes helpful to point out that delivery times may be reduced | |
442 | using Perl compared to other languages. | |
443 | .PP | |
444 | If you have a project which has a bottleneck, especially in terms of | |
445 | translation or testing, Perl almost certainly will provide a viable, | |
446 | quick solution. In conjunction with any persuasion effort, you | |
447 | should not fail to point out that Perl is used, quite extensively, and | |
448 | with extremely reliable and valuable results, at many large computer | |
449 | software and hardware companies throughout the world. In fact, | |
450 | many Unix vendors now ship Perl by default. Support is usually | |
451 | just a news-posting away, if you can't find the answer in the | |
452 | \&\fIcomprehensive\fR documentation, including this \s-1FAQ\s0. | |
453 | .PP | |
454 | See http://www.perl.org/advocacy/ for more information. | |
455 | .PP | |
456 | If you face reluctance to upgrading from an older version of perl, | |
457 | then point out that version 4 is utterly unmaintained and unsupported | |
458 | by the Perl Development Team. Another big sell for Perl5 is the large | |
459 | number of modules and extensions which greatly reduce development time | |
460 | for any given task. Also mention that the difference between version | |
461 | 4 and version 5 of Perl is like the difference between awk and \*(C+. | |
462 | (Well, \s-1OK\s0, maybe it's not quite that distinct, but you get the idea.) | |
463 | If you want support and a reasonable guarantee that what you're | |
464 | developing will continue to work in the future, then you have to run | |
465 | the supported version. As of December 2003 that means running either | |
466 | 5.8.2 (released in November 2003), or one of the older releases like | |
467 | 5.6.2 (also released in November 2003; a maintenance release to let perl | |
468 | 5.6 compile on newer systems as 5.6.1 was released in April 2001) or | |
469 | 5.005_03 (released in March 1999), | |
470 | although 5.004_05 isn't that bad if you \fBabsolutely\fR need such an old | |
471 | version (released in April 1999) for stability reasons. | |
472 | Anything older than 5.004_05 shouldn't be used. | |
473 | .PP | |
474 | Of particular note is the massive bug hunt for buffer overflow | |
475 | problems that went into the 5.004 release. All releases prior to | |
476 | that, including perl4, are considered insecure and should be upgraded | |
477 | as soon as possible. | |
478 | .PP | |
479 | In August 2000 in all Linux distributions a new security problem was | |
480 | found in the optional 'suidperl' (not built or installed by default) | |
481 | in all the Perl branches 5.6, 5.005, and 5.004, see | |
482 | http://www.cpan.org/src/5.0/sperl\-2000\-08\-05/ | |
483 | Perl maintenance releases 5.6.1 and 5.8.0 have this security hole closed. | |
484 | Most, if not all, Linux distribution have patches for this | |
485 | vulnerability available, see http://www.linuxsecurity.com/advisories/ , | |
486 | but the most recommendable way is to upgrade to at least Perl 5.6.1. | |
487 | .SH "AUTHOR AND COPYRIGHT" | |
488 | .IX Header "AUTHOR AND COPYRIGHT" | |
489 | Copyright (c) 1997\-2006 Tom Christiansen, Nathan Torkington, and | |
490 | other authors as noted. All rights reserved. | |
491 | .PP | |
492 | This documentation is free; you can redistribute it and/or modify it | |
493 | under the same terms as Perl itself. | |
494 | .PP | |
495 | Irrespective of its distribution, all code examples here are in the public | |
496 | domain. You are permitted and encouraged to use this code and any | |
497 | derivatives thereof in your own programs for fun or for profit as you | |
498 | see fit. A simple comment in the code giving credit to the \s-1FAQ\s0 would | |
499 | be courteous but is not required. |