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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLFAQ1 1" |
| 132 | .TH PERLFAQ1 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlfaq1 \- General Questions About Perl ($Revision: 1.8 $, $Date: 2002/04/07 18:46:13 $) |
| 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://perlhacker.org/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 | You should definitely use version 5. Version 4 is old, limited, and |
| 187 | no longer maintained; its last patch (4.036) was in 1992, long ago and |
| 188 | far away. Sure, it's stable, but so is anything that's dead; in fact, |
| 189 | perl4 had been called a dead, flea-bitten camel carcass. The most recent |
| 190 | production release is 5.6 (although 5.005_03 is still supported). |
| 191 | The most cutting-edge development release is 5.7. Further references |
| 192 | to the Perl language in this document refer to the production release |
| 193 | unless otherwise specified. There may be one or more official bug fixes |
| 194 | by the time you read this, and also perhaps some experimental versions |
| 195 | on the way to the next release. All releases prior to 5.004 were subject |
| 196 | to buffer overruns, a grave security issue. |
| 197 | .Sh "What are perl4 and perl5?" |
| 198 | .IX Subsection "What are perl4 and perl5?" |
| 199 | Perl4 and perl5 are informal names for different versions of the Perl |
| 200 | programming language. It's easier to say \*(L"perl5\*(R" than it is to say |
| 201 | \&\*(L"the 5(.004) release of Perl\*(R", but some people have interpreted this |
| 202 | to mean there's a language called \*(L"perl5\*(R", which isn't the case. |
| 203 | Perl5 is merely the popular name for the fifth major release (October 1994), |
| 204 | while perl4 was the fourth major release (March 1991). There was also a |
| 205 | perl1 (in January 1988), a perl2 (June 1988), and a perl3 (October 1989). |
| 206 | .PP |
| 207 | The 5.0 release is, essentially, a ground-up rewrite of the original |
| 208 | perl source code from releases 1 through 4. It has been modularized, |
| 209 | object\-oriented, tweaked, trimmed, and optimized until it almost doesn't |
| 210 | look like the old code. However, the interface is mostly the same, and |
| 211 | compatibility with previous releases is very high. |
| 212 | See \*(L"Perl4 to Perl5 Traps\*(R" in perltrap. |
| 213 | .PP |
| 214 | To avoid the \*(L"what language is perl5?\*(R" confusion, some people prefer to |
| 215 | simply use \*(L"perl\*(R" to refer to the latest version of perl and avoid using |
| 216 | \&\*(L"perl5\*(R" altogether. It's not really that big a deal, though. |
| 217 | .PP |
| 218 | See perlhist for a history of Perl revisions. |
| 219 | .Sh "What is perl6?" |
| 220 | .IX Subsection "What is perl6?" |
| 221 | At The Second O'Reilly Open Source Software Convention, Larry Wall |
| 222 | announced Perl6 development would begin in earnest. Perl6 was an oft |
| 223 | used term for Chip Salzenberg's project to rewrite Perl in \*(C+ named |
| 224 | Topaz. However, Topaz provided valuable insights to the next version |
| 225 | of Perl and its implementation, but was ultimately abandoned. |
| 226 | .PP |
| 227 | If you want to learn more about Perl6, or have a desire to help in |
| 228 | the crusade to make Perl a better place then peruse the Perl6 developers |
| 229 | page at http://dev.perl.org/perl6/ and get involved. |
| 230 | .PP |
| 231 | Perl6 is not scheduled for release yet, and Perl5 will still be supported |
| 232 | for quite awhile after its release. Do not wait for Perl6 to do whatever |
| 233 | you need to do. |
| 234 | .PP |
| 235 | \&\*(L"We're really serious about reinventing everything that needs reinventing.\*(R" |
| 236 | \&\-\-Larry Wall |
| 237 | .Sh "How stable is Perl?" |
| 238 | .IX Subsection "How stable is Perl?" |
| 239 | Production releases, which incorporate bug fixes and new functionality, |
| 240 | are widely tested before release. Since the 5.000 release, we have |
| 241 | averaged only about one production release per year. |
| 242 | .PP |
| 243 | Larry and the Perl development team occasionally make changes to the |
| 244 | internal core of the language, but all possible efforts are made toward |
| 245 | backward compatibility. While not quite all perl4 scripts run flawlessly |
| 246 | under perl5, an update to perl should nearly never invalidate a program |
| 247 | written for an earlier version of perl (barring accidental bug fixes |
| 248 | and the rare new keyword). |
| 249 | .Sh "Is Perl difficult to learn?" |
| 250 | .IX Subsection "Is Perl difficult to learn?" |
| 251 | No, Perl is easy to start learning\*(--and easy to keep learning. It looks |
| 252 | like most programming languages you're likely to have experience |
| 253 | with, so if you've ever written a C program, an awk script, a shell |
| 254 | script, or even a \s-1BASIC\s0 program, you're already partway there. |
| 255 | .PP |
| 256 | Most tasks only require a small subset of the Perl language. One of |
| 257 | the guiding mottos for Perl development is \*(L"there's more than one way |
| 258 | to do it\*(R" (\s-1TMTOWTDI\s0, sometimes pronounced \*(L"tim toady\*(R"). Perl's |
| 259 | learning curve is therefore shallow (easy to learn) and long (there's |
| 260 | a whole lot you can do if you really want). |
| 261 | .PP |
| 262 | Finally, because Perl is frequently (but not always, and certainly not by |
| 263 | definition) an interpreted language, you can write your programs and test |
| 264 | them without an intermediate compilation step, allowing you to experiment |
| 265 | and test/debug quickly and easily. This ease of experimentation flattens |
| 266 | the learning curve even more. |
| 267 | .PP |
| 268 | Things that make Perl easier to learn: Unix experience, almost any kind |
| 269 | of programming experience, an understanding of regular expressions, and |
| 270 | the ability to understand other people's code. If there's something you |
| 271 | need to do, then it's probably already been done, and a working example is |
| 272 | usually available for free. Don't forget the new perl modules, either. |
| 273 | They're discussed in Part 3 of this \s-1FAQ\s0, along with \s-1CPAN\s0, which is |
| 274 | discussed in Part 2. |
| 275 | .Sh "How does Perl compare with other languages like Java, Python, \s-1REXX\s0, Scheme, or Tcl?" |
| 276 | .IX Subsection "How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?" |
| 277 | Favorably in some areas, unfavorably in others. Precisely which areas |
| 278 | are good and bad is often a personal choice, so asking this question |
| 279 | on Usenet runs a strong risk of starting an unproductive Holy War. |
| 280 | .PP |
| 281 | Probably the best thing to do is try to write equivalent code to do a |
| 282 | set of tasks. These languages have their own newsgroups in which you |
| 283 | can learn about (but hopefully not argue about) them. |
| 284 | .PP |
| 285 | Some comparison documents can be found at http://language.perl.com/versus/ |
| 286 | if you really can't stop yourself. |
| 287 | .Sh "Can I do [task] in Perl?" |
| 288 | .IX Subsection "Can I do [task] in Perl?" |
| 289 | Perl is flexible and extensible enough for you to use on virtually any |
| 290 | task, from one-line file-processing tasks to large, elaborate systems. |
| 291 | For many people, Perl serves as a great replacement for shell scripting. |
| 292 | For others, it serves as a convenient, high-level replacement for most of |
| 293 | what they'd program in low-level languages like C or \*(C+. It's ultimately |
| 294 | up to you (and possibly your management) which tasks you'll use Perl |
| 295 | for and which you won't. |
| 296 | .PP |
| 297 | If you have a library that provides an \s-1API\s0, you can make any component |
| 298 | of it available as just another Perl function or variable using a Perl |
| 299 | extension written in C or \*(C+ and dynamically linked into your main |
| 300 | perl interpreter. You can also go the other direction, and write your |
| 301 | main program in C or \*(C+, and then link in some Perl code on the fly, |
| 302 | to create a powerful application. See perlembed. |
| 303 | .PP |
| 304 | That said, there will always be small, focused, special-purpose |
| 305 | languages dedicated to a specific problem domain that are simply more |
| 306 | convenient for certain kinds of problems. Perl tries to be all things |
| 307 | to all people, but nothing special to anyone. Examples of specialized |
| 308 | languages that come to mind include prolog and matlab. |
| 309 | .Sh "When shouldn't I program in Perl?" |
| 310 | .IX Subsection "When shouldn't I program in Perl?" |
| 311 | When your manager forbids it\*(--but do consider replacing them :\-). |
| 312 | .PP |
| 313 | Actually, one good reason is when you already have an existing |
| 314 | application written in another language that's all done (and done |
| 315 | well), or you have an application language specifically designed for a |
| 316 | certain task (e.g. prolog, make). |
| 317 | .PP |
| 318 | For various reasons, Perl is probably not well-suited for real-time |
| 319 | embedded systems, low-level operating systems development work like |
| 320 | device drivers or context-switching code, complex multi-threaded |
| 321 | shared-memory applications, or extremely large applications. You'll |
| 322 | notice that perl is not itself written in Perl. |
| 323 | .PP |
| 324 | The new, native-code compiler for Perl may eventually reduce the |
| 325 | limitations given in the previous statement to some degree, but understand |
| 326 | that Perl remains fundamentally a dynamically typed language, not |
| 327 | a statically typed one. You certainly won't be chastised if you don't |
| 328 | trust nuclear-plant or brain-surgery monitoring code to it. And Larry |
| 329 | will sleep easier, too\*(--Wall Street programs not withstanding. :\-) |
| 330 | .ie n .Sh "What's the difference between ""perl"" and ""Perl""?" |
| 331 | .el .Sh "What's the difference between ``perl'' and ``Perl''?" |
| 332 | .IX Subsection "What's the difference between perl and Perl?" |
| 333 | One bit. Oh, you weren't talking \s-1ASCII\s0? :\-) Larry now uses \*(L"Perl\*(R" to |
| 334 | signify the language proper and \*(L"perl\*(R" the implementation of it, |
| 335 | i.e. the current interpreter. Hence Tom's quip that \*(L"Nothing but perl |
| 336 | can parse Perl.\*(R" You may or may not choose to follow this usage. For |
| 337 | example, parallelism means \*(L"awk and perl\*(R" and \*(L"Python and Perl\*(R" look |
| 338 | \&\s-1OK\s0, while \*(L"awk and Perl\*(R" and \*(L"Python and perl\*(R" do not. But never |
| 339 | write \*(L"\s-1PERL\s0\*(R", because perl is not an acronym, apocryphal |
| 340 | folklore and post-facto expansions notwithstanding. |
| 341 | .Sh "Is it a Perl program or a Perl script?" |
| 342 | .IX Subsection "Is it a Perl program or a Perl script?" |
| 343 | Larry doesn't really care. He says (half in jest) that \*(L"a script is |
| 344 | what you give the actors. A program is what you give the audience.\*(R" |
| 345 | .PP |
| 346 | Originally, a script was a canned sequence of normally interactive |
| 347 | commands\*(--that is, a chat script. Something like a \s-1UUCP\s0 or \s-1PPP\s0 chat |
| 348 | script or an expect script fits the bill nicely, as do configuration |
| 349 | scripts run by a program at its start up, such \fI.cshrc\fR or \fI.ircrc\fR, |
| 350 | for example. Chat scripts were just drivers for existing programs, |
| 351 | not stand-alone programs in their own right. |
| 352 | .PP |
| 353 | A computer scientist will correctly explain that all programs are |
| 354 | interpreted and that the only question is at what level. But if you |
| 355 | ask this question of someone who isn't a computer scientist, they might |
| 356 | tell you that a \fIprogram\fR has been compiled to physical machine code |
| 357 | once and can then be run multiple times, whereas a \fIscript\fR must be |
| 358 | translated by a program each time it's used. |
| 359 | .PP |
| 360 | Perl programs are (usually) neither strictly compiled nor strictly |
| 361 | interpreted. They can be compiled to a byte-code form (something of a |
| 362 | Perl virtual machine) or to completely different languages, like C or |
| 363 | assembly language. You can't tell just by looking at it whether the |
| 364 | source is destined for a pure interpreter, a parse-tree interpreter, |
| 365 | a byte-code interpreter, or a native-code compiler, so it's hard to give |
| 366 | a definitive answer here. |
| 367 | .PP |
| 368 | Now that \*(L"script\*(R" and \*(L"scripting\*(R" are terms that have been seized by |
| 369 | unscrupulous or unknowing marketeers for their own nefarious purposes, |
| 370 | they have begun to take on strange and often pejorative meanings, |
| 371 | like \*(L"non serious\*(R" or \*(L"not real programming\*(R". Consequently, some Perl |
| 372 | programmers prefer to avoid them altogether. |
| 373 | .Sh "What is a \s-1JAPH\s0?" |
| 374 | .IX Subsection "What is a JAPH?" |
| 375 | These are the \*(L"just another perl hacker\*(R" signatures that some people |
| 376 | sign their postings with. Randal Schwartz made these famous. About |
| 377 | 100 of the earlier ones are available from |
| 378 | http://www.cpan.org/misc/japh . |
| 379 | .Sh "Where can I get a list of Larry Wall witticisms?" |
| 380 | .IX Subsection "Where can I get a list of Larry Wall witticisms?" |
| 381 | Over a hundred quips by Larry, from postings of his or source code, |
| 382 | can be found at http://www.cpan.org/misc/lwall\-quotes.txt.gz . |
| 383 | .Sh "How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?" |
| 384 | .IX Subsection "How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?" |
| 385 | If your manager or employees are wary of unsupported software, or |
| 386 | software which doesn't officially ship with your operating system, you |
| 387 | might try to appeal to their self\-interest. If programmers can be |
| 388 | more productive using and utilizing Perl constructs, functionality, |
| 389 | simplicity, and power, then the typical manager/supervisor/employee |
| 390 | may be persuaded. Regarding using Perl in general, it's also |
| 391 | sometimes helpful to point out that delivery times may be reduced |
| 392 | using Perl compared to other languages. |
| 393 | .PP |
| 394 | If you have a project which has a bottleneck, especially in terms of |
| 395 | translation or testing, Perl almost certainly will provide a viable, |
| 396 | quick solution. In conjunction with any persuasion effort, you |
| 397 | should not fail to point out that Perl is used, quite extensively, and |
| 398 | with extremely reliable and valuable results, at many large computer |
| 399 | software and hardware companies throughout the world. In fact, |
| 400 | many Unix vendors now ship Perl by default. Support is usually |
| 401 | just a news-posting away, if you can't find the answer in the |
| 402 | \&\fIcomprehensive\fR documentation, including this \s-1FAQ\s0. |
| 403 | .PP |
| 404 | See http://www.perl.org/advocacy/ for more information. |
| 405 | .PP |
| 406 | If you face reluctance to upgrading from an older version of perl, |
| 407 | then point out that version 4 is utterly unmaintained and unsupported |
| 408 | by the Perl Development Team. Another big sell for Perl5 is the large |
| 409 | number of modules and extensions which greatly reduce development time |
| 410 | for any given task. Also mention that the difference between version |
| 411 | 4 and version 5 of Perl is like the difference between awk and \*(C+. |
| 412 | (Well, \s-1OK\s0, maybe it's not quite that distinct, but you get the idea.) |
| 413 | If you want support and a reasonable guarantee that what you're |
| 414 | developing will continue to work in the future, then you have to run |
| 415 | the supported version. As of January 2002 that probably means |
| 416 | running either of the releases 5.6.1 (released in April 2001) or |
| 417 | 5.005_03 (released in March 1999), although 5.004_05 isn't that bad |
| 418 | if you \fBabsolutely\fR need such an old version (released in April 1999) |
| 419 | for stability reasons. Anything older than 5.004_05 shouldn't be used. |
| 420 | .PP |
| 421 | Of particular note is the massive bug hunt for buffer overflow |
| 422 | problems that went into the 5.004 release. All releases prior to |
| 423 | that, including perl4, are considered insecure and should be upgraded |
| 424 | as soon as possible. |
| 425 | .PP |
| 426 | In August 2000 in all Linux distributions a new security problem was |
| 427 | found in the optional 'suidperl' (not built or installed by default) |
| 428 | in all the Perl branches 5.6, 5.005, and 5.004, see |
| 429 | http://www.cpan.org/src/5.0/sperl\-2000\-08\-05/ |
| 430 | Perl maintenance releases 5.6.1 and 5.8.0 have this security hole closed. |
| 431 | Most, if not all, Linux distribution have patches for this |
| 432 | vulnerability available, see http://www.linuxsecurity.com/advisories/ , |
| 433 | but the most recommendable way is to upgrade to at least Perl 5.6.1. |
| 434 | .SH "AUTHOR AND COPYRIGHT" |
| 435 | .IX Header "AUTHOR AND COPYRIGHT" |
| 436 | Copyright (c) 1997, 1998, 1999, 2000, 2001 Tom Christiansen and Nathan |
| 437 | Torkington. All rights reserved. |
| 438 | .PP |
| 439 | This documentation is free; you can redistribute it and/or modify it |
| 440 | under the same terms as Perl itself. |
| 441 | .PP |
| 442 | Irrespective of its distribution, all code examples here are in the public |
| 443 | domain. You are permitted and encouraged to use this code and any |
| 444 | derivatives thereof in your own programs for fun or for profit as you |
| 445 | see fit. A simple comment in the code giving credit to the \s-1FAQ\s0 would |
| 446 | be courteous but is not required. |