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1 | =head1 NAME |
2 | |
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3 | perlfaq3 - Programming Tools ($Revision: 1.22 $, $Date: 1997/04/24 22:43:42 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section of the FAQ answers questions related to programmer tools |
8 | and programming support. |
9 | |
10 | =head2 How do I do (anything)? |
11 | |
12 | Have you looked at CPAN (see L<perlfaq2>)? The chances are that |
13 | someone has already written a module that can solve your problem. |
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14 | Have you read the appropriate man pages? Here's a brief index: |
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15 | |
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16 | Basics perldata, perlvar, perlsyn, perlop, perlsub |
17 | Execution perlrun, perldebug |
18 | Functions perlfunc |
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19 | Objects perlref, perlmod, perlobj, perltie |
20 | Data Structures perlref, perllol, perldsc |
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21 | Modules perlmod, perlmodlib, perlsub |
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22 | Regexps perlre, perlfunc, perlop, perllocale |
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23 | Moving to perl5 perltrap, perl |
24 | Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed |
25 | Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html |
26 | (not a man-page but still useful) |
27 | |
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28 | L<perltoc> provides a crude table of contents for the perl man page set. |
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29 | |
30 | =head2 How can I use Perl interactively? |
31 | |
32 | The typical approach uses the Perl debugger, described in the |
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33 | perldebug(1) man page, on an "empty" program, like this: |
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34 | |
35 | perl -de 42 |
36 | |
37 | Now just type in any legal Perl code, and it will be immediately |
38 | evaluated. You can also examine the symbol table, get stack |
39 | backtraces, check variable values, set breakpoints, and other |
40 | operations typically found in symbolic debuggers |
41 | |
42 | =head2 Is there a Perl shell? |
43 | |
44 | In general, no. The Shell.pm module (distributed with perl) makes |
45 | perl try commands which aren't part of the Perl language as shell |
46 | commands. perlsh from the source distribution is simplistic and |
47 | uninteresting, but may still be what you want. |
48 | |
49 | =head2 How do I debug my Perl programs? |
50 | |
51 | Have you used C<-w>? |
52 | |
53 | Have you tried C<use strict>? |
54 | |
55 | Did you check the returns of each and every system call? |
56 | |
57 | Did you read L<perltrap>? |
58 | |
59 | Have you tried the Perl debugger, described in L<perldebug>? |
60 | |
61 | =head2 How do I profile my Perl programs? |
62 | |
63 | You should get the Devel::DProf module from CPAN, and also use |
64 | Benchmark.pm from the standard distribution. Benchmark lets you time |
65 | specific portions of your code, while Devel::DProf gives detailed |
66 | breakdowns of where your code spends its time. |
67 | |
68 | =head2 How do I cross-reference my Perl programs? |
69 | |
70 | The B::Xref module, shipped with the new, alpha-release Perl compiler |
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71 | (not the general distribution prior to the 5.005 release), can be used |
72 | to generate cross-reference reports for Perl programs. |
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73 | |
74 | perl -MO=Xref[,OPTIONS] foo.pl |
75 | |
76 | =head2 Is there a pretty-printer (formatter) for Perl? |
77 | |
78 | There is no program that will reformat Perl as much as indent(1) will |
79 | do for C. The complex feedback between the scanner and the parser |
80 | (this feedback is what confuses the vgrind and emacs programs) makes it |
81 | challenging at best to write a stand-alone Perl parser. |
82 | |
83 | Of course, if you simply follow the guidelines in L<perlstyle>, you |
84 | shouldn't need to reformat. |
85 | |
86 | Your editor can and should help you with source formatting. The |
87 | perl-mode for emacs can provide a remarkable amount of help with most |
88 | (but not all) code, and even less programmable editors can provide |
89 | significant assistance. |
90 | |
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91 | If you are used to using vgrind program for printing out nice code to |
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92 | a laser printer, you can take a stab at this using |
93 | http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the |
94 | results are not particularly satisfying for sophisticated code. |
95 | |
96 | =head2 Is there a ctags for Perl? |
97 | |
98 | There's a simple one at |
99 | http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do |
100 | the trick. |
101 | |
102 | =head2 Where can I get Perl macros for vi? |
103 | |
104 | For a complete version of Tom Christiansen's vi configuration file, |
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105 | see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/toms.exrc, |
106 | the standard benchmark file for vi emulators. This runs best with nvi, |
107 | the current version of vi out of Berkeley, which incidentally can be built |
108 | with an embedded Perl interpreter -- see http://www.perl.com/CPAN/src/misc. |
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109 | |
110 | =head2 Where can I get perl-mode for emacs? |
111 | |
112 | Since Emacs version 19 patchlevel 22 or so, there have been both a |
113 | perl-mode.el and support for the perl debugger built in. These should |
114 | come with the standard Emacs 19 distribution. |
115 | |
116 | In the perl source directory, you'll find a directory called "emacs", |
117 | which contains a cperl-mode that color-codes keywords, provides |
118 | context-sensitive help, and other nifty things. |
119 | |
120 | Note that the perl-mode of emacs will have fits with "main'foo" |
121 | (single quote), and mess up the indentation and hilighting. You |
122 | should be using "main::foo", anyway. |
123 | |
124 | =head2 How can I use curses with Perl? |
125 | |
126 | The Curses module from CPAN provides a dynamically loadable object |
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127 | module interface to a curses library. A small demo can be found at the |
128 | directory http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep; |
129 | this program repeats a command and updates the screen as needed, rendering |
130 | B<rep ps axu> similar to B<top>. |
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131 | |
132 | =head2 How can I use X or Tk with Perl? |
133 | |
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134 | Tk is a completely Perl-based, object-oriented interface to the Tk toolkit |
135 | that doesn't force you to use Tcl just to get at Tk. Sx is an interface |
136 | to the Athena Widget set. Both are available from CPAN. See the |
137 | directory http://www.perl.com/CPAN/modules/by-category/08_User_Interfaces/ |
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138 | |
139 | =head2 How can I generate simple menus without using CGI or Tk? |
140 | |
141 | The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz |
142 | module, which is curses-based, can help with this. |
143 | |
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144 | =head2 What is undump? |
145 | |
146 | See the next questions. |
147 | |
148 | =head2 How can I make my Perl program run faster? |
149 | |
150 | The best way to do this is to come up with a better algorithm. |
151 | This can often make a dramatic difference. Chapter 8 in the Camel |
152 | has some efficiency tips in it you might want to look at. |
153 | |
154 | Other approaches include autoloading seldom-used Perl code. See the |
155 | AutoSplit and AutoLoader modules in the standard distribution for |
156 | that. Or you could locate the bottleneck and think about writing just |
157 | that part in C, the way we used to take bottlenecks in C code and |
158 | write them in assembler. Similar to rewriting in C is the use of |
159 | modules that have critical sections written in C (for instance, the |
160 | PDL module from CPAN). |
161 | |
162 | In some cases, it may be worth it to use the backend compiler to |
163 | produce byte code (saving compilation time) or compile into C, which |
164 | will certainly save compilation time and sometimes a small amount (but |
165 | not much) execution time. See the question about compiling your Perl |
166 | programs. |
167 | |
168 | If you're currently linking your perl executable to a shared libc.so, |
169 | you can often gain a 10-25% performance benefit by rebuilding it to |
170 | link with a static libc.a instead. This will make a bigger perl |
171 | executable, but your Perl programs (and programmers) may thank you for |
172 | it. See the F<INSTALL> file in the source distribution for more |
173 | information. |
174 | |
175 | Unsubstantiated reports allege that Perl interpreters that use sfio |
176 | outperform those that don't (for IO intensive applications). To try |
177 | this, see the F<INSTALL> file in the source distribution, especially |
178 | the "Selecting File IO mechanisms" section. |
179 | |
180 | The undump program was an old attempt to speed up your Perl program |
181 | by storing the already-compiled form to disk. This is no longer |
182 | a viable option, as it only worked on a few architectures, and |
183 | wasn't a good solution anyway. |
184 | |
185 | =head2 How can I make my Perl program take less memory? |
186 | |
187 | When it comes to time-space tradeoffs, Perl nearly always prefers to |
188 | throw memory at a problem. Scalars in Perl use more memory than |
189 | strings in C, arrays take more that, and hashes use even more. While |
190 | there's still a lot to be done, recent releases have been addressing |
191 | these issues. For example, as of 5.004, duplicate hash keys are |
192 | shared amongst all hashes using them, so require no reallocation. |
193 | |
194 | In some cases, using substr() or vec() to simulate arrays can be |
195 | highly beneficial. For example, an array of a thousand booleans will |
196 | take at least 20,000 bytes of space, but it can be turned into one |
197 | 125-byte bit vector for a considerable memory savings. The standard |
198 | Tie::SubstrHash module can also help for certain types of data |
199 | structure. If you're working with specialist data structures |
200 | (matrices, for instance) modules that implement these in C may use |
201 | less memory than equivalent Perl modules. |
202 | |
203 | Another thing to try is learning whether your Perl was compiled with |
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204 | the system malloc or with Perl's builtin malloc. Whichever one it |
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205 | is, try using the other one and see whether this makes a difference. |
206 | Information about malloc is in the F<INSTALL> file in the source |
207 | distribution. You can find out whether you are using perl's malloc by |
208 | typing C<perl -V:usemymalloc>. |
209 | |
210 | =head2 Is it unsafe to return a pointer to local data? |
211 | |
212 | No, Perl's garbage collection system takes care of this. |
213 | |
214 | sub makeone { |
215 | my @a = ( 1 .. 10 ); |
216 | return \@a; |
217 | } |
218 | |
219 | for $i ( 1 .. 10 ) { |
220 | push @many, makeone(); |
221 | } |
222 | |
223 | print $many[4][5], "\n"; |
224 | |
225 | print "@many\n"; |
226 | |
227 | =head2 How can I free an array or hash so my program shrinks? |
228 | |
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229 | You can't. Memory the system allocates to a program will in practice |
230 | never be returned to the system. That's why long-running programs |
231 | sometimes re-exec themselves. |
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232 | |
233 | However, judicious use of my() on your variables will help make sure |
234 | that they go out of scope so that Perl can free up their storage for |
235 | use in other parts of your program. (NB: my() variables also execute |
236 | about 10% faster than globals.) A global variable, of course, never |
237 | goes out of scope, so you can't get its space automatically reclaimed, |
238 | although undef()ing and/or delete()ing it will achieve the same effect. |
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239 | In general, memory allocation and de-allocation isn't something you can |
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240 | or should be worrying about much in Perl, but even this capability |
241 | (preallocation of data types) is in the works. |
242 | |
243 | =head2 How can I make my CGI script more efficient? |
244 | |
245 | Beyond the normal measures described to make general Perl programs |
246 | faster or smaller, a CGI program has additional issues. It may be run |
247 | several times per second. Given that each time it runs it will need |
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248 | to be re-compiled and will often allocate a megabyte or more of system |
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249 | memory, this can be a killer. Compiling into C B<isn't going to help |
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250 | you> because the process start-up overhead is where the bottleneck is. |
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251 | |
252 | There are at least two popular ways to avoid this overhead. One |
253 | solution involves running the Apache HTTP server (available from |
254 | http://www.apache.org/) with either of the mod_perl or mod_fastcgi |
255 | plugin modules. With mod_perl and the Apache::* modules (from CPAN), |
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256 | httpd will run with an embedded Perl interpreter which pre-compiles |
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257 | your script and then executes it within the same address space without |
258 | forking. The Apache extension also gives Perl access to the internal |
259 | server API, so modules written in Perl can do just about anything a |
260 | module written in C can. With the FCGI module (from CPAN), a Perl |
261 | executable compiled with sfio (see the F<INSTALL> file in the |
262 | distribution) and the mod_fastcgi module (available from |
263 | http://www.fastcgi.com/) each of your perl scripts becomes a permanent |
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264 | CGI daemon process. |
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265 | |
266 | Both of these solutions can have far-reaching effects on your system |
267 | and on the way you write your CGI scripts, so investigate them with |
268 | care. |
269 | |
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270 | See http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/. |
271 | |
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272 | =head2 How can I hide the source for my Perl program? |
273 | |
274 | Delete it. :-) Seriously, there are a number of (mostly |
275 | unsatisfactory) solutions with varying levels of "security". |
276 | |
277 | First of all, however, you I<can't> take away read permission, because |
278 | the source code has to be readable in order to be compiled and |
279 | interpreted. (That doesn't mean that a CGI script's source is |
280 | readable by people on the web, though.) So you have to leave the |
281 | permissions at the socially friendly 0755 level. |
282 | |
283 | Some people regard this as a security problem. If your program does |
284 | insecure things, and relies on people not knowing how to exploit those |
285 | insecurities, it is not secure. It is often possible for someone to |
286 | determine the insecure things and exploit them without viewing the |
287 | source. Security through obscurity, the name for hiding your bugs |
288 | instead of fixing them, is little security indeed. |
289 | |
290 | You can try using encryption via source filters (Filter::* from CPAN). |
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291 | But crackers might be able to decrypt it. You can try using the byte |
292 | code compiler and interpreter described below, but crackers might be |
293 | able to de-compile it. You can try using the native-code compiler |
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294 | described below, but crackers might be able to disassemble it. These |
295 | pose varying degrees of difficulty to people wanting to get at your |
296 | code, but none can definitively conceal it (this is true of every |
297 | language, not just Perl). |
298 | |
299 | If you're concerned about people profiting from your code, then the |
300 | bottom line is that nothing but a restrictive licence will give you |
301 | legal security. License your software and pepper it with threatening |
302 | statements like "This is unpublished proprietary software of XYZ Corp. |
303 | Your access to it does not give you permission to use it blah blah |
304 | blah." We are not lawyers, of course, so you should see a lawyer if |
305 | you want to be sure your licence's wording will stand up in court. |
306 | |
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307 | =head2 How can I compile my Perl program into byte code or C? |
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308 | |
309 | Malcolm Beattie has written a multifunction backend compiler, |
310 | available from CPAN, that can do both these things. It is as of |
311 | Feb-1997 in late alpha release, which means it's fun to play with if |
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312 | you're a programmer but not really for people looking for turn-key |
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313 | solutions. |
314 | |
315 | I<Please> understand that merely compiling into C does not in and of |
316 | itself guarantee that your code will run very much faster. That's |
317 | because except for lucky cases where a lot of native type inferencing |
318 | is possible, the normal Perl run time system is still present and thus |
319 | will still take just as long to run and be just as big. Most programs |
320 | save little more than compilation time, leaving execution no more than |
321 | 10-30% faster. A few rare programs actually benefit significantly |
322 | (like several times faster), but this takes some tweaking of your |
323 | code. |
324 | |
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325 | The 5.005 release of Perl itself, whose main goal is merging the various |
326 | non-Unix ports back into the one Perl source, will also have preliminary |
327 | (strictly beta) support for Malcolm's compiler and his light-weight |
328 | processes (sometimes called "threads"). |
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329 | |
330 | You'll probably be astonished to learn that the current version of the |
331 | compiler generates a compiled form of your script whose executable is |
332 | just as big as the original perl executable, and then some. That's |
333 | because as currently written, all programs are prepared for a full |
334 | eval() statement. You can tremendously reduce this cost by building a |
335 | shared libperl.so library and linking against that. See the |
336 | F<INSTALL> podfile in the perl source distribution for details. If |
337 | you link your main perl binary with this, it will make it miniscule. |
338 | For example, on one author's system, /usr/bin/perl is only 11k in |
339 | size! |
340 | |
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341 | In general, the compiler will do nothing to make a Perl program smaller, |
342 | faster, more portable, or more secure. In fact, it will usually hurt |
343 | all of those. The executable will be bigger, your VM system may take |
344 | longer to load the whole thing, the binary is fragile and hard to fix, |
345 | and compilation never stopped software piracy in the form of crackers, |
346 | viruses, or bootleggers. The real advantage of the compiler is merely |
347 | packaging, and once you see the size of what it makes (well, unless |
348 | you use a shared I<libperl.so>), you'll probably want a complete |
349 | Perl install anywayt. |
350 | |
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351 | =head2 How can I get '#!perl' to work on [MS-DOS,NT,...]? |
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352 | |
353 | For OS/2 just use |
354 | |
355 | extproc perl -S -your_switches |
356 | |
357 | as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's |
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358 | `extproc' handling). For DOS one should first invent a corresponding |
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359 | batch file, and codify it in C<ALTERNATIVE_SHEBANG> (see the |
360 | F<INSTALL> file in the source distribution for more information). |
361 | |
362 | The Win95/NT installation, when using the Activeware port of Perl, |
363 | will modify the Registry to associate the .pl extension with the perl |
364 | interpreter. If you install another port, or (eventually) build your |
365 | own Win95/NT Perl using WinGCC, then you'll have to modify the |
366 | Registry yourself. |
367 | |
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368 | Macintosh perl scripts will have the the appropriate Creator and |
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369 | Type, so that double-clicking them will invoke the perl application. |
370 | |
371 | I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just |
372 | throw the perl interpreter into your cgi-bin directory, in order to |
373 | get your scripts working for a web server. This is an EXTREMELY big |
374 | security risk. Take the time to figure out how to do it correctly. |
375 | |
376 | =head2 Can I write useful perl programs on the command line? |
377 | |
378 | Yes. Read L<perlrun> for more information. Some examples follow. |
379 | (These assume standard Unix shell quoting rules.) |
380 | |
381 | # sum first and last fields |
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382 | perl -lane 'print $F[0] + $F[-1]' * |
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383 | |
384 | # identify text files |
385 | perl -le 'for(@ARGV) {print if -f && -T _}' * |
386 | |
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387 | # remove (most) comments from C program |
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388 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
389 | |
390 | # make file a month younger than today, defeating reaper daemons |
391 | perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * |
392 | |
393 | # find first unused uid |
394 | perl -le '$i++ while getpwuid($i); print $i' |
395 | |
396 | # display reasonable manpath |
397 | echo $PATH | perl -nl -072 -e ' |
398 | s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' |
399 | |
400 | Ok, the last one was actually an obfuscated perl entry. :-) |
401 | |
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402 | =head2 Why don't perl one-liners work on my DOS/Mac/VMS system? |
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403 | |
404 | The problem is usually that the command interpreters on those systems |
405 | have rather different ideas about quoting than the Unix shells under |
406 | which the one-liners were created. On some systems, you may have to |
407 | change single-quotes to double ones, which you must I<NOT> do on Unix |
408 | or Plan9 systems. You might also have to change a single % to a %%. |
409 | |
410 | For example: |
411 | |
412 | # Unix |
413 | perl -e 'print "Hello world\n"' |
414 | |
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415 | # DOS, etc. |
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416 | perl -e "print \"Hello world\n\"" |
417 | |
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418 | # Mac |
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419 | print "Hello world\n" |
420 | (then Run "Myscript" or Shift-Command-R) |
421 | |
422 | # VMS |
423 | perl -e "print ""Hello world\n""" |
424 | |
425 | The problem is that none of this is reliable: it depends on the command |
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426 | interpreter. Under Unix, the first two often work. Under DOS, it's |
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427 | entirely possible neither works. If 4DOS was the command shell, I'd |
428 | probably have better luck like this: |
429 | |
430 | perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" |
431 | |
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432 | Under the Mac, it depends which environment you are using. The MacPerl |
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433 | shell, or MPW, is much like Unix shells in its support for several |
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434 | quoting variants, except that it makes free use of the Mac's non-ASCII |
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435 | characters as control characters. |
436 | |
437 | I'm afraid that there is no general solution to all of this. It is a |
438 | mess, pure and simple. |
439 | |
440 | [Some of this answer was contributed by Kenneth Albanowski.] |
441 | |
442 | =head2 Where can I learn about CGI or Web programming in Perl? |
443 | |
444 | For modules, get the CGI or LWP modules from CPAN. For textbooks, |
445 | see the two especially dedicated to web stuff in the question on |
446 | books. For problems and questions related to the web, like "Why |
447 | do I get 500 Errors" or "Why doesn't it run from the browser right |
448 | when it runs fine on the command line", see these sources: |
449 | |
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450 | WWW Security FAQ |
451 | http://www.w3.org/Security/Faq/ |
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452 | |
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453 | Web FAQ |
454 | http://www.boutell.com/faq/ |
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455 | |
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456 | CGI FAQ |
457 | http://www.webthing.com/page.cgi/cgifaq |
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458 | |
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459 | HTTP Spec |
460 | http://www.w3.org/pub/WWW/Protocols/HTTP/ |
461 | |
462 | HTML Spec |
463 | http://www.w3.org/TR/REC-html40/ |
464 | http://www.w3.org/pub/WWW/MarkUp/ |
465 | |
466 | CGI Spec |
467 | http://www.w3.org/CGI/ |
468 | |
469 | CGI Security FAQ |
470 | http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt |
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471 | |
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472 | |
473 | =head2 Where can I learn about object-oriented Perl programming? |
474 | |
475 | L<perltoot> is a good place to start, and you can use L<perlobj> and |
476 | L<perlbot> for reference. Perltoot didn't come out until the 5.004 |
477 | release, but you can get a copy (in pod, html, or postscript) from |
478 | http://www.perl.com/CPAN/doc/FMTEYEWTK/ . |
479 | |
480 | =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp] |
481 | |
482 | If you want to call C from Perl, start with L<perlxstut>, |
483 | moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to |
484 | call Perl from C, then read L<perlembed>, L<perlcall>, and |
485 | L<perlguts>. Don't forget that you can learn a lot from looking at |
486 | how the authors of existing extension modules wrote their code and |
487 | solved their problems. |
488 | |
489 | =head2 I've read perlembed, perlguts, etc., but I can't embed perl in |
490 | my C program, what am I doing wrong? |
491 | |
492 | Download the ExtUtils::Embed kit from CPAN and run `make test'. If |
493 | the tests pass, read the pods again and again and again. If they |
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494 | fail, see L<perlbug> and send a bugreport with the output of |
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495 | C<make test TEST_VERBOSE=1> along with C<perl -V>. |
496 | |
497 | =head2 When I tried to run my script, I got this message. What does it |
498 | mean? |
499 | |
500 | L<perldiag> has a complete list of perl's error messages and warnings, |
501 | with explanatory text. You can also use the splain program (distributed |
502 | with perl) to explain the error messages: |
503 | |
504 | perl program 2>diag.out |
505 | splain [-v] [-p] diag.out |
506 | |
507 | or change your program to explain the messages for you: |
508 | |
509 | use diagnostics; |
510 | |
511 | or |
512 | |
513 | use diagnostics -verbose; |
514 | |
515 | =head2 What's MakeMaker? |
516 | |
517 | This module (part of the standard perl distribution) is designed to |
518 | write a Makefile for an extension module from a Makefile.PL. For more |
519 | information, see L<ExtUtils::MakeMaker>. |
520 | |
521 | =head1 AUTHOR AND COPYRIGHT |
522 | |
5a964f20 |
523 | Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. |
524 | All rights reserved. |
525 | |
526 | When included as part of the Standard Version of Perl, or as part of |
527 | its complete documentation whether printed or otherwise, this work |
528 | may be distributed only under the terms of Perl's Artistic License. |
529 | Any distribution of this file or derivatives thereof I<outside> |
530 | of that package require that special arrangements be made with |
531 | copyright holder. |
532 | |
533 | Irrespective of its distribution, all code examples in this file |
534 | are hereby placed into the public domain. You are permitted and |
535 | encouraged to use this code in your own programs for fun |
536 | or for profit as you see fit. A simple comment in the code giving |
537 | credit would be courteous but is not required. |