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1 | =head1 NAME |
2 | |
3 | perltrap - Perl traps for the unwary |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | The biggest trap of all is forgetting to use the B<-w> switch; |
8 | see L<perlrun>. Making your entire program runnable under |
9 | |
10 | use strict; |
11 | |
12 | can help make your program more bullet-proof, but sometimes |
13 | it's too annoying for quick throw-away programs. |
14 | |
15 | =head2 Awk Traps |
16 | |
17 | Accustomed B<awk> users should take special note of the following: |
18 | |
19 | =over 4 |
20 | |
21 | =item * |
22 | |
23 | The English module, loaded via |
24 | |
25 | use English; |
26 | |
27 | allows you to refer to special variables (like $RS) as |
28 | though they were in B<awk>; see L<perlvar> for details. |
29 | |
30 | =item * |
31 | |
32 | Semicolons are required after all simple statements in Perl (except |
33 | at the end of a block). Newline is not a statement delimiter. |
34 | |
35 | =item * |
36 | |
37 | Curly brackets are required on C<if>s and C<while>s. |
38 | |
39 | =item * |
40 | |
41 | Variables begin with "$" or "@" in Perl. |
42 | |
43 | =item * |
44 | |
45 | Arrays index from 0. Likewise string positions in substr() and |
46 | index(). |
47 | |
48 | =item * |
49 | |
50 | You have to decide whether your array has numeric or string indices. |
51 | |
52 | =item * |
53 | |
54 | Associative array values do not spring into existence upon mere |
55 | reference. |
56 | |
57 | =item * |
58 | |
59 | You have to decide whether you want to use string or numeric |
60 | comparisons. |
61 | |
62 | =item * |
63 | |
64 | Reading an input line does not split it for you. You get to split it |
65 | yourself to an array. And split() operator has different |
66 | arguments. |
67 | |
68 | =item * |
69 | |
70 | The current input line is normally in $_, not $0. It generally does |
71 | not have the newline stripped. ($0 is the name of the program |
72 | executed.) See L<perlvar>. |
73 | |
74 | =item * |
75 | |
76 | $<I<digit>> does not refer to fields--it refers to substrings matched by |
77 | the last match pattern. |
78 | |
79 | =item * |
80 | |
81 | The print() statement does not add field and record separators unless |
82 | you set C<$,> and C<$.>. You can set $OFS and $ORS if you're using |
83 | the English module. |
84 | |
85 | =item * |
86 | |
87 | You must open your files before you print to them. |
88 | |
89 | =item * |
90 | |
91 | The range operator is "..", not comma. The comma operator works as in |
92 | C. |
93 | |
94 | =item * |
95 | |
96 | The match operator is "=~", not "~". ("~" is the one's complement |
97 | operator, as in C.) |
98 | |
99 | =item * |
100 | |
101 | The exponentiation operator is "**", not "^". "^" is the XOR |
102 | operator, as in C. (You know, one could get the feeling that B<awk> is |
103 | basically incompatible with C.) |
104 | |
105 | =item * |
106 | |
107 | The concatenation operator is ".", not the null string. (Using the |
108 | null string would render C</pat/ /pat/> unparsable, since the third slash |
109 | would be interpreted as a division operator--the tokener is in fact |
110 | slightly context sensitive for operators like "/", "?", and ">". |
111 | And in fact, "." itself can be the beginning of a number.) |
112 | |
113 | =item * |
114 | |
115 | The C<next>, C<exit>, and C<continue> keywords work differently. |
116 | |
117 | =item * |
118 | |
119 | |
120 | The following variables work differently: |
121 | |
122 | Awk Perl |
123 | ARGC $#ARGV or scalar @ARGV |
124 | ARGV[0] $0 |
125 | FILENAME $ARGV |
126 | FNR $. - something |
127 | FS (whatever you like) |
128 | NF $#Fld, or some such |
129 | NR $. |
130 | OFMT $# |
131 | OFS $, |
132 | ORS $\ |
133 | RLENGTH length($&) |
134 | RS $/ |
135 | RSTART length($`) |
136 | SUBSEP $; |
137 | |
138 | =item * |
139 | |
140 | You cannot set $RS to a pattern, only a string. |
141 | |
142 | =item * |
143 | |
144 | When in doubt, run the B<awk> construct through B<a2p> and see what it |
145 | gives you. |
146 | |
147 | =back |
148 | |
149 | =head2 C Traps |
150 | |
151 | Cerebral C programmers should take note of the following: |
152 | |
153 | =over 4 |
154 | |
155 | =item * |
156 | |
157 | Curly brackets are required on C<if>'s and C<while>'s. |
158 | |
159 | =item * |
160 | |
161 | You must use C<elsif> rather than C<else if>. |
162 | |
163 | =item * |
164 | |
165 | The C<break> and C<continue> keywords from C become in |
166 | Perl C<last> and C<next>, respectively. |
167 | Unlike in C, these do I<NOT> work within a C<do { } while> construct. |
168 | |
169 | =item * |
170 | |
171 | There's no switch statement. (But it's easy to build one on the fly.) |
172 | |
173 | =item * |
174 | |
175 | Variables begin with "$" or "@" in Perl. |
176 | |
177 | =item * |
178 | |
179 | printf() does not implement the "*" format for interpolating |
180 | field widths, but it's trivial to use interpolation of double-quoted |
181 | strings to achieve the same effect. |
182 | |
183 | =item * |
184 | |
185 | Comments begin with "#", not "/*". |
186 | |
187 | =item * |
188 | |
189 | You can't take the address of anything, although a similar operator |
190 | in Perl 5 is the backslash, which creates a reference. |
191 | |
192 | =item * |
193 | |
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194 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
195 | ends up in C<$0>. |
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196 | |
197 | =item * |
198 | |
199 | System calls such as link(), unlink(), rename(), etc. return nonzero for |
200 | success, not 0. |
201 | |
202 | =item * |
203 | |
204 | Signal handlers deal with signal names, not numbers. Use C<kill -l> |
205 | to find their names on your system. |
206 | |
207 | =back |
208 | |
209 | =head2 Sed Traps |
210 | |
211 | Seasoned B<sed> programmers should take note of the following: |
212 | |
213 | =over 4 |
214 | |
215 | =item * |
216 | |
217 | Backreferences in substitutions use "$" rather than "\". |
218 | |
219 | =item * |
220 | |
221 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes |
222 | in front. |
223 | |
224 | =item * |
225 | |
226 | The range operator is C<...>, rather than comma. |
227 | |
228 | =back |
229 | |
230 | =head2 Shell Traps |
231 | |
232 | Sharp shell programmers should take note of the following: |
233 | |
234 | =over 4 |
235 | |
236 | =item * |
237 | |
238 | The backtick operator does variable interpretation without regard to |
239 | the presence of single quotes in the command. |
240 | |
241 | =item * |
242 | |
243 | The backtick operator does no translation of the return value, unlike B<csh>. |
244 | |
245 | =item * |
246 | |
247 | Shells (especially B<csh>) do several levels of substitution on each |
248 | command line. Perl does substitution only in certain constructs |
249 | such as double quotes, backticks, angle brackets, and search patterns. |
250 | |
251 | =item * |
252 | |
253 | Shells interpret scripts a little bit at a time. Perl compiles the |
254 | entire program before executing it (except for C<BEGIN> blocks, which |
255 | execute at compile time). |
256 | |
257 | =item * |
258 | |
259 | The arguments are available via @ARGV, not $1, $2, etc. |
260 | |
261 | =item * |
262 | |
263 | The environment is not automatically made available as separate scalar |
264 | variables. |
265 | |
266 | =back |
267 | |
268 | =head2 Perl Traps |
269 | |
270 | Practicing Perl Programmers should take note of the following: |
271 | |
272 | =over 4 |
273 | |
274 | =item * |
275 | |
276 | Remember that many operations behave differently in a list |
277 | context than they do in a scalar one. See L<perldata> for details. |
278 | |
279 | =item * |
280 | |
281 | Avoid barewords if you can, especially all lower-case ones. |
282 | You can't tell just by looking at it whether a bareword is |
283 | a function or a string. By using quotes on strings and |
284 | parens on function calls, you won't ever get them confused. |
285 | |
286 | =item * |
287 | |
288 | You cannot discern from mere inspection which built-ins |
289 | are unary operators (like chop() and chdir()) |
290 | and which are list operators (like print() and unlink()). |
291 | (User-defined subroutines can B<only> be list operators, never |
292 | unary ones.) See L<perlop>. |
293 | |
294 | =item * |
295 | |
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296 | People have a hard time remembering that some functions |
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297 | default to $_, or @ARGV, or whatever, but that others which |
298 | you might expect to do not. |
299 | |
300 | =item * |
301 | |
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302 | The <FH> construct is not the name of the filehandle, it is a readline |
303 | operation on that handle. The data read is only assigned to $_ if the |
304 | file read is the sole condition in a while loop: |
305 | |
306 | while (<FH>) { } |
307 | while ($_ = <FH>) { }.. |
308 | <FH>; # data discarded! |
309 | |
310 | =item * |
311 | |
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312 | Remember not to use "C<=>" when you need "C<=~>"; |
313 | these two constructs are quite different: |
314 | |
315 | $x = /foo/; |
316 | $x =~ /foo/; |
317 | |
318 | =item * |
319 | |
320 | The C<do {}> construct isn't a real loop that you can use |
321 | loop control on. |
322 | |
323 | =item * |
324 | |
325 | Use my() for local variables whenever you can get away with |
326 | it (but see L<perlform> for where you can't). |
327 | Using local() actually gives a local value to a global |
328 | variable, which leaves you open to unforeseen side-effects |
329 | of dynamic scoping. |
330 | |
331 | =back |
332 | |
333 | =head2 Perl4 Traps |
334 | |
335 | Penitent Perl 4 Programmers should take note of the following |
336 | incompatible changes that occurred between release 4 and release 5: |
337 | |
338 | =over 4 |
339 | |
340 | =item * |
341 | |
342 | C<@> now always interpolates an array in double-quotish strings. Some programs |
343 | may now need to use backslash to protect any C<@> that shouldn't interpolate. |
344 | |
345 | =item * |
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346 | |
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347 | Barewords that used to look like strings to Perl will now look like subroutine |
348 | calls if a subroutine by that name is defined before the compiler sees them. |
349 | For example: |
350 | |
351 | sub SeeYa { die "Hasta la vista, baby!" } |
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352 | $SIG{'QUIT'} = SeeYa; |
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353 | |
354 | In Perl 4, that set the signal handler; in Perl 5, it actually calls the |
355 | function! You may use the B<-w> switch to find such places. |
356 | |
357 | =item * |
358 | |
359 | Symbols starting with C<_> are no longer forced into package C<main>, except |
360 | for $_ itself (and @_, etc.). |
361 | |
362 | =item * |
363 | |
364 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to |
365 | interpolate C<$lhs> but not C<$rhs>. |
366 | |
367 | =item * |
368 | |
369 | The second and third arguments of splice() are now evaluated in scalar |
370 | context (as the book says) rather than list context. |
371 | |
372 | =item * |
373 | |
374 | These are now semantic errors because of precedence: |
375 | |
376 | shift @list + 20; |
377 | $n = keys %map + 20; |
378 | |
379 | Because if that were to work, then this couldn't: |
380 | |
381 | sleep $dormancy + 20; |
382 | |
383 | =item * |
384 | |
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385 | The precedence of assignment operators is now the same as the precedence |
386 | of assignment. Perl 4 mistakenly gave them the precedence of the associated |
387 | operator. So you now must parenthesize them in expressions like |
388 | |
389 | /foo/ ? ($a += 2) : ($a -= 2); |
390 | |
391 | Otherwise |
392 | |
393 | /foo/ ? $a += 2 : $a -= 2; |
394 | |
395 | would be erroneously parsed as |
396 | |
397 | (/foo/ ? $a += 2 : $a) -= 2; |
398 | |
399 | On the other hand, |
400 | |
401 | $a += /foo/ ? 1 : 2; |
402 | |
403 | now works as a C programmer would expect. |
404 | |
405 | =item * |
406 | |
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407 | C<open FOO || die> is now incorrect. You need parens around the filehandle. |
408 | While temporarily supported, using such a construct will |
409 | generate a non-fatal (but non-suppressible) warning. |
410 | |
411 | =item * |
412 | |
413 | The elements of argument lists for formats are now evaluated in list |
414 | context. This means you can interpolate list values now. |
415 | |
416 | =item * |
417 | |
418 | You can't do a C<goto> into a block that is optimized away. Darn. |
419 | |
420 | =item * |
421 | |
422 | It is no longer syntactically legal to use whitespace as the name |
423 | of a variable, or as a delimiter for any kind of quote construct. |
424 | Double darn. |
425 | |
426 | =item * |
427 | |
428 | The caller() function now returns a false value in a scalar context if there |
429 | is no caller. This lets library files determine if they're being required. |
430 | |
431 | =item * |
432 | |
433 | C<m//g> now attaches its state to the searched string rather than the |
434 | regular expression. |
435 | |
436 | =item * |
437 | |
438 | C<reverse> is no longer allowed as the name of a sort subroutine. |
439 | |
440 | =item * |
441 | |
442 | B<taintperl> is no longer a separate executable. There is now a B<-T> |
443 | switch to turn on tainting when it isn't turned on automatically. |
444 | |
445 | =item * |
446 | |
447 | Double-quoted strings may no longer end with an unescaped C<$> or C<@>. |
448 | |
449 | =item * |
450 | |
451 | The archaic C<while/if> BLOCK BLOCK syntax is no longer supported. |
452 | |
453 | |
454 | =item * |
455 | |
456 | Negative array subscripts now count from the end of the array. |
457 | |
458 | =item * |
459 | |
460 | The comma operator in a scalar context is now guaranteed to give a |
461 | scalar context to its arguments. |
462 | |
463 | =item * |
464 | |
465 | The C<**> operator now binds more tightly than unary minus. |
466 | It was documented to work this way before, but didn't. |
467 | |
468 | =item * |
469 | |
470 | Setting C<$#array> lower now discards array elements. |
471 | |
472 | =item * |
473 | |
474 | delete() is not guaranteed to return the old value for tie()d arrays, |
475 | since this capability may be onerous for some modules to implement. |
476 | |
477 | =item * |
478 | |
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479 | The construct "this is $$x" used to interpolate the pid at that |
480 | point, but now tries to dereference $x. C<$$> by itself still |
481 | works fine, however. |
482 | |
483 | =item * |
484 | |
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485 | Some error messages will be different. |
486 | |
487 | =item * |
488 | |
489 | Some bugs may have been inadvertently removed. |
490 | |
491 | =back |