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68dc0745 | 1 | =head1 NAME |
2 | ||
fc36a67e | 3 | perlfaq5 - Files and Formats ($Revision: 1.22 $, $Date: 1997/04/24 22:44:02 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | This section deals with I/O and the "f" issues: filehandles, flushing, | |
8 | formats, and footers. | |
9 | ||
5a964f20 | 10 | =head2 How do I flush/unbuffer an output filehandle? Why must I do this? |
68dc0745 | 11 | |
12 | The C standard I/O library (stdio) normally buffers characters sent to | |
13 | devices. This is done for efficiency reasons, so that there isn't a | |
14 | system call for each byte. Any time you use print() or write() in | |
15 | Perl, you go though this buffering. syswrite() circumvents stdio and | |
16 | buffering. | |
17 | ||
5a964f20 | 18 | In most stdio implementations, the type of output buffering and the size of |
68dc0745 | 19 | the buffer varies according to the type of device. Disk files are block |
20 | buffered, often with a buffer size of more than 2k. Pipes and sockets | |
21 | are often buffered with a buffer size between 1/2 and 2k. Serial devices | |
22 | (e.g. modems, terminals) are normally line-buffered, and stdio sends | |
23 | the entire line when it gets the newline. | |
24 | ||
25 | Perl does not support truly unbuffered output (except insofar as you can | |
26 | C<syswrite(OUT, $char, 1)>). What it does instead support is "command | |
27 | buffering", in which a physical write is performed after every output | |
28 | command. This isn't as hard on your system as unbuffering, but does | |
29 | get the output where you want it when you want it. | |
30 | ||
31 | If you expect characters to get to your device when you print them there, | |
5a964f20 TC |
32 | you'll want to autoflush its handle. |
33 | Use select() and the C<$|> variable to control autoflushing | |
34 | (see L<perlvar/$|> and L<perlfunc/select>): | |
35 | ||
36 | $old_fh = select(OUTPUT_HANDLE); | |
37 | $| = 1; | |
38 | select($old_fh); | |
39 | ||
40 | Or using the traditional idiom: | |
41 | ||
42 | select((select(OUTPUT_HANDLE), $| = 1)[0]); | |
43 | ||
44 | Or if don't mind slowly loading several thousand lines of module code | |
45 | just because you're afraid of the C<$|> variable: | |
68dc0745 | 46 | |
47 | use FileHandle; | |
5a964f20 | 48 | open(DEV, "+</dev/tty"); # ceci n'est pas une pipe |
68dc0745 | 49 | DEV->autoflush(1); |
50 | ||
51 | or the newer IO::* modules: | |
52 | ||
53 | use IO::Handle; | |
54 | open(DEV, ">/dev/printer"); # but is this? | |
55 | DEV->autoflush(1); | |
56 | ||
57 | or even this: | |
58 | ||
59 | use IO::Socket; # this one is kinda a pipe? | |
60 | $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com', | |
61 | PeerPort => 'http(80)', | |
62 | Proto => 'tcp'); | |
63 | die "$!" unless $sock; | |
64 | ||
65 | $sock->autoflush(); | |
5a964f20 TC |
66 | print $sock "GET / HTTP/1.0" . "\015\012" x 2; |
67 | $document = join('', <$sock>); | |
68dc0745 | 68 | print "DOC IS: $document\n"; |
69 | ||
5a964f20 TC |
70 | Note the bizarrely hardcoded carriage return and newline in their octal |
71 | equivalents. This is the ONLY way (currently) to assure a proper flush | |
72 | on all platforms, including Macintosh. That the way things work in | |
73 | network programming: you really should specify the exact bit pattern | |
74 | on the network line terminator. In practice, C<"\n\n"> often works, | |
75 | but this is not portable. | |
68dc0745 | 76 | |
5a964f20 | 77 | See L<perlfaq9> for other examples of fetching URLs over the web. |
68dc0745 | 78 | |
79 | =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file? | |
80 | ||
81 | Although humans have an easy time thinking of a text file as being a | |
82 | sequence of lines that operates much like a stack of playing cards -- | |
83 | or punch cards -- computers usually see the text file as a sequence of | |
84 | bytes. In general, there's no direct way for Perl to seek to a | |
85 | particular line of a file, insert text into a file, or remove text | |
86 | from a file. | |
87 | ||
5a964f20 TC |
88 | (There are exceptions in special circumstances. You can add or remove at |
89 | the very end of the file. Another is replacing a sequence of bytes with | |
90 | another sequence of the same length. Another is using the C<$DB_RECNO> | |
91 | array bindings as documented in L<DB_File>. Yet another is manipulating | |
92 | files with all lines the same length.) | |
68dc0745 | 93 | |
94 | The general solution is to create a temporary copy of the text file with | |
5a964f20 TC |
95 | the changes you want, then copy that over the original. This assumes |
96 | no locking. | |
68dc0745 | 97 | |
98 | $old = $file; | |
99 | $new = "$file.tmp.$$"; | |
100 | $bak = "$file.bak"; | |
101 | ||
102 | open(OLD, "< $old") or die "can't open $old: $!"; | |
103 | open(NEW, "> $new") or die "can't open $new: $!"; | |
104 | ||
105 | # Correct typos, preserving case | |
106 | while (<OLD>) { | |
107 | s/\b(p)earl\b/${1}erl/i; | |
108 | (print NEW $_) or die "can't write to $new: $!"; | |
109 | } | |
110 | ||
111 | close(OLD) or die "can't close $old: $!"; | |
112 | close(NEW) or die "can't close $new: $!"; | |
113 | ||
114 | rename($old, $bak) or die "can't rename $old to $bak: $!"; | |
115 | rename($new, $old) or die "can't rename $new to $old: $!"; | |
116 | ||
117 | Perl can do this sort of thing for you automatically with the C<-i> | |
46fc3d4c | 118 | command-line switch or the closely-related C<$^I> variable (see |
68dc0745 | 119 | L<perlrun> for more details). Note that |
120 | C<-i> may require a suffix on some non-Unix systems; see the | |
121 | platform-specific documentation that came with your port. | |
122 | ||
123 | # Renumber a series of tests from the command line | |
124 | perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t | |
125 | ||
126 | # form a script | |
127 | local($^I, @ARGV) = ('.bak', glob("*.c")); | |
128 | while (<>) { | |
129 | if ($. == 1) { | |
130 | print "This line should appear at the top of each file\n"; | |
131 | } | |
132 | s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case | |
133 | print; | |
134 | close ARGV if eof; # Reset $. | |
135 | } | |
136 | ||
137 | If you need to seek to an arbitrary line of a file that changes | |
138 | infrequently, you could build up an index of byte positions of where | |
139 | the line ends are in the file. If the file is large, an index of | |
140 | every tenth or hundredth line end would allow you to seek and read | |
141 | fairly efficiently. If the file is sorted, try the look.pl library | |
142 | (part of the standard perl distribution). | |
143 | ||
144 | In the unique case of deleting lines at the end of a file, you | |
145 | can use tell() and truncate(). The following code snippet deletes | |
146 | the last line of a file without making a copy or reading the | |
147 | whole file into memory: | |
148 | ||
149 | open (FH, "+< $file"); | |
54310121 | 150 | while ( <FH> ) { $addr = tell(FH) unless eof(FH) } |
68dc0745 | 151 | truncate(FH, $addr); |
152 | ||
153 | Error checking is left as an exercise for the reader. | |
154 | ||
155 | =head2 How do I count the number of lines in a file? | |
156 | ||
157 | One fairly efficient way is to count newlines in the file. The | |
158 | following program uses a feature of tr///, as documented in L<perlop>. | |
159 | If your text file doesn't end with a newline, then it's not really a | |
160 | proper text file, so this may report one fewer line than you expect. | |
161 | ||
162 | $lines = 0; | |
163 | open(FILE, $filename) or die "Can't open `$filename': $!"; | |
164 | while (sysread FILE, $buffer, 4096) { | |
165 | $lines += ($buffer =~ tr/\n//); | |
166 | } | |
167 | close FILE; | |
168 | ||
5a964f20 TC |
169 | This assumes no funny games with newline translations. |
170 | ||
68dc0745 | 171 | =head2 How do I make a temporary file name? |
172 | ||
5a964f20 TC |
173 | Use the C<new_tmpfile> class method from the IO::File module to get a |
174 | filehandle opened for reading and writing. Use this if you don't | |
175 | need to know the file's name. | |
68dc0745 | 176 | |
68dc0745 | 177 | use IO::File; |
5a964f20 TC |
178 | $fh = IO::File->new_tmpfile() |
179 | or die "Unable to make new temporary file: $!"; | |
180 | ||
181 | Or you can use the C<tmpnam> function from the POSIX module to get a | |
182 | filename that you then open yourself. Use this if you do need to know | |
183 | the file's name. | |
184 | ||
185 | use Fcntl; | |
186 | use POSIX qw(tmpnam); | |
187 | ||
188 | # try new temporary filenames until we get one that didn't already | |
189 | # exist; the check should be unnecessary, but you can't be too careful | |
190 | do { $name = tmpnam() } | |
191 | until sysopen(FH, $name, O_RDWR|O_CREAT|O_EXCL); | |
192 | ||
193 | # install atexit-style handler so that when we exit or die, | |
194 | # we automatically delete this temporary file | |
195 | END { unlink($name) or die "Couldn't unlink $name : $!" } | |
196 | ||
197 | # now go on to use the file ... | |
198 | ||
199 | If you're committed to doing this by hand, use the process ID and/or | |
200 | the current time-value. If you need to have many temporary files in | |
201 | one process, use a counter: | |
202 | ||
203 | BEGIN { | |
68dc0745 | 204 | use Fcntl; |
205 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP}; | |
206 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); | |
207 | sub temp_file { | |
5a964f20 | 208 | local *FH; |
68dc0745 | 209 | my $count = 0; |
5a964f20 | 210 | until (defined(fileno(FH)) || $count++ > 100) { |
68dc0745 | 211 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
5a964f20 | 212 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
68dc0745 | 213 | } |
5a964f20 TC |
214 | if (defined(fileno(FH)) |
215 | return (*FH, $base_name); | |
68dc0745 | 216 | } else { |
217 | return (); | |
218 | } | |
219 | } | |
220 | } | |
221 | ||
68dc0745 | 222 | =head2 How can I manipulate fixed-record-length files? |
223 | ||
5a964f20 TC |
224 | The most efficient way is using pack() and unpack(). This is faster than |
225 | using substr() when take many, many strings. It is slower for just a few. | |
226 | ||
227 | Here is a sample chunk of code to break up and put back together again | |
228 | some fixed-format input lines, in this case from the output of a normal, | |
229 | Berkeley-style ps: | |
68dc0745 | 230 | |
231 | # sample input line: | |
232 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what | |
233 | $PS_T = 'A6 A4 A7 A5 A*'; | |
234 | open(PS, "ps|"); | |
5a964f20 | 235 | print scalar <PS>; |
68dc0745 | 236 | while (<PS>) { |
237 | ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_); | |
238 | for $var (qw!pid tt stat time command!) { | |
239 | print "$var: <$$var>\n"; | |
240 | } | |
241 | print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command), | |
242 | "\n"; | |
243 | } | |
244 | ||
5a964f20 TC |
245 | We've used C<$$var> in a way that forbidden by C<use strict 'refs'>. |
246 | That is, we've promoted a string to a scalar variable reference using | |
247 | symbolic references. This is ok in small programs, but doesn't scale | |
248 | well. It also only works on global variables, not lexicals. | |
249 | ||
68dc0745 | 250 | =head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? |
251 | ||
5a964f20 TC |
252 | The fastest, simplest, and most direct way is to localize the typeglob |
253 | of the filehandle in question: | |
68dc0745 | 254 | |
5a964f20 | 255 | local *TmpHandle; |
68dc0745 | 256 | |
5a964f20 TC |
257 | Typeglobs are fast (especially compared with the alternatives) and |
258 | reasonably easy to use, but they also have one subtle drawback. If you | |
259 | had, for example, a function named TmpHandle(), or a variable named | |
260 | %TmpHandle, you just hid it from yourself. | |
68dc0745 | 261 | |
68dc0745 | 262 | sub findme { |
5a964f20 TC |
263 | local *HostFile; |
264 | open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!"; | |
265 | local $_; # <- VERY IMPORTANT | |
266 | while (<HostFile>) { | |
68dc0745 | 267 | print if /\b127\.(0\.0\.)?1\b/; |
268 | } | |
5a964f20 TC |
269 | # *HostFile automatically closes/disappears here |
270 | } | |
271 | ||
272 | Here's how to use this in a loop to open and store a bunch of | |
273 | filehandles. We'll use as values of the hash an ordered | |
274 | pair to make it easy to sort the hash in insertion order. | |
275 | ||
276 | @names = qw(motd termcap passwd hosts); | |
277 | my $i = 0; | |
278 | foreach $filename (@names) { | |
279 | local *FH; | |
280 | open(FH, "/etc/$filename") || die "$filename: $!"; | |
281 | $file{$filename} = [ $i++, *FH ]; | |
68dc0745 | 282 | } |
283 | ||
5a964f20 TC |
284 | # Using the filehandles in the array |
285 | foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) { | |
286 | my $fh = $file{$name}[1]; | |
287 | my $line = <$fh>; | |
288 | print "$name $. $line"; | |
289 | } | |
290 | ||
291 | If you want to create many, anonymous handles, you should check out the | |
292 | Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent | |
293 | code with Symbol::gensym, which is reasonably light-weight: | |
294 | ||
295 | foreach $filename (@names) { | |
296 | use Symbol; | |
297 | my $fh = gensym(); | |
298 | open($fh, "/etc/$filename") || die "open /etc/$filename: $!"; | |
299 | $file{$filename} = [ $i++, $fh ]; | |
300 | } | |
68dc0745 | 301 | |
5a964f20 TC |
302 | Or here using the semi-object-oriented FileHandle, which certainly isn't |
303 | light-weight: | |
46fc3d4c | 304 | |
305 | use FileHandle; | |
306 | ||
46fc3d4c | 307 | foreach $filename (@names) { |
5a964f20 TC |
308 | my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!"; |
309 | $file{$filename} = [ $i++, $fh ]; | |
46fc3d4c | 310 | } |
311 | ||
5a964f20 TC |
312 | Please understand that whether the filehandle happens to be a (probably |
313 | localized) typeglob or an anonymous handle from one of the modules, | |
314 | in no way affects the bizarre rules for managing indirect handles. | |
315 | See the next question. | |
316 | ||
317 | =head2 How can I use a filehandle indirectly? | |
318 | ||
319 | An indirect filehandle is using something other than a symbol | |
320 | in a place that a filehandle is expected. Here are ways | |
321 | to get those: | |
322 | ||
323 | $fh = SOME_FH; # bareword is strict-subs hostile | |
324 | $fh = "SOME_FH"; # strict-refs hostile; same package only | |
325 | $fh = *SOME_FH; # typeglob | |
326 | $fh = \*SOME_FH; # ref to typeglob (bless-able) | |
327 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob | |
328 | ||
329 | Or to use the C<new> method from the FileHandle or IO modules to | |
330 | create an anonymous filehandle, store that in a scalar variable, | |
331 | and use it as though it were a normal filehandle. | |
332 | ||
333 | use FileHandle; | |
334 | $fh = FileHandle->new(); | |
335 | ||
336 | use IO::Handle; # 5.004 or higher | |
337 | $fh = IO::Handle->new(); | |
338 | ||
339 | Then use any of those as you would a normal filehandle. Anywhere that | |
340 | Perl is expecting a filehandle, an indirect filehandle may be used | |
341 | instead. An indirect filehandle is just a scalar variable that contains | |
342 | a filehandle. Functions like C<print>, C<open>, C<seek>, or the functions or | |
343 | the C<E<lt>FHE<gt>> diamond operator will accept either a read filehandle | |
344 | or a scalar variable containing one: | |
345 | ||
346 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); | |
347 | print $ofh "Type it: "; | |
348 | $got = <$ifh> | |
349 | print $efh "What was that: $got"; | |
350 | ||
351 | Of you're passing a filehandle to a function, you can write | |
352 | the function in two ways: | |
353 | ||
354 | sub accept_fh { | |
355 | my $fh = shift; | |
356 | print $fh "Sending to indirect filehandle\n"; | |
46fc3d4c | 357 | } |
358 | ||
5a964f20 | 359 | Or it can localize a typeglob and use the filehandle directly: |
46fc3d4c | 360 | |
5a964f20 TC |
361 | sub accept_fh { |
362 | local *FH = shift; | |
363 | print FH "Sending to localized filehandle\n"; | |
46fc3d4c | 364 | } |
365 | ||
5a964f20 TC |
366 | Both styles work with either objects or typeglobs of real filehandles. |
367 | (They might also work with strings under some circumstances, but this | |
368 | is risky.) | |
369 | ||
370 | accept_fh(*STDOUT); | |
371 | accept_fh($handle); | |
372 | ||
373 | In the examples above, we assigned the filehandle to a scalar variable | |
374 | before using it. That is because only simple scalar variables, | |
375 | not expressions or subscripts into hashes or arrays, can be used with | |
376 | built-ins like C<print>, C<printf>, or the diamond operator. These are | |
377 | illegal and won't even compile: | |
378 | ||
379 | @fd = (*STDIN, *STDOUT, *STDERR); | |
380 | print $fd[1] "Type it: "; # WRONG | |
381 | $got = <$fd[0]> # WRONG | |
382 | print $fd[2] "What was that: $got"; # WRONG | |
383 | ||
384 | With C<print> and C<printf>, you get around this by using a block and | |
385 | an expression where you would place the filehandle: | |
386 | ||
387 | print { $fd[1] } "funny stuff\n"; | |
388 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; | |
389 | # Pity the poor deadbeef. | |
390 | ||
391 | That block is a proper block like any other, so you can put more | |
392 | complicated code there. This sends the message out to one of two places: | |
393 | ||
394 | $ok = -x "/bin/cat"; | |
395 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; | |
396 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; | |
397 | ||
398 | This approach of treating C<print> and C<printf> like object methods | |
399 | calls doesn't work for the diamond operator. That's because it's a | |
400 | real operator, not just a function with a comma-less argument. Assuming | |
401 | you've been storing typeglobs in your structure as we did above, you | |
402 | can use the built-in function named C<readline> to reads a record just | |
403 | as C<E<lt>E<gt>> does. Given the initialization shown above for @fd, this | |
404 | would work, but only because readline() require a typeglob. It doesn't | |
405 | work with objects or strings, which might be a bug we haven't fixed yet. | |
406 | ||
407 | $got = readline($fd[0]); | |
408 | ||
409 | Let it be noted that the flakiness of indirect filehandles is not | |
410 | related to whether they're strings, typeglobs, objects, or anything else. | |
411 | It's the syntax of the fundamental operators. Playing the object | |
412 | game doesn't help you at all here. | |
46fc3d4c | 413 | |
68dc0745 | 414 | =head2 How can I set up a footer format to be used with write()? |
415 | ||
54310121 | 416 | There's no builtin way to do this, but L<perlform> has a couple of |
68dc0745 | 417 | techniques to make it possible for the intrepid hacker. |
418 | ||
419 | =head2 How can I write() into a string? | |
420 | ||
421 | See L<perlform> for an swrite() function. | |
422 | ||
423 | =head2 How can I output my numbers with commas added? | |
424 | ||
425 | This one will do it for you: | |
426 | ||
427 | sub commify { | |
428 | local $_ = shift; | |
429 | 1 while s/^(-?\d+)(\d{3})/$1,$2/; | |
430 | return $_; | |
431 | } | |
432 | ||
433 | $n = 23659019423.2331; | |
434 | print "GOT: ", commify($n), "\n"; | |
435 | ||
436 | GOT: 23,659,019,423.2331 | |
437 | ||
438 | You can't just: | |
439 | ||
440 | s/^(-?\d+)(\d{3})/$1,$2/g; | |
441 | ||
442 | because you have to put the comma in and then recalculate your | |
443 | position. | |
444 | ||
46fc3d4c | 445 | Alternatively, this commifies all numbers in a line regardless of |
446 | whether they have decimal portions, are preceded by + or -, or | |
447 | whatever: | |
448 | ||
449 | # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca> | |
450 | sub commify { | |
451 | my $input = shift; | |
452 | $input = reverse $input; | |
453 | $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g; | |
454 | return reverse $input; | |
455 | } | |
456 | ||
68dc0745 | 457 | =head2 How can I translate tildes (~) in a filename? |
458 | ||
459 | Use the E<lt>E<gt> (glob()) operator, documented in L<perlfunc>. This | |
460 | requires that you have a shell installed that groks tildes, meaning | |
461 | csh or tcsh or (some versions of) ksh, and thus may have portability | |
462 | problems. The Glob::KGlob module (available from CPAN) gives more | |
463 | portable glob functionality. | |
464 | ||
465 | Within Perl, you may use this directly: | |
466 | ||
467 | $filename =~ s{ | |
468 | ^ ~ # find a leading tilde | |
469 | ( # save this in $1 | |
470 | [^/] # a non-slash character | |
471 | * # repeated 0 or more times (0 means me) | |
472 | ) | |
473 | }{ | |
474 | $1 | |
475 | ? (getpwnam($1))[7] | |
476 | : ( $ENV{HOME} || $ENV{LOGDIR} ) | |
477 | }ex; | |
478 | ||
5a964f20 | 479 | =head2 How come when I open a file read-write it wipes it out? |
68dc0745 | 480 | |
481 | Because you're using something like this, which truncates the file and | |
482 | I<then> gives you read-write access: | |
483 | ||
5a964f20 | 484 | open(FH, "+> /path/name"); # WRONG (almost always) |
68dc0745 | 485 | |
486 | Whoops. You should instead use this, which will fail if the file | |
5a964f20 TC |
487 | doesn't exist. Using "E<gt>" always clobbers or creates. |
488 | Using "E<lt>" never does either. The "+" doesn't change this. | |
68dc0745 | 489 | |
5a964f20 TC |
490 | Here are examples of many kinds of file opens. Those using sysopen() |
491 | all assume | |
68dc0745 | 492 | |
5a964f20 | 493 | use Fcntl; |
68dc0745 | 494 | |
5a964f20 | 495 | To open file for reading: |
68dc0745 | 496 | |
5a964f20 TC |
497 | open(FH, "< $path") || die $!; |
498 | sysopen(FH, $path, O_RDONLY) || die $!; | |
499 | ||
500 | To open file for writing, create new file if needed or else truncate old file: | |
501 | ||
502 | open(FH, "> $path") || die $!; | |
503 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; | |
504 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; | |
505 | ||
506 | To open file for writing, create new file, file must not exist: | |
507 | ||
508 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; | |
509 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; | |
510 | ||
511 | To open file for appending, create if necessary: | |
512 | ||
513 | open(FH, ">> $path") || die $!; | |
514 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; | |
515 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; | |
516 | ||
517 | To open file for appending, file must exist: | |
518 | ||
519 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; | |
520 | ||
521 | To open file for update, file must exist: | |
522 | ||
523 | open(FH, "+< $path") || die $!; | |
524 | sysopen(FH, $path, O_RDWR) || die $!; | |
525 | ||
526 | To open file for update, create file if necessary: | |
527 | ||
528 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; | |
529 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; | |
530 | ||
531 | To open file for update, file must not exist: | |
532 | ||
533 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; | |
534 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; | |
535 | ||
536 | To open a file without blocking, creating if necessary: | |
537 | ||
538 | sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT) | |
539 | or die "can't open /tmp/somefile: $!": | |
540 | ||
541 | Be warned that neither creation nor deletion of files is guaranteed to | |
542 | be an atomic operation over NFS. That is, two processes might both | |
543 | successful create or unlink the same file! Therefore O_EXCL | |
544 | isn't so exclusive as you might wish. | |
68dc0745 | 545 | |
546 | =head2 Why do I sometimes get an "Argument list too long" when I use <*>? | |
547 | ||
548 | The C<E<lt>E<gt>> operator performs a globbing operation (see above). | |
549 | By default glob() forks csh(1) to do the actual glob expansion, but | |
550 | csh can't handle more than 127 items and so gives the error message | |
551 | C<Argument list too long>. People who installed tcsh as csh won't | |
552 | have this problem, but their users may be surprised by it. | |
553 | ||
554 | To get around this, either do the glob yourself with C<Dirhandle>s and | |
555 | patterns, or use a module like Glob::KGlob, one that doesn't use the | |
556 | shell to do globbing. | |
557 | ||
558 | =head2 Is there a leak/bug in glob()? | |
559 | ||
560 | Due to the current implementation on some operating systems, when you | |
561 | use the glob() function or its angle-bracket alias in a scalar | |
562 | context, you may cause a leak and/or unpredictable behavior. It's | |
563 | best therefore to use glob() only in list context. | |
564 | ||
565 | =head2 How can I open a file with a leading "E<gt>" or trailing blanks? | |
566 | ||
567 | Normally perl ignores trailing blanks in filenames, and interprets | |
568 | certain leading characters (or a trailing "|") to mean something | |
569 | special. To avoid this, you might want to use a routine like this. | |
570 | It makes incomplete pathnames into explicit relative ones, and tacks a | |
571 | trailing null byte on the name to make perl leave it alone: | |
572 | ||
573 | sub safe_filename { | |
574 | local $_ = shift; | |
575 | return m#^/# | |
576 | ? "$_\0" | |
577 | : "./$_\0"; | |
578 | } | |
579 | ||
580 | $fn = safe_filename("<<<something really wicked "); | |
581 | open(FH, "> $fn") or "couldn't open $fn: $!"; | |
582 | ||
583 | You could also use the sysopen() function (see L<perlfunc/sysopen>). | |
584 | ||
585 | =head2 How can I reliably rename a file? | |
586 | ||
587 | Well, usually you just use Perl's rename() function. But that may | |
588 | not work everywhere, in particular, renaming files across file systems. | |
589 | If your operating system supports a mv(1) program or its moral equivalent, | |
590 | this works: | |
591 | ||
592 | rename($old, $new) or system("mv", $old, $new); | |
593 | ||
594 | It may be more compelling to use the File::Copy module instead. You | |
595 | just copy to the new file to the new name (checking return values), | |
596 | then delete the old one. This isn't really the same semantics as a | |
597 | real rename(), though, which preserves metainformation like | |
598 | permissions, timestamps, inode info, etc. | |
599 | ||
5a964f20 TC |
600 | The newer version of File::Copy export a move() function. |
601 | ||
68dc0745 | 602 | =head2 How can I lock a file? |
603 | ||
54310121 | 604 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 | 605 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
606 | later), and lockf(3) if neither of the two previous system calls exists. | |
607 | On some systems, it may even use a different form of native locking. | |
608 | Here are some gotchas with Perl's flock(): | |
609 | ||
610 | =over 4 | |
611 | ||
612 | =item 1 | |
613 | ||
614 | Produces a fatal error if none of the three system calls (or their | |
615 | close equivalent) exists. | |
616 | ||
617 | =item 2 | |
618 | ||
619 | lockf(3) does not provide shared locking, and requires that the | |
620 | filehandle be open for writing (or appending, or read/writing). | |
621 | ||
622 | =item 3 | |
623 | ||
624 | Some versions of flock() can't lock files over a network (e.g. on NFS | |
625 | file systems), so you'd need to force the use of fcntl(2) when you | |
626 | build Perl. See the flock entry of L<perlfunc>, and the F<INSTALL> | |
627 | file in the source distribution for information on building Perl to do | |
628 | this. | |
629 | ||
630 | =back | |
631 | ||
68dc0745 | 632 | =head2 What can't I just open(FH, ">file.lock")? |
633 | ||
634 | A common bit of code B<NOT TO USE> is this: | |
635 | ||
636 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE | |
637 | open(LCK, "> file.lock"); # THIS BROKEN CODE | |
638 | ||
639 | This is a classic race condition: you take two steps to do something | |
640 | which must be done in one. That's why computer hardware provides an | |
641 | atomic test-and-set instruction. In theory, this "ought" to work: | |
642 | ||
5a964f20 | 643 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
68dc0745 | 644 | or die "can't open file.lock: $!": |
645 | ||
646 | except that lamentably, file creation (and deletion) is not atomic | |
647 | over NFS, so this won't work (at least, not every time) over the net. | |
46fc3d4c | 648 | Various schemes involving involving link() have been suggested, but |
649 | these tend to involve busy-wait, which is also subdesirable. | |
68dc0745 | 650 | |
fc36a67e | 651 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
68dc0745 | 652 | |
46fc3d4c | 653 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 TC |
654 | They don't count number of hits, they're a waste of time, and they serve |
655 | only to stroke the writer's vanity. Better to pick a random number. | |
656 | It's more realistic. | |
68dc0745 | 657 | |
5a964f20 | 658 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 | 659 | |
660 | use Fcntl; | |
5a964f20 | 661 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
68dc0745 | 662 | flock(FH, 2) or die "can't flock numfile: $!"; |
663 | $num = <FH> || 0; | |
664 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; | |
665 | truncate(FH, 0) or die "can't truncate numfile: $!"; | |
666 | (print FH $num+1, "\n") or die "can't write numfile: $!"; | |
667 | # DO NOT UNLOCK THIS UNTIL YOU CLOSE | |
668 | close FH or die "can't close numfile: $!"; | |
669 | ||
46fc3d4c | 670 | Here's a much better web-page hit counter: |
68dc0745 | 671 | |
672 | $hits = int( (time() - 850_000_000) / rand(1_000) ); | |
673 | ||
674 | If the count doesn't impress your friends, then the code might. :-) | |
675 | ||
676 | =head2 How do I randomly update a binary file? | |
677 | ||
678 | If you're just trying to patch a binary, in many cases something as | |
679 | simple as this works: | |
680 | ||
681 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs | |
682 | ||
683 | However, if you have fixed sized records, then you might do something more | |
684 | like this: | |
685 | ||
686 | $RECSIZE = 220; # size of record, in bytes | |
687 | $recno = 37; # which record to update | |
688 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; | |
689 | seek(FH, $recno * $RECSIZE, 0); | |
690 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; | |
691 | # munge the record | |
692 | seek(FH, $recno * $RECSIZE, 0); | |
693 | print FH $record; | |
694 | close FH; | |
695 | ||
696 | Locking and error checking are left as an exercise for the reader. | |
697 | Don't forget them, or you'll be quite sorry. | |
698 | ||
68dc0745 | 699 | =head2 How do I get a file's timestamp in perl? |
700 | ||
701 | If you want to retrieve the time at which the file was last read, | |
46fc3d4c | 702 | written, or had its meta-data (owner, etc) changed, you use the B<-M>, |
68dc0745 | 703 | B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These |
704 | retrieve the age of the file (measured against the start-time of your | |
705 | program) in days as a floating point number. To retrieve the "raw" | |
706 | time in seconds since the epoch, you would call the stat function, | |
707 | then use localtime(), gmtime(), or POSIX::strftime() to convert this | |
708 | into human-readable form. | |
709 | ||
710 | Here's an example: | |
711 | ||
712 | $write_secs = (stat($file))[9]; | |
713 | print "file $file updated at ", scalar(localtime($file)), "\n"; | |
714 | ||
715 | If you prefer something more legible, use the File::stat module | |
716 | (part of the standard distribution in version 5.004 and later): | |
717 | ||
718 | use File::stat; | |
719 | use Time::localtime; | |
720 | $date_string = ctime(stat($file)->mtime); | |
721 | print "file $file updated at $date_string\n"; | |
722 | ||
723 | Error checking is left as an exercise for the reader. | |
724 | ||
725 | =head2 How do I set a file's timestamp in perl? | |
726 | ||
727 | You use the utime() function documented in L<perlfunc/utime>. | |
728 | By way of example, here's a little program that copies the | |
729 | read and write times from its first argument to all the rest | |
730 | of them. | |
731 | ||
732 | if (@ARGV < 2) { | |
733 | die "usage: cptimes timestamp_file other_files ...\n"; | |
734 | } | |
735 | $timestamp = shift; | |
736 | ($atime, $mtime) = (stat($timestamp))[8,9]; | |
737 | utime $atime, $mtime, @ARGV; | |
738 | ||
739 | Error checking is left as an exercise for the reader. | |
740 | ||
741 | Note that utime() currently doesn't work correctly with Win95/NT | |
742 | ports. A bug has been reported. Check it carefully before using | |
743 | it on those platforms. | |
744 | ||
745 | =head2 How do I print to more than one file at once? | |
746 | ||
747 | If you only have to do this once, you can do this: | |
748 | ||
749 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } | |
750 | ||
751 | To connect up to one filehandle to several output filehandles, it's | |
752 | easiest to use the tee(1) program if you have it, and let it take care | |
753 | of the multiplexing: | |
754 | ||
755 | open (FH, "| tee file1 file2 file3"); | |
756 | ||
5a964f20 TC |
757 | Or even: |
758 | ||
759 | # make STDOUT go to three files, plus original STDOUT | |
760 | open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n"; | |
761 | print "whatever\n" or die "Writing: $!\n"; | |
762 | close(STDOUT) or die "Closing: $!\n"; | |
68dc0745 | 763 | |
5a964f20 TC |
764 | Otherwise you'll have to write your own multiplexing print |
765 | function -- or your own tee program -- or use Tom Christiansen's, | |
766 | at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is | |
767 | written in Perl and offers much greater functionality | |
768 | than the stock version. | |
68dc0745 | 769 | |
770 | =head2 How can I read in a file by paragraphs? | |
771 | ||
772 | Use the C<$\> variable (see L<perlvar> for details). You can either | |
773 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, | |
774 | for instance, gets treated as two paragraphs and not three), or | |
775 | C<"\n\n"> to accept empty paragraphs. | |
776 | ||
777 | =head2 How can I read a single character from a file? From the keyboard? | |
778 | ||
779 | You can use the builtin C<getc()> function for most filehandles, but | |
780 | it won't (easily) work on a terminal device. For STDIN, either use | |
781 | the Term::ReadKey module from CPAN, or use the sample code in | |
782 | L<perlfunc/getc>. | |
783 | ||
784 | If your system supports POSIX, you can use the following code, which | |
785 | you'll note turns off echo processing as well. | |
786 | ||
787 | #!/usr/bin/perl -w | |
788 | use strict; | |
789 | $| = 1; | |
790 | for (1..4) { | |
791 | my $got; | |
792 | print "gimme: "; | |
793 | $got = getone(); | |
794 | print "--> $got\n"; | |
795 | } | |
796 | exit; | |
797 | ||
798 | BEGIN { | |
799 | use POSIX qw(:termios_h); | |
800 | ||
801 | my ($term, $oterm, $echo, $noecho, $fd_stdin); | |
802 | ||
803 | $fd_stdin = fileno(STDIN); | |
804 | ||
805 | $term = POSIX::Termios->new(); | |
806 | $term->getattr($fd_stdin); | |
807 | $oterm = $term->getlflag(); | |
808 | ||
809 | $echo = ECHO | ECHOK | ICANON; | |
810 | $noecho = $oterm & ~$echo; | |
811 | ||
812 | sub cbreak { | |
813 | $term->setlflag($noecho); | |
814 | $term->setcc(VTIME, 1); | |
815 | $term->setattr($fd_stdin, TCSANOW); | |
816 | } | |
817 | ||
818 | sub cooked { | |
819 | $term->setlflag($oterm); | |
820 | $term->setcc(VTIME, 0); | |
821 | $term->setattr($fd_stdin, TCSANOW); | |
822 | } | |
823 | ||
824 | sub getone { | |
825 | my $key = ''; | |
826 | cbreak(); | |
827 | sysread(STDIN, $key, 1); | |
828 | cooked(); | |
829 | return $key; | |
830 | } | |
831 | ||
832 | } | |
833 | ||
834 | END { cooked() } | |
835 | ||
836 | The Term::ReadKey module from CPAN may be easier to use: | |
837 | ||
838 | use Term::ReadKey; | |
839 | open(TTY, "</dev/tty"); | |
840 | print "Gimme a char: "; | |
841 | ReadMode "raw"; | |
842 | $key = ReadKey 0, *TTY; | |
843 | ReadMode "normal"; | |
844 | printf "\nYou said %s, char number %03d\n", | |
845 | $key, ord $key; | |
846 | ||
46fc3d4c | 847 | For DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following: |
68dc0745 | 848 | |
849 | To put the PC in "raw" mode, use ioctl with some magic numbers gleaned | |
850 | from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes | |
851 | across the net every so often): | |
852 | ||
853 | $old_ioctl = ioctl(STDIN,0,0); # Gets device info | |
854 | $old_ioctl &= 0xff; | |
855 | ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5 | |
856 | ||
857 | Then to read a single character: | |
858 | ||
859 | sysread(STDIN,$c,1); # Read a single character | |
860 | ||
861 | And to put the PC back to "cooked" mode: | |
862 | ||
863 | ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode. | |
864 | ||
865 | So now you have $c. If C<ord($c) == 0>, you have a two byte code, which | |
866 | means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>, | |
867 | and that value tells you what combination it was according to this | |
868 | table: | |
869 | ||
870 | # PC 2-byte keycodes = ^@ + the following: | |
871 | ||
872 | # HEX KEYS | |
873 | # --- ---- | |
874 | # 0F SHF TAB | |
875 | # 10-19 ALT QWERTYUIOP | |
876 | # 1E-26 ALT ASDFGHJKL | |
877 | # 2C-32 ALT ZXCVBNM | |
878 | # 3B-44 F1-F10 | |
879 | # 47-49 HOME,UP,PgUp | |
880 | # 4B LEFT | |
881 | # 4D RIGHT | |
882 | # 4F-53 END,DOWN,PgDn,Ins,Del | |
883 | # 54-5D SHF F1-F10 | |
884 | # 5E-67 CTR F1-F10 | |
885 | # 68-71 ALT F1-F10 | |
886 | # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME | |
887 | # 78-83 ALT 1234567890-= | |
888 | # 84 CTR PgUp | |
889 | ||
890 | This is all trial and error I did a long time ago, I hope I'm reading the | |
891 | file that worked. | |
892 | ||
893 | =head2 How can I tell if there's a character waiting on a filehandle? | |
894 | ||
5a964f20 TC |
895 | The very first thing you should do is look into getting the Term::ReadKey |
896 | extension from CPAN. It now even has limited support for closed, proprietary | |
897 | (read: not open systems, not POSIX, not Unix, etc) systems. | |
898 | ||
899 | You should also check out the Frequently Asked Questions list in | |
68dc0745 | 900 | comp.unix.* for things like this: the answer is essentially the same. |
901 | It's very system dependent. Here's one solution that works on BSD | |
902 | systems: | |
903 | ||
904 | sub key_ready { | |
905 | my($rin, $nfd); | |
906 | vec($rin, fileno(STDIN), 1) = 1; | |
907 | return $nfd = select($rin,undef,undef,0); | |
908 | } | |
909 | ||
5a964f20 TC |
910 | If you want to find out how many characters are waiting, |
911 | there's also the FIONREAD ioctl call to be looked at. | |
68dc0745 | 912 | |
5a964f20 TC |
913 | The I<h2ph> tool that comes with Perl tries to convert C include |
914 | files to Perl code, which can be C<require>d. FIONREAD ends | |
915 | up defined as a function in the I<sys/ioctl.ph> file: | |
68dc0745 | 916 | |
5a964f20 | 917 | require 'sys/ioctl.ph'; |
68dc0745 | 918 | |
5a964f20 TC |
919 | $size = pack("L", 0); |
920 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; | |
921 | $size = unpack("L", $size); | |
68dc0745 | 922 | |
5a964f20 TC |
923 | If I<h2ph> wasn't installed or doesn't work for you, you can |
924 | I<grep> the include files by hand: | |
68dc0745 | 925 | |
5a964f20 TC |
926 | % grep FIONREAD /usr/include/*/* |
927 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B | |
68dc0745 | 928 | |
5a964f20 | 929 | Or write a small C program using the editor of champions: |
68dc0745 | 930 | |
5a964f20 TC |
931 | % cat > fionread.c |
932 | #include <sys/ioctl.h> | |
933 | main() { | |
934 | printf("%#08x\n", FIONREAD); | |
935 | } | |
936 | ^D | |
937 | % cc -o fionread fionread | |
938 | % ./fionread | |
939 | 0x4004667f | |
940 | ||
941 | And then hard-code it, leaving porting as an exercise to your successor. | |
942 | ||
943 | $FIONREAD = 0x4004667f; # XXX: opsys dependent | |
944 | ||
945 | $size = pack("L", 0); | |
946 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; | |
947 | $size = unpack("L", $size); | |
948 | ||
949 | FIONREAD requires a filehandle connected to a stream, meaning sockets, | |
950 | pipes, and tty devices work, but I<not> files. | |
68dc0745 | 951 | |
952 | =head2 How do I do a C<tail -f> in perl? | |
953 | ||
954 | First try | |
955 | ||
956 | seek(GWFILE, 0, 1); | |
957 | ||
958 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, | |
959 | but it does clear the end-of-file condition on the handle, so that the | |
960 | next <GWFILE> makes Perl try again to read something. | |
961 | ||
962 | If that doesn't work (it relies on features of your stdio implementation), | |
963 | then you need something more like this: | |
964 | ||
965 | for (;;) { | |
966 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { | |
967 | # search for some stuff and put it into files | |
968 | } | |
969 | # sleep for a while | |
970 | seek(GWFILE, $curpos, 0); # seek to where we had been | |
971 | } | |
972 | ||
973 | If this still doesn't work, look into the POSIX module. POSIX defines | |
974 | the clearerr() method, which can remove the end of file condition on a | |
975 | filehandle. The method: read until end of file, clearerr(), read some | |
976 | more. Lather, rinse, repeat. | |
977 | ||
978 | =head2 How do I dup() a filehandle in Perl? | |
979 | ||
980 | If you check L<perlfunc/open>, you'll see that several of the ways | |
981 | to call open() should do the trick. For example: | |
982 | ||
983 | open(LOG, ">>/tmp/logfile"); | |
984 | open(STDERR, ">&LOG"); | |
985 | ||
986 | Or even with a literal numeric descriptor: | |
987 | ||
988 | $fd = $ENV{MHCONTEXTFD}; | |
989 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) | |
990 | ||
5a964f20 TC |
991 | Note that "E<lt>&STDIN" makes a copy, but "E<lt>&=STDIN" make |
992 | an alias. That means if you close an aliased handle, all | |
993 | aliases become inaccessible. This is not true with | |
994 | a copied one. | |
995 | ||
996 | Error checking, as always, has been left as an exercise for the reader. | |
68dc0745 | 997 | |
998 | =head2 How do I close a file descriptor by number? | |
999 | ||
1000 | This should rarely be necessary, as the Perl close() function is to be | |
1001 | used for things that Perl opened itself, even if it was a dup of a | |
1002 | numeric descriptor, as with MHCONTEXT above. But if you really have | |
1003 | to, you may be able to do this: | |
1004 | ||
1005 | require 'sys/syscall.ph'; | |
1006 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric | |
1007 | die "can't sysclose $fd: $!" unless $rc == -1; | |
1008 | ||
46fc3d4c | 1009 | =head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work? |
68dc0745 | 1010 | |
1011 | Whoops! You just put a tab and a formfeed into that filename! | |
1012 | Remember that within double quoted strings ("like\this"), the | |
1013 | backslash is an escape character. The full list of these is in | |
1014 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't | |
1015 | have a file called "c:(tab)emp(formfeed)oo" or | |
46fc3d4c | 1016 | "c:(tab)emp(formfeed)oo.exe" on your DOS filesystem. |
68dc0745 | 1017 | |
1018 | Either single-quote your strings, or (preferably) use forward slashes. | |
46fc3d4c | 1019 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 | 1020 | have treated C</> and C<\> the same in a path, you might as well use the |
1021 | one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++, | |
1022 | awk, Tcl, Java, or Python, just to mention a few. | |
1023 | ||
1024 | =head2 Why doesn't glob("*.*") get all the files? | |
1025 | ||
1026 | Because even on non-Unix ports, Perl's glob function follows standard | |
46fc3d4c | 1027 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
5a964f20 | 1028 | files. This makes glob() portable. |
68dc0745 | 1029 | |
1030 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? | |
1031 | ||
1032 | This is elaborately and painstakingly described in the "Far More Than | |
7b8d334a | 1033 | You Ever Wanted To Know" in |
68dc0745 | 1034 | http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms . |
1035 | ||
1036 | The executive summary: learn how your filesystem works. The | |
1037 | permissions on a file say what can happen to the data in that file. | |
1038 | The permissions on a directory say what can happen to the list of | |
1039 | files in that directory. If you delete a file, you're removing its | |
1040 | name from the directory (so the operation depends on the permissions | |
1041 | of the directory, not of the file). If you try to write to the file, | |
1042 | the permissions of the file govern whether you're allowed to. | |
1043 | ||
1044 | =head2 How do I select a random line from a file? | |
1045 | ||
1046 | Here's an algorithm from the Camel Book: | |
1047 | ||
1048 | srand; | |
1049 | rand($.) < 1 && ($line = $_) while <>; | |
1050 | ||
1051 | This has a significant advantage in space over reading the whole | |
5a964f20 TC |
1052 | file in. A simple proof by induction is available upon |
1053 | request if you doubt its correctness. | |
68dc0745 | 1054 | |
1055 | =head1 AUTHOR AND COPYRIGHT | |
1056 | ||
5a964f20 TC |
1057 | Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. |
1058 | All rights reserved. | |
1059 | ||
1060 | When included as part of the Standard Version of Perl, or as part of | |
1061 | its complete documentation whether printed or otherwise, this work | |
1062 | may be distributed only under the terms of Perl's Artistic License. | |
1063 | Any distribution of this file or derivatives thereof I<outside> | |
1064 | of that package require that special arrangements be made with | |
1065 | copyright holder. | |
1066 | ||
1067 | Irrespective of its distribution, all code examples in this file | |
1068 | are hereby placed into the public domain. You are permitted and | |
1069 | encouraged to use this code in your own programs for fun | |
1070 | or for profit as you see fit. A simple comment in the code giving | |
1071 | credit would be courteous but is not required. |