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1 | =head1 NAME |
2 | |
3 | perlopentut - tutorial on opening things in Perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | Perl has two simple, built-in ways to open files: the shell way for |
8 | convenience, and the C way for precision. The choice is yours. |
9 | |
10 | =head1 Open E<agrave> la shell |
11 | |
12 | Perl's C<open> function was designed to mimic the way command-line |
13 | redirection in the shell works. Here are some basic examples |
14 | from the shell: |
15 | |
16 | $ myprogram file1 file2 file3 |
17 | $ myprogram < inputfile |
18 | $ myprogram > outputfile |
19 | $ myprogram >> outputfile |
20 | $ myprogram | otherprogram |
21 | $ otherprogram | myprogram |
22 | |
23 | And here are some more advanced examples: |
24 | |
25 | $ otherprogram | myprogram f1 - f2 |
26 | $ otherprogram 2>&1 | myprogram - |
27 | $ myprogram <&3 |
28 | $ myprogram >&4 |
29 | |
30 | Programmers accustomed to constructs like those above can take comfort |
31 | in learning that Perl directly supports these familiar constructs using |
32 | virtually the same syntax as the shell. |
33 | |
34 | =head2 Simple Opens |
35 | |
36 | The C<open> function takes two arguments: the first is a filehandle, |
37 | and the second is a single string comprising both what to open and how |
38 | to open it. C<open> returns true when it works, and when it fails, |
39 | returns a false value and sets the special variable $! to reflect |
40 | the system error. If the filehandle was previously opened, it will |
41 | be implicitly closed first. |
42 | |
43 | For example: |
44 | |
45 | open(INFO, "datafile") || die("can't open datafile: $!"); |
46 | open(INFO, "< datafile") || die("can't open datafile: $!"); |
47 | open(RESULTS,"> runstats") || die("can't open runstats: $!"); |
48 | open(LOG, ">> logfile ") || die("can't open logfile: $!"); |
49 | |
50 | If you prefer the low-punctuation version, you could write that this way: |
51 | |
52 | open INFO, "< datafile" or die "can't open datafile: $!"; |
53 | open RESULTS,"> runstats" or die "can't open runstats: $!"; |
54 | open LOG, ">> logfile " or die "can't open logfile: $!"; |
55 | |
56 | A few things to notice. First, the leading less-than is optional. |
57 | If omitted, Perl assumes that you want to open the file for reading. |
58 | |
59 | The other important thing to notice is that, just as in the shell, |
60 | any white space before or after the filename is ignored. This is good, |
61 | because you wouldn't want these to do different things: |
62 | |
63 | open INFO, "<datafile" |
64 | open INFO, "< datafile" |
65 | open INFO, "< datafile" |
66 | |
67 | Ignoring surround whitespace also helps for when you read a filename in |
68 | from a different file, and forget to trim it before opening: |
69 | |
70 | $filename = <INFO>; # oops, \n still there |
71 | open(EXTRA, "< $filename") || die "can't open $filename: $!"; |
72 | |
73 | This is not a bug, but a feature. Because C<open> mimics the shell in |
74 | its style of using redirection arrows to specify how to open the file, it |
75 | also does so with respect to extra white space around the filename itself |
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76 | as well. For accessing files with naughty names, see |
77 | L<"Dispelling the Dweomer">. |
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78 | |
79 | =head2 Pipe Opens |
80 | |
81 | In C, when you want to open a file using the standard I/O library, |
82 | you use the C<fopen> function, but when opening a pipe, you use the |
83 | C<popen> function. But in the shell, you just use a different redirection |
84 | character. That's also the case for Perl. The C<open> call |
85 | remains the same--just its argument differs. |
86 | |
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87 | If the leading character is a pipe symbol, C<open> starts up a new |
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88 | command and open a write-only filehandle leading into that command. |
89 | This lets you write into that handle and have what you write show up on |
90 | that command's standard input. For example: |
91 | |
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92 | open(PRINTER, "| lpr -Plp1") || die "can't run lpr: $!"; |
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93 | print PRINTER "stuff\n"; |
94 | close(PRINTER) || die "can't close lpr: $!"; |
95 | |
96 | If the trailing character is a pipe, you start up a new command and open a |
97 | read-only filehandle leading out of that command. This lets whatever that |
98 | command writes to its standard output show up on your handle for reading. |
99 | For example: |
100 | |
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101 | open(NET, "netstat -i -n |") || die "can't fun netstat: $!"; |
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102 | while (<NET>) { } # do something with input |
103 | close(NET) || die "can't close netstat: $!"; |
104 | |
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105 | What happens if you try to open a pipe to or from a non-existent |
106 | command? If possible, Perl will detect the failure and set C<$!> as |
107 | usual. But if the command contains special shell characters, such as |
108 | C<E<gt>> or C<*>, called 'metacharacters', Perl does not execute the |
109 | command directly. Instead, Perl runs the shell, which then tries to |
110 | run the command. This means that it's the shell that gets the error |
111 | indication. In such a case, the C<open> call will only indicate |
112 | failure if Perl can't even run the shell. See L<perlfaq8/"How can I |
113 | capture STDERR from an external command?"> to see how to cope with |
114 | this. There's also an explanation in L<perlipc>. |
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115 | |
116 | If you would like to open a bidirectional pipe, the IPC::Open2 |
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117 | library will handle this for you. Check out |
118 | L<perlipc/"Bidirectional Communication with Another Process"> |
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119 | |
120 | =head2 The Minus File |
121 | |
122 | Again following the lead of the standard shell utilities, Perl's |
123 | C<open> function treats a file whose name is a single minus, "-", in a |
124 | special way. If you open minus for reading, it really means to access |
125 | the standard input. If you open minus for writing, it really means to |
126 | access the standard output. |
127 | |
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128 | If minus can be used as the default input or default output, what happens |
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129 | if you open a pipe into or out of minus? What's the default command it |
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130 | would run? The same script as you're currently running! This is actually |
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131 | a stealth C<fork> hidden inside an C<open> call. See |
132 | L<perlipc/"Safe Pipe Opens"> for details. |
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133 | |
134 | =head2 Mixing Reads and Writes |
135 | |
136 | It is possible to specify both read and write access. All you do is |
137 | add a "+" symbol in front of the redirection. But as in the shell, |
138 | using a less-than on a file never creates a new file; it only opens an |
139 | existing one. On the other hand, using a greater-than always clobbers |
140 | (truncates to zero length) an existing file, or creates a brand-new one |
141 | if there isn't an old one. Adding a "+" for read-write doesn't affect |
142 | whether it only works on existing files or always clobbers existing ones. |
143 | |
144 | open(WTMP, "+< /usr/adm/wtmp") |
145 | || die "can't open /usr/adm/wtmp: $!"; |
146 | |
147 | open(SCREEN, "+> /tmp/lkscreen") |
148 | || die "can't open /tmp/lkscreen: $!"; |
149 | |
150 | open(LOGFILE, "+>> /tmp/applog" |
151 | || die "can't open /tmp/applog: $!"; |
152 | |
153 | The first one won't create a new file, and the second one will always |
154 | clobber an old one. The third one will create a new file if necessary |
155 | and not clobber an old one, and it will allow you to read at any point |
156 | in the file, but all writes will always go to the end. In short, |
157 | the first case is substantially more common than the second and third |
158 | cases, which are almost always wrong. (If you know C, the plus in |
159 | Perl's C<open> is historically derived from the one in C's fopen(3S), |
160 | which it ultimately calls.) |
161 | |
162 | In fact, when it comes to updating a file, unless you're working on |
163 | a binary file as in the WTMP case above, you probably don't want to |
164 | use this approach for updating. Instead, Perl's B<-i> flag comes to |
165 | the rescue. The following command takes all the C, C++, or yacc source |
166 | or header files and changes all their foo's to bar's, leaving |
167 | the old version in the original file name with a ".orig" tacked |
168 | on the end: |
169 | |
170 | $ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy] |
171 | |
172 | This is a short cut for some renaming games that are really |
173 | the best way to update textfiles. See the second question in |
174 | L<perlfaq5> for more details. |
175 | |
176 | =head2 Filters |
177 | |
178 | One of the most common uses for C<open> is one you never |
179 | even notice. When you process the ARGV filehandle using |
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180 | C<< <ARGV> >>, Perl actually does an implicit open |
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181 | on each file in @ARGV. Thus a program called like this: |
182 | |
183 | $ myprogram file1 file2 file3 |
184 | |
185 | Can have all its files opened and processed one at a time |
186 | using a construct no more complex than: |
187 | |
188 | while (<>) { |
189 | # do something with $_ |
190 | } |
191 | |
192 | If @ARGV is empty when the loop first begins, Perl pretends you've opened |
193 | up minus, that is, the standard input. In fact, $ARGV, the currently |
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194 | open file during C<< <ARGV> >> processing, is even set to "-" |
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195 | in these circumstances. |
196 | |
197 | You are welcome to pre-process your @ARGV before starting the loop to |
198 | make sure it's to your liking. One reason to do this might be to remove |
199 | command options beginning with a minus. While you can always roll the |
200 | simple ones by hand, the Getopts modules are good for this. |
201 | |
202 | use Getopt::Std; |
203 | |
204 | # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o |
205 | getopts("vDo:"); |
206 | |
207 | # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o} |
208 | getopts("vDo:", \%args); |
209 | |
210 | Or the standard Getopt::Long module to permit named arguments: |
211 | |
212 | use Getopt::Long; |
213 | GetOptions( "verbose" => \$verbose, # --verbose |
214 | "Debug" => \$debug, # --Debug |
215 | "output=s" => \$output ); |
216 | # --output=somestring or --output somestring |
217 | |
218 | Another reason for preprocessing arguments is to make an empty |
219 | argument list default to all files: |
220 | |
221 | @ARGV = glob("*") unless @ARGV; |
222 | |
223 | You could even filter out all but plain, text files. This is a bit |
224 | silent, of course, and you might prefer to mention them on the way. |
225 | |
226 | @ARGV = grep { -f && -T } @ARGV; |
227 | |
228 | If you're using the B<-n> or B<-p> command-line options, you |
229 | should put changes to @ARGV in a C<BEGIN{}> block. |
230 | |
231 | Remember that a normal C<open> has special properties, in that it might |
232 | call fopen(3S) or it might called popen(3S), depending on what its |
233 | argument looks like; that's why it's sometimes called "magic open". |
234 | Here's an example: |
235 | |
236 | $pwdinfo = `domainname` =~ /^(\(none\))?$/ |
237 | ? '< /etc/passwd' |
238 | : 'ypcat passwd |'; |
239 | |
240 | open(PWD, $pwdinfo) |
241 | or die "can't open $pwdinfo: $!"; |
242 | |
243 | This sort of thing also comes into play in filter processing. Because |
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244 | C<< <ARGV> >> processing employs the normal, shell-style Perl C<open>, |
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245 | it respects all the special things we've already seen: |
246 | |
247 | $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile |
248 | |
249 | That program will read from the file F<f1>, the process F<cmd1>, standard |
250 | input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command, |
251 | and finally the F<f3> file. |
252 | |
253 | Yes, this also means that if you have a file named "-" (and so on) in |
254 | your directory, that they won't be processed as literal files by C<open>. |
255 | You'll need to pass them as "./-" much as you would for the I<rm> program. |
256 | Or you could use C<sysopen> as described below. |
257 | |
258 | One of the more interesting applications is to change files of a certain |
259 | name into pipes. For example, to autoprocess gzipped or compressed |
260 | files by decompressing them with I<gzip>: |
261 | |
262 | @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV; |
263 | |
264 | Or, if you have the I<GET> program installed from LWP, |
265 | you can fetch URLs before processing them: |
266 | |
267 | @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV; |
268 | |
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269 | It's not for nothing that this is called magic C<< <ARGV> >>. |
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270 | Pretty nifty, eh? |
271 | |
272 | =head1 Open E<agrave> la C |
273 | |
274 | If you want the convenience of the shell, then Perl's C<open> is |
275 | definitely the way to go. On the other hand, if you want finer precision |
276 | than C's simplistic fopen(3S) provides, then you should look to Perl's |
277 | C<sysopen>, which is a direct hook into the open(2) system call. |
278 | That does mean it's a bit more involved, but that's the price of |
279 | precision. |
280 | |
281 | C<sysopen> takes 3 (or 4) arguments. |
282 | |
283 | sysopen HANDLE, PATH, FLAGS, [MASK] |
284 | |
285 | The HANDLE argument is a filehandle just as with C<open>. The PATH is |
286 | a literal path, one that doesn't pay attention to any greater-thans or |
287 | less-thans or pipes or minuses, nor ignore white space. If it's there, |
288 | it's part of the path. The FLAGS argument contains one or more values |
289 | derived from the Fcntl module that have been or'd together using the |
290 | bitwise "|" operator. The final argument, the MASK, is optional; if |
291 | present, it is combined with the user's current umask for the creation |
292 | mode of the file. You should usually omit this. |
293 | |
294 | Although the traditional values of read-only, write-only, and read-write |
295 | are 0, 1, and 2 respectively, this is known not to hold true on some |
296 | systems. Instead, it's best to load in the appropriate constants first |
297 | from the Fcntl module, which supplies the following standard flags: |
298 | |
299 | O_RDONLY Read only |
300 | O_WRONLY Write only |
301 | O_RDWR Read and write |
302 | O_CREAT Create the file if it doesn't exist |
303 | O_EXCL Fail if the file already exists |
304 | O_APPEND Append to the file |
305 | O_TRUNC Truncate the file |
306 | O_NONBLOCK Non-blocking access |
307 | |
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308 | Less common flags that are sometimes available on some operating |
309 | systems include C<O_BINARY>, C<O_TEXT>, C<O_SHLOCK>, C<O_EXLOCK>, |
310 | C<O_DEFER>, C<O_SYNC>, C<O_ASYNC>, C<O_DSYNC>, C<O_RSYNC>, |
311 | C<O_NOCTTY>, C<O_NDELAY> and C<O_LARGEFILE>. Consult your open(2) |
312 | manpage or its local equivalent for details. (Note: starting from |
313 | Perl release 5.6 the O_LARGEFILE flag, if available, is automatically |
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314 | added to the sysopen() flags because large files are the default.) |
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315 | |
316 | Here's how to use C<sysopen> to emulate the simple C<open> calls we had |
317 | before. We'll omit the C<|| die $!> checks for clarity, but make sure |
318 | you always check the return values in real code. These aren't quite |
319 | the same, since C<open> will trim leading and trailing white space, |
320 | but you'll get the idea: |
321 | |
322 | To open a file for reading: |
323 | |
324 | open(FH, "< $path"); |
325 | sysopen(FH, $path, O_RDONLY); |
326 | |
327 | To open a file for writing, creating a new file if needed or else truncating |
328 | an old file: |
329 | |
330 | open(FH, "> $path"); |
331 | sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT); |
332 | |
333 | To open a file for appending, creating one if necessary: |
334 | |
335 | open(FH, ">> $path"); |
336 | sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT); |
337 | |
338 | To open a file for update, where the file must already exist: |
339 | |
340 | open(FH, "+< $path"); |
341 | sysopen(FH, $path, O_RDWR); |
342 | |
343 | And here are things you can do with C<sysopen> that you cannot do with |
344 | a regular C<open>. As you see, it's just a matter of controlling the |
345 | flags in the third argument. |
346 | |
347 | To open a file for writing, creating a new file which must not previously |
348 | exist: |
349 | |
350 | sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT); |
351 | |
352 | To open a file for appending, where that file must already exist: |
353 | |
354 | sysopen(FH, $path, O_WRONLY | O_APPEND); |
355 | |
356 | To open a file for update, creating a new file if necessary: |
357 | |
358 | sysopen(FH, $path, O_RDWR | O_CREAT); |
359 | |
360 | To open a file for update, where that file must not already exist: |
361 | |
362 | sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT); |
363 | |
364 | To open a file without blocking, creating one if necessary: |
365 | |
366 | sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT); |
367 | |
368 | =head2 Permissions E<agrave> la mode |
369 | |
370 | If you omit the MASK argument to C<sysopen>, Perl uses the octal value |
371 | 0666. The normal MASK to use for executables and directories should |
372 | be 0777, and for anything else, 0666. |
373 | |
374 | Why so permissive? Well, it isn't really. The MASK will be modified |
375 | by your process's current C<umask>. A umask is a number representing |
376 | I<disabled> permissions bits; that is, bits that will not be turned on |
377 | in the created files' permissions field. |
378 | |
379 | For example, if your C<umask> were 027, then the 020 part would |
380 | disable the group from writing, and the 007 part would disable others |
381 | from reading, writing, or executing. Under these conditions, passing |
382 | C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027> |
383 | is 0640. |
384 | |
385 | You should seldom use the MASK argument to C<sysopen()>. That takes |
386 | away the user's freedom to choose what permission new files will have. |
387 | Denying choice is almost always a bad thing. One exception would be for |
388 | cases where sensitive or private data is being stored, such as with mail |
389 | folders, cookie files, and internal temporary files. |
390 | |
391 | =head1 Obscure Open Tricks |
392 | |
393 | =head2 Re-Opening Files (dups) |
394 | |
395 | Sometimes you already have a filehandle open, and want to make another |
396 | handle that's a duplicate of the first one. In the shell, we place an |
397 | ampersand in front of a file descriptor number when doing redirections. |
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398 | For example, C<< 2>&1 >> makes descriptor 2 (that's STDERR in Perl) |
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399 | be redirected into descriptor 1 (which is usually Perl's STDOUT). |
400 | The same is essentially true in Perl: a filename that begins with an |
401 | ampersand is treated instead as a file descriptor if a number, or as a |
402 | filehandle if a string. |
403 | |
404 | open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!"; |
405 | open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!"; |
406 | |
407 | That means that if a function is expecting a filename, but you don't |
408 | want to give it a filename because you already have the file open, you |
409 | can just pass the filehandle with a leading ampersand. It's best to |
410 | use a fully qualified handle though, just in case the function happens |
411 | to be in a different package: |
412 | |
413 | somefunction("&main::LOGFILE"); |
414 | |
415 | This way if somefunction() is planning on opening its argument, it can |
416 | just use the already opened handle. This differs from passing a handle, |
417 | because with a handle, you don't open the file. Here you have something |
418 | you can pass to open. |
419 | |
420 | If you have one of those tricky, newfangled I/O objects that the C++ |
421 | folks are raving about, then this doesn't work because those aren't a |
422 | proper filehandle in the native Perl sense. You'll have to use fileno() |
423 | to pull out the proper descriptor number, assuming you can: |
424 | |
425 | use IO::Socket; |
426 | $handle = IO::Socket::INET->new("www.perl.com:80"); |
427 | $fd = $handle->fileno; |
428 | somefunction("&$fd"); # not an indirect function call |
429 | |
430 | It can be easier (and certainly will be faster) just to use real |
431 | filehandles though: |
432 | |
433 | use IO::Socket; |
434 | local *REMOTE = IO::Socket::INET->new("www.perl.com:80"); |
435 | die "can't connect" unless defined(fileno(REMOTE)); |
436 | somefunction("&main::REMOTE"); |
437 | |
438 | If the filehandle or descriptor number is preceded not just with a simple |
439 | "&" but rather with a "&=" combination, then Perl will not create a |
440 | completely new descriptor opened to the same place using the dup(2) |
441 | system call. Instead, it will just make something of an alias to the |
442 | existing one using the fdopen(3S) library call This is slightly more |
443 | parsimonious of systems resources, although this is less a concern |
444 | these days. Here's an example of that: |
445 | |
446 | $fd = $ENV{"MHCONTEXTFD"}; |
447 | open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!"; |
448 | |
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449 | If you're using magic C<< <ARGV> >>, you could even pass in as a |
450 | command line argument in @ARGV something like C<"<&=$MHCONTEXTFD">, |
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451 | but we've never seen anyone actually do this. |
452 | |
453 | =head2 Dispelling the Dweomer |
454 | |
455 | Perl is more of a DWIMmer language than something like Java--where DWIM |
456 | is an acronym for "do what I mean". But this principle sometimes leads |
457 | to more hidden magic than one knows what to do with. In this way, Perl |
458 | is also filled with I<dweomer>, an obscure word meaning an enchantment. |
459 | Sometimes, Perl's DWIMmer is just too much like dweomer for comfort. |
460 | |
461 | If magic C<open> is a bit too magical for you, you don't have to turn |
462 | to C<sysopen>. To open a file with arbitrary weird characters in |
463 | it, it's necessary to protect any leading and trailing whitespace. |
464 | Leading whitespace is protected by inserting a C<"./"> in front of a |
465 | filename that starts with whitespace. Trailing whitespace is protected |
466 | by appending an ASCII NUL byte (C<"\0">) at the end off the string. |
467 | |
468 | $file =~ s#^(\s)#./$1#; |
469 | open(FH, "< $file\0") || die "can't open $file: $!"; |
470 | |
471 | This assumes, of course, that your system considers dot the current |
472 | working directory, slash the directory separator, and disallows ASCII |
473 | NULs within a valid filename. Most systems follow these conventions, |
474 | including all POSIX systems as well as proprietary Microsoft systems. |
475 | The only vaguely popular system that doesn't work this way is the |
476 | proprietary Macintosh system, which uses a colon where the rest of us |
477 | use a slash. Maybe C<sysopen> isn't such a bad idea after all. |
478 | |
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479 | If you want to use C<< <ARGV> >> processing in a totally boring |
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480 | and non-magical way, you could do this first: |
481 | |
482 | # "Sam sat on the ground and put his head in his hands. |
483 | # 'I wish I had never come here, and I don't want to see |
484 | # no more magic,' he said, and fell silent." |
485 | for (@ARGV) { |
486 | s#^([^./])#./$1#; |
487 | $_ .= "\0"; |
488 | } |
489 | while (<>) { |
490 | # now process $_ |
491 | } |
492 | |
493 | But be warned that users will not appreciate being unable to use "-" |
494 | to mean standard input, per the standard convention. |
495 | |
496 | =head2 Paths as Opens |
497 | |
498 | You've probably noticed how Perl's C<warn> and C<die> functions can |
499 | produce messages like: |
500 | |
1761cee5 |
501 | Some warning at scriptname line 29, <FH> line 7. |
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502 | |
503 | That's because you opened a filehandle FH, and had read in seven records |
504 | from it. But what was the name of the file, not the handle? |
505 | |
506 | If you aren't running with C<strict refs>, or if you've turn them off |
507 | temporarily, then all you have to do is this: |
508 | |
509 | open($path, "< $path") || die "can't open $path: $!"; |
510 | while (<$path>) { |
511 | # whatever |
512 | } |
513 | |
514 | Since you're using the pathname of the file as its handle, |
515 | you'll get warnings more like |
516 | |
1761cee5 |
517 | Some warning at scriptname line 29, </etc/motd> line 7. |
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518 | |
519 | =head2 Single Argument Open |
520 | |
521 | Remember how we said that Perl's open took two arguments? That was a |
522 | passive prevarication. You see, it can also take just one argument. |
523 | If and only if the variable is a global variable, not a lexical, you |
524 | can pass C<open> just one argument, the filehandle, and it will |
525 | get the path from the global scalar variable of the same name. |
526 | |
527 | $FILE = "/etc/motd"; |
528 | open FILE or die "can't open $FILE: $!"; |
529 | while (<FILE>) { |
530 | # whatever |
531 | } |
532 | |
533 | Why is this here? Someone has to cater to the hysterical porpoises. |
534 | It's something that's been in Perl since the very beginning, if not |
535 | before. |
536 | |
537 | =head2 Playing with STDIN and STDOUT |
538 | |
539 | One clever move with STDOUT is to explicitly close it when you're done |
540 | with the program. |
541 | |
542 | END { close(STDOUT) || die "can't close stdout: $!" } |
543 | |
544 | If you don't do this, and your program fills up the disk partition due |
545 | to a command line redirection, it won't report the error exit with a |
546 | failure status. |
547 | |
548 | You don't have to accept the STDIN and STDOUT you were given. You are |
549 | welcome to reopen them if you'd like. |
550 | |
551 | open(STDIN, "< datafile") |
552 | || die "can't open datafile: $!"; |
553 | |
554 | open(STDOUT, "> output") |
555 | || die "can't open output: $!"; |
556 | |
557 | And then these can be read directly or passed on to subprocesses. |
558 | This makes it look as though the program were initially invoked |
559 | with those redirections from the command line. |
560 | |
561 | It's probably more interesting to connect these to pipes. For example: |
562 | |
563 | $pager = $ENV{PAGER} || "(less || more)"; |
564 | open(STDOUT, "| $pager") |
565 | || die "can't fork a pager: $!"; |
566 | |
567 | This makes it appear as though your program were called with its stdout |
568 | already piped into your pager. You can also use this kind of thing |
569 | in conjunction with an implicit fork to yourself. You might do this |
570 | if you would rather handle the post processing in your own program, |
571 | just in a different process: |
572 | |
573 | head(100); |
574 | while (<>) { |
575 | print; |
576 | } |
577 | |
578 | sub head { |
579 | my $lines = shift || 20; |
580 | return unless $pid = open(STDOUT, "|-"); |
581 | die "cannot fork: $!" unless defined $pid; |
582 | while (<STDIN>) { |
583 | print; |
584 | last if --$lines < 0; |
585 | } |
586 | exit; |
587 | } |
588 | |
589 | This technique can be applied to repeatedly push as many filters on your |
590 | output stream as you wish. |
591 | |
592 | =head1 Other I/O Issues |
593 | |
594 | These topics aren't really arguments related to C<open> or C<sysopen>, |
595 | but they do affect what you do with your open files. |
596 | |
597 | =head2 Opening Non-File Files |
598 | |
599 | When is a file not a file? Well, you could say when it exists but |
600 | isn't a plain file. We'll check whether it's a symbolic link first, |
601 | just in case. |
602 | |
603 | if (-l $file || ! -f _) { |
604 | print "$file is not a plain file\n"; |
605 | } |
606 | |
607 | What other kinds of files are there than, well, files? Directories, |
608 | symbolic links, named pipes, Unix-domain sockets, and block and character |
609 | devices. Those are all files, too--just not I<plain> files. This isn't |
610 | the same issue as being a text file. Not all text files are plain files. |
611 | Not all plain files are textfiles. That's why there are separate C<-f> |
612 | and C<-T> file tests. |
613 | |
614 | To open a directory, you should use the C<opendir> function, then |
615 | process it with C<readdir>, carefully restoring the directory |
616 | name if necessary: |
617 | |
618 | opendir(DIR, $dirname) or die "can't opendir $dirname: $!"; |
619 | while (defined($file = readdir(DIR))) { |
620 | # do something with "$dirname/$file" |
621 | } |
622 | closedir(DIR); |
623 | |
624 | If you want to process directories recursively, it's better to use the |
625 | File::Find module. For example, this prints out all files recursively, |
626 | add adds a slash to their names if the file is a directory. |
627 | |
628 | @ARGV = qw(.) unless @ARGV; |
629 | use File::Find; |
630 | find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV; |
631 | |
632 | This finds all bogus symbolic links beneath a particular directory: |
633 | |
634 | find sub { print "$File::Find::name\n" if -l && !-e }, $dir; |
635 | |
636 | As you see, with symbolic links, you can just pretend that it is |
637 | what it points to. Or, if you want to know I<what> it points to, then |
638 | C<readlink> is called for: |
639 | |
640 | if (-l $file) { |
641 | if (defined($whither = readlink($file))) { |
642 | print "$file points to $whither\n"; |
643 | } else { |
644 | print "$file points nowhere: $!\n"; |
645 | } |
646 | } |
647 | |
648 | Named pipes are a different matter. You pretend they're regular files, |
649 | but their opens will normally block until there is both a reader and |
650 | a writer. You can read more about them in L<perlipc/"Named Pipes">. |
651 | Unix-domain sockets are rather different beasts as well; they're |
652 | described in L<perlipc/"Unix-Domain TCP Clients and Servers">. |
653 | |
654 | When it comes to opening devices, it can be easy and it can tricky. |
655 | We'll assume that if you're opening up a block device, you know what |
656 | you're doing. The character devices are more interesting. These are |
657 | typically used for modems, mice, and some kinds of printers. This is |
658 | described in L<perlfaq8/"How do I read and write the serial port?"> |
659 | It's often enough to open them carefully: |
660 | |
661 | sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY) |
662 | # (O_NOCTTY no longer needed on POSIX systems) |
663 | or die "can't open /dev/ttyS1: $!"; |
664 | open(TTYOUT, "+>&TTYIN") |
665 | or die "can't dup TTYIN: $!"; |
666 | |
667 | $ofh = select(TTYOUT); $| = 1; select($ofh); |
668 | |
669 | print TTYOUT "+++at\015"; |
670 | $answer = <TTYIN>; |
671 | |
672 | With descriptors that you haven't opened using C<sysopen>, such as a |
673 | socket, you can set them to be non-blocking using C<fcntl>: |
674 | |
675 | use Fcntl; |
676 | fcntl(Connection, F_SETFL, O_NONBLOCK) |
677 | or die "can't set non blocking: $!"; |
678 | |
679 | Rather than losing yourself in a morass of twisting, turning C<ioctl>s, |
680 | all dissimilar, if you're going to manipulate ttys, it's best to |
681 | make calls out to the stty(1) program if you have it, or else use the |
682 | portable POSIX interface. To figure this all out, you'll need to read the |
683 | termios(3) manpage, which describes the POSIX interface to tty devices, |
684 | and then L<POSIX>, which describes Perl's interface to POSIX. There are |
685 | also some high-level modules on CPAN that can help you with these games. |
686 | Check out Term::ReadKey and Term::ReadLine. |
687 | |
688 | What else can you open? To open a connection using sockets, you won't use |
13a2d996 |
689 | one of Perl's two open functions. See |
690 | L<perlipc/"Sockets: Client/Server Communication"> for that. Here's an |
691 | example. Once you have it, you can use FH as a bidirectional filehandle. |
f8284313 |
692 | |
693 | use IO::Socket; |
694 | local *FH = IO::Socket::INET->new("www.perl.com:80"); |
695 | |
696 | For opening up a URL, the LWP modules from CPAN are just what |
697 | the doctor ordered. There's no filehandle interface, but |
698 | it's still easy to get the contents of a document: |
699 | |
700 | use LWP::Simple; |
6cecdcac |
701 | $doc = get('http://www.linpro.no/lwp/'); |
f8284313 |
702 | |
703 | =head2 Binary Files |
704 | |
705 | On certain legacy systems with what could charitably be called terminally |
706 | convoluted (some would say broken) I/O models, a file isn't a file--at |
707 | least, not with respect to the C standard I/O library. On these old |
708 | systems whose libraries (but not kernels) distinguish between text and |
709 | binary streams, to get files to behave properly you'll have to bend over |
710 | backwards to avoid nasty problems. On such infelicitous systems, sockets |
711 | and pipes are already opened in binary mode, and there is currently no |
712 | way to turn that off. With files, you have more options. |
713 | |
714 | Another option is to use the C<binmode> function on the appropriate |
715 | handles before doing regular I/O on them: |
716 | |
717 | binmode(STDIN); |
718 | binmode(STDOUT); |
719 | while (<STDIN>) { print } |
720 | |
721 | Passing C<sysopen> a non-standard flag option will also open the file in |
722 | binary mode on those systems that support it. This is the equivalent of |
723 | opening the file normally, then calling C<binmode>ing on the handle. |
724 | |
725 | sysopen(BINDAT, "records.data", O_RDWR | O_BINARY) |
726 | || die "can't open records.data: $!"; |
727 | |
728 | Now you can use C<read> and C<print> on that handle without worrying |
729 | about the system non-standard I/O library breaking your data. It's not |
730 | a pretty picture, but then, legacy systems seldom are. CP/M will be |
731 | with us until the end of days, and after. |
732 | |
733 | On systems with exotic I/O systems, it turns out that, astonishingly |
734 | enough, even unbuffered I/O using C<sysread> and C<syswrite> might do |
735 | sneaky data mutilation behind your back. |
736 | |
737 | while (sysread(WHENCE, $buf, 1024)) { |
738 | syswrite(WHITHER, $buf, length($buf)); |
739 | } |
740 | |
741 | Depending on the vicissitudes of your runtime system, even these calls |
742 | may need C<binmode> or C<O_BINARY> first. Systems known to be free of |
743 | such difficulties include Unix, the Mac OS, Plan9, and Inferno. |
744 | |
745 | =head2 File Locking |
746 | |
747 | In a multitasking environment, you may need to be careful not to collide |
748 | with other processes who want to do I/O on the same files as others |
749 | are working on. You'll often need shared or exclusive locks |
750 | on files for reading and writing respectively. You might just |
751 | pretend that only exclusive locks exist. |
752 | |
753 | Never use the existence of a file C<-e $file> as a locking indication, |
754 | because there is a race condition between the test for the existence of |
755 | the file and its creation. Atomicity is critical. |
756 | |
757 | Perl's most portable locking interface is via the C<flock> function, |
758 | whose simplicity is emulated on systems that don't directly support it, |
759 | such as SysV or WindowsNT. The underlying semantics may affect how |
760 | it all works, so you should learn how C<flock> is implemented on your |
761 | system's port of Perl. |
762 | |
763 | File locking I<does not> lock out another process that would like to |
764 | do I/O. A file lock only locks out others trying to get a lock, not |
765 | processes trying to do I/O. Because locks are advisory, if one process |
766 | uses locking and another doesn't, all bets are off. |
767 | |
768 | By default, the C<flock> call will block until a lock is granted. |
769 | A request for a shared lock will be granted as soon as there is no |
d1be9408 |
770 | exclusive locker. A request for an exclusive lock will be granted as |
f8284313 |
771 | soon as there is no locker of any kind. Locks are on file descriptors, |
772 | not file names. You can't lock a file until you open it, and you can't |
773 | hold on to a lock once the file has been closed. |
774 | |
775 | Here's how to get a blocking shared lock on a file, typically used |
776 | for reading: |
777 | |
778 | use 5.004; |
779 | use Fcntl qw(:DEFAULT :flock); |
780 | open(FH, "< filename") or die "can't open filename: $!"; |
781 | flock(FH, LOCK_SH) or die "can't lock filename: $!"; |
782 | # now read from FH |
783 | |
784 | You can get a non-blocking lock by using C<LOCK_NB>. |
785 | |
786 | flock(FH, LOCK_SH | LOCK_NB) |
787 | or die "can't lock filename: $!"; |
788 | |
789 | This can be useful for producing more user-friendly behaviour by warning |
790 | if you're going to be blocking: |
791 | |
792 | use 5.004; |
793 | use Fcntl qw(:DEFAULT :flock); |
794 | open(FH, "< filename") or die "can't open filename: $!"; |
795 | unless (flock(FH, LOCK_SH | LOCK_NB)) { |
796 | $| = 1; |
797 | print "Waiting for lock..."; |
798 | flock(FH, LOCK_SH) or die "can't lock filename: $!"; |
799 | print "got it.\n" |
800 | } |
801 | # now read from FH |
802 | |
803 | To get an exclusive lock, typically used for writing, you have to be |
804 | careful. We C<sysopen> the file so it can be locked before it gets |
805 | emptied. You can get a nonblocking version using C<LOCK_EX | LOCK_NB>. |
806 | |
807 | use 5.004; |
808 | use Fcntl qw(:DEFAULT :flock); |
809 | sysopen(FH, "filename", O_WRONLY | O_CREAT) |
810 | or die "can't open filename: $!"; |
811 | flock(FH, LOCK_EX) |
812 | or die "can't lock filename: $!"; |
813 | truncate(FH, 0) |
814 | or die "can't truncate filename: $!"; |
815 | # now write to FH |
816 | |
817 | Finally, due to the uncounted millions who cannot be dissuaded from |
818 | wasting cycles on useless vanity devices called hit counters, here's |
819 | how to increment a number in a file safely: |
820 | |
821 | use Fcntl qw(:DEFAULT :flock); |
822 | |
823 | sysopen(FH, "numfile", O_RDWR | O_CREAT) |
824 | or die "can't open numfile: $!"; |
825 | # autoflush FH |
826 | $ofh = select(FH); $| = 1; select ($ofh); |
827 | flock(FH, LOCK_EX) |
828 | or die "can't write-lock numfile: $!"; |
829 | |
830 | $num = <FH> || 0; |
831 | seek(FH, 0, 0) |
832 | or die "can't rewind numfile : $!"; |
833 | print FH $num+1, "\n" |
834 | or die "can't write numfile: $!"; |
835 | |
836 | truncate(FH, tell(FH)) |
837 | or die "can't truncate numfile: $!"; |
838 | close(FH) |
839 | or die "can't close numfile: $!"; |
840 | |
841 | =head1 SEE ALSO |
842 | |
843 | The C<open> and C<sysopen> function in perlfunc(1); |
844 | the standard open(2), dup(2), fopen(3), and fdopen(3) manpages; |
845 | the POSIX documentation. |
846 | |
847 | =head1 AUTHOR and COPYRIGHT |
848 | |
849 | Copyright 1998 Tom Christiansen. |
850 | |
5a7beb56 |
851 | This documentation is free; you can redistribute it and/or modify it |
852 | under the same terms as Perl itself. |
f8284313 |
853 | |
854 | Irrespective of its distribution, all code examples in these files are |
855 | hereby placed into the public domain. You are permitted and |
856 | encouraged to use this code in your own programs for fun or for profit |
857 | as you see fit. A simple comment in the code giving credit would be |
858 | courteous but is not required. |
859 | |
860 | =head1 HISTORY |
861 | |
862 | First release: Sat Jan 9 08:09:11 MST 1999 |