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1 | =head1 NAME |
2 | |
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3 | perlfaq5 - Files and Formats ($Revision: 7875 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section deals with I/O and the "f" issues: filehandles, flushing, |
8 | formats, and footers. |
9 | |
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10 | =head2 How do I flush/unbuffer an output filehandle? Why must I do this? |
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11 | X<flush> X<buffer> X<unbuffer> X<autoflush> |
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12 | |
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13 | Perl does not support truly unbuffered output (except |
14 | insofar as you can C<syswrite(OUT, $char, 1)>), although it |
15 | does support is "command buffering", in which a physical |
16 | write is performed after every output command. |
17 | |
18 | The C standard I/O library (stdio) normally buffers |
19 | characters sent to devices so that there isn't a system call |
20 | for each byte. In most stdio implementations, the type of |
21 | output buffering and the size of the buffer varies according |
22 | to the type of device. Perl's print() and write() functions |
23 | normally buffer output, while syswrite() bypasses buffering |
24 | all together. |
25 | |
26 | If you want your output to be sent immediately when you |
27 | execute print() or write() (for instance, for some network |
28 | protocols), you must set the handle's autoflush flag. This |
29 | flag is the Perl variable $| and when it is set to a true |
30 | value, Perl will flush the handle's buffer after each |
31 | print() or write(). Setting $| affects buffering only for |
32 | the currently selected default file handle. You choose this |
33 | handle with the one argument select() call (see |
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34 | L<perlvar/$E<verbar>> and L<perlfunc/select>). |
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35 | |
36 | Use select() to choose the desired handle, then set its |
37 | per-filehandle variables. |
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38 | |
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39 | $old_fh = select(OUTPUT_HANDLE); |
40 | $| = 1; |
41 | select($old_fh); |
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42 | |
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43 | Some modules offer object-oriented access to handles and their |
44 | variables, although they may be overkill if this is the only |
45 | thing you do with them. You can use IO::Handle: |
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46 | |
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47 | use IO::Handle; |
48 | open(DEV, ">/dev/printer"); # but is this? |
49 | DEV->autoflush(1); |
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50 | |
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51 | or IO::Socket: |
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52 | |
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53 | use IO::Socket; # this one is kinda a pipe? |
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54 | my $sock = IO::Socket::INET->new( 'www.example.com:80' ); |
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55 | |
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56 | $sock->autoflush(); |
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57 | |
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58 | =head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file? |
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59 | X<file, editing> |
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60 | |
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61 | (contributed by brian d foy) |
62 | |
63 | The basic idea of inserting, changing, or deleting a line from a text |
64 | file involves reading and printing the file to the point you want to |
65 | make the change, making the change, then reading and printing the rest |
66 | of the file. Perl doesn't provide random access to lines (especially |
67 | since the record input separator, C<$/>, is mutable), although modules |
68 | such as C<Tie::File> can fake it. |
69 | |
70 | A Perl program to do these tasks takes the basic form of opening a |
71 | file, printing its lines, then closing the file: |
72 | |
73 | open my $in, '<', $file or die "Can't read old file: $!"; |
74 | open my $out, '>', "$file.new" or die "Can't write new file: $!"; |
75 | |
76 | while( <$in> ) |
77 | { |
78 | print $out $_; |
79 | } |
80 | |
81 | close $out; |
82 | |
83 | Within that basic form, add the parts that you need to insert, change, |
84 | or delete lines. |
85 | |
86 | To prepend lines to the beginning, print those lines before you enter |
87 | the loop that prints the existing lines. |
88 | |
89 | open my $in, '<', $file or die "Can't read old file: $!"; |
90 | open my $out, '>', "$file.new" or die "Can't write new file: $!"; |
91 | |
92 | print "# Add this line to the top\n"; # <--- HERE'S THE MAGIC |
93 | |
94 | while( <$in> ) |
95 | { |
96 | print $out $_; |
97 | } |
98 | |
99 | close $out; |
100 | |
101 | To change existing lines, insert the code to modify the lines inside |
102 | the C<while> loop. In this case, the code finds all lowercased |
103 | versions of "perl" and uppercases them. The happens for every line, so |
104 | be sure that you're supposed to do that on every line! |
105 | |
106 | open my $in, '<', $file or die "Can't read old file: $!"; |
107 | open my $out, '>', "$file.new" or die "Can't write new file: $!"; |
108 | |
109 | print "# Add this line to the top\n"; |
110 | |
111 | while( <$in> ) |
112 | { |
113 | s/\b(perl)\b/Perl/g; |
114 | print $out $_; |
115 | } |
116 | |
117 | close $out; |
118 | |
119 | To change only a particular line, the input line number, C<$.>, is |
120 | useful. Use C<next> to skip all lines up to line 5, make a change and |
121 | print the result, then stop further processing with C<last>. |
122 | |
123 | while( <$in> ) |
124 | { |
125 | next unless $. == 5; |
126 | s/\b(perl)\b/Perl/g; |
127 | print $out $_; |
128 | last; |
129 | } |
130 | |
131 | To skip lines, use the looping controls. The C<next> in this example |
132 | skips comment lines, and the C<last> stops all processing once it |
133 | encounters either C<__END__> or C<__DATA__>. |
134 | |
135 | while( <$in> ) |
136 | { |
137 | next if /^\s+#/; # skip comment lines |
138 | last if /^__(END|DATA)__$/; # stop at end of code marker |
139 | print $out $_; |
140 | } |
141 | |
142 | Do the same sort of thing to delete a particular line by using C<next> |
143 | to skip the lines you don't want to show up in the output. This |
144 | example skips every fifth line: |
145 | |
146 | while( <$in> ) |
147 | { |
148 | next unless $. % 5; |
149 | print $out $_; |
150 | } |
151 | |
152 | If, for some odd reason, you really want to see the whole file at once |
153 | rather than processing line by line, you can slurp it in (as long as |
154 | you can fit the whole thing in memory!): |
155 | |
156 | open my $in, '<', $file or die "Can't read old file: $!" |
157 | open my $out, '>', "$file.new" or die "Can't write new file: $!"; |
158 | |
159 | my @lines = do { local $/; <$in> }; # slurp! |
160 | |
161 | # do your magic here |
162 | |
163 | print $out @lines; |
164 | |
165 | Modules such as C<File::Slurp> and C<Tie::File> can help with that |
166 | too. If you can, however, avoid reading the entire file at once. Perl |
167 | won't give that memory back to the operating system until the process |
168 | finishes. |
169 | |
170 | You can also use Perl one-liners to modify a file in-place. The |
171 | following changes all 'Fred' to 'Barney' in F<inFile.txt>, overwriting |
172 | the file with the new contents. With the C<-p> switch, Perl wraps a |
173 | C<while> loop around the code you specify with C<-e>, and C<-i> turns |
174 | on in-place editing. The current line is in C<$_>. With C<-p>, Perl |
175 | automatically prints the value of C<$_> at the end of the loop. See |
176 | L<perlrun> for more details. |
177 | |
178 | perl -pi -e 's/Fred/Barney/' inFile.txt |
179 | |
180 | To make a backup of C<inFile.txt>, give C<-i> a file extension to add: |
181 | |
182 | perl -pi.bak -e 's/Fred/Barney/' inFile.txt |
183 | |
184 | To change only the fifth line, you can add a test checking C<$.>, the |
185 | input line number, then only perform the operation when the test |
186 | passes: |
187 | |
188 | perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt |
189 | |
190 | To add lines before a certain line, you can add a line (or lines!) |
191 | before Perl prints C<$_>: |
192 | |
193 | perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt |
194 | |
195 | You can even add a line to the beginning of a file, since the current |
196 | line prints at the end of the loop: |
197 | |
198 | perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt |
199 | |
200 | To insert a line after one already in the file, use the C<-n> switch. |
201 | It's just like C<-p> except that it doesn't print C<$_> at the end of |
202 | the loop, so you have to do that yourself. In this case, print C<$_> |
203 | first, then print the line that you want to add. |
204 | |
205 | perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt |
206 | |
207 | To delete lines, only print the ones that you want. |
208 | |
209 | perl -ni -e 'print unless /d/' inFile.txt |
210 | |
211 | ... or ... |
212 | |
213 | perl -pi -e 'next unless /d/' inFile.txt |
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214 | |
215 | =head2 How do I count the number of lines in a file? |
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216 | X<file, counting lines> X<lines> X<line> |
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217 | |
218 | One fairly efficient way is to count newlines in the file. The |
219 | following program uses a feature of tr///, as documented in L<perlop>. |
220 | If your text file doesn't end with a newline, then it's not really a |
221 | proper text file, so this may report one fewer line than you expect. |
222 | |
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223 | $lines = 0; |
224 | open(FILE, $filename) or die "Can't open `$filename': $!"; |
225 | while (sysread FILE, $buffer, 4096) { |
226 | $lines += ($buffer =~ tr/\n//); |
227 | } |
228 | close FILE; |
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229 | |
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230 | This assumes no funny games with newline translations. |
231 | |
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232 | =head2 How can I use Perl's C<-i> option from within a program? |
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233 | X<-i> X<in-place> |
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234 | |
235 | C<-i> sets the value of Perl's C<$^I> variable, which in turn affects |
236 | the behavior of C<< <> >>; see L<perlrun> for more details. By |
237 | modifying the appropriate variables directly, you can get the same |
238 | behavior within a larger program. For example: |
239 | |
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240 | # ... |
241 | { |
242 | local($^I, @ARGV) = ('.orig', glob("*.c")); |
243 | while (<>) { |
244 | if ($. == 1) { |
245 | print "This line should appear at the top of each file\n"; |
246 | } |
247 | s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case |
248 | print; |
249 | close ARGV if eof; # Reset $. |
250 | } |
251 | } |
252 | # $^I and @ARGV return to their old values here |
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253 | |
254 | This block modifies all the C<.c> files in the current directory, |
255 | leaving a backup of the original data from each file in a new |
256 | C<.c.orig> file. |
257 | |
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258 | =head2 How can I copy a file? |
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259 | X<copy> X<file, copy> |
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260 | |
261 | (contributed by brian d foy) |
262 | |
263 | Use the File::Copy module. It comes with Perl and can do a |
264 | true copy across file systems, and it does its magic in |
265 | a portable fashion. |
266 | |
267 | use File::Copy; |
268 | |
269 | copy( $original, $new_copy ) or die "Copy failed: $!"; |
270 | |
271 | If you can't use File::Copy, you'll have to do the work yourself: |
272 | open the original file, open the destination file, then print |
273 | to the destination file as you read the original. |
274 | |
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275 | =head2 How do I make a temporary file name? |
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276 | X<file, temporary> |
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277 | |
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278 | If you don't need to know the name of the file, you can use C<open()> |
279 | with C<undef> in place of the file name. The C<open()> function |
280 | creates an anonymous temporary file. |
281 | |
282 | open my $tmp, '+>', undef or die $!; |
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283 | |
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284 | Otherwise, you can use the File::Temp module. |
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285 | |
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286 | use File::Temp qw/ tempfile tempdir /; |
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287 | |
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288 | $dir = tempdir( CLEANUP => 1 ); |
289 | ($fh, $filename) = tempfile( DIR => $dir ); |
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290 | |
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291 | # or if you don't need to know the filename |
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292 | |
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293 | $fh = tempfile( DIR => $dir ); |
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294 | |
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295 | The File::Temp has been a standard module since Perl 5.6.1. If you |
296 | don't have a modern enough Perl installed, use the C<new_tmpfile> |
297 | class method from the IO::File module to get a filehandle opened for |
298 | reading and writing. Use it if you don't need to know the file's name: |
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299 | |
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300 | use IO::File; |
301 | $fh = IO::File->new_tmpfile() |
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302 | or die "Unable to make new temporary file: $!"; |
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303 | |
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304 | If you're committed to creating a temporary file by hand, use the |
305 | process ID and/or the current time-value. If you need to have many |
306 | temporary files in one process, use a counter: |
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307 | |
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308 | BEGIN { |
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309 | use Fcntl; |
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310 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; |
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311 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); |
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312 | |
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313 | sub temp_file { |
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314 | local *FH; |
315 | my $count = 0; |
316 | until (defined(fileno(FH)) || $count++ > 100) { |
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317 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
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318 | # O_EXCL is required for security reasons. |
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319 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
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320 | } |
321 | |
322 | if (defined(fileno(FH)) |
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323 | return (*FH, $base_name); |
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324 | } |
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325 | else { |
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326 | return (); |
327 | } |
328 | } |
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329 | } |
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330 | |
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331 | =head2 How can I manipulate fixed-record-length files? |
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332 | X<fixed-length> X<file, fixed-length records> |
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333 | |
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334 | The most efficient way is using L<pack()|perlfunc/"pack"> and |
335 | L<unpack()|perlfunc/"unpack">. This is faster than using |
336 | L<substr()|perlfunc/"substr"> when taking many, many strings. It is |
337 | slower for just a few. |
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338 | |
339 | Here is a sample chunk of code to break up and put back together again |
340 | some fixed-format input lines, in this case from the output of a normal, |
341 | Berkeley-style ps: |
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342 | |
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343 | # sample input line: |
344 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what |
345 | my $PS_T = 'A6 A4 A7 A5 A*'; |
346 | open my $ps, '-|', 'ps'; |
347 | print scalar <$ps>; |
348 | my @fields = qw( pid tt stat time command ); |
349 | while (<$ps>) { |
350 | my %process; |
351 | @process{@fields} = unpack($PS_T, $_); |
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352 | for my $field ( @fields ) { |
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353 | print "$field: <$process{$field}>\n"; |
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354 | } |
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355 | print 'line=', pack($PS_T, @process{@fields} ), "\n"; |
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356 | } |
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357 | |
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358 | We've used a hash slice in order to easily handle the fields of each row. |
359 | Storing the keys in an array means it's easy to operate on them as a |
360 | group or loop over them with for. It also avoids polluting the program |
361 | with global variables and using symbolic references. |
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362 | |
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363 | =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? |
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364 | X<filehandle, local> X<filehandle, passing> X<filehandle, reference> |
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365 | |
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366 | As of perl5.6, open() autovivifies file and directory handles |
367 | as references if you pass it an uninitialized scalar variable. |
368 | You can then pass these references just like any other scalar, |
369 | and use them in the place of named handles. |
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370 | |
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371 | open my $fh, $file_name; |
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372 | |
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373 | open local $fh, $file_name; |
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374 | |
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375 | print $fh "Hello World!\n"; |
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376 | |
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377 | process_file( $fh ); |
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378 | |
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379 | If you like, you can store these filehandles in an array or a hash. |
380 | If you access them directly, they aren't simple scalars and you |
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381 | need to give C<print> a little help by placing the filehandle |
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382 | reference in braces. Perl can only figure it out on its own when |
383 | the filehandle reference is a simple scalar. |
384 | |
385 | my @fhs = ( $fh1, $fh2, $fh3 ); |
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386 | |
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387 | for( $i = 0; $i <= $#fhs; $i++ ) { |
388 | print {$fhs[$i]} "just another Perl answer, \n"; |
389 | } |
390 | |
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391 | Before perl5.6, you had to deal with various typeglob idioms |
392 | which you may see in older code. |
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393 | |
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394 | open FILE, "> $filename"; |
395 | process_typeglob( *FILE ); |
396 | process_reference( \*FILE ); |
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397 | |
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398 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
399 | sub process_reference { local $fh = shift; print $fh "Reference!" } |
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400 | |
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401 | If you want to create many anonymous handles, you should |
402 | check out the Symbol or IO::Handle modules. |
5a964f20 |
403 | |
404 | =head2 How can I use a filehandle indirectly? |
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405 | X<filehandle, indirect> |
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406 | |
407 | An indirect filehandle is using something other than a symbol |
408 | in a place that a filehandle is expected. Here are ways |
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409 | to get indirect filehandles: |
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410 | |
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411 | $fh = SOME_FH; # bareword is strict-subs hostile |
412 | $fh = "SOME_FH"; # strict-refs hostile; same package only |
413 | $fh = *SOME_FH; # typeglob |
414 | $fh = \*SOME_FH; # ref to typeglob (bless-able) |
415 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob |
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416 | |
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417 | Or, you can use the C<new> method from one of the IO::* modules to |
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418 | create an anonymous filehandle, store that in a scalar variable, |
419 | and use it as though it were a normal filehandle. |
420 | |
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421 | use IO::Handle; # 5.004 or higher |
422 | $fh = IO::Handle->new(); |
5a964f20 |
423 | |
424 | Then use any of those as you would a normal filehandle. Anywhere that |
425 | Perl is expecting a filehandle, an indirect filehandle may be used |
426 | instead. An indirect filehandle is just a scalar variable that contains |
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427 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
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428 | the C<< <FH> >> diamond operator will accept either a named filehandle |
5a964f20 |
429 | or a scalar variable containing one: |
430 | |
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431 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); |
432 | print $ofh "Type it: "; |
433 | $got = <$ifh> |
434 | print $efh "What was that: $got"; |
5a964f20 |
435 | |
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436 | If you're passing a filehandle to a function, you can write |
5a964f20 |
437 | the function in two ways: |
438 | |
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439 | sub accept_fh { |
440 | my $fh = shift; |
441 | print $fh "Sending to indirect filehandle\n"; |
442 | } |
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443 | |
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444 | Or it can localize a typeglob and use the filehandle directly: |
46fc3d4c |
445 | |
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446 | sub accept_fh { |
447 | local *FH = shift; |
448 | print FH "Sending to localized filehandle\n"; |
449 | } |
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450 | |
5a964f20 |
451 | Both styles work with either objects or typeglobs of real filehandles. |
452 | (They might also work with strings under some circumstances, but this |
453 | is risky.) |
454 | |
500071f4 |
455 | accept_fh(*STDOUT); |
456 | accept_fh($handle); |
5a964f20 |
457 | |
458 | In the examples above, we assigned the filehandle to a scalar variable |
a6dd486b |
459 | before using it. That is because only simple scalar variables, not |
460 | expressions or subscripts of hashes or arrays, can be used with |
461 | built-ins like C<print>, C<printf>, or the diamond operator. Using |
8305e449 |
462 | something other than a simple scalar variable as a filehandle is |
5a964f20 |
463 | illegal and won't even compile: |
464 | |
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465 | @fd = (*STDIN, *STDOUT, *STDERR); |
466 | print $fd[1] "Type it: "; # WRONG |
467 | $got = <$fd[0]> # WRONG |
468 | print $fd[2] "What was that: $got"; # WRONG |
5a964f20 |
469 | |
470 | With C<print> and C<printf>, you get around this by using a block and |
471 | an expression where you would place the filehandle: |
472 | |
500071f4 |
473 | print { $fd[1] } "funny stuff\n"; |
474 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; |
475 | # Pity the poor deadbeef. |
5a964f20 |
476 | |
477 | That block is a proper block like any other, so you can put more |
478 | complicated code there. This sends the message out to one of two places: |
479 | |
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480 | $ok = -x "/bin/cat"; |
481 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
482 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
5a964f20 |
483 | |
484 | This approach of treating C<print> and C<printf> like object methods |
485 | calls doesn't work for the diamond operator. That's because it's a |
486 | real operator, not just a function with a comma-less argument. Assuming |
487 | you've been storing typeglobs in your structure as we did above, you |
c90536be |
488 | can use the built-in function named C<readline> to read a record just |
c47ff5f1 |
489 | as C<< <> >> does. Given the initialization shown above for @fd, this |
c90536be |
490 | would work, but only because readline() requires a typeglob. It doesn't |
5a964f20 |
491 | work with objects or strings, which might be a bug we haven't fixed yet. |
492 | |
500071f4 |
493 | $got = readline($fd[0]); |
5a964f20 |
494 | |
495 | Let it be noted that the flakiness of indirect filehandles is not |
496 | related to whether they're strings, typeglobs, objects, or anything else. |
497 | It's the syntax of the fundamental operators. Playing the object |
498 | game doesn't help you at all here. |
46fc3d4c |
499 | |
68dc0745 |
500 | =head2 How can I set up a footer format to be used with write()? |
d74e8afc |
501 | X<footer> |
68dc0745 |
502 | |
54310121 |
503 | There's no builtin way to do this, but L<perlform> has a couple of |
68dc0745 |
504 | techniques to make it possible for the intrepid hacker. |
505 | |
506 | =head2 How can I write() into a string? |
d74e8afc |
507 | X<write, into a string> |
68dc0745 |
508 | |
65acb1b1 |
509 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
68dc0745 |
510 | |
511 | =head2 How can I output my numbers with commas added? |
d74e8afc |
512 | X<number, commify> |
68dc0745 |
513 | |
b68463f7 |
514 | (contributed by brian d foy and Benjamin Goldberg) |
515 | |
516 | You can use L<Number::Format> to separate places in a number. |
517 | It handles locale information for those of you who want to insert |
518 | full stops instead (or anything else that they want to use, |
519 | really). |
520 | |
49d635f9 |
521 | This subroutine will add commas to your number: |
522 | |
523 | sub commify { |
500071f4 |
524 | local $_ = shift; |
525 | 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; |
526 | return $_; |
527 | } |
49d635f9 |
528 | |
529 | This regex from Benjamin Goldberg will add commas to numbers: |
68dc0745 |
530 | |
500071f4 |
531 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
68dc0745 |
532 | |
49d635f9 |
533 | It is easier to see with comments: |
68dc0745 |
534 | |
500071f4 |
535 | s/( |
536 | ^[-+]? # beginning of number. |
537 | \d+? # first digits before first comma |
538 | (?= # followed by, (but not included in the match) : |
539 | (?>(?:\d{3})+) # some positive multiple of three digits. |
540 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. |
541 | ) |
542 | | # or: |
543 | \G\d{3} # after the last group, get three digits |
544 | (?=\d) # but they have to have more digits after them. |
545 | )/$1,/xg; |
46fc3d4c |
546 | |
68dc0745 |
547 | =head2 How can I translate tildes (~) in a filename? |
d74e8afc |
548 | X<tilde> X<tilde expansion> |
68dc0745 |
549 | |
575cc754 |
550 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
551 | versions of Perl require that you have a shell installed that groks |
552 | tildes. Recent perl versions have this feature built in. The |
d6260402 |
553 | File::KGlob module (available from CPAN) gives more portable glob |
575cc754 |
554 | functionality. |
68dc0745 |
555 | |
556 | Within Perl, you may use this directly: |
557 | |
558 | $filename =~ s{ |
559 | ^ ~ # find a leading tilde |
560 | ( # save this in $1 |
561 | [^/] # a non-slash character |
562 | * # repeated 0 or more times (0 means me) |
563 | ) |
564 | }{ |
565 | $1 |
566 | ? (getpwnam($1))[7] |
567 | : ( $ENV{HOME} || $ENV{LOGDIR} ) |
568 | }ex; |
569 | |
5a964f20 |
570 | =head2 How come when I open a file read-write it wipes it out? |
d74e8afc |
571 | X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating> |
68dc0745 |
572 | |
573 | Because you're using something like this, which truncates the file and |
574 | I<then> gives you read-write access: |
575 | |
500071f4 |
576 | open(FH, "+> /path/name"); # WRONG (almost always) |
68dc0745 |
577 | |
578 | Whoops. You should instead use this, which will fail if the file |
197aec24 |
579 | doesn't exist. |
d92eb7b0 |
580 | |
500071f4 |
581 | open(FH, "+< /path/name"); # open for update |
d92eb7b0 |
582 | |
c47ff5f1 |
583 | Using ">" always clobbers or creates. Using "<" never does |
d92eb7b0 |
584 | either. The "+" doesn't change this. |
68dc0745 |
585 | |
5a964f20 |
586 | Here are examples of many kinds of file opens. Those using sysopen() |
587 | all assume |
68dc0745 |
588 | |
500071f4 |
589 | use Fcntl; |
68dc0745 |
590 | |
5a964f20 |
591 | To open file for reading: |
68dc0745 |
592 | |
500071f4 |
593 | open(FH, "< $path") || die $!; |
594 | sysopen(FH, $path, O_RDONLY) || die $!; |
5a964f20 |
595 | |
596 | To open file for writing, create new file if needed or else truncate old file: |
597 | |
500071f4 |
598 | open(FH, "> $path") || die $!; |
599 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; |
600 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; |
5a964f20 |
601 | |
602 | To open file for writing, create new file, file must not exist: |
603 | |
500071f4 |
604 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; |
605 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
606 | |
607 | To open file for appending, create if necessary: |
608 | |
500071f4 |
609 | open(FH, ">> $path") || die $!; |
610 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; |
611 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; |
5a964f20 |
612 | |
613 | To open file for appending, file must exist: |
614 | |
500071f4 |
615 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; |
5a964f20 |
616 | |
617 | To open file for update, file must exist: |
618 | |
500071f4 |
619 | open(FH, "+< $path") || die $!; |
620 | sysopen(FH, $path, O_RDWR) || die $!; |
5a964f20 |
621 | |
622 | To open file for update, create file if necessary: |
623 | |
500071f4 |
624 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; |
625 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; |
5a964f20 |
626 | |
627 | To open file for update, file must not exist: |
628 | |
500071f4 |
629 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; |
630 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
631 | |
632 | To open a file without blocking, creating if necessary: |
633 | |
500071f4 |
634 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
2359510d |
635 | or die "can't open /foo/somefile: $!": |
5a964f20 |
636 | |
637 | Be warned that neither creation nor deletion of files is guaranteed to |
638 | be an atomic operation over NFS. That is, two processes might both |
a6dd486b |
639 | successfully create or unlink the same file! Therefore O_EXCL |
640 | isn't as exclusive as you might wish. |
68dc0745 |
641 | |
87275199 |
642 | See also the new L<perlopentut> if you have it (new for 5.6). |
65acb1b1 |
643 | |
04d666b1 |
644 | =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? |
d74e8afc |
645 | X<argument list too long> |
68dc0745 |
646 | |
c47ff5f1 |
647 | The C<< <> >> operator performs a globbing operation (see above). |
3a4b19e4 |
648 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
649 | csh(1) to do the actual glob expansion, but |
68dc0745 |
650 | csh can't handle more than 127 items and so gives the error message |
651 | C<Argument list too long>. People who installed tcsh as csh won't |
652 | have this problem, but their users may be surprised by it. |
653 | |
3a4b19e4 |
654 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
d6260402 |
655 | yourself with readdir() and patterns, or use a module like File::KGlob, |
3a4b19e4 |
656 | one that doesn't use the shell to do globbing. |
68dc0745 |
657 | |
658 | =head2 Is there a leak/bug in glob()? |
d74e8afc |
659 | X<glob> |
68dc0745 |
660 | |
661 | Due to the current implementation on some operating systems, when you |
662 | use the glob() function or its angle-bracket alias in a scalar |
a6dd486b |
663 | context, you may cause a memory leak and/or unpredictable behavior. It's |
68dc0745 |
664 | best therefore to use glob() only in list context. |
665 | |
c47ff5f1 |
666 | =head2 How can I open a file with a leading ">" or trailing blanks? |
d74e8afc |
667 | X<filename, special characters> |
68dc0745 |
668 | |
b68463f7 |
669 | (contributed by Brian McCauley) |
68dc0745 |
670 | |
b68463f7 |
671 | The special two argument form of Perl's open() function ignores |
672 | trailing blanks in filenames and infers the mode from certain leading |
673 | characters (or a trailing "|"). In older versions of Perl this was the |
674 | only version of open() and so it is prevalent in old code and books. |
65acb1b1 |
675 | |
b68463f7 |
676 | Unless you have a particular reason to use the two argument form you |
677 | should use the three argument form of open() which does not treat any |
678 | charcters in the filename as special. |
58103a2e |
679 | |
881bdbd4 |
680 | open FILE, "<", " file "; # filename is " file " |
681 | open FILE, ">", ">file"; # filename is ">file" |
65acb1b1 |
682 | |
68dc0745 |
683 | =head2 How can I reliably rename a file? |
d74e8afc |
684 | X<rename> X<mv> X<move> X<file, rename> X<ren> |
68dc0745 |
685 | |
49d635f9 |
686 | If your operating system supports a proper mv(1) utility or its |
687 | functional equivalent, this works: |
68dc0745 |
688 | |
500071f4 |
689 | rename($old, $new) or system("mv", $old, $new); |
68dc0745 |
690 | |
d2321c93 |
691 | It may be more portable to use the File::Copy module instead. |
692 | You just copy to the new file to the new name (checking return |
693 | values), then delete the old one. This isn't really the same |
694 | semantically as a rename(), which preserves meta-information like |
68dc0745 |
695 | permissions, timestamps, inode info, etc. |
696 | |
d2321c93 |
697 | Newer versions of File::Copy export a move() function. |
5a964f20 |
698 | |
68dc0745 |
699 | =head2 How can I lock a file? |
d74e8afc |
700 | X<lock> X<file, lock> X<flock> |
68dc0745 |
701 | |
54310121 |
702 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 |
703 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
704 | later), and lockf(3) if neither of the two previous system calls exists. |
705 | On some systems, it may even use a different form of native locking. |
706 | Here are some gotchas with Perl's flock(): |
707 | |
708 | =over 4 |
709 | |
710 | =item 1 |
711 | |
712 | Produces a fatal error if none of the three system calls (or their |
713 | close equivalent) exists. |
714 | |
715 | =item 2 |
716 | |
717 | lockf(3) does not provide shared locking, and requires that the |
718 | filehandle be open for writing (or appending, or read/writing). |
719 | |
720 | =item 3 |
721 | |
d92eb7b0 |
722 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
723 | systems), so you'd need to force the use of fcntl(2) when you build Perl. |
a6dd486b |
724 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 |
725 | and the F<INSTALL> file in the source distribution for information on |
726 | building Perl to do this. |
727 | |
728 | Two potentially non-obvious but traditional flock semantics are that |
a6dd486b |
729 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 |
730 | I<merely advisory>. Such discretionary locks are more flexible, but |
731 | offer fewer guarantees. This means that files locked with flock() may |
732 | be modified by programs that do not also use flock(). Cars that stop |
733 | for red lights get on well with each other, but not with cars that don't |
734 | stop for red lights. See the perlport manpage, your port's specific |
735 | documentation, or your system-specific local manpages for details. It's |
736 | best to assume traditional behavior if you're writing portable programs. |
a6dd486b |
737 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 |
738 | for your own system's idiosyncrasies (sometimes called "features"). |
739 | Slavish adherence to portability concerns shouldn't get in the way of |
740 | your getting your job done.) |
68dc0745 |
741 | |
197aec24 |
742 | For more information on file locking, see also |
13a2d996 |
743 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 |
744 | |
68dc0745 |
745 | =back |
746 | |
04d666b1 |
747 | =head2 Why can't I just open(FH, "E<gt>file.lock")? |
d74e8afc |
748 | X<lock, lockfile race condition> |
68dc0745 |
749 | |
750 | A common bit of code B<NOT TO USE> is this: |
751 | |
500071f4 |
752 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE |
753 | open(LCK, "> file.lock"); # THIS BROKEN CODE |
68dc0745 |
754 | |
755 | This is a classic race condition: you take two steps to do something |
756 | which must be done in one. That's why computer hardware provides an |
757 | atomic test-and-set instruction. In theory, this "ought" to work: |
758 | |
500071f4 |
759 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
9b55d3ab |
760 | or die "can't open file.lock: $!"; |
68dc0745 |
761 | |
762 | except that lamentably, file creation (and deletion) is not atomic |
763 | over NFS, so this won't work (at least, not every time) over the net. |
65acb1b1 |
764 | Various schemes involving link() have been suggested, but |
46fc3d4c |
765 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 |
766 | |
fc36a67e |
767 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
d74e8afc |
768 | X<counter> X<file, counter> |
68dc0745 |
769 | |
46fc3d4c |
770 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 |
771 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b |
772 | only to stroke the writer's vanity. It's better to pick a random number; |
773 | they're more realistic. |
68dc0745 |
774 | |
5a964f20 |
775 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 |
776 | |
500071f4 |
777 | use Fcntl qw(:DEFAULT :flock); |
778 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
779 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
780 | $num = <FH> || 0; |
781 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; |
782 | truncate(FH, 0) or die "can't truncate numfile: $!"; |
783 | (print FH $num+1, "\n") or die "can't write numfile: $!"; |
784 | close FH or die "can't close numfile: $!"; |
68dc0745 |
785 | |
46fc3d4c |
786 | Here's a much better web-page hit counter: |
68dc0745 |
787 | |
500071f4 |
788 | $hits = int( (time() - 850_000_000) / rand(1_000) ); |
68dc0745 |
789 | |
790 | If the count doesn't impress your friends, then the code might. :-) |
791 | |
f52f3be2 |
792 | =head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking? |
d74e8afc |
793 | X<append> X<file, append> |
05caf3a7 |
794 | |
795 | If you are on a system that correctly implements flock() and you use the |
796 | example appending code from "perldoc -f flock" everything will be OK |
797 | even if the OS you are on doesn't implement append mode correctly (if |
798 | such a system exists.) So if you are happy to restrict yourself to OSs |
799 | that implement flock() (and that's not really much of a restriction) |
800 | then that is what you should do. |
801 | |
802 | If you know you are only going to use a system that does correctly |
803 | implement appending (i.e. not Win32) then you can omit the seek() from |
804 | the above code. |
805 | |
806 | If you know you are only writing code to run on an OS and filesystem that |
807 | does implement append mode correctly (a local filesystem on a modern |
808 | Unix for example), and you keep the file in block-buffered mode and you |
809 | write less than one buffer-full of output between each manual flushing |
8305e449 |
810 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 |
811 | the end of the file in one chunk without getting intermingled with |
812 | anyone else's output. You can also use the syswrite() function which is |
813 | simply a wrapper around your systems write(2) system call. |
814 | |
815 | There is still a small theoretical chance that a signal will interrupt |
816 | the system level write() operation before completion. There is also a |
817 | possibility that some STDIO implementations may call multiple system |
818 | level write()s even if the buffer was empty to start. There may be some |
819 | systems where this probability is reduced to zero. |
820 | |
68dc0745 |
821 | =head2 How do I randomly update a binary file? |
d74e8afc |
822 | X<file, binary patch> |
68dc0745 |
823 | |
824 | If you're just trying to patch a binary, in many cases something as |
825 | simple as this works: |
826 | |
500071f4 |
827 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs |
68dc0745 |
828 | |
829 | However, if you have fixed sized records, then you might do something more |
830 | like this: |
831 | |
500071f4 |
832 | $RECSIZE = 220; # size of record, in bytes |
833 | $recno = 37; # which record to update |
834 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; |
835 | seek(FH, $recno * $RECSIZE, 0); |
836 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; |
837 | # munge the record |
838 | seek(FH, -$RECSIZE, 1); |
839 | print FH $record; |
840 | close FH; |
68dc0745 |
841 | |
842 | Locking and error checking are left as an exercise for the reader. |
a6dd486b |
843 | Don't forget them or you'll be quite sorry. |
68dc0745 |
844 | |
68dc0745 |
845 | =head2 How do I get a file's timestamp in perl? |
d74e8afc |
846 | X<timestamp> X<file, timestamp> |
68dc0745 |
847 | |
881bdbd4 |
848 | If you want to retrieve the time at which the file was last |
849 | read, written, or had its meta-data (owner, etc) changed, |
a05e4845 |
850 | you use the B<-A>, B<-M>, or B<-C> file test operations as |
881bdbd4 |
851 | documented in L<perlfunc>. These retrieve the age of the |
852 | file (measured against the start-time of your program) in |
853 | days as a floating point number. Some platforms may not have |
854 | all of these times. See L<perlport> for details. To |
855 | retrieve the "raw" time in seconds since the epoch, you |
856 | would call the stat function, then use localtime(), |
857 | gmtime(), or POSIX::strftime() to convert this into |
858 | human-readable form. |
68dc0745 |
859 | |
860 | Here's an example: |
861 | |
500071f4 |
862 | $write_secs = (stat($file))[9]; |
863 | printf "file %s updated at %s\n", $file, |
c8db1d39 |
864 | scalar localtime($write_secs); |
68dc0745 |
865 | |
866 | If you prefer something more legible, use the File::stat module |
867 | (part of the standard distribution in version 5.004 and later): |
868 | |
500071f4 |
869 | # error checking left as an exercise for reader. |
870 | use File::stat; |
871 | use Time::localtime; |
872 | $date_string = ctime(stat($file)->mtime); |
873 | print "file $file updated at $date_string\n"; |
68dc0745 |
874 | |
65acb1b1 |
875 | The POSIX::strftime() approach has the benefit of being, |
876 | in theory, independent of the current locale. See L<perllocale> |
877 | for details. |
68dc0745 |
878 | |
879 | =head2 How do I set a file's timestamp in perl? |
d74e8afc |
880 | X<timestamp> X<file, timestamp> |
68dc0745 |
881 | |
882 | You use the utime() function documented in L<perlfunc/utime>. |
883 | By way of example, here's a little program that copies the |
884 | read and write times from its first argument to all the rest |
885 | of them. |
886 | |
500071f4 |
887 | if (@ARGV < 2) { |
888 | die "usage: cptimes timestamp_file other_files ...\n"; |
889 | } |
890 | $timestamp = shift; |
891 | ($atime, $mtime) = (stat($timestamp))[8,9]; |
892 | utime $atime, $mtime, @ARGV; |
68dc0745 |
893 | |
65acb1b1 |
894 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 |
895 | |
19a1cd16 |
896 | The perldoc for utime also has an example that has the same |
897 | effect as touch(1) on files that I<already exist>. |
898 | |
899 | Certain file systems have a limited ability to store the times |
900 | on a file at the expected level of precision. For example, the |
901 | FAT and HPFS filesystem are unable to create dates on files with |
902 | a finer granularity than two seconds. This is a limitation of |
903 | the filesystems, not of utime(). |
68dc0745 |
904 | |
905 | =head2 How do I print to more than one file at once? |
d74e8afc |
906 | X<print, to multiple files> |
68dc0745 |
907 | |
49d635f9 |
908 | To connect one filehandle to several output filehandles, |
909 | you can use the IO::Tee or Tie::FileHandle::Multiplex modules. |
68dc0745 |
910 | |
49d635f9 |
911 | If you only have to do this once, you can print individually |
912 | to each filehandle. |
68dc0745 |
913 | |
500071f4 |
914 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
5a964f20 |
915 | |
49d635f9 |
916 | =head2 How can I read in an entire file all at once? |
d74e8afc |
917 | X<slurp> X<file, slurping> |
68dc0745 |
918 | |
49d635f9 |
919 | You can use the File::Slurp module to do it in one step. |
68dc0745 |
920 | |
49d635f9 |
921 | use File::Slurp; |
197aec24 |
922 | |
49d635f9 |
923 | $all_of_it = read_file($filename); # entire file in scalar |
500071f4 |
924 | @all_lines = read_file($filename); # one line perl element |
d92eb7b0 |
925 | |
926 | The customary Perl approach for processing all the lines in a file is to |
927 | do so one line at a time: |
928 | |
500071f4 |
929 | open (INPUT, $file) || die "can't open $file: $!"; |
930 | while (<INPUT>) { |
931 | chomp; |
932 | # do something with $_ |
933 | } |
934 | close(INPUT) || die "can't close $file: $!"; |
d92eb7b0 |
935 | |
936 | This is tremendously more efficient than reading the entire file into |
937 | memory as an array of lines and then processing it one element at a time, |
a6dd486b |
938 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 |
939 | you see someone do this: |
940 | |
500071f4 |
941 | @lines = <INPUT>; |
d92eb7b0 |
942 | |
30852c57 |
943 | you should think long and hard about why you need everything loaded at |
944 | once. It's just not a scalable solution. You might also find it more |
945 | fun to use the standard Tie::File module, or the DB_File module's |
946 | $DB_RECNO bindings, which allow you to tie an array to a file so that |
947 | accessing an element the array actually accesses the corresponding |
948 | line in the file. |
d92eb7b0 |
949 | |
f05bbc40 |
950 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 |
951 | |
500071f4 |
952 | { |
d92eb7b0 |
953 | local(*INPUT, $/); |
954 | open (INPUT, $file) || die "can't open $file: $!"; |
955 | $var = <INPUT>; |
500071f4 |
956 | } |
d92eb7b0 |
957 | |
197aec24 |
958 | That temporarily undefs your record separator, and will automatically |
d92eb7b0 |
959 | close the file at block exit. If the file is already open, just use this: |
960 | |
500071f4 |
961 | $var = do { local $/; <INPUT> }; |
d92eb7b0 |
962 | |
f05bbc40 |
963 | For ordinary files you can also use the read function. |
964 | |
965 | read( INPUT, $var, -s INPUT ); |
966 | |
967 | The third argument tests the byte size of the data on the INPUT filehandle |
968 | and reads that many bytes into the buffer $var. |
969 | |
68dc0745 |
970 | =head2 How can I read in a file by paragraphs? |
d74e8afc |
971 | X<file, reading by paragraphs> |
68dc0745 |
972 | |
65acb1b1 |
973 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 |
974 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
975 | for instance, gets treated as two paragraphs and not three), or |
976 | C<"\n\n"> to accept empty paragraphs. |
977 | |
197aec24 |
978 | Note that a blank line must have no blanks in it. Thus |
c4db748a |
979 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 |
980 | |
68dc0745 |
981 | =head2 How can I read a single character from a file? From the keyboard? |
d74e8afc |
982 | X<getc> X<file, reading one character at a time> |
68dc0745 |
983 | |
984 | You can use the builtin C<getc()> function for most filehandles, but |
985 | it won't (easily) work on a terminal device. For STDIN, either use |
a6dd486b |
986 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 |
987 | L<perlfunc/getc>. |
988 | |
65acb1b1 |
989 | If your system supports the portable operating system programming |
990 | interface (POSIX), you can use the following code, which you'll note |
991 | turns off echo processing as well. |
68dc0745 |
992 | |
500071f4 |
993 | #!/usr/bin/perl -w |
994 | use strict; |
995 | $| = 1; |
996 | for (1..4) { |
997 | my $got; |
998 | print "gimme: "; |
999 | $got = getone(); |
1000 | print "--> $got\n"; |
1001 | } |
68dc0745 |
1002 | exit; |
1003 | |
500071f4 |
1004 | BEGIN { |
68dc0745 |
1005 | use POSIX qw(:termios_h); |
1006 | |
1007 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
1008 | |
1009 | $fd_stdin = fileno(STDIN); |
1010 | |
1011 | $term = POSIX::Termios->new(); |
1012 | $term->getattr($fd_stdin); |
1013 | $oterm = $term->getlflag(); |
1014 | |
1015 | $echo = ECHO | ECHOK | ICANON; |
1016 | $noecho = $oterm & ~$echo; |
1017 | |
1018 | sub cbreak { |
500071f4 |
1019 | $term->setlflag($noecho); |
1020 | $term->setcc(VTIME, 1); |
1021 | $term->setattr($fd_stdin, TCSANOW); |
1022 | } |
ac9dac7f |
1023 | |
68dc0745 |
1024 | sub cooked { |
500071f4 |
1025 | $term->setlflag($oterm); |
1026 | $term->setcc(VTIME, 0); |
1027 | $term->setattr($fd_stdin, TCSANOW); |
1028 | } |
68dc0745 |
1029 | |
1030 | sub getone { |
500071f4 |
1031 | my $key = ''; |
1032 | cbreak(); |
1033 | sysread(STDIN, $key, 1); |
1034 | cooked(); |
1035 | return $key; |
1036 | } |
68dc0745 |
1037 | |
500071f4 |
1038 | } |
68dc0745 |
1039 | |
500071f4 |
1040 | END { cooked() } |
68dc0745 |
1041 | |
a6dd486b |
1042 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 |
1043 | include also support for non-portable systems as well. |
68dc0745 |
1044 | |
500071f4 |
1045 | use Term::ReadKey; |
1046 | open(TTY, "</dev/tty"); |
1047 | print "Gimme a char: "; |
1048 | ReadMode "raw"; |
1049 | $key = ReadKey 0, *TTY; |
1050 | ReadMode "normal"; |
1051 | printf "\nYou said %s, char number %03d\n", |
1052 | $key, ord $key; |
68dc0745 |
1053 | |
65acb1b1 |
1054 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 |
1055 | |
5a964f20 |
1056 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 |
1057 | extension from CPAN. As we mentioned earlier, it now even has limited |
1058 | support for non-portable (read: not open systems, closed, proprietary, |
1059 | not POSIX, not Unix, etc) systems. |
5a964f20 |
1060 | |
1061 | You should also check out the Frequently Asked Questions list in |
68dc0745 |
1062 | comp.unix.* for things like this: the answer is essentially the same. |
1063 | It's very system dependent. Here's one solution that works on BSD |
1064 | systems: |
1065 | |
500071f4 |
1066 | sub key_ready { |
1067 | my($rin, $nfd); |
1068 | vec($rin, fileno(STDIN), 1) = 1; |
1069 | return $nfd = select($rin,undef,undef,0); |
1070 | } |
68dc0745 |
1071 | |
65acb1b1 |
1072 | If you want to find out how many characters are waiting, there's |
1073 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that |
1074 | comes with Perl tries to convert C include files to Perl code, which |
1075 | can be C<require>d. FIONREAD ends up defined as a function in the |
1076 | I<sys/ioctl.ph> file: |
68dc0745 |
1077 | |
500071f4 |
1078 | require 'sys/ioctl.ph'; |
68dc0745 |
1079 | |
500071f4 |
1080 | $size = pack("L", 0); |
1081 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; |
1082 | $size = unpack("L", $size); |
68dc0745 |
1083 | |
5a964f20 |
1084 | If I<h2ph> wasn't installed or doesn't work for you, you can |
1085 | I<grep> the include files by hand: |
68dc0745 |
1086 | |
500071f4 |
1087 | % grep FIONREAD /usr/include/*/* |
1088 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B |
68dc0745 |
1089 | |
5a964f20 |
1090 | Or write a small C program using the editor of champions: |
68dc0745 |
1091 | |
500071f4 |
1092 | % cat > fionread.c |
1093 | #include <sys/ioctl.h> |
1094 | main() { |
1095 | printf("%#08x\n", FIONREAD); |
1096 | } |
1097 | ^D |
1098 | % cc -o fionread fionread.c |
1099 | % ./fionread |
1100 | 0x4004667f |
5a964f20 |
1101 | |
8305e449 |
1102 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 |
1103 | |
500071f4 |
1104 | $FIONREAD = 0x4004667f; # XXX: opsys dependent |
5a964f20 |
1105 | |
500071f4 |
1106 | $size = pack("L", 0); |
1107 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; |
1108 | $size = unpack("L", $size); |
5a964f20 |
1109 | |
a6dd486b |
1110 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 |
1111 | pipes, and tty devices work, but I<not> files. |
68dc0745 |
1112 | |
1113 | =head2 How do I do a C<tail -f> in perl? |
ac9dac7f |
1114 | X<tail> X<IO::Handle> X<File::Tail> X<clearerr> |
68dc0745 |
1115 | |
1116 | First try |
1117 | |
500071f4 |
1118 | seek(GWFILE, 0, 1); |
68dc0745 |
1119 | |
1120 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, |
1121 | but it does clear the end-of-file condition on the handle, so that the |
ac9dac7f |
1122 | next C<< <GWFILE> >> makes Perl try again to read something. |
68dc0745 |
1123 | |
1124 | If that doesn't work (it relies on features of your stdio implementation), |
1125 | then you need something more like this: |
1126 | |
1127 | for (;;) { |
1128 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { |
1129 | # search for some stuff and put it into files |
1130 | } |
1131 | # sleep for a while |
1132 | seek(GWFILE, $curpos, 0); # seek to where we had been |
1133 | } |
1134 | |
ac9dac7f |
1135 | If this still doesn't work, look into the C<clearerr> method |
1136 | from C<IO::Handle>, which resets the error and end-of-file states |
1137 | on the handle. |
68dc0745 |
1138 | |
ac9dac7f |
1139 | There's also a C<File::Tail> module from CPAN. |
65acb1b1 |
1140 | |
68dc0745 |
1141 | =head2 How do I dup() a filehandle in Perl? |
d74e8afc |
1142 | X<dup> |
68dc0745 |
1143 | |
1144 | If you check L<perlfunc/open>, you'll see that several of the ways |
1145 | to call open() should do the trick. For example: |
1146 | |
500071f4 |
1147 | open(LOG, ">>/foo/logfile"); |
1148 | open(STDERR, ">&LOG"); |
68dc0745 |
1149 | |
1150 | Or even with a literal numeric descriptor: |
1151 | |
1152 | $fd = $ENV{MHCONTEXTFD}; |
1153 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) |
1154 | |
c47ff5f1 |
1155 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 |
1156 | an alias. That means if you close an aliased handle, all |
197aec24 |
1157 | aliases become inaccessible. This is not true with |
5a964f20 |
1158 | a copied one. |
1159 | |
1160 | Error checking, as always, has been left as an exercise for the reader. |
68dc0745 |
1161 | |
1162 | =head2 How do I close a file descriptor by number? |
d74e8afc |
1163 | X<file, closing file descriptors> |
68dc0745 |
1164 | |
1165 | This should rarely be necessary, as the Perl close() function is to be |
1166 | used for things that Perl opened itself, even if it was a dup of a |
a6dd486b |
1167 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 |
1168 | to, you may be able to do this: |
1169 | |
500071f4 |
1170 | require 'sys/syscall.ph'; |
1171 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric |
1172 | die "can't sysclose $fd: $!" unless $rc == -1; |
68dc0745 |
1173 | |
a6dd486b |
1174 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 |
1175 | |
500071f4 |
1176 | { |
197aec24 |
1177 | local *F; |
d92eb7b0 |
1178 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
1179 | close F; |
500071f4 |
1180 | } |
d92eb7b0 |
1181 | |
883f1635 |
1182 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
d74e8afc |
1183 | X<filename, DOS issues> |
68dc0745 |
1184 | |
1185 | Whoops! You just put a tab and a formfeed into that filename! |
1186 | Remember that within double quoted strings ("like\this"), the |
1187 | backslash is an escape character. The full list of these is in |
1188 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't |
1189 | have a file called "c:(tab)emp(formfeed)oo" or |
65acb1b1 |
1190 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 |
1191 | |
1192 | Either single-quote your strings, or (preferably) use forward slashes. |
46fc3d4c |
1193 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 |
1194 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b |
1195 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 |
1196 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
1197 | are more portable, too. |
68dc0745 |
1198 | |
1199 | =head2 Why doesn't glob("*.*") get all the files? |
d74e8afc |
1200 | X<glob> |
68dc0745 |
1201 | |
1202 | Because even on non-Unix ports, Perl's glob function follows standard |
46fc3d4c |
1203 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 |
1204 | files. This makes glob() portable even to legacy systems. Your |
1205 | port may include proprietary globbing functions as well. Check its |
1206 | documentation for details. |
68dc0745 |
1207 | |
1208 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? |
1209 | |
06a5f41f |
1210 | This is elaborately and painstakingly described in the |
1211 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To |
49d635f9 |
1212 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . |
68dc0745 |
1213 | |
1214 | The executive summary: learn how your filesystem works. The |
1215 | permissions on a file say what can happen to the data in that file. |
1216 | The permissions on a directory say what can happen to the list of |
1217 | files in that directory. If you delete a file, you're removing its |
1218 | name from the directory (so the operation depends on the permissions |
1219 | of the directory, not of the file). If you try to write to the file, |
1220 | the permissions of the file govern whether you're allowed to. |
1221 | |
1222 | =head2 How do I select a random line from a file? |
d74e8afc |
1223 | X<file, selecting a random line> |
68dc0745 |
1224 | |
1225 | Here's an algorithm from the Camel Book: |
1226 | |
500071f4 |
1227 | srand; |
1228 | rand($.) < 1 && ($line = $_) while <>; |
68dc0745 |
1229 | |
49d635f9 |
1230 | This has a significant advantage in space over reading the whole file |
1231 | in. You can find a proof of this method in I<The Art of Computer |
1232 | Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. |
1233 | |
1234 | You can use the File::Random module which provides a function |
1235 | for that algorithm: |
1236 | |
1237 | use File::Random qw/random_line/; |
1238 | my $line = random_line($filename); |
1239 | |
1240 | Another way is to use the Tie::File module, which treats the entire |
1241 | file as an array. Simply access a random array element. |
68dc0745 |
1242 | |
65acb1b1 |
1243 | =head2 Why do I get weird spaces when I print an array of lines? |
1244 | |
1245 | Saying |
1246 | |
500071f4 |
1247 | print "@lines\n"; |
65acb1b1 |
1248 | |
1249 | joins together the elements of C<@lines> with a space between them. |
1250 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above |
a6dd486b |
1251 | statement would print |
65acb1b1 |
1252 | |
500071f4 |
1253 | little fluffy clouds |
65acb1b1 |
1254 | |
1255 | but if each element of C<@lines> was a line of text, ending a newline |
1256 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: |
1257 | |
500071f4 |
1258 | little |
1259 | fluffy |
1260 | clouds |
65acb1b1 |
1261 | |
1262 | If your array contains lines, just print them: |
1263 | |
500071f4 |
1264 | print @lines; |
1265 | |
1266 | =head1 REVISION |
1267 | |
e573f903 |
1268 | Revision: $Revision: 7875 $ |
500071f4 |
1269 | |
e573f903 |
1270 | Date: $Date: 2006-10-04 22:39:26 +0200 (mer, 04 oct 2006) $ |
500071f4 |
1271 | |
1272 | See L<perlfaq> for source control details and availability. |
65acb1b1 |
1273 | |
68dc0745 |
1274 | =head1 AUTHOR AND COPYRIGHT |
1275 | |
58103a2e |
1276 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced |
1277 | other authors as noted. All rights reserved. |
5a964f20 |
1278 | |
5a7beb56 |
1279 | This documentation is free; you can redistribute it and/or modify it |
1280 | under the same terms as Perl itself. |
c8db1d39 |
1281 | |
87275199 |
1282 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
1283 | domain. You are permitted and encouraged to use this code and any |
1284 | derivatives thereof in your own programs for fun or for profit as you |
1285 | see fit. A simple comment in the code giving credit to the FAQ would |
1286 | be courteous but is not required. |