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1 | =head1 NAME |
2 | |
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3 | perlfaq5 - Files and Formats ($Revision: 1.17 $, $Date: 2002/05/23 19:33:50 $) |
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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 | |
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12 | Perl does not support truly unbuffered output (except |
13 | insofar as you can C<syswrite(OUT, $char, 1)>), although it |
14 | does support is "command buffering", in which a physical |
15 | write is performed after every output command. |
16 | |
17 | The C standard I/O library (stdio) normally buffers |
18 | characters sent to devices so that there isn't a system call |
19 | for each byte. In most stdio implementations, the type of |
20 | output buffering and the size of the buffer varies according |
21 | to the type of device. Perl's print() and write() functions |
22 | normally buffer output, while syswrite() bypasses buffering |
23 | all together. |
24 | |
25 | If you want your output to be sent immediately when you |
26 | execute print() or write() (for instance, for some network |
27 | protocols), you must set the handle's autoflush flag. This |
28 | flag is the Perl variable $| and when it is set to a true |
29 | value, Perl will flush the handle's buffer after each |
30 | print() or write(). Setting $| affects buffering only for |
31 | the currently selected default file handle. You choose this |
32 | handle with the one argument select() call (see |
33 | L<perlvar/$|> and L<perlfunc/select>). |
34 | |
35 | Use select() to choose the desired handle, then set its |
36 | per-filehandle variables. |
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37 | |
38 | $old_fh = select(OUTPUT_HANDLE); |
39 | $| = 1; |
40 | select($old_fh); |
41 | |
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42 | Some idioms can handle this in a single statement: |
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43 | |
44 | select((select(OUTPUT_HANDLE), $| = 1)[0]); |
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45 | |
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46 | $| = 1, select $_ for select OUTPUT_HANDLE; |
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47 | |
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48 | Some modules offer object-oriented access to handles and their |
49 | variables, although they may be overkill if this is the only |
50 | thing you do with them. You can use IO::Handle: |
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51 | |
52 | use IO::Handle; |
53 | open(DEV, ">/dev/printer"); # but is this? |
54 | DEV->autoflush(1); |
55 | |
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56 | or IO::Socket: |
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57 | |
58 | use IO::Socket; # this one is kinda a pipe? |
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59 | my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ; |
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60 | |
61 | $sock->autoflush(); |
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62 | |
63 | =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file? |
64 | |
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65 | Use the Tie::File module, which is included in the standard |
66 | distribution since Perl 5.8.0. |
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67 | |
68 | =head2 How do I count the number of lines in a file? |
69 | |
70 | One fairly efficient way is to count newlines in the file. The |
71 | following program uses a feature of tr///, as documented in L<perlop>. |
72 | If your text file doesn't end with a newline, then it's not really a |
73 | proper text file, so this may report one fewer line than you expect. |
74 | |
75 | $lines = 0; |
76 | open(FILE, $filename) or die "Can't open `$filename': $!"; |
77 | while (sysread FILE, $buffer, 4096) { |
78 | $lines += ($buffer =~ tr/\n//); |
79 | } |
80 | close FILE; |
81 | |
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82 | This assumes no funny games with newline translations. |
83 | |
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84 | =head2 How do I make a temporary file name? |
85 | |
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86 | Use the File::Temp module, see L<File::Temp> for more information. |
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87 | |
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88 | use File::Temp qw/ tempfile tempdir /; |
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89 | |
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90 | $dir = tempdir( CLEANUP => 1 ); |
91 | ($fh, $filename) = tempfile( DIR => $dir ); |
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92 | |
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93 | # or if you don't need to know the filename |
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94 | |
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95 | $fh = tempfile( DIR => $dir ); |
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96 | |
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97 | The File::Temp has been a standard module since Perl 5.6.1. If you |
98 | don't have a modern enough Perl installed, use the C<new_tmpfile> |
99 | class method from the IO::File module to get a filehandle opened for |
100 | reading and writing. Use it if you don't need to know the file's name: |
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101 | |
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102 | use IO::File; |
103 | $fh = IO::File->new_tmpfile() |
104 | or die "Unable to make new temporary file: $!"; |
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105 | |
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106 | If you're committed to creating a temporary file by hand, use the |
107 | process ID and/or the current time-value. If you need to have many |
108 | temporary files in one process, use a counter: |
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109 | |
110 | BEGIN { |
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111 | use Fcntl; |
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112 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; |
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113 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); |
114 | sub temp_file { |
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115 | local *FH; |
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116 | my $count = 0; |
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117 | until (defined(fileno(FH)) || $count++ > 100) { |
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118 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
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119 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
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120 | } |
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121 | if (defined(fileno(FH)) |
122 | return (*FH, $base_name); |
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123 | } else { |
124 | return (); |
125 | } |
126 | } |
127 | } |
128 | |
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129 | =head2 How can I manipulate fixed-record-length files? |
130 | |
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131 | The most efficient way is using pack() and unpack(). This is faster than |
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132 | using substr() when taking many, many strings. It is slower for just a few. |
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133 | |
134 | Here is a sample chunk of code to break up and put back together again |
135 | some fixed-format input lines, in this case from the output of a normal, |
136 | Berkeley-style ps: |
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137 | |
138 | # sample input line: |
139 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what |
140 | $PS_T = 'A6 A4 A7 A5 A*'; |
141 | open(PS, "ps|"); |
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142 | print scalar <PS>; |
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143 | while (<PS>) { |
144 | ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_); |
145 | for $var (qw!pid tt stat time command!) { |
146 | print "$var: <$$var>\n"; |
147 | } |
148 | print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command), |
149 | "\n"; |
150 | } |
151 | |
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152 | We've used C<$$var> in a way that forbidden by C<use strict 'refs'>. |
153 | That is, we've promoted a string to a scalar variable reference using |
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154 | symbolic references. This is okay in small programs, but doesn't scale |
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155 | well. It also only works on global variables, not lexicals. |
156 | |
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157 | =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? |
158 | |
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159 | As of perl5.6, open() autovivifies file and directory handles |
160 | as references if you pass it an uninitialized scalar variable. |
161 | You can then pass these references just like any other scalar, |
162 | and use them in the place of named handles. |
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163 | |
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164 | open my $fh, $file_name; |
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165 | |
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166 | open local $fh, $file_name; |
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167 | |
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168 | print $fh "Hello World!\n"; |
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169 | |
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170 | process_file( $fh ); |
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171 | |
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172 | Before perl5.6, you had to deal with various typeglob idioms |
173 | which you may see in older code. |
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174 | |
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175 | open FILE, "> $filename"; |
176 | process_typeglob( *FILE ); |
177 | process_reference( \*FILE ); |
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178 | |
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179 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
180 | sub process_reference { local $fh = shift; print $fh "Reference!" } |
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181 | |
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182 | If you want to create many anonymous handles, you should |
183 | check out the Symbol or IO::Handle modules. |
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184 | |
185 | =head2 How can I use a filehandle indirectly? |
186 | |
187 | An indirect filehandle is using something other than a symbol |
188 | in a place that a filehandle is expected. Here are ways |
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189 | to get indirect filehandles: |
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190 | |
191 | $fh = SOME_FH; # bareword is strict-subs hostile |
192 | $fh = "SOME_FH"; # strict-refs hostile; same package only |
193 | $fh = *SOME_FH; # typeglob |
194 | $fh = \*SOME_FH; # ref to typeglob (bless-able) |
195 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob |
196 | |
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197 | Or, you can use the C<new> method from one of the IO::* modules to |
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198 | create an anonymous filehandle, store that in a scalar variable, |
199 | and use it as though it were a normal filehandle. |
200 | |
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201 | use IO::Handle; # 5.004 or higher |
202 | $fh = IO::Handle->new(); |
203 | |
204 | Then use any of those as you would a normal filehandle. Anywhere that |
205 | Perl is expecting a filehandle, an indirect filehandle may be used |
206 | instead. An indirect filehandle is just a scalar variable that contains |
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207 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
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208 | the C<< <FH> >> diamond operator will accept either a named filehandle |
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209 | or a scalar variable containing one: |
210 | |
211 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); |
212 | print $ofh "Type it: "; |
213 | $got = <$ifh> |
214 | print $efh "What was that: $got"; |
215 | |
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216 | If you're passing a filehandle to a function, you can write |
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217 | the function in two ways: |
218 | |
219 | sub accept_fh { |
220 | my $fh = shift; |
221 | print $fh "Sending to indirect filehandle\n"; |
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222 | } |
223 | |
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224 | Or it can localize a typeglob and use the filehandle directly: |
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225 | |
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226 | sub accept_fh { |
227 | local *FH = shift; |
228 | print FH "Sending to localized filehandle\n"; |
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229 | } |
230 | |
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231 | Both styles work with either objects or typeglobs of real filehandles. |
232 | (They might also work with strings under some circumstances, but this |
233 | is risky.) |
234 | |
235 | accept_fh(*STDOUT); |
236 | accept_fh($handle); |
237 | |
238 | In the examples above, we assigned the filehandle to a scalar variable |
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239 | before using it. That is because only simple scalar variables, not |
240 | expressions or subscripts of hashes or arrays, can be used with |
241 | built-ins like C<print>, C<printf>, or the diamond operator. Using |
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242 | something other than a simple scalar variable as a filehandle is |
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243 | illegal and won't even compile: |
244 | |
245 | @fd = (*STDIN, *STDOUT, *STDERR); |
246 | print $fd[1] "Type it: "; # WRONG |
247 | $got = <$fd[0]> # WRONG |
248 | print $fd[2] "What was that: $got"; # WRONG |
249 | |
250 | With C<print> and C<printf>, you get around this by using a block and |
251 | an expression where you would place the filehandle: |
252 | |
253 | print { $fd[1] } "funny stuff\n"; |
254 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; |
255 | # Pity the poor deadbeef. |
256 | |
257 | That block is a proper block like any other, so you can put more |
258 | complicated code there. This sends the message out to one of two places: |
259 | |
260 | $ok = -x "/bin/cat"; |
261 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
262 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
263 | |
264 | This approach of treating C<print> and C<printf> like object methods |
265 | calls doesn't work for the diamond operator. That's because it's a |
266 | real operator, not just a function with a comma-less argument. Assuming |
267 | you've been storing typeglobs in your structure as we did above, you |
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268 | can use the built-in function named C<readline> to read a record just |
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269 | as C<< <> >> does. Given the initialization shown above for @fd, this |
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270 | would work, but only because readline() requires a typeglob. It doesn't |
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271 | work with objects or strings, which might be a bug we haven't fixed yet. |
272 | |
273 | $got = readline($fd[0]); |
274 | |
275 | Let it be noted that the flakiness of indirect filehandles is not |
276 | related to whether they're strings, typeglobs, objects, or anything else. |
277 | It's the syntax of the fundamental operators. Playing the object |
278 | game doesn't help you at all here. |
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279 | |
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280 | =head2 How can I set up a footer format to be used with write()? |
281 | |
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282 | There's no builtin way to do this, but L<perlform> has a couple of |
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283 | techniques to make it possible for the intrepid hacker. |
284 | |
285 | =head2 How can I write() into a string? |
286 | |
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287 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
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288 | |
289 | =head2 How can I output my numbers with commas added? |
290 | |
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291 | This one from Benjamin Goldberg will do it for you: |
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292 | |
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293 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
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294 | |
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295 | or written verbosely: |
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296 | |
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297 | s/( |
298 | ^[-+]? # beginning of number. |
299 | \d{1,3}? # first digits before first comma |
300 | (?= # followed by, (but not included in the match) : |
301 | (?>(?:\d{3})+) # some positive multiple of three digits. |
302 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. |
303 | ) |
304 | | # or: |
305 | \G\d{3} # after the last group, get three digits |
306 | (?=\d) # but they have to have more digits after them. |
307 | )/$1,/xg; |
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308 | |
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309 | =head2 How can I translate tildes (~) in a filename? |
310 | |
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311 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
312 | versions of Perl require that you have a shell installed that groks |
313 | tildes. Recent perl versions have this feature built in. The |
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314 | File::KGlob module (available from CPAN) gives more portable glob |
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315 | functionality. |
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316 | |
317 | Within Perl, you may use this directly: |
318 | |
319 | $filename =~ s{ |
320 | ^ ~ # find a leading tilde |
321 | ( # save this in $1 |
322 | [^/] # a non-slash character |
323 | * # repeated 0 or more times (0 means me) |
324 | ) |
325 | }{ |
326 | $1 |
327 | ? (getpwnam($1))[7] |
328 | : ( $ENV{HOME} || $ENV{LOGDIR} ) |
329 | }ex; |
330 | |
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331 | =head2 How come when I open a file read-write it wipes it out? |
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332 | |
333 | Because you're using something like this, which truncates the file and |
334 | I<then> gives you read-write access: |
335 | |
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336 | open(FH, "+> /path/name"); # WRONG (almost always) |
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337 | |
338 | Whoops. You should instead use this, which will fail if the file |
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339 | doesn't exist. |
340 | |
341 | open(FH, "+< /path/name"); # open for update |
342 | |
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343 | Using ">" always clobbers or creates. Using "<" never does |
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344 | either. The "+" doesn't change this. |
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345 | |
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346 | Here are examples of many kinds of file opens. Those using sysopen() |
347 | all assume |
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348 | |
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349 | use Fcntl; |
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350 | |
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351 | To open file for reading: |
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352 | |
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353 | open(FH, "< $path") || die $!; |
354 | sysopen(FH, $path, O_RDONLY) || die $!; |
355 | |
356 | To open file for writing, create new file if needed or else truncate old file: |
357 | |
358 | open(FH, "> $path") || die $!; |
359 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; |
360 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; |
361 | |
362 | To open file for writing, create new file, file must not exist: |
363 | |
364 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; |
365 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; |
366 | |
367 | To open file for appending, create if necessary: |
368 | |
369 | open(FH, ">> $path") || die $!; |
370 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; |
371 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; |
372 | |
373 | To open file for appending, file must exist: |
374 | |
375 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; |
376 | |
377 | To open file for update, file must exist: |
378 | |
379 | open(FH, "+< $path") || die $!; |
380 | sysopen(FH, $path, O_RDWR) || die $!; |
381 | |
382 | To open file for update, create file if necessary: |
383 | |
384 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; |
385 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; |
386 | |
387 | To open file for update, file must not exist: |
388 | |
389 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; |
390 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; |
391 | |
392 | To open a file without blocking, creating if necessary: |
393 | |
394 | sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
395 | or die "can't open /tmp/somefile: $!": |
396 | |
397 | Be warned that neither creation nor deletion of files is guaranteed to |
398 | be an atomic operation over NFS. That is, two processes might both |
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399 | successfully create or unlink the same file! Therefore O_EXCL |
400 | isn't as exclusive as you might wish. |
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401 | |
87275199 |
402 | See also the new L<perlopentut> if you have it (new for 5.6). |
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403 | |
c47ff5f1 |
404 | =head2 Why do I sometimes get an "Argument list too long" when I use <*>? |
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405 | |
c47ff5f1 |
406 | The C<< <> >> operator performs a globbing operation (see above). |
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407 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
408 | csh(1) to do the actual glob expansion, but |
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409 | csh can't handle more than 127 items and so gives the error message |
410 | C<Argument list too long>. People who installed tcsh as csh won't |
411 | have this problem, but their users may be surprised by it. |
412 | |
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413 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
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414 | yourself with readdir() and patterns, or use a module like File::KGlob, |
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415 | one that doesn't use the shell to do globbing. |
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416 | |
417 | =head2 Is there a leak/bug in glob()? |
418 | |
419 | Due to the current implementation on some operating systems, when you |
420 | use the glob() function or its angle-bracket alias in a scalar |
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421 | context, you may cause a memory leak and/or unpredictable behavior. It's |
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422 | best therefore to use glob() only in list context. |
423 | |
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424 | =head2 How can I open a file with a leading ">" or trailing blanks? |
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425 | |
426 | Normally perl ignores trailing blanks in filenames, and interprets |
427 | certain leading characters (or a trailing "|") to mean something |
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428 | special. |
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429 | |
881bdbd4 |
430 | The three argument form of open() lets you specify the mode |
431 | separately from the filename. The open() function treats |
432 | special mode characters and whitespace in the filename as |
433 | literals |
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434 | |
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435 | open FILE, "<", " file "; # filename is " file " |
436 | open FILE, ">", ">file"; # filename is ">file" |
65acb1b1 |
437 | |
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438 | It may be a lot clearer to use sysopen(), though: |
65acb1b1 |
439 | |
440 | use Fcntl; |
441 | $badpath = "<<<something really wicked "; |
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442 | sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC) |
65acb1b1 |
443 | or die "can't open $badpath: $!"; |
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444 | |
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445 | =head2 How can I reliably rename a file? |
446 | |
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447 | If your operating system supports a proper mv(1) utility or its functional |
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448 | equivalent, this works: |
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449 | |
450 | rename($old, $new) or system("mv", $old, $new); |
451 | |
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452 | It may be more portable to use the File::Copy module instead. |
453 | You just copy to the new file to the new name (checking return |
454 | values), then delete the old one. This isn't really the same |
455 | semantically as a rename(), which preserves meta-information like |
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456 | permissions, timestamps, inode info, etc. |
457 | |
d2321c93 |
458 | Newer versions of File::Copy export a move() function. |
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459 | |
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460 | =head2 How can I lock a file? |
461 | |
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462 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
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463 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
464 | later), and lockf(3) if neither of the two previous system calls exists. |
465 | On some systems, it may even use a different form of native locking. |
466 | Here are some gotchas with Perl's flock(): |
467 | |
468 | =over 4 |
469 | |
470 | =item 1 |
471 | |
472 | Produces a fatal error if none of the three system calls (or their |
473 | close equivalent) exists. |
474 | |
475 | =item 2 |
476 | |
477 | lockf(3) does not provide shared locking, and requires that the |
478 | filehandle be open for writing (or appending, or read/writing). |
479 | |
480 | =item 3 |
481 | |
d92eb7b0 |
482 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
483 | systems), so you'd need to force the use of fcntl(2) when you build Perl. |
a6dd486b |
484 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 |
485 | and the F<INSTALL> file in the source distribution for information on |
486 | building Perl to do this. |
487 | |
488 | Two potentially non-obvious but traditional flock semantics are that |
a6dd486b |
489 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 |
490 | I<merely advisory>. Such discretionary locks are more flexible, but |
491 | offer fewer guarantees. This means that files locked with flock() may |
492 | be modified by programs that do not also use flock(). Cars that stop |
493 | for red lights get on well with each other, but not with cars that don't |
494 | stop for red lights. See the perlport manpage, your port's specific |
495 | documentation, or your system-specific local manpages for details. It's |
496 | best to assume traditional behavior if you're writing portable programs. |
a6dd486b |
497 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 |
498 | for your own system's idiosyncrasies (sometimes called "features"). |
499 | Slavish adherence to portability concerns shouldn't get in the way of |
500 | your getting your job done.) |
68dc0745 |
501 | |
13a2d996 |
502 | For more information on file locking, see also |
503 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 |
504 | |
68dc0745 |
505 | =back |
506 | |
65acb1b1 |
507 | =head2 Why can't I just open(FH, ">file.lock")? |
68dc0745 |
508 | |
509 | A common bit of code B<NOT TO USE> is this: |
510 | |
511 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE |
512 | open(LCK, "> file.lock"); # THIS BROKEN CODE |
513 | |
514 | This is a classic race condition: you take two steps to do something |
515 | which must be done in one. That's why computer hardware provides an |
516 | atomic test-and-set instruction. In theory, this "ought" to work: |
517 | |
5a964f20 |
518 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
68dc0745 |
519 | or die "can't open file.lock: $!": |
520 | |
521 | except that lamentably, file creation (and deletion) is not atomic |
522 | over NFS, so this won't work (at least, not every time) over the net. |
65acb1b1 |
523 | Various schemes involving link() have been suggested, but |
46fc3d4c |
524 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 |
525 | |
fc36a67e |
526 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
68dc0745 |
527 | |
46fc3d4c |
528 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 |
529 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b |
530 | only to stroke the writer's vanity. It's better to pick a random number; |
531 | they're more realistic. |
68dc0745 |
532 | |
5a964f20 |
533 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 |
534 | |
e2c57c3e |
535 | use Fcntl qw(:DEFAULT :flock); |
5a964f20 |
536 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
65acb1b1 |
537 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
68dc0745 |
538 | $num = <FH> || 0; |
539 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; |
540 | truncate(FH, 0) or die "can't truncate numfile: $!"; |
541 | (print FH $num+1, "\n") or die "can't write numfile: $!"; |
68dc0745 |
542 | close FH or die "can't close numfile: $!"; |
543 | |
46fc3d4c |
544 | Here's a much better web-page hit counter: |
68dc0745 |
545 | |
546 | $hits = int( (time() - 850_000_000) / rand(1_000) ); |
547 | |
548 | If the count doesn't impress your friends, then the code might. :-) |
549 | |
f52f3be2 |
550 | =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? |
05caf3a7 |
551 | |
552 | If you are on a system that correctly implements flock() and you use the |
553 | example appending code from "perldoc -f flock" everything will be OK |
554 | even if the OS you are on doesn't implement append mode correctly (if |
555 | such a system exists.) So if you are happy to restrict yourself to OSs |
556 | that implement flock() (and that's not really much of a restriction) |
557 | then that is what you should do. |
558 | |
559 | If you know you are only going to use a system that does correctly |
560 | implement appending (i.e. not Win32) then you can omit the seek() from |
561 | the above code. |
562 | |
563 | If you know you are only writing code to run on an OS and filesystem that |
564 | does implement append mode correctly (a local filesystem on a modern |
565 | Unix for example), and you keep the file in block-buffered mode and you |
566 | write less than one buffer-full of output between each manual flushing |
8305e449 |
567 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 |
568 | the end of the file in one chunk without getting intermingled with |
569 | anyone else's output. You can also use the syswrite() function which is |
570 | simply a wrapper around your systems write(2) system call. |
571 | |
572 | There is still a small theoretical chance that a signal will interrupt |
573 | the system level write() operation before completion. There is also a |
574 | possibility that some STDIO implementations may call multiple system |
575 | level write()s even if the buffer was empty to start. There may be some |
576 | systems where this probability is reduced to zero. |
577 | |
68dc0745 |
578 | =head2 How do I randomly update a binary file? |
579 | |
580 | If you're just trying to patch a binary, in many cases something as |
581 | simple as this works: |
582 | |
583 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs |
584 | |
585 | However, if you have fixed sized records, then you might do something more |
586 | like this: |
587 | |
588 | $RECSIZE = 220; # size of record, in bytes |
589 | $recno = 37; # which record to update |
590 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; |
591 | seek(FH, $recno * $RECSIZE, 0); |
592 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; |
593 | # munge the record |
65acb1b1 |
594 | seek(FH, -$RECSIZE, 1); |
68dc0745 |
595 | print FH $record; |
596 | close FH; |
597 | |
598 | Locking and error checking are left as an exercise for the reader. |
a6dd486b |
599 | Don't forget them or you'll be quite sorry. |
68dc0745 |
600 | |
68dc0745 |
601 | =head2 How do I get a file's timestamp in perl? |
602 | |
881bdbd4 |
603 | If you want to retrieve the time at which the file was last |
604 | read, written, or had its meta-data (owner, etc) changed, |
605 | you use the B<-M>, B<-A>, or B<-C> file test operations as |
606 | documented in L<perlfunc>. These retrieve the age of the |
607 | file (measured against the start-time of your program) in |
608 | days as a floating point number. Some platforms may not have |
609 | all of these times. See L<perlport> for details. To |
610 | retrieve the "raw" time in seconds since the epoch, you |
611 | would call the stat function, then use localtime(), |
612 | gmtime(), or POSIX::strftime() to convert this into |
613 | human-readable form. |
68dc0745 |
614 | |
615 | Here's an example: |
616 | |
617 | $write_secs = (stat($file))[9]; |
c8db1d39 |
618 | printf "file %s updated at %s\n", $file, |
619 | scalar localtime($write_secs); |
68dc0745 |
620 | |
621 | If you prefer something more legible, use the File::stat module |
622 | (part of the standard distribution in version 5.004 and later): |
623 | |
65acb1b1 |
624 | # error checking left as an exercise for reader. |
68dc0745 |
625 | use File::stat; |
626 | use Time::localtime; |
627 | $date_string = ctime(stat($file)->mtime); |
628 | print "file $file updated at $date_string\n"; |
629 | |
65acb1b1 |
630 | The POSIX::strftime() approach has the benefit of being, |
631 | in theory, independent of the current locale. See L<perllocale> |
632 | for details. |
68dc0745 |
633 | |
634 | =head2 How do I set a file's timestamp in perl? |
635 | |
636 | You use the utime() function documented in L<perlfunc/utime>. |
637 | By way of example, here's a little program that copies the |
638 | read and write times from its first argument to all the rest |
639 | of them. |
640 | |
641 | if (@ARGV < 2) { |
642 | die "usage: cptimes timestamp_file other_files ...\n"; |
643 | } |
644 | $timestamp = shift; |
645 | ($atime, $mtime) = (stat($timestamp))[8,9]; |
646 | utime $atime, $mtime, @ARGV; |
647 | |
65acb1b1 |
648 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 |
649 | |
650 | Note that utime() currently doesn't work correctly with Win95/NT |
651 | ports. A bug has been reported. Check it carefully before using |
a6dd486b |
652 | utime() on those platforms. |
68dc0745 |
653 | |
654 | =head2 How do I print to more than one file at once? |
655 | |
656 | If you only have to do this once, you can do this: |
657 | |
658 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
659 | |
660 | To connect up to one filehandle to several output filehandles, it's |
661 | easiest to use the tee(1) program if you have it, and let it take care |
662 | of the multiplexing: |
663 | |
664 | open (FH, "| tee file1 file2 file3"); |
665 | |
5a964f20 |
666 | Or even: |
667 | |
668 | # make STDOUT go to three files, plus original STDOUT |
669 | open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n"; |
670 | print "whatever\n" or die "Writing: $!\n"; |
671 | close(STDOUT) or die "Closing: $!\n"; |
68dc0745 |
672 | |
5a964f20 |
673 | Otherwise you'll have to write your own multiplexing print |
a6dd486b |
674 | function--or your own tee program--or use Tom Christiansen's, |
a93751fa |
675 | at http://www.cpan.org/authors/id/TOMC/scripts/tct.gz , which is |
5a964f20 |
676 | written in Perl and offers much greater functionality |
677 | than the stock version. |
68dc0745 |
678 | |
d92eb7b0 |
679 | =head2 How can I read in an entire file all at once? |
680 | |
681 | The customary Perl approach for processing all the lines in a file is to |
682 | do so one line at a time: |
683 | |
684 | open (INPUT, $file) || die "can't open $file: $!"; |
685 | while (<INPUT>) { |
686 | chomp; |
687 | # do something with $_ |
688 | } |
689 | close(INPUT) || die "can't close $file: $!"; |
690 | |
691 | This is tremendously more efficient than reading the entire file into |
692 | memory as an array of lines and then processing it one element at a time, |
a6dd486b |
693 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 |
694 | you see someone do this: |
695 | |
696 | @lines = <INPUT>; |
697 | |
30852c57 |
698 | you should think long and hard about why you need everything loaded at |
699 | once. It's just not a scalable solution. You might also find it more |
700 | fun to use the standard Tie::File module, or the DB_File module's |
701 | $DB_RECNO bindings, which allow you to tie an array to a file so that |
702 | accessing an element the array actually accesses the corresponding |
703 | line in the file. |
d92eb7b0 |
704 | |
f05bbc40 |
705 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 |
706 | |
707 | { |
708 | local(*INPUT, $/); |
709 | open (INPUT, $file) || die "can't open $file: $!"; |
710 | $var = <INPUT>; |
711 | } |
712 | |
713 | That temporarily undefs your record separator, and will automatically |
714 | close the file at block exit. If the file is already open, just use this: |
715 | |
716 | $var = do { local $/; <INPUT> }; |
717 | |
f05bbc40 |
718 | For ordinary files you can also use the read function. |
719 | |
720 | read( INPUT, $var, -s INPUT ); |
721 | |
722 | The third argument tests the byte size of the data on the INPUT filehandle |
723 | and reads that many bytes into the buffer $var. |
724 | |
68dc0745 |
725 | =head2 How can I read in a file by paragraphs? |
726 | |
65acb1b1 |
727 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 |
728 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
729 | for instance, gets treated as two paragraphs and not three), or |
730 | C<"\n\n"> to accept empty paragraphs. |
731 | |
d05ac700 |
732 | Note that a blank line must have no blanks in it. Thus |
c4db748a |
733 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 |
734 | |
68dc0745 |
735 | =head2 How can I read a single character from a file? From the keyboard? |
736 | |
737 | You can use the builtin C<getc()> function for most filehandles, but |
738 | it won't (easily) work on a terminal device. For STDIN, either use |
a6dd486b |
739 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 |
740 | L<perlfunc/getc>. |
741 | |
65acb1b1 |
742 | If your system supports the portable operating system programming |
743 | interface (POSIX), you can use the following code, which you'll note |
744 | turns off echo processing as well. |
68dc0745 |
745 | |
746 | #!/usr/bin/perl -w |
747 | use strict; |
748 | $| = 1; |
749 | for (1..4) { |
750 | my $got; |
751 | print "gimme: "; |
752 | $got = getone(); |
753 | print "--> $got\n"; |
754 | } |
755 | exit; |
756 | |
757 | BEGIN { |
758 | use POSIX qw(:termios_h); |
759 | |
760 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
761 | |
762 | $fd_stdin = fileno(STDIN); |
763 | |
764 | $term = POSIX::Termios->new(); |
765 | $term->getattr($fd_stdin); |
766 | $oterm = $term->getlflag(); |
767 | |
768 | $echo = ECHO | ECHOK | ICANON; |
769 | $noecho = $oterm & ~$echo; |
770 | |
771 | sub cbreak { |
772 | $term->setlflag($noecho); |
773 | $term->setcc(VTIME, 1); |
774 | $term->setattr($fd_stdin, TCSANOW); |
775 | } |
776 | |
777 | sub cooked { |
778 | $term->setlflag($oterm); |
779 | $term->setcc(VTIME, 0); |
780 | $term->setattr($fd_stdin, TCSANOW); |
781 | } |
782 | |
783 | sub getone { |
784 | my $key = ''; |
785 | cbreak(); |
786 | sysread(STDIN, $key, 1); |
787 | cooked(); |
788 | return $key; |
789 | } |
790 | |
791 | } |
792 | |
793 | END { cooked() } |
794 | |
a6dd486b |
795 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 |
796 | include also support for non-portable systems as well. |
68dc0745 |
797 | |
798 | use Term::ReadKey; |
799 | open(TTY, "</dev/tty"); |
800 | print "Gimme a char: "; |
801 | ReadMode "raw"; |
802 | $key = ReadKey 0, *TTY; |
803 | ReadMode "normal"; |
804 | printf "\nYou said %s, char number %03d\n", |
805 | $key, ord $key; |
806 | |
65acb1b1 |
807 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 |
808 | |
5a964f20 |
809 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 |
810 | extension from CPAN. As we mentioned earlier, it now even has limited |
811 | support for non-portable (read: not open systems, closed, proprietary, |
812 | not POSIX, not Unix, etc) systems. |
5a964f20 |
813 | |
814 | You should also check out the Frequently Asked Questions list in |
68dc0745 |
815 | comp.unix.* for things like this: the answer is essentially the same. |
816 | It's very system dependent. Here's one solution that works on BSD |
817 | systems: |
818 | |
819 | sub key_ready { |
820 | my($rin, $nfd); |
821 | vec($rin, fileno(STDIN), 1) = 1; |
822 | return $nfd = select($rin,undef,undef,0); |
823 | } |
824 | |
65acb1b1 |
825 | If you want to find out how many characters are waiting, there's |
826 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that |
827 | comes with Perl tries to convert C include files to Perl code, which |
828 | can be C<require>d. FIONREAD ends up defined as a function in the |
829 | I<sys/ioctl.ph> file: |
68dc0745 |
830 | |
5a964f20 |
831 | require 'sys/ioctl.ph'; |
68dc0745 |
832 | |
5a964f20 |
833 | $size = pack("L", 0); |
834 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; |
835 | $size = unpack("L", $size); |
68dc0745 |
836 | |
5a964f20 |
837 | If I<h2ph> wasn't installed or doesn't work for you, you can |
838 | I<grep> the include files by hand: |
68dc0745 |
839 | |
5a964f20 |
840 | % grep FIONREAD /usr/include/*/* |
841 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B |
68dc0745 |
842 | |
5a964f20 |
843 | Or write a small C program using the editor of champions: |
68dc0745 |
844 | |
5a964f20 |
845 | % cat > fionread.c |
846 | #include <sys/ioctl.h> |
847 | main() { |
848 | printf("%#08x\n", FIONREAD); |
849 | } |
850 | ^D |
65acb1b1 |
851 | % cc -o fionread fionread.c |
5a964f20 |
852 | % ./fionread |
853 | 0x4004667f |
854 | |
8305e449 |
855 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 |
856 | |
857 | $FIONREAD = 0x4004667f; # XXX: opsys dependent |
858 | |
859 | $size = pack("L", 0); |
860 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; |
861 | $size = unpack("L", $size); |
862 | |
a6dd486b |
863 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 |
864 | pipes, and tty devices work, but I<not> files. |
68dc0745 |
865 | |
866 | =head2 How do I do a C<tail -f> in perl? |
867 | |
868 | First try |
869 | |
870 | seek(GWFILE, 0, 1); |
871 | |
872 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, |
873 | but it does clear the end-of-file condition on the handle, so that the |
874 | next <GWFILE> makes Perl try again to read something. |
875 | |
876 | If that doesn't work (it relies on features of your stdio implementation), |
877 | then you need something more like this: |
878 | |
879 | for (;;) { |
880 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { |
881 | # search for some stuff and put it into files |
882 | } |
883 | # sleep for a while |
884 | seek(GWFILE, $curpos, 0); # seek to where we had been |
885 | } |
886 | |
887 | If this still doesn't work, look into the POSIX module. POSIX defines |
888 | the clearerr() method, which can remove the end of file condition on a |
889 | filehandle. The method: read until end of file, clearerr(), read some |
890 | more. Lather, rinse, repeat. |
891 | |
65acb1b1 |
892 | There's also a File::Tail module from CPAN. |
893 | |
68dc0745 |
894 | =head2 How do I dup() a filehandle in Perl? |
895 | |
896 | If you check L<perlfunc/open>, you'll see that several of the ways |
897 | to call open() should do the trick. For example: |
898 | |
899 | open(LOG, ">>/tmp/logfile"); |
900 | open(STDERR, ">&LOG"); |
901 | |
902 | Or even with a literal numeric descriptor: |
903 | |
904 | $fd = $ENV{MHCONTEXTFD}; |
905 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) |
906 | |
c47ff5f1 |
907 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 |
908 | an alias. That means if you close an aliased handle, all |
909 | aliases become inaccessible. This is not true with |
910 | a copied one. |
911 | |
912 | Error checking, as always, has been left as an exercise for the reader. |
68dc0745 |
913 | |
914 | =head2 How do I close a file descriptor by number? |
915 | |
916 | This should rarely be necessary, as the Perl close() function is to be |
917 | used for things that Perl opened itself, even if it was a dup of a |
a6dd486b |
918 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 |
919 | to, you may be able to do this: |
920 | |
921 | require 'sys/syscall.ph'; |
922 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric |
923 | die "can't sysclose $fd: $!" unless $rc == -1; |
924 | |
a6dd486b |
925 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 |
926 | |
927 | { |
928 | local *F; |
929 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
930 | close F; |
931 | } |
932 | |
883f1635 |
933 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
68dc0745 |
934 | |
935 | Whoops! You just put a tab and a formfeed into that filename! |
936 | Remember that within double quoted strings ("like\this"), the |
937 | backslash is an escape character. The full list of these is in |
938 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't |
939 | have a file called "c:(tab)emp(formfeed)oo" or |
65acb1b1 |
940 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 |
941 | |
942 | Either single-quote your strings, or (preferably) use forward slashes. |
46fc3d4c |
943 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 |
944 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b |
945 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 |
946 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
947 | are more portable, too. |
68dc0745 |
948 | |
949 | =head2 Why doesn't glob("*.*") get all the files? |
950 | |
951 | Because even on non-Unix ports, Perl's glob function follows standard |
46fc3d4c |
952 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 |
953 | files. This makes glob() portable even to legacy systems. Your |
954 | port may include proprietary globbing functions as well. Check its |
955 | documentation for details. |
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956 | |
957 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? |
958 | |
06a5f41f |
959 | This is elaborately and painstakingly described in the |
960 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To |
961 | Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz . |
68dc0745 |
962 | |
963 | The executive summary: learn how your filesystem works. The |
964 | permissions on a file say what can happen to the data in that file. |
965 | The permissions on a directory say what can happen to the list of |
966 | files in that directory. If you delete a file, you're removing its |
967 | name from the directory (so the operation depends on the permissions |
968 | of the directory, not of the file). If you try to write to the file, |
969 | the permissions of the file govern whether you're allowed to. |
970 | |
971 | =head2 How do I select a random line from a file? |
972 | |
973 | Here's an algorithm from the Camel Book: |
974 | |
975 | srand; |
976 | rand($.) < 1 && ($line = $_) while <>; |
977 | |
978 | This has a significant advantage in space over reading the whole |
5a964f20 |
979 | file in. A simple proof by induction is available upon |
a6dd486b |
980 | request if you doubt the algorithm's correctness. |
68dc0745 |
981 | |
65acb1b1 |
982 | =head2 Why do I get weird spaces when I print an array of lines? |
983 | |
984 | Saying |
985 | |
986 | print "@lines\n"; |
987 | |
988 | joins together the elements of C<@lines> with a space between them. |
989 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above |
a6dd486b |
990 | statement would print |
65acb1b1 |
991 | |
992 | little fluffy clouds |
993 | |
994 | but if each element of C<@lines> was a line of text, ending a newline |
995 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: |
996 | |
997 | little |
998 | fluffy |
999 | clouds |
1000 | |
1001 | If your array contains lines, just print them: |
1002 | |
1003 | print @lines; |
1004 | |
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1005 | =head1 AUTHOR AND COPYRIGHT |
1006 | |
0bc0ad85 |
1007 | Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. |
5a964f20 |
1008 | All rights reserved. |
1009 | |
5a7beb56 |
1010 | This documentation is free; you can redistribute it and/or modify it |
1011 | under the same terms as Perl itself. |
c8db1d39 |
1012 | |
87275199 |
1013 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
1014 | domain. You are permitted and encouraged to use this code and any |
1015 | derivatives thereof in your own programs for fun or for profit as you |
1016 | see fit. A simple comment in the code giving credit to the FAQ would |
1017 | be courteous but is not required. |