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1 | =head1 NAME |
2 | |
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3 | perlfaq5 - Files and Formats ($Revision: 6019 $) |
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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 $| affects buffering only for |
32 | the currently selected default file handle. You choose this |
33 | handle with the one argument select() call (see |
197aec24 |
34 | L<perlvar/$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? |
4358a253 |
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 | |
58 | =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? |
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59 | X<file, editing> |
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60 | |
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61 | Use the Tie::File module, which is included in the standard |
62 | distribution since Perl 5.8.0. |
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63 | |
64 | =head2 How do I count the number of lines in a file? |
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65 | X<file, counting lines> X<lines> X<line> |
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66 | |
67 | One fairly efficient way is to count newlines in the file. The |
68 | following program uses a feature of tr///, as documented in L<perlop>. |
69 | If your text file doesn't end with a newline, then it's not really a |
70 | proper text file, so this may report one fewer line than you expect. |
71 | |
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72 | $lines = 0; |
73 | open(FILE, $filename) or die "Can't open `$filename': $!"; |
74 | while (sysread FILE, $buffer, 4096) { |
75 | $lines += ($buffer =~ tr/\n//); |
76 | } |
77 | close FILE; |
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78 | |
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79 | This assumes no funny games with newline translations. |
80 | |
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81 | =head2 How can I use Perl's C<-i> option from within a program? |
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82 | X<-i> X<in-place> |
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83 | |
84 | C<-i> sets the value of Perl's C<$^I> variable, which in turn affects |
85 | the behavior of C<< <> >>; see L<perlrun> for more details. By |
86 | modifying the appropriate variables directly, you can get the same |
87 | behavior within a larger program. For example: |
88 | |
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89 | # ... |
90 | { |
91 | local($^I, @ARGV) = ('.orig', glob("*.c")); |
92 | while (<>) { |
93 | if ($. == 1) { |
94 | print "This line should appear at the top of each file\n"; |
95 | } |
96 | s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case |
97 | print; |
98 | close ARGV if eof; # Reset $. |
99 | } |
100 | } |
101 | # $^I and @ARGV return to their old values here |
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102 | |
103 | This block modifies all the C<.c> files in the current directory, |
104 | leaving a backup of the original data from each file in a new |
105 | C<.c.orig> file. |
106 | |
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107 | =head2 How can I copy a file? |
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108 | X<copy> X<file, copy> |
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109 | |
110 | (contributed by brian d foy) |
111 | |
112 | Use the File::Copy module. It comes with Perl and can do a |
113 | true copy across file systems, and it does its magic in |
114 | a portable fashion. |
115 | |
116 | use File::Copy; |
117 | |
118 | copy( $original, $new_copy ) or die "Copy failed: $!"; |
119 | |
120 | If you can't use File::Copy, you'll have to do the work yourself: |
121 | open the original file, open the destination file, then print |
122 | to the destination file as you read the original. |
123 | |
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124 | =head2 How do I make a temporary file name? |
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125 | X<file, temporary> |
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126 | |
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127 | If you don't need to know the name of the file, you can use C<open()> |
128 | with C<undef> in place of the file name. The C<open()> function |
129 | creates an anonymous temporary file. |
130 | |
131 | open my $tmp, '+>', undef or die $!; |
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132 | |
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133 | Otherwise, you can use the File::Temp module. |
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134 | |
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135 | use File::Temp qw/ tempfile tempdir /; |
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136 | |
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137 | $dir = tempdir( CLEANUP => 1 ); |
138 | ($fh, $filename) = tempfile( DIR => $dir ); |
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139 | |
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140 | # or if you don't need to know the filename |
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141 | |
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142 | $fh = tempfile( DIR => $dir ); |
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143 | |
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144 | The File::Temp has been a standard module since Perl 5.6.1. If you |
145 | don't have a modern enough Perl installed, use the C<new_tmpfile> |
146 | class method from the IO::File module to get a filehandle opened for |
147 | reading and writing. Use it if you don't need to know the file's name: |
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148 | |
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149 | use IO::File; |
150 | $fh = IO::File->new_tmpfile() |
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151 | or die "Unable to make new temporary file: $!"; |
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152 | |
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153 | If you're committed to creating a temporary file by hand, use the |
154 | process ID and/or the current time-value. If you need to have many |
155 | temporary files in one process, use a counter: |
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156 | |
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157 | BEGIN { |
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158 | use Fcntl; |
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159 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; |
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160 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); |
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161 | |
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162 | sub temp_file { |
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163 | local *FH; |
164 | my $count = 0; |
165 | until (defined(fileno(FH)) || $count++ > 100) { |
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166 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
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167 | # O_EXCL is required for security reasons. |
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168 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
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169 | } |
170 | |
171 | if (defined(fileno(FH)) |
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172 | return (*FH, $base_name); |
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173 | } |
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174 | else { |
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175 | return (); |
176 | } |
177 | } |
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178 | } |
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179 | |
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180 | =head2 How can I manipulate fixed-record-length files? |
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181 | X<fixed-length> X<file, fixed-length records> |
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182 | |
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183 | The most efficient way is using L<pack()|perlfunc/"pack"> and |
184 | L<unpack()|perlfunc/"unpack">. This is faster than using |
185 | L<substr()|perlfunc/"substr"> when taking many, many strings. It is |
186 | slower for just a few. |
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187 | |
188 | Here is a sample chunk of code to break up and put back together again |
189 | some fixed-format input lines, in this case from the output of a normal, |
190 | Berkeley-style ps: |
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191 | |
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192 | # sample input line: |
193 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what |
194 | my $PS_T = 'A6 A4 A7 A5 A*'; |
195 | open my $ps, '-|', 'ps'; |
196 | print scalar <$ps>; |
197 | my @fields = qw( pid tt stat time command ); |
198 | while (<$ps>) { |
199 | my %process; |
200 | @process{@fields} = unpack($PS_T, $_); |
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201 | for my $field ( @fields ) { |
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202 | print "$field: <$process{$field}>\n"; |
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203 | } |
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204 | print 'line=', pack($PS_T, @process{@fields} ), "\n"; |
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205 | } |
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206 | |
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207 | We've used a hash slice in order to easily handle the fields of each row. |
208 | Storing the keys in an array means it's easy to operate on them as a |
209 | group or loop over them with for. It also avoids polluting the program |
210 | with global variables and using symbolic references. |
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211 | |
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212 | =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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213 | X<filehandle, local> X<filehandle, passing> X<filehandle, reference> |
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214 | |
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215 | As of perl5.6, open() autovivifies file and directory handles |
216 | as references if you pass it an uninitialized scalar variable. |
217 | You can then pass these references just like any other scalar, |
218 | and use them in the place of named handles. |
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219 | |
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220 | open my $fh, $file_name; |
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221 | |
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222 | open local $fh, $file_name; |
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223 | |
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224 | print $fh "Hello World!\n"; |
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225 | |
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226 | process_file( $fh ); |
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227 | |
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228 | If you like, you can store these filehandles in an array or a hash. |
229 | If you access them directly, they aren't simple scalars and you |
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230 | need to give C<print> a little help by placing the filehandle |
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231 | reference in braces. Perl can only figure it out on its own when |
232 | the filehandle reference is a simple scalar. |
233 | |
234 | my @fhs = ( $fh1, $fh2, $fh3 ); |
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235 | |
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236 | for( $i = 0; $i <= $#fhs; $i++ ) { |
237 | print {$fhs[$i]} "just another Perl answer, \n"; |
238 | } |
239 | |
240 | |
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241 | Before perl5.6, you had to deal with various typeglob idioms |
242 | which you may see in older code. |
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243 | |
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244 | open FILE, "> $filename"; |
245 | process_typeglob( *FILE ); |
246 | process_reference( \*FILE ); |
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247 | |
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248 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
249 | sub process_reference { local $fh = shift; print $fh "Reference!" } |
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250 | |
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251 | If you want to create many anonymous handles, you should |
252 | check out the Symbol or IO::Handle modules. |
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253 | |
254 | =head2 How can I use a filehandle indirectly? |
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255 | X<filehandle, indirect> |
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256 | |
257 | An indirect filehandle is using something other than a symbol |
258 | in a place that a filehandle is expected. Here are ways |
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259 | to get indirect filehandles: |
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260 | |
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261 | $fh = SOME_FH; # bareword is strict-subs hostile |
262 | $fh = "SOME_FH"; # strict-refs hostile; same package only |
263 | $fh = *SOME_FH; # typeglob |
264 | $fh = \*SOME_FH; # ref to typeglob (bless-able) |
265 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob |
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266 | |
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267 | Or, you can use the C<new> method from one of the IO::* modules to |
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268 | create an anonymous filehandle, store that in a scalar variable, |
269 | and use it as though it were a normal filehandle. |
270 | |
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271 | use IO::Handle; # 5.004 or higher |
272 | $fh = IO::Handle->new(); |
5a964f20 |
273 | |
274 | Then use any of those as you would a normal filehandle. Anywhere that |
275 | Perl is expecting a filehandle, an indirect filehandle may be used |
276 | instead. An indirect filehandle is just a scalar variable that contains |
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277 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
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278 | the C<< <FH> >> diamond operator will accept either a named filehandle |
5a964f20 |
279 | or a scalar variable containing one: |
280 | |
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281 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); |
282 | print $ofh "Type it: "; |
283 | $got = <$ifh> |
284 | print $efh "What was that: $got"; |
5a964f20 |
285 | |
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286 | If you're passing a filehandle to a function, you can write |
5a964f20 |
287 | the function in two ways: |
288 | |
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289 | sub accept_fh { |
290 | my $fh = shift; |
291 | print $fh "Sending to indirect filehandle\n"; |
292 | } |
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293 | |
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294 | Or it can localize a typeglob and use the filehandle directly: |
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295 | |
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296 | sub accept_fh { |
297 | local *FH = shift; |
298 | print FH "Sending to localized filehandle\n"; |
299 | } |
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300 | |
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301 | Both styles work with either objects or typeglobs of real filehandles. |
302 | (They might also work with strings under some circumstances, but this |
303 | is risky.) |
304 | |
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305 | accept_fh(*STDOUT); |
306 | accept_fh($handle); |
5a964f20 |
307 | |
308 | In the examples above, we assigned the filehandle to a scalar variable |
a6dd486b |
309 | before using it. That is because only simple scalar variables, not |
310 | expressions or subscripts of hashes or arrays, can be used with |
311 | built-ins like C<print>, C<printf>, or the diamond operator. Using |
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312 | something other than a simple scalar variable as a filehandle is |
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313 | illegal and won't even compile: |
314 | |
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315 | @fd = (*STDIN, *STDOUT, *STDERR); |
316 | print $fd[1] "Type it: "; # WRONG |
317 | $got = <$fd[0]> # WRONG |
318 | print $fd[2] "What was that: $got"; # WRONG |
5a964f20 |
319 | |
320 | With C<print> and C<printf>, you get around this by using a block and |
321 | an expression where you would place the filehandle: |
322 | |
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323 | print { $fd[1] } "funny stuff\n"; |
324 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; |
325 | # Pity the poor deadbeef. |
5a964f20 |
326 | |
327 | That block is a proper block like any other, so you can put more |
328 | complicated code there. This sends the message out to one of two places: |
329 | |
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330 | $ok = -x "/bin/cat"; |
331 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
332 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
5a964f20 |
333 | |
334 | This approach of treating C<print> and C<printf> like object methods |
335 | calls doesn't work for the diamond operator. That's because it's a |
336 | real operator, not just a function with a comma-less argument. Assuming |
337 | you've been storing typeglobs in your structure as we did above, you |
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338 | can use the built-in function named C<readline> to read a record just |
c47ff5f1 |
339 | as C<< <> >> does. Given the initialization shown above for @fd, this |
c90536be |
340 | would work, but only because readline() requires a typeglob. It doesn't |
5a964f20 |
341 | work with objects or strings, which might be a bug we haven't fixed yet. |
342 | |
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343 | $got = readline($fd[0]); |
5a964f20 |
344 | |
345 | Let it be noted that the flakiness of indirect filehandles is not |
346 | related to whether they're strings, typeglobs, objects, or anything else. |
347 | It's the syntax of the fundamental operators. Playing the object |
348 | game doesn't help you at all here. |
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349 | |
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350 | =head2 How can I set up a footer format to be used with write()? |
d74e8afc |
351 | X<footer> |
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352 | |
54310121 |
353 | There's no builtin way to do this, but L<perlform> has a couple of |
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354 | techniques to make it possible for the intrepid hacker. |
355 | |
356 | =head2 How can I write() into a string? |
d74e8afc |
357 | X<write, into a string> |
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358 | |
65acb1b1 |
359 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
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360 | |
361 | =head2 How can I output my numbers with commas added? |
d74e8afc |
362 | X<number, commify> |
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363 | |
b68463f7 |
364 | (contributed by brian d foy and Benjamin Goldberg) |
365 | |
366 | You can use L<Number::Format> to separate places in a number. |
367 | It handles locale information for those of you who want to insert |
368 | full stops instead (or anything else that they want to use, |
369 | really). |
370 | |
49d635f9 |
371 | This subroutine will add commas to your number: |
372 | |
373 | sub commify { |
500071f4 |
374 | local $_ = shift; |
375 | 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; |
376 | return $_; |
377 | } |
49d635f9 |
378 | |
379 | This regex from Benjamin Goldberg will add commas to numbers: |
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380 | |
500071f4 |
381 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
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382 | |
49d635f9 |
383 | It is easier to see with comments: |
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384 | |
500071f4 |
385 | s/( |
386 | ^[-+]? # beginning of number. |
387 | \d+? # first digits before first comma |
388 | (?= # followed by, (but not included in the match) : |
389 | (?>(?:\d{3})+) # some positive multiple of three digits. |
390 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. |
391 | ) |
392 | | # or: |
393 | \G\d{3} # after the last group, get three digits |
394 | (?=\d) # but they have to have more digits after them. |
395 | )/$1,/xg; |
46fc3d4c |
396 | |
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397 | =head2 How can I translate tildes (~) in a filename? |
d74e8afc |
398 | X<tilde> X<tilde expansion> |
68dc0745 |
399 | |
575cc754 |
400 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
401 | versions of Perl require that you have a shell installed that groks |
402 | tildes. Recent perl versions have this feature built in. The |
d6260402 |
403 | File::KGlob module (available from CPAN) gives more portable glob |
575cc754 |
404 | functionality. |
68dc0745 |
405 | |
406 | Within Perl, you may use this directly: |
407 | |
408 | $filename =~ s{ |
409 | ^ ~ # find a leading tilde |
410 | ( # save this in $1 |
411 | [^/] # a non-slash character |
412 | * # repeated 0 or more times (0 means me) |
413 | ) |
414 | }{ |
415 | $1 |
416 | ? (getpwnam($1))[7] |
417 | : ( $ENV{HOME} || $ENV{LOGDIR} ) |
418 | }ex; |
419 | |
5a964f20 |
420 | =head2 How come when I open a file read-write it wipes it out? |
d74e8afc |
421 | X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating> |
68dc0745 |
422 | |
423 | Because you're using something like this, which truncates the file and |
424 | I<then> gives you read-write access: |
425 | |
500071f4 |
426 | open(FH, "+> /path/name"); # WRONG (almost always) |
68dc0745 |
427 | |
428 | Whoops. You should instead use this, which will fail if the file |
197aec24 |
429 | doesn't exist. |
d92eb7b0 |
430 | |
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431 | open(FH, "+< /path/name"); # open for update |
d92eb7b0 |
432 | |
c47ff5f1 |
433 | Using ">" always clobbers or creates. Using "<" never does |
d92eb7b0 |
434 | either. The "+" doesn't change this. |
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435 | |
5a964f20 |
436 | Here are examples of many kinds of file opens. Those using sysopen() |
437 | all assume |
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438 | |
500071f4 |
439 | use Fcntl; |
68dc0745 |
440 | |
5a964f20 |
441 | To open file for reading: |
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442 | |
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443 | open(FH, "< $path") || die $!; |
444 | sysopen(FH, $path, O_RDONLY) || die $!; |
5a964f20 |
445 | |
446 | To open file for writing, create new file if needed or else truncate old file: |
447 | |
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448 | open(FH, "> $path") || die $!; |
449 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; |
450 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; |
5a964f20 |
451 | |
452 | To open file for writing, create new file, file must not exist: |
453 | |
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454 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; |
455 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
456 | |
457 | To open file for appending, create if necessary: |
458 | |
500071f4 |
459 | open(FH, ">> $path") || die $!; |
460 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; |
461 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; |
5a964f20 |
462 | |
463 | To open file for appending, file must exist: |
464 | |
500071f4 |
465 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; |
5a964f20 |
466 | |
467 | To open file for update, file must exist: |
468 | |
500071f4 |
469 | open(FH, "+< $path") || die $!; |
470 | sysopen(FH, $path, O_RDWR) || die $!; |
5a964f20 |
471 | |
472 | To open file for update, create file if necessary: |
473 | |
500071f4 |
474 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; |
475 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; |
5a964f20 |
476 | |
477 | To open file for update, file must not exist: |
478 | |
500071f4 |
479 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; |
480 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
481 | |
482 | To open a file without blocking, creating if necessary: |
483 | |
500071f4 |
484 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
2359510d |
485 | or die "can't open /foo/somefile: $!": |
5a964f20 |
486 | |
487 | Be warned that neither creation nor deletion of files is guaranteed to |
488 | be an atomic operation over NFS. That is, two processes might both |
a6dd486b |
489 | successfully create or unlink the same file! Therefore O_EXCL |
490 | isn't as exclusive as you might wish. |
68dc0745 |
491 | |
87275199 |
492 | See also the new L<perlopentut> if you have it (new for 5.6). |
65acb1b1 |
493 | |
04d666b1 |
494 | =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? |
d74e8afc |
495 | X<argument list too long> |
68dc0745 |
496 | |
c47ff5f1 |
497 | The C<< <> >> operator performs a globbing operation (see above). |
3a4b19e4 |
498 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
499 | csh(1) to do the actual glob expansion, but |
68dc0745 |
500 | csh can't handle more than 127 items and so gives the error message |
501 | C<Argument list too long>. People who installed tcsh as csh won't |
502 | have this problem, but their users may be surprised by it. |
503 | |
3a4b19e4 |
504 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
d6260402 |
505 | yourself with readdir() and patterns, or use a module like File::KGlob, |
3a4b19e4 |
506 | one that doesn't use the shell to do globbing. |
68dc0745 |
507 | |
508 | =head2 Is there a leak/bug in glob()? |
d74e8afc |
509 | X<glob> |
68dc0745 |
510 | |
511 | Due to the current implementation on some operating systems, when you |
512 | use the glob() function or its angle-bracket alias in a scalar |
a6dd486b |
513 | context, you may cause a memory leak and/or unpredictable behavior. It's |
68dc0745 |
514 | best therefore to use glob() only in list context. |
515 | |
c47ff5f1 |
516 | =head2 How can I open a file with a leading ">" or trailing blanks? |
d74e8afc |
517 | X<filename, special characters> |
68dc0745 |
518 | |
b68463f7 |
519 | (contributed by Brian McCauley) |
68dc0745 |
520 | |
b68463f7 |
521 | The special two argument form of Perl's open() function ignores |
522 | trailing blanks in filenames and infers the mode from certain leading |
523 | characters (or a trailing "|"). In older versions of Perl this was the |
524 | only version of open() and so it is prevalent in old code and books. |
65acb1b1 |
525 | |
b68463f7 |
526 | Unless you have a particular reason to use the two argument form you |
527 | should use the three argument form of open() which does not treat any |
528 | charcters in the filename as special. |
58103a2e |
529 | |
881bdbd4 |
530 | open FILE, "<", " file "; # filename is " file " |
531 | open FILE, ">", ">file"; # filename is ">file" |
65acb1b1 |
532 | |
68dc0745 |
533 | =head2 How can I reliably rename a file? |
d74e8afc |
534 | X<rename> X<mv> X<move> X<file, rename> X<ren> |
68dc0745 |
535 | |
49d635f9 |
536 | If your operating system supports a proper mv(1) utility or its |
537 | functional equivalent, this works: |
68dc0745 |
538 | |
500071f4 |
539 | rename($old, $new) or system("mv", $old, $new); |
68dc0745 |
540 | |
d2321c93 |
541 | It may be more portable to use the File::Copy module instead. |
542 | You just copy to the new file to the new name (checking return |
543 | values), then delete the old one. This isn't really the same |
544 | semantically as a rename(), which preserves meta-information like |
68dc0745 |
545 | permissions, timestamps, inode info, etc. |
546 | |
d2321c93 |
547 | Newer versions of File::Copy export a move() function. |
5a964f20 |
548 | |
68dc0745 |
549 | =head2 How can I lock a file? |
d74e8afc |
550 | X<lock> X<file, lock> X<flock> |
68dc0745 |
551 | |
54310121 |
552 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 |
553 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
554 | later), and lockf(3) if neither of the two previous system calls exists. |
555 | On some systems, it may even use a different form of native locking. |
556 | Here are some gotchas with Perl's flock(): |
557 | |
558 | =over 4 |
559 | |
560 | =item 1 |
561 | |
562 | Produces a fatal error if none of the three system calls (or their |
563 | close equivalent) exists. |
564 | |
565 | =item 2 |
566 | |
567 | lockf(3) does not provide shared locking, and requires that the |
568 | filehandle be open for writing (or appending, or read/writing). |
569 | |
570 | =item 3 |
571 | |
d92eb7b0 |
572 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
573 | systems), so you'd need to force the use of fcntl(2) when you build Perl. |
a6dd486b |
574 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 |
575 | and the F<INSTALL> file in the source distribution for information on |
576 | building Perl to do this. |
577 | |
578 | Two potentially non-obvious but traditional flock semantics are that |
a6dd486b |
579 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 |
580 | I<merely advisory>. Such discretionary locks are more flexible, but |
581 | offer fewer guarantees. This means that files locked with flock() may |
582 | be modified by programs that do not also use flock(). Cars that stop |
583 | for red lights get on well with each other, but not with cars that don't |
584 | stop for red lights. See the perlport manpage, your port's specific |
585 | documentation, or your system-specific local manpages for details. It's |
586 | best to assume traditional behavior if you're writing portable programs. |
a6dd486b |
587 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 |
588 | for your own system's idiosyncrasies (sometimes called "features"). |
589 | Slavish adherence to portability concerns shouldn't get in the way of |
590 | your getting your job done.) |
68dc0745 |
591 | |
197aec24 |
592 | For more information on file locking, see also |
13a2d996 |
593 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 |
594 | |
68dc0745 |
595 | =back |
596 | |
04d666b1 |
597 | =head2 Why can't I just open(FH, "E<gt>file.lock")? |
d74e8afc |
598 | X<lock, lockfile race condition> |
68dc0745 |
599 | |
600 | A common bit of code B<NOT TO USE> is this: |
601 | |
500071f4 |
602 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE |
603 | open(LCK, "> file.lock"); # THIS BROKEN CODE |
68dc0745 |
604 | |
605 | This is a classic race condition: you take two steps to do something |
606 | which must be done in one. That's why computer hardware provides an |
607 | atomic test-and-set instruction. In theory, this "ought" to work: |
608 | |
500071f4 |
609 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
9b55d3ab |
610 | or die "can't open file.lock: $!"; |
68dc0745 |
611 | |
612 | except that lamentably, file creation (and deletion) is not atomic |
613 | over NFS, so this won't work (at least, not every time) over the net. |
65acb1b1 |
614 | Various schemes involving link() have been suggested, but |
46fc3d4c |
615 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 |
616 | |
fc36a67e |
617 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
d74e8afc |
618 | X<counter> X<file, counter> |
68dc0745 |
619 | |
46fc3d4c |
620 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 |
621 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b |
622 | only to stroke the writer's vanity. It's better to pick a random number; |
623 | they're more realistic. |
68dc0745 |
624 | |
5a964f20 |
625 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 |
626 | |
500071f4 |
627 | use Fcntl qw(:DEFAULT :flock); |
628 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
629 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
630 | $num = <FH> || 0; |
631 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; |
632 | truncate(FH, 0) or die "can't truncate numfile: $!"; |
633 | (print FH $num+1, "\n") or die "can't write numfile: $!"; |
634 | close FH or die "can't close numfile: $!"; |
68dc0745 |
635 | |
46fc3d4c |
636 | Here's a much better web-page hit counter: |
68dc0745 |
637 | |
500071f4 |
638 | $hits = int( (time() - 850_000_000) / rand(1_000) ); |
68dc0745 |
639 | |
640 | If the count doesn't impress your friends, then the code might. :-) |
641 | |
f52f3be2 |
642 | =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 |
643 | X<append> X<file, append> |
05caf3a7 |
644 | |
645 | If you are on a system that correctly implements flock() and you use the |
646 | example appending code from "perldoc -f flock" everything will be OK |
647 | even if the OS you are on doesn't implement append mode correctly (if |
648 | such a system exists.) So if you are happy to restrict yourself to OSs |
649 | that implement flock() (and that's not really much of a restriction) |
650 | then that is what you should do. |
651 | |
652 | If you know you are only going to use a system that does correctly |
653 | implement appending (i.e. not Win32) then you can omit the seek() from |
654 | the above code. |
655 | |
656 | If you know you are only writing code to run on an OS and filesystem that |
657 | does implement append mode correctly (a local filesystem on a modern |
658 | Unix for example), and you keep the file in block-buffered mode and you |
659 | write less than one buffer-full of output between each manual flushing |
8305e449 |
660 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 |
661 | the end of the file in one chunk without getting intermingled with |
662 | anyone else's output. You can also use the syswrite() function which is |
663 | simply a wrapper around your systems write(2) system call. |
664 | |
665 | There is still a small theoretical chance that a signal will interrupt |
666 | the system level write() operation before completion. There is also a |
667 | possibility that some STDIO implementations may call multiple system |
668 | level write()s even if the buffer was empty to start. There may be some |
669 | systems where this probability is reduced to zero. |
670 | |
68dc0745 |
671 | =head2 How do I randomly update a binary file? |
d74e8afc |
672 | X<file, binary patch> |
68dc0745 |
673 | |
674 | If you're just trying to patch a binary, in many cases something as |
675 | simple as this works: |
676 | |
500071f4 |
677 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs |
68dc0745 |
678 | |
679 | However, if you have fixed sized records, then you might do something more |
680 | like this: |
681 | |
500071f4 |
682 | $RECSIZE = 220; # size of record, in bytes |
683 | $recno = 37; # which record to update |
684 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; |
685 | seek(FH, $recno * $RECSIZE, 0); |
686 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; |
687 | # munge the record |
688 | seek(FH, -$RECSIZE, 1); |
689 | print FH $record; |
690 | close FH; |
68dc0745 |
691 | |
692 | Locking and error checking are left as an exercise for the reader. |
a6dd486b |
693 | Don't forget them or you'll be quite sorry. |
68dc0745 |
694 | |
68dc0745 |
695 | =head2 How do I get a file's timestamp in perl? |
d74e8afc |
696 | X<timestamp> X<file, timestamp> |
68dc0745 |
697 | |
881bdbd4 |
698 | If you want to retrieve the time at which the file was last |
699 | read, written, or had its meta-data (owner, etc) changed, |
a05e4845 |
700 | you use the B<-A>, B<-M>, or B<-C> file test operations as |
881bdbd4 |
701 | documented in L<perlfunc>. These retrieve the age of the |
702 | file (measured against the start-time of your program) in |
703 | days as a floating point number. Some platforms may not have |
704 | all of these times. See L<perlport> for details. To |
705 | retrieve the "raw" time in seconds since the epoch, you |
706 | would call the stat function, then use localtime(), |
707 | gmtime(), or POSIX::strftime() to convert this into |
708 | human-readable form. |
68dc0745 |
709 | |
710 | Here's an example: |
711 | |
500071f4 |
712 | $write_secs = (stat($file))[9]; |
713 | printf "file %s updated at %s\n", $file, |
c8db1d39 |
714 | scalar localtime($write_secs); |
68dc0745 |
715 | |
716 | If you prefer something more legible, use the File::stat module |
717 | (part of the standard distribution in version 5.004 and later): |
718 | |
500071f4 |
719 | # error checking left as an exercise for reader. |
720 | use File::stat; |
721 | use Time::localtime; |
722 | $date_string = ctime(stat($file)->mtime); |
723 | print "file $file updated at $date_string\n"; |
68dc0745 |
724 | |
65acb1b1 |
725 | The POSIX::strftime() approach has the benefit of being, |
726 | in theory, independent of the current locale. See L<perllocale> |
727 | for details. |
68dc0745 |
728 | |
729 | =head2 How do I set a file's timestamp in perl? |
d74e8afc |
730 | X<timestamp> X<file, timestamp> |
68dc0745 |
731 | |
732 | You use the utime() function documented in L<perlfunc/utime>. |
733 | By way of example, here's a little program that copies the |
734 | read and write times from its first argument to all the rest |
735 | of them. |
736 | |
500071f4 |
737 | if (@ARGV < 2) { |
738 | die "usage: cptimes timestamp_file other_files ...\n"; |
739 | } |
740 | $timestamp = shift; |
741 | ($atime, $mtime) = (stat($timestamp))[8,9]; |
742 | utime $atime, $mtime, @ARGV; |
68dc0745 |
743 | |
65acb1b1 |
744 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 |
745 | |
19a1cd16 |
746 | The perldoc for utime also has an example that has the same |
747 | effect as touch(1) on files that I<already exist>. |
748 | |
749 | Certain file systems have a limited ability to store the times |
750 | on a file at the expected level of precision. For example, the |
751 | FAT and HPFS filesystem are unable to create dates on files with |
752 | a finer granularity than two seconds. This is a limitation of |
753 | the filesystems, not of utime(). |
68dc0745 |
754 | |
755 | =head2 How do I print to more than one file at once? |
d74e8afc |
756 | X<print, to multiple files> |
68dc0745 |
757 | |
49d635f9 |
758 | To connect one filehandle to several output filehandles, |
759 | you can use the IO::Tee or Tie::FileHandle::Multiplex modules. |
68dc0745 |
760 | |
49d635f9 |
761 | If you only have to do this once, you can print individually |
762 | to each filehandle. |
68dc0745 |
763 | |
500071f4 |
764 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
5a964f20 |
765 | |
49d635f9 |
766 | =head2 How can I read in an entire file all at once? |
d74e8afc |
767 | X<slurp> X<file, slurping> |
68dc0745 |
768 | |
49d635f9 |
769 | You can use the File::Slurp module to do it in one step. |
68dc0745 |
770 | |
49d635f9 |
771 | use File::Slurp; |
197aec24 |
772 | |
49d635f9 |
773 | $all_of_it = read_file($filename); # entire file in scalar |
500071f4 |
774 | @all_lines = read_file($filename); # one line perl element |
d92eb7b0 |
775 | |
776 | The customary Perl approach for processing all the lines in a file is to |
777 | do so one line at a time: |
778 | |
500071f4 |
779 | open (INPUT, $file) || die "can't open $file: $!"; |
780 | while (<INPUT>) { |
781 | chomp; |
782 | # do something with $_ |
783 | } |
784 | close(INPUT) || die "can't close $file: $!"; |
d92eb7b0 |
785 | |
786 | This is tremendously more efficient than reading the entire file into |
787 | memory as an array of lines and then processing it one element at a time, |
a6dd486b |
788 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 |
789 | you see someone do this: |
790 | |
500071f4 |
791 | @lines = <INPUT>; |
d92eb7b0 |
792 | |
30852c57 |
793 | you should think long and hard about why you need everything loaded at |
794 | once. It's just not a scalable solution. You might also find it more |
795 | fun to use the standard Tie::File module, or the DB_File module's |
796 | $DB_RECNO bindings, which allow you to tie an array to a file so that |
797 | accessing an element the array actually accesses the corresponding |
798 | line in the file. |
d92eb7b0 |
799 | |
f05bbc40 |
800 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 |
801 | |
500071f4 |
802 | { |
d92eb7b0 |
803 | local(*INPUT, $/); |
804 | open (INPUT, $file) || die "can't open $file: $!"; |
805 | $var = <INPUT>; |
500071f4 |
806 | } |
d92eb7b0 |
807 | |
197aec24 |
808 | That temporarily undefs your record separator, and will automatically |
d92eb7b0 |
809 | close the file at block exit. If the file is already open, just use this: |
810 | |
500071f4 |
811 | $var = do { local $/; <INPUT> }; |
d92eb7b0 |
812 | |
f05bbc40 |
813 | For ordinary files you can also use the read function. |
814 | |
815 | read( INPUT, $var, -s INPUT ); |
816 | |
817 | The third argument tests the byte size of the data on the INPUT filehandle |
818 | and reads that many bytes into the buffer $var. |
819 | |
68dc0745 |
820 | =head2 How can I read in a file by paragraphs? |
d74e8afc |
821 | X<file, reading by paragraphs> |
68dc0745 |
822 | |
65acb1b1 |
823 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 |
824 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
825 | for instance, gets treated as two paragraphs and not three), or |
826 | C<"\n\n"> to accept empty paragraphs. |
827 | |
197aec24 |
828 | Note that a blank line must have no blanks in it. Thus |
c4db748a |
829 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 |
830 | |
68dc0745 |
831 | =head2 How can I read a single character from a file? From the keyboard? |
d74e8afc |
832 | X<getc> X<file, reading one character at a time> |
68dc0745 |
833 | |
834 | You can use the builtin C<getc()> function for most filehandles, but |
835 | it won't (easily) work on a terminal device. For STDIN, either use |
a6dd486b |
836 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 |
837 | L<perlfunc/getc>. |
838 | |
65acb1b1 |
839 | If your system supports the portable operating system programming |
840 | interface (POSIX), you can use the following code, which you'll note |
841 | turns off echo processing as well. |
68dc0745 |
842 | |
500071f4 |
843 | #!/usr/bin/perl -w |
844 | use strict; |
845 | $| = 1; |
846 | for (1..4) { |
847 | my $got; |
848 | print "gimme: "; |
849 | $got = getone(); |
850 | print "--> $got\n"; |
851 | } |
68dc0745 |
852 | exit; |
853 | |
500071f4 |
854 | BEGIN { |
68dc0745 |
855 | use POSIX qw(:termios_h); |
856 | |
857 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
858 | |
859 | $fd_stdin = fileno(STDIN); |
860 | |
861 | $term = POSIX::Termios->new(); |
862 | $term->getattr($fd_stdin); |
863 | $oterm = $term->getlflag(); |
864 | |
865 | $echo = ECHO | ECHOK | ICANON; |
866 | $noecho = $oterm & ~$echo; |
867 | |
868 | sub cbreak { |
500071f4 |
869 | $term->setlflag($noecho); |
870 | $term->setcc(VTIME, 1); |
871 | $term->setattr($fd_stdin, TCSANOW); |
872 | } |
ac9dac7f |
873 | |
68dc0745 |
874 | sub cooked { |
500071f4 |
875 | $term->setlflag($oterm); |
876 | $term->setcc(VTIME, 0); |
877 | $term->setattr($fd_stdin, TCSANOW); |
878 | } |
68dc0745 |
879 | |
880 | sub getone { |
500071f4 |
881 | my $key = ''; |
882 | cbreak(); |
883 | sysread(STDIN, $key, 1); |
884 | cooked(); |
885 | return $key; |
886 | } |
68dc0745 |
887 | |
500071f4 |
888 | } |
68dc0745 |
889 | |
500071f4 |
890 | END { cooked() } |
68dc0745 |
891 | |
a6dd486b |
892 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 |
893 | include also support for non-portable systems as well. |
68dc0745 |
894 | |
500071f4 |
895 | use Term::ReadKey; |
896 | open(TTY, "</dev/tty"); |
897 | print "Gimme a char: "; |
898 | ReadMode "raw"; |
899 | $key = ReadKey 0, *TTY; |
900 | ReadMode "normal"; |
901 | printf "\nYou said %s, char number %03d\n", |
902 | $key, ord $key; |
68dc0745 |
903 | |
65acb1b1 |
904 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 |
905 | |
5a964f20 |
906 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 |
907 | extension from CPAN. As we mentioned earlier, it now even has limited |
908 | support for non-portable (read: not open systems, closed, proprietary, |
909 | not POSIX, not Unix, etc) systems. |
5a964f20 |
910 | |
911 | You should also check out the Frequently Asked Questions list in |
68dc0745 |
912 | comp.unix.* for things like this: the answer is essentially the same. |
913 | It's very system dependent. Here's one solution that works on BSD |
914 | systems: |
915 | |
500071f4 |
916 | sub key_ready { |
917 | my($rin, $nfd); |
918 | vec($rin, fileno(STDIN), 1) = 1; |
919 | return $nfd = select($rin,undef,undef,0); |
920 | } |
68dc0745 |
921 | |
65acb1b1 |
922 | If you want to find out how many characters are waiting, there's |
923 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that |
924 | comes with Perl tries to convert C include files to Perl code, which |
925 | can be C<require>d. FIONREAD ends up defined as a function in the |
926 | I<sys/ioctl.ph> file: |
68dc0745 |
927 | |
500071f4 |
928 | require 'sys/ioctl.ph'; |
68dc0745 |
929 | |
500071f4 |
930 | $size = pack("L", 0); |
931 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; |
932 | $size = unpack("L", $size); |
68dc0745 |
933 | |
5a964f20 |
934 | If I<h2ph> wasn't installed or doesn't work for you, you can |
935 | I<grep> the include files by hand: |
68dc0745 |
936 | |
500071f4 |
937 | % grep FIONREAD /usr/include/*/* |
938 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B |
68dc0745 |
939 | |
5a964f20 |
940 | Or write a small C program using the editor of champions: |
68dc0745 |
941 | |
500071f4 |
942 | % cat > fionread.c |
943 | #include <sys/ioctl.h> |
944 | main() { |
945 | printf("%#08x\n", FIONREAD); |
946 | } |
947 | ^D |
948 | % cc -o fionread fionread.c |
949 | % ./fionread |
950 | 0x4004667f |
5a964f20 |
951 | |
8305e449 |
952 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 |
953 | |
500071f4 |
954 | $FIONREAD = 0x4004667f; # XXX: opsys dependent |
5a964f20 |
955 | |
500071f4 |
956 | $size = pack("L", 0); |
957 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; |
958 | $size = unpack("L", $size); |
5a964f20 |
959 | |
a6dd486b |
960 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 |
961 | pipes, and tty devices work, but I<not> files. |
68dc0745 |
962 | |
963 | =head2 How do I do a C<tail -f> in perl? |
ac9dac7f |
964 | X<tail> X<IO::Handle> X<File::Tail> X<clearerr> |
68dc0745 |
965 | |
966 | First try |
967 | |
500071f4 |
968 | seek(GWFILE, 0, 1); |
68dc0745 |
969 | |
970 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, |
971 | but it does clear the end-of-file condition on the handle, so that the |
ac9dac7f |
972 | next C<< <GWFILE> >> makes Perl try again to read something. |
68dc0745 |
973 | |
974 | If that doesn't work (it relies on features of your stdio implementation), |
975 | then you need something more like this: |
976 | |
977 | for (;;) { |
978 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { |
979 | # search for some stuff and put it into files |
980 | } |
981 | # sleep for a while |
982 | seek(GWFILE, $curpos, 0); # seek to where we had been |
983 | } |
984 | |
ac9dac7f |
985 | If this still doesn't work, look into the C<clearerr> method |
986 | from C<IO::Handle>, which resets the error and end-of-file states |
987 | on the handle. |
68dc0745 |
988 | |
ac9dac7f |
989 | There's also a C<File::Tail> module from CPAN. |
65acb1b1 |
990 | |
68dc0745 |
991 | =head2 How do I dup() a filehandle in Perl? |
d74e8afc |
992 | X<dup> |
68dc0745 |
993 | |
994 | If you check L<perlfunc/open>, you'll see that several of the ways |
995 | to call open() should do the trick. For example: |
996 | |
500071f4 |
997 | open(LOG, ">>/foo/logfile"); |
998 | open(STDERR, ">&LOG"); |
68dc0745 |
999 | |
1000 | Or even with a literal numeric descriptor: |
1001 | |
1002 | $fd = $ENV{MHCONTEXTFD}; |
1003 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) |
1004 | |
c47ff5f1 |
1005 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 |
1006 | an alias. That means if you close an aliased handle, all |
197aec24 |
1007 | aliases become inaccessible. This is not true with |
5a964f20 |
1008 | a copied one. |
1009 | |
1010 | Error checking, as always, has been left as an exercise for the reader. |
68dc0745 |
1011 | |
1012 | =head2 How do I close a file descriptor by number? |
d74e8afc |
1013 | X<file, closing file descriptors> |
68dc0745 |
1014 | |
1015 | This should rarely be necessary, as the Perl close() function is to be |
1016 | used for things that Perl opened itself, even if it was a dup of a |
a6dd486b |
1017 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 |
1018 | to, you may be able to do this: |
1019 | |
500071f4 |
1020 | require 'sys/syscall.ph'; |
1021 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric |
1022 | die "can't sysclose $fd: $!" unless $rc == -1; |
68dc0745 |
1023 | |
a6dd486b |
1024 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 |
1025 | |
500071f4 |
1026 | { |
197aec24 |
1027 | local *F; |
d92eb7b0 |
1028 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
1029 | close F; |
500071f4 |
1030 | } |
d92eb7b0 |
1031 | |
883f1635 |
1032 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
d74e8afc |
1033 | X<filename, DOS issues> |
68dc0745 |
1034 | |
1035 | Whoops! You just put a tab and a formfeed into that filename! |
1036 | Remember that within double quoted strings ("like\this"), the |
1037 | backslash is an escape character. The full list of these is in |
1038 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't |
1039 | have a file called "c:(tab)emp(formfeed)oo" or |
65acb1b1 |
1040 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 |
1041 | |
1042 | Either single-quote your strings, or (preferably) use forward slashes. |
46fc3d4c |
1043 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 |
1044 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b |
1045 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 |
1046 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
1047 | are more portable, too. |
68dc0745 |
1048 | |
1049 | =head2 Why doesn't glob("*.*") get all the files? |
d74e8afc |
1050 | X<glob> |
68dc0745 |
1051 | |
1052 | Because even on non-Unix ports, Perl's glob function follows standard |
46fc3d4c |
1053 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 |
1054 | files. This makes glob() portable even to legacy systems. Your |
1055 | port may include proprietary globbing functions as well. Check its |
1056 | documentation for details. |
68dc0745 |
1057 | |
1058 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? |
1059 | |
06a5f41f |
1060 | This is elaborately and painstakingly described in the |
1061 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To |
49d635f9 |
1062 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . |
68dc0745 |
1063 | |
1064 | The executive summary: learn how your filesystem works. The |
1065 | permissions on a file say what can happen to the data in that file. |
1066 | The permissions on a directory say what can happen to the list of |
1067 | files in that directory. If you delete a file, you're removing its |
1068 | name from the directory (so the operation depends on the permissions |
1069 | of the directory, not of the file). If you try to write to the file, |
1070 | the permissions of the file govern whether you're allowed to. |
1071 | |
1072 | =head2 How do I select a random line from a file? |
d74e8afc |
1073 | X<file, selecting a random line> |
68dc0745 |
1074 | |
1075 | Here's an algorithm from the Camel Book: |
1076 | |
500071f4 |
1077 | srand; |
1078 | rand($.) < 1 && ($line = $_) while <>; |
68dc0745 |
1079 | |
49d635f9 |
1080 | This has a significant advantage in space over reading the whole file |
1081 | in. You can find a proof of this method in I<The Art of Computer |
1082 | Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. |
1083 | |
1084 | You can use the File::Random module which provides a function |
1085 | for that algorithm: |
1086 | |
1087 | use File::Random qw/random_line/; |
1088 | my $line = random_line($filename); |
1089 | |
1090 | Another way is to use the Tie::File module, which treats the entire |
1091 | file as an array. Simply access a random array element. |
68dc0745 |
1092 | |
65acb1b1 |
1093 | =head2 Why do I get weird spaces when I print an array of lines? |
1094 | |
1095 | Saying |
1096 | |
500071f4 |
1097 | print "@lines\n"; |
65acb1b1 |
1098 | |
1099 | joins together the elements of C<@lines> with a space between them. |
1100 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above |
a6dd486b |
1101 | statement would print |
65acb1b1 |
1102 | |
500071f4 |
1103 | little fluffy clouds |
65acb1b1 |
1104 | |
1105 | but if each element of C<@lines> was a line of text, ending a newline |
1106 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: |
1107 | |
500071f4 |
1108 | little |
1109 | fluffy |
1110 | clouds |
65acb1b1 |
1111 | |
1112 | If your array contains lines, just print them: |
1113 | |
500071f4 |
1114 | print @lines; |
1115 | |
1116 | =head1 REVISION |
1117 | |
ac9dac7f |
1118 | Revision: $Revision: 6019 $ |
500071f4 |
1119 | |
ac9dac7f |
1120 | Date: $Date: 2006-05-04 19:04:31 +0200 (jeu, 04 mai 2006) $ |
500071f4 |
1121 | |
1122 | See L<perlfaq> for source control details and availability. |
65acb1b1 |
1123 | |
68dc0745 |
1124 | =head1 AUTHOR AND COPYRIGHT |
1125 | |
58103a2e |
1126 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced |
1127 | other authors as noted. All rights reserved. |
5a964f20 |
1128 | |
5a7beb56 |
1129 | This documentation is free; you can redistribute it and/or modify it |
1130 | under the same terms as Perl itself. |
c8db1d39 |
1131 | |
87275199 |
1132 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
1133 | domain. You are permitted and encouraged to use this code and any |
1134 | derivatives thereof in your own programs for fun or for profit as you |
1135 | see fit. A simple comment in the code giving credit to the FAQ would |
1136 | be courteous but is not required. |