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1 | =head1 NAME |
2 | |
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3 | perlfaq5 - Files and Formats ($Revision: 3606 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section deals with I/O and the "f" issues: filehandles, flushing, |
8 | formats, and footers. |
9 | |
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10 | =head2 How do I flush/unbuffer an output filehandle? Why must I do this? |
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11 | X<flush> X<buffer> X<unbuffer> X<autoflush> |
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12 | |
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13 | Perl does not support truly unbuffered output (except |
14 | insofar as you can C<syswrite(OUT, $char, 1)>), although it |
15 | does support is "command buffering", in which a physical |
16 | write is performed after every output command. |
17 | |
18 | The C standard I/O library (stdio) normally buffers |
19 | characters sent to devices so that there isn't a system call |
20 | for each byte. In most stdio implementations, the type of |
21 | output buffering and the size of the buffer varies according |
22 | to the type of device. Perl's print() and write() functions |
23 | normally buffer output, while syswrite() bypasses buffering |
24 | all together. |
25 | |
26 | If you want your output to be sent immediately when you |
27 | execute print() or write() (for instance, for some network |
28 | protocols), you must set the handle's autoflush flag. This |
29 | flag is the Perl variable $| and when it is set to a true |
30 | value, Perl will flush the handle's buffer after each |
31 | print() or write(). Setting $| affects buffering only for |
32 | the currently selected default file handle. You choose this |
33 | handle with the one argument select() call (see |
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34 | L<perlvar/$E<verbar>> and L<perlfunc/select>). |
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35 | |
36 | Use select() to choose the desired handle, then set its |
37 | per-filehandle variables. |
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38 | |
500071f4 |
39 | $old_fh = select(OUTPUT_HANDLE); |
40 | $| = 1; |
41 | select($old_fh); |
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42 | |
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43 | Some idioms can handle this in a single statement: |
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44 | |
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45 | select((select(OUTPUT_HANDLE), $| = 1)[0]); |
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46 | |
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47 | $| = 1, select $_ for select OUTPUT_HANDLE; |
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48 | |
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49 | Some modules offer object-oriented access to handles and their |
50 | variables, although they may be overkill if this is the only |
51 | thing you do with them. You can use IO::Handle: |
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52 | |
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53 | use IO::Handle; |
54 | open(DEV, ">/dev/printer"); # but is this? |
55 | DEV->autoflush(1); |
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56 | |
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57 | or IO::Socket: |
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58 | |
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59 | use IO::Socket; # this one is kinda a pipe? |
4358a253 |
60 | my $sock = IO::Socket::INET->new( 'www.example.com:80' ); |
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61 | |
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62 | $sock->autoflush(); |
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63 | |
64 | =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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65 | X<file, editing> |
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66 | |
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67 | Use the Tie::File module, which is included in the standard |
68 | distribution since Perl 5.8.0. |
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69 | |
70 | =head2 How do I count the number of lines in a file? |
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71 | X<file, counting lines> X<lines> X<line> |
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72 | |
73 | One fairly efficient way is to count newlines in the file. The |
74 | following program uses a feature of tr///, as documented in L<perlop>. |
75 | If your text file doesn't end with a newline, then it's not really a |
76 | proper text file, so this may report one fewer line than you expect. |
77 | |
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78 | $lines = 0; |
79 | open(FILE, $filename) or die "Can't open `$filename': $!"; |
80 | while (sysread FILE, $buffer, 4096) { |
81 | $lines += ($buffer =~ tr/\n//); |
82 | } |
83 | close FILE; |
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84 | |
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85 | This assumes no funny games with newline translations. |
86 | |
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87 | =head2 How can I use Perl's C<-i> option from within a program? |
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88 | X<-i> X<in-place> |
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89 | |
90 | C<-i> sets the value of Perl's C<$^I> variable, which in turn affects |
91 | the behavior of C<< <> >>; see L<perlrun> for more details. By |
92 | modifying the appropriate variables directly, you can get the same |
93 | behavior within a larger program. For example: |
94 | |
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95 | # ... |
96 | { |
97 | local($^I, @ARGV) = ('.orig', glob("*.c")); |
98 | while (<>) { |
99 | if ($. == 1) { |
100 | print "This line should appear at the top of each file\n"; |
101 | } |
102 | s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case |
103 | print; |
104 | close ARGV if eof; # Reset $. |
105 | } |
106 | } |
107 | # $^I and @ARGV return to their old values here |
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108 | |
109 | This block modifies all the C<.c> files in the current directory, |
110 | leaving a backup of the original data from each file in a new |
111 | C<.c.orig> file. |
112 | |
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113 | =head2 How can I copy a file? |
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114 | X<copy> X<file, copy> |
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115 | |
116 | (contributed by brian d foy) |
117 | |
118 | Use the File::Copy module. It comes with Perl and can do a |
119 | true copy across file systems, and it does its magic in |
120 | a portable fashion. |
121 | |
122 | use File::Copy; |
123 | |
124 | copy( $original, $new_copy ) or die "Copy failed: $!"; |
125 | |
126 | If you can't use File::Copy, you'll have to do the work yourself: |
127 | open the original file, open the destination file, then print |
128 | to the destination file as you read the original. |
129 | |
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130 | =head2 How do I make a temporary file name? |
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131 | X<file, temporary> |
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132 | |
7678cced |
133 | If you don't need to know the name of the file, you can use C<open()> |
134 | with C<undef> in place of the file name. The C<open()> function |
135 | creates an anonymous temporary file. |
136 | |
137 | open my $tmp, '+>', undef or die $!; |
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138 | |
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139 | Otherwise, you can use the File::Temp module. |
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140 | |
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141 | use File::Temp qw/ tempfile tempdir /; |
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142 | |
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143 | $dir = tempdir( CLEANUP => 1 ); |
144 | ($fh, $filename) = tempfile( DIR => $dir ); |
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145 | |
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146 | # or if you don't need to know the filename |
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147 | |
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148 | $fh = tempfile( DIR => $dir ); |
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149 | |
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150 | The File::Temp has been a standard module since Perl 5.6.1. If you |
151 | don't have a modern enough Perl installed, use the C<new_tmpfile> |
152 | class method from the IO::File module to get a filehandle opened for |
153 | reading and writing. Use it if you don't need to know the file's name: |
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154 | |
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155 | use IO::File; |
156 | $fh = IO::File->new_tmpfile() |
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157 | or die "Unable to make new temporary file: $!"; |
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158 | |
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159 | If you're committed to creating a temporary file by hand, use the |
160 | process ID and/or the current time-value. If you need to have many |
161 | temporary files in one process, use a counter: |
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162 | |
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163 | BEGIN { |
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164 | use Fcntl; |
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165 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; |
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166 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); |
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167 | |
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168 | sub temp_file { |
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169 | local *FH; |
170 | my $count = 0; |
171 | until (defined(fileno(FH)) || $count++ > 100) { |
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172 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
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173 | # O_EXCL is required for security reasons. |
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174 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
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175 | } |
176 | |
177 | if (defined(fileno(FH)) |
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178 | return (*FH, $base_name); |
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179 | } |
180 | else { |
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181 | return (); |
182 | } |
183 | } |
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184 | } |
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185 | |
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186 | =head2 How can I manipulate fixed-record-length files? |
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187 | X<fixed-length> X<file, fixed-length records> |
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188 | |
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189 | The most efficient way is using L<pack()|perlfunc/"pack"> and |
190 | L<unpack()|perlfunc/"unpack">. This is faster than using |
191 | L<substr()|perlfunc/"substr"> when taking many, many strings. It is |
192 | slower for just a few. |
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193 | |
194 | Here is a sample chunk of code to break up and put back together again |
195 | some fixed-format input lines, in this case from the output of a normal, |
196 | Berkeley-style ps: |
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197 | |
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198 | # sample input line: |
199 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what |
200 | my $PS_T = 'A6 A4 A7 A5 A*'; |
201 | open my $ps, '-|', 'ps'; |
202 | print scalar <$ps>; |
203 | my @fields = qw( pid tt stat time command ); |
204 | while (<$ps>) { |
205 | my %process; |
206 | @process{@fields} = unpack($PS_T, $_); |
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207 | for my $field ( @fields ) { |
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208 | print "$field: <$process{$field}>\n"; |
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209 | } |
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210 | print 'line=', pack($PS_T, @process{@fields} ), "\n"; |
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211 | } |
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212 | |
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213 | We've used a hash slice in order to easily handle the fields of each row. |
214 | Storing the keys in an array means it's easy to operate on them as a |
215 | group or loop over them with for. It also avoids polluting the program |
216 | with global variables and using symbolic references. |
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217 | |
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218 | =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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219 | X<filehandle, local> X<filehandle, passing> X<filehandle, reference> |
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220 | |
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221 | As of perl5.6, open() autovivifies file and directory handles |
222 | as references if you pass it an uninitialized scalar variable. |
223 | You can then pass these references just like any other scalar, |
224 | and use them in the place of named handles. |
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225 | |
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226 | open my $fh, $file_name; |
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227 | |
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228 | open local $fh, $file_name; |
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229 | |
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230 | print $fh "Hello World!\n"; |
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231 | |
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232 | process_file( $fh ); |
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233 | |
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234 | If you like, you can store these filehandles in an array or a hash. |
235 | If you access them directly, they aren't simple scalars and you |
236 | need to give C<print> a little help by placing the filehandle |
237 | reference in braces. Perl can only figure it out on its own when |
238 | the filehandle reference is a simple scalar. |
239 | |
240 | my @fhs = ( $fh1, $fh2, $fh3 ); |
241 | |
242 | for( $i = 0; $i <= $#fhs; $i++ ) { |
243 | print {$fhs[$i]} "just another Perl answer, \n"; |
244 | } |
245 | |
246 | |
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247 | Before perl5.6, you had to deal with various typeglob idioms |
248 | which you may see in older code. |
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249 | |
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250 | open FILE, "> $filename"; |
251 | process_typeglob( *FILE ); |
252 | process_reference( \*FILE ); |
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253 | |
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254 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
255 | sub process_reference { local $fh = shift; print $fh "Reference!" } |
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256 | |
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257 | If you want to create many anonymous handles, you should |
258 | check out the Symbol or IO::Handle modules. |
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259 | |
260 | =head2 How can I use a filehandle indirectly? |
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261 | X<filehandle, indirect> |
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262 | |
263 | An indirect filehandle is using something other than a symbol |
264 | in a place that a filehandle is expected. Here are ways |
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265 | to get indirect filehandles: |
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266 | |
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267 | $fh = SOME_FH; # bareword is strict-subs hostile |
268 | $fh = "SOME_FH"; # strict-refs hostile; same package only |
269 | $fh = *SOME_FH; # typeglob |
270 | $fh = \*SOME_FH; # ref to typeglob (bless-able) |
271 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob |
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272 | |
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273 | Or, you can use the C<new> method from one of the IO::* modules to |
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274 | create an anonymous filehandle, store that in a scalar variable, |
275 | and use it as though it were a normal filehandle. |
276 | |
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277 | use IO::Handle; # 5.004 or higher |
278 | $fh = IO::Handle->new(); |
5a964f20 |
279 | |
280 | Then use any of those as you would a normal filehandle. Anywhere that |
281 | Perl is expecting a filehandle, an indirect filehandle may be used |
282 | instead. An indirect filehandle is just a scalar variable that contains |
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283 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
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284 | the C<< <FH> >> diamond operator will accept either a named filehandle |
5a964f20 |
285 | or a scalar variable containing one: |
286 | |
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287 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); |
288 | print $ofh "Type it: "; |
289 | $got = <$ifh> |
290 | print $efh "What was that: $got"; |
5a964f20 |
291 | |
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292 | If you're passing a filehandle to a function, you can write |
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293 | the function in two ways: |
294 | |
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295 | sub accept_fh { |
296 | my $fh = shift; |
297 | print $fh "Sending to indirect filehandle\n"; |
298 | } |
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299 | |
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300 | Or it can localize a typeglob and use the filehandle directly: |
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301 | |
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302 | sub accept_fh { |
303 | local *FH = shift; |
304 | print FH "Sending to localized filehandle\n"; |
305 | } |
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306 | |
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307 | Both styles work with either objects or typeglobs of real filehandles. |
308 | (They might also work with strings under some circumstances, but this |
309 | is risky.) |
310 | |
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311 | accept_fh(*STDOUT); |
312 | accept_fh($handle); |
5a964f20 |
313 | |
314 | In the examples above, we assigned the filehandle to a scalar variable |
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315 | before using it. That is because only simple scalar variables, not |
316 | expressions or subscripts of hashes or arrays, can be used with |
317 | built-ins like C<print>, C<printf>, or the diamond operator. Using |
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318 | something other than a simple scalar variable as a filehandle is |
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319 | illegal and won't even compile: |
320 | |
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321 | @fd = (*STDIN, *STDOUT, *STDERR); |
322 | print $fd[1] "Type it: "; # WRONG |
323 | $got = <$fd[0]> # WRONG |
324 | print $fd[2] "What was that: $got"; # WRONG |
5a964f20 |
325 | |
326 | With C<print> and C<printf>, you get around this by using a block and |
327 | an expression where you would place the filehandle: |
328 | |
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329 | print { $fd[1] } "funny stuff\n"; |
330 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; |
331 | # Pity the poor deadbeef. |
5a964f20 |
332 | |
333 | That block is a proper block like any other, so you can put more |
334 | complicated code there. This sends the message out to one of two places: |
335 | |
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336 | $ok = -x "/bin/cat"; |
337 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
338 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
5a964f20 |
339 | |
340 | This approach of treating C<print> and C<printf> like object methods |
341 | calls doesn't work for the diamond operator. That's because it's a |
342 | real operator, not just a function with a comma-less argument. Assuming |
343 | you've been storing typeglobs in your structure as we did above, you |
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344 | can use the built-in function named C<readline> to read a record just |
c47ff5f1 |
345 | as C<< <> >> does. Given the initialization shown above for @fd, this |
c90536be |
346 | would work, but only because readline() requires a typeglob. It doesn't |
5a964f20 |
347 | work with objects or strings, which might be a bug we haven't fixed yet. |
348 | |
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349 | $got = readline($fd[0]); |
5a964f20 |
350 | |
351 | Let it be noted that the flakiness of indirect filehandles is not |
352 | related to whether they're strings, typeglobs, objects, or anything else. |
353 | It's the syntax of the fundamental operators. Playing the object |
354 | game doesn't help you at all here. |
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355 | |
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356 | =head2 How can I set up a footer format to be used with write()? |
d74e8afc |
357 | X<footer> |
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358 | |
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359 | There's no builtin way to do this, but L<perlform> has a couple of |
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360 | techniques to make it possible for the intrepid hacker. |
361 | |
362 | =head2 How can I write() into a string? |
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363 | X<write, into a string> |
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364 | |
65acb1b1 |
365 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
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366 | |
367 | =head2 How can I output my numbers with commas added? |
d74e8afc |
368 | X<number, commify> |
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369 | |
b68463f7 |
370 | (contributed by brian d foy and Benjamin Goldberg) |
371 | |
372 | You can use L<Number::Format> to separate places in a number. |
373 | It handles locale information for those of you who want to insert |
374 | full stops instead (or anything else that they want to use, |
375 | really). |
376 | |
49d635f9 |
377 | This subroutine will add commas to your number: |
378 | |
379 | sub commify { |
500071f4 |
380 | local $_ = shift; |
381 | 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; |
382 | return $_; |
383 | } |
49d635f9 |
384 | |
385 | This regex from Benjamin Goldberg will add commas to numbers: |
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386 | |
500071f4 |
387 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
68dc0745 |
388 | |
49d635f9 |
389 | It is easier to see with comments: |
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390 | |
500071f4 |
391 | s/( |
392 | ^[-+]? # beginning of number. |
393 | \d+? # first digits before first comma |
394 | (?= # followed by, (but not included in the match) : |
395 | (?>(?:\d{3})+) # some positive multiple of three digits. |
396 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. |
397 | ) |
398 | | # or: |
399 | \G\d{3} # after the last group, get three digits |
400 | (?=\d) # but they have to have more digits after them. |
401 | )/$1,/xg; |
46fc3d4c |
402 | |
68dc0745 |
403 | =head2 How can I translate tildes (~) in a filename? |
d74e8afc |
404 | X<tilde> X<tilde expansion> |
68dc0745 |
405 | |
575cc754 |
406 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
407 | versions of Perl require that you have a shell installed that groks |
408 | tildes. Recent perl versions have this feature built in. The |
d6260402 |
409 | File::KGlob module (available from CPAN) gives more portable glob |
575cc754 |
410 | functionality. |
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411 | |
412 | Within Perl, you may use this directly: |
413 | |
414 | $filename =~ s{ |
415 | ^ ~ # find a leading tilde |
416 | ( # save this in $1 |
417 | [^/] # a non-slash character |
418 | * # repeated 0 or more times (0 means me) |
419 | ) |
420 | }{ |
421 | $1 |
422 | ? (getpwnam($1))[7] |
423 | : ( $ENV{HOME} || $ENV{LOGDIR} ) |
424 | }ex; |
425 | |
5a964f20 |
426 | =head2 How come when I open a file read-write it wipes it out? |
d74e8afc |
427 | X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating> |
68dc0745 |
428 | |
429 | Because you're using something like this, which truncates the file and |
430 | I<then> gives you read-write access: |
431 | |
500071f4 |
432 | open(FH, "+> /path/name"); # WRONG (almost always) |
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433 | |
434 | Whoops. You should instead use this, which will fail if the file |
197aec24 |
435 | doesn't exist. |
d92eb7b0 |
436 | |
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437 | open(FH, "+< /path/name"); # open for update |
d92eb7b0 |
438 | |
c47ff5f1 |
439 | Using ">" always clobbers or creates. Using "<" never does |
d92eb7b0 |
440 | either. The "+" doesn't change this. |
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441 | |
5a964f20 |
442 | Here are examples of many kinds of file opens. Those using sysopen() |
443 | all assume |
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444 | |
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445 | use Fcntl; |
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446 | |
5a964f20 |
447 | To open file for reading: |
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448 | |
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449 | open(FH, "< $path") || die $!; |
450 | sysopen(FH, $path, O_RDONLY) || die $!; |
5a964f20 |
451 | |
452 | To open file for writing, create new file if needed or else truncate old file: |
453 | |
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454 | open(FH, "> $path") || die $!; |
455 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; |
456 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; |
5a964f20 |
457 | |
458 | To open file for writing, create new file, file must not exist: |
459 | |
500071f4 |
460 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; |
461 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
462 | |
463 | To open file for appending, create if necessary: |
464 | |
500071f4 |
465 | open(FH, ">> $path") || die $!; |
466 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; |
467 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; |
5a964f20 |
468 | |
469 | To open file for appending, file must exist: |
470 | |
500071f4 |
471 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; |
5a964f20 |
472 | |
473 | To open file for update, file must exist: |
474 | |
500071f4 |
475 | open(FH, "+< $path") || die $!; |
476 | sysopen(FH, $path, O_RDWR) || die $!; |
5a964f20 |
477 | |
478 | To open file for update, create file if necessary: |
479 | |
500071f4 |
480 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; |
481 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; |
5a964f20 |
482 | |
483 | To open file for update, file must not exist: |
484 | |
500071f4 |
485 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; |
486 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; |
5a964f20 |
487 | |
488 | To open a file without blocking, creating if necessary: |
489 | |
500071f4 |
490 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
2359510d |
491 | or die "can't open /foo/somefile: $!": |
5a964f20 |
492 | |
493 | Be warned that neither creation nor deletion of files is guaranteed to |
494 | be an atomic operation over NFS. That is, two processes might both |
a6dd486b |
495 | successfully create or unlink the same file! Therefore O_EXCL |
496 | isn't as exclusive as you might wish. |
68dc0745 |
497 | |
87275199 |
498 | See also the new L<perlopentut> if you have it (new for 5.6). |
65acb1b1 |
499 | |
04d666b1 |
500 | =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? |
d74e8afc |
501 | X<argument list too long> |
68dc0745 |
502 | |
c47ff5f1 |
503 | The C<< <> >> operator performs a globbing operation (see above). |
3a4b19e4 |
504 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
505 | csh(1) to do the actual glob expansion, but |
68dc0745 |
506 | csh can't handle more than 127 items and so gives the error message |
507 | C<Argument list too long>. People who installed tcsh as csh won't |
508 | have this problem, but their users may be surprised by it. |
509 | |
3a4b19e4 |
510 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
d6260402 |
511 | yourself with readdir() and patterns, or use a module like File::KGlob, |
3a4b19e4 |
512 | one that doesn't use the shell to do globbing. |
68dc0745 |
513 | |
514 | =head2 Is there a leak/bug in glob()? |
d74e8afc |
515 | X<glob> |
68dc0745 |
516 | |
517 | Due to the current implementation on some operating systems, when you |
518 | use the glob() function or its angle-bracket alias in a scalar |
a6dd486b |
519 | context, you may cause a memory leak and/or unpredictable behavior. It's |
68dc0745 |
520 | best therefore to use glob() only in list context. |
521 | |
c47ff5f1 |
522 | =head2 How can I open a file with a leading ">" or trailing blanks? |
d74e8afc |
523 | X<filename, special characters> |
68dc0745 |
524 | |
b68463f7 |
525 | (contributed by Brian McCauley) |
68dc0745 |
526 | |
b68463f7 |
527 | The special two argument form of Perl's open() function ignores |
528 | trailing blanks in filenames and infers the mode from certain leading |
529 | characters (or a trailing "|"). In older versions of Perl this was the |
530 | only version of open() and so it is prevalent in old code and books. |
65acb1b1 |
531 | |
b68463f7 |
532 | Unless you have a particular reason to use the two argument form you |
533 | should use the three argument form of open() which does not treat any |
534 | charcters in the filename as special. |
58103a2e |
535 | |
881bdbd4 |
536 | open FILE, "<", " file "; # filename is " file " |
537 | open FILE, ">", ">file"; # filename is ">file" |
65acb1b1 |
538 | |
68dc0745 |
539 | =head2 How can I reliably rename a file? |
d74e8afc |
540 | X<rename> X<mv> X<move> X<file, rename> X<ren> |
68dc0745 |
541 | |
49d635f9 |
542 | If your operating system supports a proper mv(1) utility or its |
543 | functional equivalent, this works: |
68dc0745 |
544 | |
500071f4 |
545 | rename($old, $new) or system("mv", $old, $new); |
68dc0745 |
546 | |
d2321c93 |
547 | It may be more portable to use the File::Copy module instead. |
548 | You just copy to the new file to the new name (checking return |
549 | values), then delete the old one. This isn't really the same |
550 | semantically as a rename(), which preserves meta-information like |
68dc0745 |
551 | permissions, timestamps, inode info, etc. |
552 | |
d2321c93 |
553 | Newer versions of File::Copy export a move() function. |
5a964f20 |
554 | |
68dc0745 |
555 | =head2 How can I lock a file? |
d74e8afc |
556 | X<lock> X<file, lock> X<flock> |
68dc0745 |
557 | |
54310121 |
558 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 |
559 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
560 | later), and lockf(3) if neither of the two previous system calls exists. |
561 | On some systems, it may even use a different form of native locking. |
562 | Here are some gotchas with Perl's flock(): |
563 | |
564 | =over 4 |
565 | |
566 | =item 1 |
567 | |
568 | Produces a fatal error if none of the three system calls (or their |
569 | close equivalent) exists. |
570 | |
571 | =item 2 |
572 | |
573 | lockf(3) does not provide shared locking, and requires that the |
574 | filehandle be open for writing (or appending, or read/writing). |
575 | |
576 | =item 3 |
577 | |
d92eb7b0 |
578 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
579 | systems), so you'd need to force the use of fcntl(2) when you build Perl. |
a6dd486b |
580 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 |
581 | and the F<INSTALL> file in the source distribution for information on |
582 | building Perl to do this. |
583 | |
584 | Two potentially non-obvious but traditional flock semantics are that |
a6dd486b |
585 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 |
586 | I<merely advisory>. Such discretionary locks are more flexible, but |
587 | offer fewer guarantees. This means that files locked with flock() may |
588 | be modified by programs that do not also use flock(). Cars that stop |
589 | for red lights get on well with each other, but not with cars that don't |
590 | stop for red lights. See the perlport manpage, your port's specific |
591 | documentation, or your system-specific local manpages for details. It's |
592 | best to assume traditional behavior if you're writing portable programs. |
a6dd486b |
593 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 |
594 | for your own system's idiosyncrasies (sometimes called "features"). |
595 | Slavish adherence to portability concerns shouldn't get in the way of |
596 | your getting your job done.) |
68dc0745 |
597 | |
197aec24 |
598 | For more information on file locking, see also |
13a2d996 |
599 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 |
600 | |
68dc0745 |
601 | =back |
602 | |
04d666b1 |
603 | =head2 Why can't I just open(FH, "E<gt>file.lock")? |
d74e8afc |
604 | X<lock, lockfile race condition> |
68dc0745 |
605 | |
606 | A common bit of code B<NOT TO USE> is this: |
607 | |
500071f4 |
608 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE |
609 | open(LCK, "> file.lock"); # THIS BROKEN CODE |
68dc0745 |
610 | |
611 | This is a classic race condition: you take two steps to do something |
612 | which must be done in one. That's why computer hardware provides an |
613 | atomic test-and-set instruction. In theory, this "ought" to work: |
614 | |
500071f4 |
615 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
9b55d3ab |
616 | or die "can't open file.lock: $!"; |
68dc0745 |
617 | |
618 | except that lamentably, file creation (and deletion) is not atomic |
619 | over NFS, so this won't work (at least, not every time) over the net. |
65acb1b1 |
620 | Various schemes involving link() have been suggested, but |
46fc3d4c |
621 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 |
622 | |
fc36a67e |
623 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
d74e8afc |
624 | X<counter> X<file, counter> |
68dc0745 |
625 | |
46fc3d4c |
626 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 |
627 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b |
628 | only to stroke the writer's vanity. It's better to pick a random number; |
629 | they're more realistic. |
68dc0745 |
630 | |
5a964f20 |
631 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 |
632 | |
500071f4 |
633 | use Fcntl qw(:DEFAULT :flock); |
634 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
635 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
636 | $num = <FH> || 0; |
637 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; |
638 | truncate(FH, 0) or die "can't truncate numfile: $!"; |
639 | (print FH $num+1, "\n") or die "can't write numfile: $!"; |
640 | close FH or die "can't close numfile: $!"; |
68dc0745 |
641 | |
46fc3d4c |
642 | Here's a much better web-page hit counter: |
68dc0745 |
643 | |
500071f4 |
644 | $hits = int( (time() - 850_000_000) / rand(1_000) ); |
68dc0745 |
645 | |
646 | If the count doesn't impress your friends, then the code might. :-) |
647 | |
f52f3be2 |
648 | =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 |
649 | X<append> X<file, append> |
05caf3a7 |
650 | |
651 | If you are on a system that correctly implements flock() and you use the |
652 | example appending code from "perldoc -f flock" everything will be OK |
653 | even if the OS you are on doesn't implement append mode correctly (if |
654 | such a system exists.) So if you are happy to restrict yourself to OSs |
655 | that implement flock() (and that's not really much of a restriction) |
656 | then that is what you should do. |
657 | |
658 | If you know you are only going to use a system that does correctly |
659 | implement appending (i.e. not Win32) then you can omit the seek() from |
660 | the above code. |
661 | |
662 | If you know you are only writing code to run on an OS and filesystem that |
663 | does implement append mode correctly (a local filesystem on a modern |
664 | Unix for example), and you keep the file in block-buffered mode and you |
665 | write less than one buffer-full of output between each manual flushing |
8305e449 |
666 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 |
667 | the end of the file in one chunk without getting intermingled with |
668 | anyone else's output. You can also use the syswrite() function which is |
669 | simply a wrapper around your systems write(2) system call. |
670 | |
671 | There is still a small theoretical chance that a signal will interrupt |
672 | the system level write() operation before completion. There is also a |
673 | possibility that some STDIO implementations may call multiple system |
674 | level write()s even if the buffer was empty to start. There may be some |
675 | systems where this probability is reduced to zero. |
676 | |
68dc0745 |
677 | =head2 How do I randomly update a binary file? |
d74e8afc |
678 | X<file, binary patch> |
68dc0745 |
679 | |
680 | If you're just trying to patch a binary, in many cases something as |
681 | simple as this works: |
682 | |
500071f4 |
683 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs |
68dc0745 |
684 | |
685 | However, if you have fixed sized records, then you might do something more |
686 | like this: |
687 | |
500071f4 |
688 | $RECSIZE = 220; # size of record, in bytes |
689 | $recno = 37; # which record to update |
690 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; |
691 | seek(FH, $recno * $RECSIZE, 0); |
692 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; |
693 | # munge the record |
694 | seek(FH, -$RECSIZE, 1); |
695 | print FH $record; |
696 | close FH; |
68dc0745 |
697 | |
698 | Locking and error checking are left as an exercise for the reader. |
a6dd486b |
699 | Don't forget them or you'll be quite sorry. |
68dc0745 |
700 | |
68dc0745 |
701 | =head2 How do I get a file's timestamp in perl? |
d74e8afc |
702 | X<timestamp> X<file, timestamp> |
68dc0745 |
703 | |
881bdbd4 |
704 | If you want to retrieve the time at which the file was last |
705 | read, written, or had its meta-data (owner, etc) changed, |
a05e4845 |
706 | you use the B<-A>, B<-M>, or B<-C> file test operations as |
881bdbd4 |
707 | documented in L<perlfunc>. These retrieve the age of the |
708 | file (measured against the start-time of your program) in |
709 | days as a floating point number. Some platforms may not have |
710 | all of these times. See L<perlport> for details. To |
711 | retrieve the "raw" time in seconds since the epoch, you |
712 | would call the stat function, then use localtime(), |
713 | gmtime(), or POSIX::strftime() to convert this into |
714 | human-readable form. |
68dc0745 |
715 | |
716 | Here's an example: |
717 | |
500071f4 |
718 | $write_secs = (stat($file))[9]; |
719 | printf "file %s updated at %s\n", $file, |
c8db1d39 |
720 | scalar localtime($write_secs); |
68dc0745 |
721 | |
722 | If you prefer something more legible, use the File::stat module |
723 | (part of the standard distribution in version 5.004 and later): |
724 | |
500071f4 |
725 | # error checking left as an exercise for reader. |
726 | use File::stat; |
727 | use Time::localtime; |
728 | $date_string = ctime(stat($file)->mtime); |
729 | print "file $file updated at $date_string\n"; |
68dc0745 |
730 | |
65acb1b1 |
731 | The POSIX::strftime() approach has the benefit of being, |
732 | in theory, independent of the current locale. See L<perllocale> |
733 | for details. |
68dc0745 |
734 | |
735 | =head2 How do I set a file's timestamp in perl? |
d74e8afc |
736 | X<timestamp> X<file, timestamp> |
68dc0745 |
737 | |
738 | You use the utime() function documented in L<perlfunc/utime>. |
739 | By way of example, here's a little program that copies the |
740 | read and write times from its first argument to all the rest |
741 | of them. |
742 | |
500071f4 |
743 | if (@ARGV < 2) { |
744 | die "usage: cptimes timestamp_file other_files ...\n"; |
745 | } |
746 | $timestamp = shift; |
747 | ($atime, $mtime) = (stat($timestamp))[8,9]; |
748 | utime $atime, $mtime, @ARGV; |
68dc0745 |
749 | |
65acb1b1 |
750 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 |
751 | |
19a1cd16 |
752 | The perldoc for utime also has an example that has the same |
753 | effect as touch(1) on files that I<already exist>. |
754 | |
755 | Certain file systems have a limited ability to store the times |
756 | on a file at the expected level of precision. For example, the |
757 | FAT and HPFS filesystem are unable to create dates on files with |
758 | a finer granularity than two seconds. This is a limitation of |
759 | the filesystems, not of utime(). |
68dc0745 |
760 | |
761 | =head2 How do I print to more than one file at once? |
d74e8afc |
762 | X<print, to multiple files> |
68dc0745 |
763 | |
49d635f9 |
764 | To connect one filehandle to several output filehandles, |
765 | you can use the IO::Tee or Tie::FileHandle::Multiplex modules. |
68dc0745 |
766 | |
49d635f9 |
767 | If you only have to do this once, you can print individually |
768 | to each filehandle. |
68dc0745 |
769 | |
500071f4 |
770 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
5a964f20 |
771 | |
49d635f9 |
772 | =head2 How can I read in an entire file all at once? |
d74e8afc |
773 | X<slurp> X<file, slurping> |
68dc0745 |
774 | |
49d635f9 |
775 | You can use the File::Slurp module to do it in one step. |
68dc0745 |
776 | |
49d635f9 |
777 | use File::Slurp; |
197aec24 |
778 | |
49d635f9 |
779 | $all_of_it = read_file($filename); # entire file in scalar |
500071f4 |
780 | @all_lines = read_file($filename); # one line perl element |
d92eb7b0 |
781 | |
782 | The customary Perl approach for processing all the lines in a file is to |
783 | do so one line at a time: |
784 | |
500071f4 |
785 | open (INPUT, $file) || die "can't open $file: $!"; |
786 | while (<INPUT>) { |
787 | chomp; |
788 | # do something with $_ |
789 | } |
790 | close(INPUT) || die "can't close $file: $!"; |
d92eb7b0 |
791 | |
792 | This is tremendously more efficient than reading the entire file into |
793 | memory as an array of lines and then processing it one element at a time, |
a6dd486b |
794 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 |
795 | you see someone do this: |
796 | |
500071f4 |
797 | @lines = <INPUT>; |
d92eb7b0 |
798 | |
30852c57 |
799 | you should think long and hard about why you need everything loaded at |
800 | once. It's just not a scalable solution. You might also find it more |
801 | fun to use the standard Tie::File module, or the DB_File module's |
802 | $DB_RECNO bindings, which allow you to tie an array to a file so that |
803 | accessing an element the array actually accesses the corresponding |
804 | line in the file. |
d92eb7b0 |
805 | |
f05bbc40 |
806 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 |
807 | |
500071f4 |
808 | { |
d92eb7b0 |
809 | local(*INPUT, $/); |
810 | open (INPUT, $file) || die "can't open $file: $!"; |
811 | $var = <INPUT>; |
500071f4 |
812 | } |
d92eb7b0 |
813 | |
197aec24 |
814 | That temporarily undefs your record separator, and will automatically |
d92eb7b0 |
815 | close the file at block exit. If the file is already open, just use this: |
816 | |
500071f4 |
817 | $var = do { local $/; <INPUT> }; |
d92eb7b0 |
818 | |
f05bbc40 |
819 | For ordinary files you can also use the read function. |
820 | |
821 | read( INPUT, $var, -s INPUT ); |
822 | |
823 | The third argument tests the byte size of the data on the INPUT filehandle |
824 | and reads that many bytes into the buffer $var. |
825 | |
68dc0745 |
826 | =head2 How can I read in a file by paragraphs? |
d74e8afc |
827 | X<file, reading by paragraphs> |
68dc0745 |
828 | |
65acb1b1 |
829 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 |
830 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
831 | for instance, gets treated as two paragraphs and not three), or |
832 | C<"\n\n"> to accept empty paragraphs. |
833 | |
197aec24 |
834 | Note that a blank line must have no blanks in it. Thus |
c4db748a |
835 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 |
836 | |
68dc0745 |
837 | =head2 How can I read a single character from a file? From the keyboard? |
d74e8afc |
838 | X<getc> X<file, reading one character at a time> |
68dc0745 |
839 | |
840 | You can use the builtin C<getc()> function for most filehandles, but |
841 | it won't (easily) work on a terminal device. For STDIN, either use |
a6dd486b |
842 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 |
843 | L<perlfunc/getc>. |
844 | |
65acb1b1 |
845 | If your system supports the portable operating system programming |
846 | interface (POSIX), you can use the following code, which you'll note |
847 | turns off echo processing as well. |
68dc0745 |
848 | |
500071f4 |
849 | #!/usr/bin/perl -w |
850 | use strict; |
851 | $| = 1; |
852 | for (1..4) { |
853 | my $got; |
854 | print "gimme: "; |
855 | $got = getone(); |
856 | print "--> $got\n"; |
857 | } |
68dc0745 |
858 | exit; |
859 | |
500071f4 |
860 | BEGIN { |
68dc0745 |
861 | use POSIX qw(:termios_h); |
862 | |
863 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
864 | |
865 | $fd_stdin = fileno(STDIN); |
866 | |
867 | $term = POSIX::Termios->new(); |
868 | $term->getattr($fd_stdin); |
869 | $oterm = $term->getlflag(); |
870 | |
871 | $echo = ECHO | ECHOK | ICANON; |
872 | $noecho = $oterm & ~$echo; |
873 | |
874 | sub cbreak { |
500071f4 |
875 | $term->setlflag($noecho); |
876 | $term->setcc(VTIME, 1); |
877 | $term->setattr($fd_stdin, TCSANOW); |
878 | } |
879 | |
68dc0745 |
880 | sub cooked { |
500071f4 |
881 | $term->setlflag($oterm); |
882 | $term->setcc(VTIME, 0); |
883 | $term->setattr($fd_stdin, TCSANOW); |
884 | } |
68dc0745 |
885 | |
886 | sub getone { |
500071f4 |
887 | my $key = ''; |
888 | cbreak(); |
889 | sysread(STDIN, $key, 1); |
890 | cooked(); |
891 | return $key; |
892 | } |
68dc0745 |
893 | |
500071f4 |
894 | } |
68dc0745 |
895 | |
500071f4 |
896 | END { cooked() } |
68dc0745 |
897 | |
a6dd486b |
898 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 |
899 | include also support for non-portable systems as well. |
68dc0745 |
900 | |
500071f4 |
901 | use Term::ReadKey; |
902 | open(TTY, "</dev/tty"); |
903 | print "Gimme a char: "; |
904 | ReadMode "raw"; |
905 | $key = ReadKey 0, *TTY; |
906 | ReadMode "normal"; |
907 | printf "\nYou said %s, char number %03d\n", |
908 | $key, ord $key; |
68dc0745 |
909 | |
65acb1b1 |
910 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 |
911 | |
5a964f20 |
912 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 |
913 | extension from CPAN. As we mentioned earlier, it now even has limited |
914 | support for non-portable (read: not open systems, closed, proprietary, |
915 | not POSIX, not Unix, etc) systems. |
5a964f20 |
916 | |
917 | You should also check out the Frequently Asked Questions list in |
68dc0745 |
918 | comp.unix.* for things like this: the answer is essentially the same. |
919 | It's very system dependent. Here's one solution that works on BSD |
920 | systems: |
921 | |
500071f4 |
922 | sub key_ready { |
923 | my($rin, $nfd); |
924 | vec($rin, fileno(STDIN), 1) = 1; |
925 | return $nfd = select($rin,undef,undef,0); |
926 | } |
68dc0745 |
927 | |
65acb1b1 |
928 | If you want to find out how many characters are waiting, there's |
929 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that |
930 | comes with Perl tries to convert C include files to Perl code, which |
931 | can be C<require>d. FIONREAD ends up defined as a function in the |
932 | I<sys/ioctl.ph> file: |
68dc0745 |
933 | |
500071f4 |
934 | require 'sys/ioctl.ph'; |
68dc0745 |
935 | |
500071f4 |
936 | $size = pack("L", 0); |
937 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; |
938 | $size = unpack("L", $size); |
68dc0745 |
939 | |
5a964f20 |
940 | If I<h2ph> wasn't installed or doesn't work for you, you can |
941 | I<grep> the include files by hand: |
68dc0745 |
942 | |
500071f4 |
943 | % grep FIONREAD /usr/include/*/* |
944 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B |
68dc0745 |
945 | |
5a964f20 |
946 | Or write a small C program using the editor of champions: |
68dc0745 |
947 | |
500071f4 |
948 | % cat > fionread.c |
949 | #include <sys/ioctl.h> |
950 | main() { |
951 | printf("%#08x\n", FIONREAD); |
952 | } |
953 | ^D |
954 | % cc -o fionread fionread.c |
955 | % ./fionread |
956 | 0x4004667f |
5a964f20 |
957 | |
8305e449 |
958 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 |
959 | |
500071f4 |
960 | $FIONREAD = 0x4004667f; # XXX: opsys dependent |
5a964f20 |
961 | |
500071f4 |
962 | $size = pack("L", 0); |
963 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; |
964 | $size = unpack("L", $size); |
5a964f20 |
965 | |
a6dd486b |
966 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 |
967 | pipes, and tty devices work, but I<not> files. |
68dc0745 |
968 | |
969 | =head2 How do I do a C<tail -f> in perl? |
d74e8afc |
970 | X<tail> |
68dc0745 |
971 | |
972 | First try |
973 | |
500071f4 |
974 | seek(GWFILE, 0, 1); |
68dc0745 |
975 | |
976 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, |
977 | but it does clear the end-of-file condition on the handle, so that the |
978 | next <GWFILE> makes Perl try again to read something. |
979 | |
980 | If that doesn't work (it relies on features of your stdio implementation), |
981 | then you need something more like this: |
982 | |
983 | for (;;) { |
984 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { |
985 | # search for some stuff and put it into files |
986 | } |
987 | # sleep for a while |
988 | seek(GWFILE, $curpos, 0); # seek to where we had been |
989 | } |
990 | |
991 | If this still doesn't work, look into the POSIX module. POSIX defines |
992 | the clearerr() method, which can remove the end of file condition on a |
993 | filehandle. The method: read until end of file, clearerr(), read some |
994 | more. Lather, rinse, repeat. |
995 | |
65acb1b1 |
996 | There's also a File::Tail module from CPAN. |
997 | |
68dc0745 |
998 | =head2 How do I dup() a filehandle in Perl? |
d74e8afc |
999 | X<dup> |
68dc0745 |
1000 | |
1001 | If you check L<perlfunc/open>, you'll see that several of the ways |
1002 | to call open() should do the trick. For example: |
1003 | |
500071f4 |
1004 | open(LOG, ">>/foo/logfile"); |
1005 | open(STDERR, ">&LOG"); |
68dc0745 |
1006 | |
1007 | Or even with a literal numeric descriptor: |
1008 | |
1009 | $fd = $ENV{MHCONTEXTFD}; |
1010 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) |
1011 | |
c47ff5f1 |
1012 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 |
1013 | an alias. That means if you close an aliased handle, all |
197aec24 |
1014 | aliases become inaccessible. This is not true with |
5a964f20 |
1015 | a copied one. |
1016 | |
1017 | Error checking, as always, has been left as an exercise for the reader. |
68dc0745 |
1018 | |
1019 | =head2 How do I close a file descriptor by number? |
d74e8afc |
1020 | X<file, closing file descriptors> |
68dc0745 |
1021 | |
1022 | This should rarely be necessary, as the Perl close() function is to be |
1023 | used for things that Perl opened itself, even if it was a dup of a |
a6dd486b |
1024 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 |
1025 | to, you may be able to do this: |
1026 | |
500071f4 |
1027 | require 'sys/syscall.ph'; |
1028 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric |
1029 | die "can't sysclose $fd: $!" unless $rc == -1; |
68dc0745 |
1030 | |
a6dd486b |
1031 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 |
1032 | |
500071f4 |
1033 | { |
197aec24 |
1034 | local *F; |
d92eb7b0 |
1035 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
1036 | close F; |
500071f4 |
1037 | } |
d92eb7b0 |
1038 | |
883f1635 |
1039 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
d74e8afc |
1040 | X<filename, DOS issues> |
68dc0745 |
1041 | |
1042 | Whoops! You just put a tab and a formfeed into that filename! |
1043 | Remember that within double quoted strings ("like\this"), the |
1044 | backslash is an escape character. The full list of these is in |
1045 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't |
1046 | have a file called "c:(tab)emp(formfeed)oo" or |
65acb1b1 |
1047 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 |
1048 | |
1049 | Either single-quote your strings, or (preferably) use forward slashes. |
46fc3d4c |
1050 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 |
1051 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b |
1052 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 |
1053 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
1054 | are more portable, too. |
68dc0745 |
1055 | |
1056 | =head2 Why doesn't glob("*.*") get all the files? |
d74e8afc |
1057 | X<glob> |
68dc0745 |
1058 | |
1059 | Because even on non-Unix ports, Perl's glob function follows standard |
46fc3d4c |
1060 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 |
1061 | files. This makes glob() portable even to legacy systems. Your |
1062 | port may include proprietary globbing functions as well. Check its |
1063 | documentation for details. |
68dc0745 |
1064 | |
1065 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? |
1066 | |
06a5f41f |
1067 | This is elaborately and painstakingly described in the |
1068 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To |
49d635f9 |
1069 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . |
68dc0745 |
1070 | |
1071 | The executive summary: learn how your filesystem works. The |
1072 | permissions on a file say what can happen to the data in that file. |
1073 | The permissions on a directory say what can happen to the list of |
1074 | files in that directory. If you delete a file, you're removing its |
1075 | name from the directory (so the operation depends on the permissions |
1076 | of the directory, not of the file). If you try to write to the file, |
1077 | the permissions of the file govern whether you're allowed to. |
1078 | |
1079 | =head2 How do I select a random line from a file? |
d74e8afc |
1080 | X<file, selecting a random line> |
68dc0745 |
1081 | |
1082 | Here's an algorithm from the Camel Book: |
1083 | |
500071f4 |
1084 | srand; |
1085 | rand($.) < 1 && ($line = $_) while <>; |
68dc0745 |
1086 | |
49d635f9 |
1087 | This has a significant advantage in space over reading the whole file |
1088 | in. You can find a proof of this method in I<The Art of Computer |
1089 | Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. |
1090 | |
1091 | You can use the File::Random module which provides a function |
1092 | for that algorithm: |
1093 | |
1094 | use File::Random qw/random_line/; |
1095 | my $line = random_line($filename); |
1096 | |
1097 | Another way is to use the Tie::File module, which treats the entire |
1098 | file as an array. Simply access a random array element. |
68dc0745 |
1099 | |
65acb1b1 |
1100 | =head2 Why do I get weird spaces when I print an array of lines? |
1101 | |
1102 | Saying |
1103 | |
500071f4 |
1104 | print "@lines\n"; |
65acb1b1 |
1105 | |
1106 | joins together the elements of C<@lines> with a space between them. |
1107 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above |
a6dd486b |
1108 | statement would print |
65acb1b1 |
1109 | |
500071f4 |
1110 | little fluffy clouds |
65acb1b1 |
1111 | |
1112 | but if each element of C<@lines> was a line of text, ending a newline |
1113 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: |
1114 | |
500071f4 |
1115 | little |
1116 | fluffy |
1117 | clouds |
65acb1b1 |
1118 | |
1119 | If your array contains lines, just print them: |
1120 | |
500071f4 |
1121 | print @lines; |
1122 | |
1123 | =head1 REVISION |
1124 | |
1125 | Revision: $Revision: 3606 $ |
1126 | |
1127 | Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $ |
1128 | |
1129 | See L<perlfaq> for source control details and availability. |
65acb1b1 |
1130 | |
68dc0745 |
1131 | =head1 AUTHOR AND COPYRIGHT |
1132 | |
58103a2e |
1133 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and |
7678cced |
1134 | other authors as noted. All rights reserved. |
5a964f20 |
1135 | |
5a7beb56 |
1136 | This documentation is free; you can redistribute it and/or modify it |
1137 | under the same terms as Perl itself. |
c8db1d39 |
1138 | |
87275199 |
1139 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
1140 | domain. You are permitted and encouraged to use this code and any |
1141 | derivatives thereof in your own programs for fun or for profit as you |
1142 | see fit. A simple comment in the code giving credit to the FAQ would |
1143 | be courteous but is not required. |