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1 | =head1 NAME |
2 | |
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3 | perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 2005/10/13 19:49:13 $) |
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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 | |
39 | $old_fh = select(OUTPUT_HANDLE); |
40 | $| = 1; |
41 | select($old_fh); |
42 | |
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43 | Some idioms can handle this in a single statement: |
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44 | |
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 | |
53 | use IO::Handle; |
54 | open(DEV, ">/dev/printer"); # but is this? |
55 | DEV->autoflush(1); |
56 | |
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57 | or IO::Socket: |
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58 | |
59 | use IO::Socket; # this one is kinda a pipe? |
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60 | my $sock = IO::Socket::INET->new( 'www.example.com:80' ); |
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61 | |
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 | |
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; |
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<$^I> variable, which in turn affects |
91 | the behavior of C<< <> >>; see L<perlrun> for more details. By |
92 | modifying the appropriate variables directly, you can get the same |
93 | behavior within a larger program. For example: |
94 | |
95 | # ... |
96 | { |
97 | local($^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 |
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 | |
7678cced |
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 | |
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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 | |
16394a69 |
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() |
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 | |
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()); |
167 | sub temp_file { |
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168 | local *FH; |
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169 | my $count = 0; |
5a964f20 |
170 | until (defined(fileno(FH)) || $count++ > 100) { |
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171 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
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172 | # O_EXCL is required for security reasons. |
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173 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
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174 | } |
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175 | if (defined(fileno(FH)) |
176 | return (*FH, $base_name); |
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177 | } else { |
178 | return (); |
179 | } |
180 | } |
181 | } |
182 | |
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183 | =head2 How can I manipulate fixed-record-length files? |
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184 | X<fixed-length> X<file, fixed-length records> |
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185 | |
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186 | The most efficient way is using L<pack()|perlfunc/"pack"> and |
187 | L<unpack()|perlfunc/"unpack">. This is faster than using |
188 | L<substr()|perlfunc/"substr"> when taking many, many strings. It is |
189 | slower for just a few. |
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190 | |
191 | Here is a sample chunk of code to break up and put back together again |
192 | some fixed-format input lines, in this case from the output of a normal, |
193 | Berkeley-style ps: |
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194 | |
195 | # sample input line: |
196 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what |
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197 | my $PS_T = 'A6 A4 A7 A5 A*'; |
198 | open my $ps, '-|', 'ps'; |
199 | print scalar <$ps>; |
200 | my @fields = qw( pid tt stat time command ); |
201 | while (<$ps>) { |
202 | my %process; |
203 | @process{@fields} = unpack($PS_T, $_); |
204 | for my $field ( @fields ) { |
205 | print "$field: <$process{$field}>\n"; |
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206 | } |
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207 | print 'line=', pack($PS_T, @process{@fields} ), "\n"; |
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208 | } |
209 | |
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210 | We've used a hash slice in order to easily handle the fields of each row. |
211 | Storing the keys in an array means it's easy to operate on them as a |
212 | group or loop over them with for. It also avoids polluting the program |
213 | with global variables and using symbolic references. |
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214 | |
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215 | =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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216 | X<filehandle, local> X<filehandle, passing> X<filehandle, reference> |
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217 | |
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218 | As of perl5.6, open() autovivifies file and directory handles |
219 | as references if you pass it an uninitialized scalar variable. |
220 | You can then pass these references just like any other scalar, |
221 | and use them in the place of named handles. |
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222 | |
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223 | open my $fh, $file_name; |
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224 | |
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225 | open local $fh, $file_name; |
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226 | |
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227 | print $fh "Hello World!\n"; |
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228 | |
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229 | process_file( $fh ); |
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230 | |
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231 | Before perl5.6, you had to deal with various typeglob idioms |
232 | which you may see in older code. |
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233 | |
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234 | open FILE, "> $filename"; |
235 | process_typeglob( *FILE ); |
236 | process_reference( \*FILE ); |
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237 | |
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238 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
239 | sub process_reference { local $fh = shift; print $fh "Reference!" } |
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240 | |
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241 | If you want to create many anonymous handles, you should |
242 | check out the Symbol or IO::Handle modules. |
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243 | |
244 | =head2 How can I use a filehandle indirectly? |
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245 | X<filehandle, indirect> |
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246 | |
247 | An indirect filehandle is using something other than a symbol |
248 | in a place that a filehandle is expected. Here are ways |
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249 | to get indirect filehandles: |
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250 | |
251 | $fh = SOME_FH; # bareword is strict-subs hostile |
252 | $fh = "SOME_FH"; # strict-refs hostile; same package only |
253 | $fh = *SOME_FH; # typeglob |
254 | $fh = \*SOME_FH; # ref to typeglob (bless-able) |
255 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob |
256 | |
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257 | Or, you can use the C<new> method from one of the IO::* modules to |
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258 | create an anonymous filehandle, store that in a scalar variable, |
259 | and use it as though it were a normal filehandle. |
260 | |
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261 | use IO::Handle; # 5.004 or higher |
262 | $fh = IO::Handle->new(); |
263 | |
264 | Then use any of those as you would a normal filehandle. Anywhere that |
265 | Perl is expecting a filehandle, an indirect filehandle may be used |
266 | instead. An indirect filehandle is just a scalar variable that contains |
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267 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
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268 | the C<< <FH> >> diamond operator will accept either a named filehandle |
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269 | or a scalar variable containing one: |
270 | |
271 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); |
272 | print $ofh "Type it: "; |
273 | $got = <$ifh> |
274 | print $efh "What was that: $got"; |
275 | |
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276 | If you're passing a filehandle to a function, you can write |
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277 | the function in two ways: |
278 | |
279 | sub accept_fh { |
280 | my $fh = shift; |
281 | print $fh "Sending to indirect filehandle\n"; |
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282 | } |
283 | |
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284 | Or it can localize a typeglob and use the filehandle directly: |
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285 | |
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286 | sub accept_fh { |
287 | local *FH = shift; |
288 | print FH "Sending to localized filehandle\n"; |
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289 | } |
290 | |
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291 | Both styles work with either objects or typeglobs of real filehandles. |
292 | (They might also work with strings under some circumstances, but this |
293 | is risky.) |
294 | |
295 | accept_fh(*STDOUT); |
296 | accept_fh($handle); |
297 | |
298 | In the examples above, we assigned the filehandle to a scalar variable |
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299 | before using it. That is because only simple scalar variables, not |
300 | expressions or subscripts of hashes or arrays, can be used with |
301 | built-ins like C<print>, C<printf>, or the diamond operator. Using |
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302 | something other than a simple scalar variable as a filehandle is |
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303 | illegal and won't even compile: |
304 | |
305 | @fd = (*STDIN, *STDOUT, *STDERR); |
306 | print $fd[1] "Type it: "; # WRONG |
307 | $got = <$fd[0]> # WRONG |
308 | print $fd[2] "What was that: $got"; # WRONG |
309 | |
310 | With C<print> and C<printf>, you get around this by using a block and |
311 | an expression where you would place the filehandle: |
312 | |
313 | print { $fd[1] } "funny stuff\n"; |
314 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; |
315 | # Pity the poor deadbeef. |
316 | |
317 | That block is a proper block like any other, so you can put more |
318 | complicated code there. This sends the message out to one of two places: |
319 | |
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320 | $ok = -x "/bin/cat"; |
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321 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
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322 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
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323 | |
324 | This approach of treating C<print> and C<printf> like object methods |
325 | calls doesn't work for the diamond operator. That's because it's a |
326 | real operator, not just a function with a comma-less argument. Assuming |
327 | you've been storing typeglobs in your structure as we did above, you |
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328 | can use the built-in function named C<readline> to read a record just |
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329 | as C<< <> >> does. Given the initialization shown above for @fd, this |
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330 | would work, but only because readline() requires a typeglob. It doesn't |
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331 | work with objects or strings, which might be a bug we haven't fixed yet. |
332 | |
333 | $got = readline($fd[0]); |
334 | |
335 | Let it be noted that the flakiness of indirect filehandles is not |
336 | related to whether they're strings, typeglobs, objects, or anything else. |
337 | It's the syntax of the fundamental operators. Playing the object |
338 | game doesn't help you at all here. |
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339 | |
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340 | =head2 How can I set up a footer format to be used with write()? |
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341 | X<footer> |
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342 | |
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343 | There's no builtin way to do this, but L<perlform> has a couple of |
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344 | techniques to make it possible for the intrepid hacker. |
345 | |
346 | =head2 How can I write() into a string? |
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347 | X<write, into a string> |
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348 | |
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349 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
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350 | |
351 | =head2 How can I output my numbers with commas added? |
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352 | X<number, commify> |
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353 | |
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354 | (contributed by brian d foy and Benjamin Goldberg) |
355 | |
356 | You can use L<Number::Format> to separate places in a number. |
357 | It handles locale information for those of you who want to insert |
358 | full stops instead (or anything else that they want to use, |
359 | really). |
360 | |
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361 | This subroutine will add commas to your number: |
362 | |
363 | sub commify { |
364 | local $_ = shift; |
365 | 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; |
366 | return $_; |
367 | } |
368 | |
369 | This regex from Benjamin Goldberg will add commas to numbers: |
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370 | |
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371 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
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372 | |
49d635f9 |
373 | It is easier to see with comments: |
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374 | |
881bdbd4 |
375 | s/( |
376 | ^[-+]? # beginning of number. |
7678cced |
377 | \d+? # first digits before first comma |
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378 | (?= # followed by, (but not included in the match) : |
379 | (?>(?:\d{3})+) # some positive multiple of three digits. |
380 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. |
381 | ) |
382 | | # or: |
383 | \G\d{3} # after the last group, get three digits |
384 | (?=\d) # but they have to have more digits after them. |
385 | )/$1,/xg; |
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386 | |
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387 | =head2 How can I translate tildes (~) in a filename? |
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388 | X<tilde> X<tilde expansion> |
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389 | |
575cc754 |
390 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
391 | versions of Perl require that you have a shell installed that groks |
392 | tildes. Recent perl versions have this feature built in. The |
d6260402 |
393 | File::KGlob module (available from CPAN) gives more portable glob |
575cc754 |
394 | functionality. |
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395 | |
396 | Within Perl, you may use this directly: |
397 | |
398 | $filename =~ s{ |
399 | ^ ~ # find a leading tilde |
400 | ( # save this in $1 |
401 | [^/] # a non-slash character |
402 | * # repeated 0 or more times (0 means me) |
403 | ) |
404 | }{ |
405 | $1 |
406 | ? (getpwnam($1))[7] |
407 | : ( $ENV{HOME} || $ENV{LOGDIR} ) |
408 | }ex; |
409 | |
5a964f20 |
410 | =head2 How come when I open a file read-write it wipes it out? |
d74e8afc |
411 | X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating> |
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412 | |
413 | Because you're using something like this, which truncates the file and |
414 | I<then> gives you read-write access: |
415 | |
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416 | open(FH, "+> /path/name"); # WRONG (almost always) |
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417 | |
418 | Whoops. You should instead use this, which will fail if the file |
197aec24 |
419 | doesn't exist. |
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420 | |
421 | open(FH, "+< /path/name"); # open for update |
422 | |
c47ff5f1 |
423 | Using ">" always clobbers or creates. Using "<" never does |
d92eb7b0 |
424 | either. The "+" doesn't change this. |
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425 | |
5a964f20 |
426 | Here are examples of many kinds of file opens. Those using sysopen() |
427 | all assume |
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428 | |
5a964f20 |
429 | use Fcntl; |
68dc0745 |
430 | |
5a964f20 |
431 | To open file for reading: |
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432 | |
5a964f20 |
433 | open(FH, "< $path") || die $!; |
434 | sysopen(FH, $path, O_RDONLY) || die $!; |
435 | |
436 | To open file for writing, create new file if needed or else truncate old file: |
437 | |
438 | open(FH, "> $path") || die $!; |
439 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; |
440 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; |
441 | |
442 | To open file for writing, create new file, file must not exist: |
443 | |
444 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; |
445 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; |
446 | |
447 | To open file for appending, create if necessary: |
448 | |
449 | open(FH, ">> $path") || die $!; |
450 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; |
451 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; |
452 | |
453 | To open file for appending, file must exist: |
454 | |
455 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; |
456 | |
457 | To open file for update, file must exist: |
458 | |
459 | open(FH, "+< $path") || die $!; |
460 | sysopen(FH, $path, O_RDWR) || die $!; |
461 | |
462 | To open file for update, create file if necessary: |
463 | |
464 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; |
465 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; |
466 | |
467 | To open file for update, file must not exist: |
468 | |
469 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; |
470 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; |
471 | |
472 | To open a file without blocking, creating if necessary: |
473 | |
2359510d |
474 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
475 | or die "can't open /foo/somefile: $!": |
5a964f20 |
476 | |
477 | Be warned that neither creation nor deletion of files is guaranteed to |
478 | be an atomic operation over NFS. That is, two processes might both |
a6dd486b |
479 | successfully create or unlink the same file! Therefore O_EXCL |
480 | isn't as exclusive as you might wish. |
68dc0745 |
481 | |
87275199 |
482 | See also the new L<perlopentut> if you have it (new for 5.6). |
65acb1b1 |
483 | |
04d666b1 |
484 | =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? |
d74e8afc |
485 | X<argument list too long> |
68dc0745 |
486 | |
c47ff5f1 |
487 | The C<< <> >> operator performs a globbing operation (see above). |
3a4b19e4 |
488 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
489 | csh(1) to do the actual glob expansion, but |
68dc0745 |
490 | csh can't handle more than 127 items and so gives the error message |
491 | C<Argument list too long>. People who installed tcsh as csh won't |
492 | have this problem, but their users may be surprised by it. |
493 | |
3a4b19e4 |
494 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
d6260402 |
495 | yourself with readdir() and patterns, or use a module like File::KGlob, |
3a4b19e4 |
496 | one that doesn't use the shell to do globbing. |
68dc0745 |
497 | |
498 | =head2 Is there a leak/bug in glob()? |
d74e8afc |
499 | X<glob> |
68dc0745 |
500 | |
501 | Due to the current implementation on some operating systems, when you |
502 | use the glob() function or its angle-bracket alias in a scalar |
a6dd486b |
503 | context, you may cause a memory leak and/or unpredictable behavior. It's |
68dc0745 |
504 | best therefore to use glob() only in list context. |
505 | |
c47ff5f1 |
506 | =head2 How can I open a file with a leading ">" or trailing blanks? |
d74e8afc |
507 | X<filename, special characters> |
68dc0745 |
508 | |
b68463f7 |
509 | (contributed by Brian McCauley) |
68dc0745 |
510 | |
b68463f7 |
511 | The special two argument form of Perl's open() function ignores |
512 | trailing blanks in filenames and infers the mode from certain leading |
513 | characters (or a trailing "|"). In older versions of Perl this was the |
514 | only version of open() and so it is prevalent in old code and books. |
65acb1b1 |
515 | |
b68463f7 |
516 | Unless you have a particular reason to use the two argument form you |
517 | should use the three argument form of open() which does not treat any |
518 | charcters in the filename as special. |
519 | |
881bdbd4 |
520 | open FILE, "<", " file "; # filename is " file " |
521 | open FILE, ">", ">file"; # filename is ">file" |
65acb1b1 |
522 | |
68dc0745 |
523 | =head2 How can I reliably rename a file? |
d74e8afc |
524 | X<rename> X<mv> X<move> X<file, rename> X<ren> |
68dc0745 |
525 | |
49d635f9 |
526 | If your operating system supports a proper mv(1) utility or its |
527 | functional equivalent, this works: |
68dc0745 |
528 | |
529 | rename($old, $new) or system("mv", $old, $new); |
530 | |
d2321c93 |
531 | It may be more portable to use the File::Copy module instead. |
532 | You just copy to the new file to the new name (checking return |
533 | values), then delete the old one. This isn't really the same |
534 | semantically as a rename(), which preserves meta-information like |
68dc0745 |
535 | permissions, timestamps, inode info, etc. |
536 | |
d2321c93 |
537 | Newer versions of File::Copy export a move() function. |
5a964f20 |
538 | |
68dc0745 |
539 | =head2 How can I lock a file? |
d74e8afc |
540 | X<lock> X<file, lock> X<flock> |
68dc0745 |
541 | |
54310121 |
542 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 |
543 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
544 | later), and lockf(3) if neither of the two previous system calls exists. |
545 | On some systems, it may even use a different form of native locking. |
546 | Here are some gotchas with Perl's flock(): |
547 | |
548 | =over 4 |
549 | |
550 | =item 1 |
551 | |
552 | Produces a fatal error if none of the three system calls (or their |
553 | close equivalent) exists. |
554 | |
555 | =item 2 |
556 | |
557 | lockf(3) does not provide shared locking, and requires that the |
558 | filehandle be open for writing (or appending, or read/writing). |
559 | |
560 | =item 3 |
561 | |
d92eb7b0 |
562 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
563 | systems), so you'd need to force the use of fcntl(2) when you build Perl. |
a6dd486b |
564 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 |
565 | and the F<INSTALL> file in the source distribution for information on |
566 | building Perl to do this. |
567 | |
568 | Two potentially non-obvious but traditional flock semantics are that |
a6dd486b |
569 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 |
570 | I<merely advisory>. Such discretionary locks are more flexible, but |
571 | offer fewer guarantees. This means that files locked with flock() may |
572 | be modified by programs that do not also use flock(). Cars that stop |
573 | for red lights get on well with each other, but not with cars that don't |
574 | stop for red lights. See the perlport manpage, your port's specific |
575 | documentation, or your system-specific local manpages for details. It's |
576 | best to assume traditional behavior if you're writing portable programs. |
a6dd486b |
577 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 |
578 | for your own system's idiosyncrasies (sometimes called "features"). |
579 | Slavish adherence to portability concerns shouldn't get in the way of |
580 | your getting your job done.) |
68dc0745 |
581 | |
197aec24 |
582 | For more information on file locking, see also |
13a2d996 |
583 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 |
584 | |
68dc0745 |
585 | =back |
586 | |
04d666b1 |
587 | =head2 Why can't I just open(FH, "E<gt>file.lock")? |
d74e8afc |
588 | X<lock, lockfile race condition> |
68dc0745 |
589 | |
590 | A common bit of code B<NOT TO USE> is this: |
591 | |
592 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE |
593 | open(LCK, "> file.lock"); # THIS BROKEN CODE |
594 | |
595 | This is a classic race condition: you take two steps to do something |
596 | which must be done in one. That's why computer hardware provides an |
597 | atomic test-and-set instruction. In theory, this "ought" to work: |
598 | |
5a964f20 |
599 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
9b55d3ab |
600 | or die "can't open file.lock: $!"; |
68dc0745 |
601 | |
602 | except that lamentably, file creation (and deletion) is not atomic |
603 | over NFS, so this won't work (at least, not every time) over the net. |
65acb1b1 |
604 | Various schemes involving link() have been suggested, but |
46fc3d4c |
605 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 |
606 | |
fc36a67e |
607 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
d74e8afc |
608 | X<counter> X<file, counter> |
68dc0745 |
609 | |
46fc3d4c |
610 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 |
611 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b |
612 | only to stroke the writer's vanity. It's better to pick a random number; |
613 | they're more realistic. |
68dc0745 |
614 | |
5a964f20 |
615 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 |
616 | |
e2c57c3e |
617 | use Fcntl qw(:DEFAULT :flock); |
5a964f20 |
618 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
65acb1b1 |
619 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
68dc0745 |
620 | $num = <FH> || 0; |
621 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; |
622 | truncate(FH, 0) or die "can't truncate numfile: $!"; |
623 | (print FH $num+1, "\n") or die "can't write numfile: $!"; |
68dc0745 |
624 | close FH or die "can't close numfile: $!"; |
625 | |
46fc3d4c |
626 | Here's a much better web-page hit counter: |
68dc0745 |
627 | |
628 | $hits = int( (time() - 850_000_000) / rand(1_000) ); |
629 | |
630 | If the count doesn't impress your friends, then the code might. :-) |
631 | |
f52f3be2 |
632 | =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 |
633 | X<append> X<file, append> |
05caf3a7 |
634 | |
635 | If you are on a system that correctly implements flock() and you use the |
636 | example appending code from "perldoc -f flock" everything will be OK |
637 | even if the OS you are on doesn't implement append mode correctly (if |
638 | such a system exists.) So if you are happy to restrict yourself to OSs |
639 | that implement flock() (and that's not really much of a restriction) |
640 | then that is what you should do. |
641 | |
642 | If you know you are only going to use a system that does correctly |
643 | implement appending (i.e. not Win32) then you can omit the seek() from |
644 | the above code. |
645 | |
646 | If you know you are only writing code to run on an OS and filesystem that |
647 | does implement append mode correctly (a local filesystem on a modern |
648 | Unix for example), and you keep the file in block-buffered mode and you |
649 | write less than one buffer-full of output between each manual flushing |
8305e449 |
650 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 |
651 | the end of the file in one chunk without getting intermingled with |
652 | anyone else's output. You can also use the syswrite() function which is |
653 | simply a wrapper around your systems write(2) system call. |
654 | |
655 | There is still a small theoretical chance that a signal will interrupt |
656 | the system level write() operation before completion. There is also a |
657 | possibility that some STDIO implementations may call multiple system |
658 | level write()s even if the buffer was empty to start. There may be some |
659 | systems where this probability is reduced to zero. |
660 | |
68dc0745 |
661 | =head2 How do I randomly update a binary file? |
d74e8afc |
662 | X<file, binary patch> |
68dc0745 |
663 | |
664 | If you're just trying to patch a binary, in many cases something as |
665 | simple as this works: |
666 | |
667 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs |
668 | |
669 | However, if you have fixed sized records, then you might do something more |
670 | like this: |
671 | |
672 | $RECSIZE = 220; # size of record, in bytes |
673 | $recno = 37; # which record to update |
674 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; |
675 | seek(FH, $recno * $RECSIZE, 0); |
676 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; |
677 | # munge the record |
65acb1b1 |
678 | seek(FH, -$RECSIZE, 1); |
68dc0745 |
679 | print FH $record; |
680 | close FH; |
681 | |
682 | Locking and error checking are left as an exercise for the reader. |
a6dd486b |
683 | Don't forget them or you'll be quite sorry. |
68dc0745 |
684 | |
68dc0745 |
685 | =head2 How do I get a file's timestamp in perl? |
d74e8afc |
686 | X<timestamp> X<file, timestamp> |
68dc0745 |
687 | |
881bdbd4 |
688 | If you want to retrieve the time at which the file was last |
689 | read, written, or had its meta-data (owner, etc) changed, |
690 | you use the B<-M>, B<-A>, or B<-C> file test operations as |
691 | documented in L<perlfunc>. These retrieve the age of the |
692 | file (measured against the start-time of your program) in |
693 | days as a floating point number. Some platforms may not have |
694 | all of these times. See L<perlport> for details. To |
695 | retrieve the "raw" time in seconds since the epoch, you |
696 | would call the stat function, then use localtime(), |
697 | gmtime(), or POSIX::strftime() to convert this into |
698 | human-readable form. |
68dc0745 |
699 | |
700 | Here's an example: |
701 | |
702 | $write_secs = (stat($file))[9]; |
c8db1d39 |
703 | printf "file %s updated at %s\n", $file, |
704 | scalar localtime($write_secs); |
68dc0745 |
705 | |
706 | If you prefer something more legible, use the File::stat module |
707 | (part of the standard distribution in version 5.004 and later): |
708 | |
65acb1b1 |
709 | # error checking left as an exercise for reader. |
68dc0745 |
710 | use File::stat; |
711 | use Time::localtime; |
712 | $date_string = ctime(stat($file)->mtime); |
713 | print "file $file updated at $date_string\n"; |
714 | |
65acb1b1 |
715 | The POSIX::strftime() approach has the benefit of being, |
716 | in theory, independent of the current locale. See L<perllocale> |
717 | for details. |
68dc0745 |
718 | |
719 | =head2 How do I set a file's timestamp in perl? |
d74e8afc |
720 | X<timestamp> X<file, timestamp> |
68dc0745 |
721 | |
722 | You use the utime() function documented in L<perlfunc/utime>. |
723 | By way of example, here's a little program that copies the |
724 | read and write times from its first argument to all the rest |
725 | of them. |
726 | |
727 | if (@ARGV < 2) { |
728 | die "usage: cptimes timestamp_file other_files ...\n"; |
729 | } |
730 | $timestamp = shift; |
731 | ($atime, $mtime) = (stat($timestamp))[8,9]; |
732 | utime $atime, $mtime, @ARGV; |
733 | |
65acb1b1 |
734 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 |
735 | |
19a1cd16 |
736 | The perldoc for utime also has an example that has the same |
737 | effect as touch(1) on files that I<already exist>. |
738 | |
739 | Certain file systems have a limited ability to store the times |
740 | on a file at the expected level of precision. For example, the |
741 | FAT and HPFS filesystem are unable to create dates on files with |
742 | a finer granularity than two seconds. This is a limitation of |
743 | the filesystems, not of utime(). |
68dc0745 |
744 | |
745 | =head2 How do I print to more than one file at once? |
d74e8afc |
746 | X<print, to multiple files> |
68dc0745 |
747 | |
49d635f9 |
748 | To connect one filehandle to several output filehandles, |
749 | you can use the IO::Tee or Tie::FileHandle::Multiplex modules. |
68dc0745 |
750 | |
49d635f9 |
751 | If you only have to do this once, you can print individually |
752 | to each filehandle. |
68dc0745 |
753 | |
49d635f9 |
754 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
5a964f20 |
755 | |
49d635f9 |
756 | =head2 How can I read in an entire file all at once? |
d74e8afc |
757 | X<slurp> X<file, slurping> |
68dc0745 |
758 | |
49d635f9 |
759 | You can use the File::Slurp module to do it in one step. |
68dc0745 |
760 | |
49d635f9 |
761 | use File::Slurp; |
197aec24 |
762 | |
49d635f9 |
763 | $all_of_it = read_file($filename); # entire file in scalar |
764 | @all_lines = read_file($filename); # one line perl element |
d92eb7b0 |
765 | |
766 | The customary Perl approach for processing all the lines in a file is to |
767 | do so one line at a time: |
768 | |
769 | open (INPUT, $file) || die "can't open $file: $!"; |
770 | while (<INPUT>) { |
771 | chomp; |
772 | # do something with $_ |
197aec24 |
773 | } |
d92eb7b0 |
774 | close(INPUT) || die "can't close $file: $!"; |
775 | |
776 | This is tremendously more efficient than reading the entire file into |
777 | memory as an array of lines and then processing it one element at a time, |
a6dd486b |
778 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 |
779 | you see someone do this: |
780 | |
781 | @lines = <INPUT>; |
782 | |
30852c57 |
783 | you should think long and hard about why you need everything loaded at |
784 | once. It's just not a scalable solution. You might also find it more |
785 | fun to use the standard Tie::File module, or the DB_File module's |
786 | $DB_RECNO bindings, which allow you to tie an array to a file so that |
787 | accessing an element the array actually accesses the corresponding |
788 | line in the file. |
d92eb7b0 |
789 | |
f05bbc40 |
790 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 |
791 | |
792 | { |
793 | local(*INPUT, $/); |
794 | open (INPUT, $file) || die "can't open $file: $!"; |
795 | $var = <INPUT>; |
796 | } |
797 | |
197aec24 |
798 | That temporarily undefs your record separator, and will automatically |
d92eb7b0 |
799 | close the file at block exit. If the file is already open, just use this: |
800 | |
801 | $var = do { local $/; <INPUT> }; |
802 | |
f05bbc40 |
803 | For ordinary files you can also use the read function. |
804 | |
805 | read( INPUT, $var, -s INPUT ); |
806 | |
807 | The third argument tests the byte size of the data on the INPUT filehandle |
808 | and reads that many bytes into the buffer $var. |
809 | |
68dc0745 |
810 | =head2 How can I read in a file by paragraphs? |
d74e8afc |
811 | X<file, reading by paragraphs> |
68dc0745 |
812 | |
65acb1b1 |
813 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 |
814 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
815 | for instance, gets treated as two paragraphs and not three), or |
816 | C<"\n\n"> to accept empty paragraphs. |
817 | |
197aec24 |
818 | Note that a blank line must have no blanks in it. Thus |
c4db748a |
819 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 |
820 | |
68dc0745 |
821 | =head2 How can I read a single character from a file? From the keyboard? |
d74e8afc |
822 | X<getc> X<file, reading one character at a time> |
68dc0745 |
823 | |
824 | You can use the builtin C<getc()> function for most filehandles, but |
825 | it won't (easily) work on a terminal device. For STDIN, either use |
a6dd486b |
826 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 |
827 | L<perlfunc/getc>. |
828 | |
65acb1b1 |
829 | If your system supports the portable operating system programming |
830 | interface (POSIX), you can use the following code, which you'll note |
831 | turns off echo processing as well. |
68dc0745 |
832 | |
833 | #!/usr/bin/perl -w |
834 | use strict; |
835 | $| = 1; |
836 | for (1..4) { |
837 | my $got; |
838 | print "gimme: "; |
839 | $got = getone(); |
840 | print "--> $got\n"; |
841 | } |
842 | exit; |
843 | |
844 | BEGIN { |
845 | use POSIX qw(:termios_h); |
846 | |
847 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
848 | |
849 | $fd_stdin = fileno(STDIN); |
850 | |
851 | $term = POSIX::Termios->new(); |
852 | $term->getattr($fd_stdin); |
853 | $oterm = $term->getlflag(); |
854 | |
855 | $echo = ECHO | ECHOK | ICANON; |
856 | $noecho = $oterm & ~$echo; |
857 | |
858 | sub cbreak { |
859 | $term->setlflag($noecho); |
860 | $term->setcc(VTIME, 1); |
861 | $term->setattr($fd_stdin, TCSANOW); |
862 | } |
863 | |
864 | sub cooked { |
865 | $term->setlflag($oterm); |
866 | $term->setcc(VTIME, 0); |
867 | $term->setattr($fd_stdin, TCSANOW); |
868 | } |
869 | |
870 | sub getone { |
871 | my $key = ''; |
872 | cbreak(); |
873 | sysread(STDIN, $key, 1); |
874 | cooked(); |
875 | return $key; |
876 | } |
877 | |
878 | } |
879 | |
880 | END { cooked() } |
881 | |
a6dd486b |
882 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 |
883 | include also support for non-portable systems as well. |
68dc0745 |
884 | |
885 | use Term::ReadKey; |
886 | open(TTY, "</dev/tty"); |
887 | print "Gimme a char: "; |
888 | ReadMode "raw"; |
889 | $key = ReadKey 0, *TTY; |
890 | ReadMode "normal"; |
891 | printf "\nYou said %s, char number %03d\n", |
892 | $key, ord $key; |
893 | |
65acb1b1 |
894 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 |
895 | |
5a964f20 |
896 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 |
897 | extension from CPAN. As we mentioned earlier, it now even has limited |
898 | support for non-portable (read: not open systems, closed, proprietary, |
899 | not POSIX, not Unix, etc) systems. |
5a964f20 |
900 | |
901 | You should also check out the Frequently Asked Questions list in |
68dc0745 |
902 | comp.unix.* for things like this: the answer is essentially the same. |
903 | It's very system dependent. Here's one solution that works on BSD |
904 | systems: |
905 | |
906 | sub key_ready { |
907 | my($rin, $nfd); |
908 | vec($rin, fileno(STDIN), 1) = 1; |
909 | return $nfd = select($rin,undef,undef,0); |
910 | } |
911 | |
65acb1b1 |
912 | If you want to find out how many characters are waiting, there's |
913 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that |
914 | comes with Perl tries to convert C include files to Perl code, which |
915 | can be C<require>d. FIONREAD ends up defined as a function in the |
916 | I<sys/ioctl.ph> file: |
68dc0745 |
917 | |
5a964f20 |
918 | require 'sys/ioctl.ph'; |
68dc0745 |
919 | |
5a964f20 |
920 | $size = pack("L", 0); |
921 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; |
922 | $size = unpack("L", $size); |
68dc0745 |
923 | |
5a964f20 |
924 | If I<h2ph> wasn't installed or doesn't work for you, you can |
925 | I<grep> the include files by hand: |
68dc0745 |
926 | |
5a964f20 |
927 | % grep FIONREAD /usr/include/*/* |
928 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B |
68dc0745 |
929 | |
5a964f20 |
930 | Or write a small C program using the editor of champions: |
68dc0745 |
931 | |
5a964f20 |
932 | % cat > fionread.c |
933 | #include <sys/ioctl.h> |
934 | main() { |
935 | printf("%#08x\n", FIONREAD); |
936 | } |
937 | ^D |
65acb1b1 |
938 | % cc -o fionread fionread.c |
5a964f20 |
939 | % ./fionread |
940 | 0x4004667f |
941 | |
8305e449 |
942 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 |
943 | |
944 | $FIONREAD = 0x4004667f; # XXX: opsys dependent |
945 | |
946 | $size = pack("L", 0); |
947 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; |
948 | $size = unpack("L", $size); |
949 | |
a6dd486b |
950 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 |
951 | pipes, and tty devices work, but I<not> files. |
68dc0745 |
952 | |
953 | =head2 How do I do a C<tail -f> in perl? |
d74e8afc |
954 | X<tail> |
68dc0745 |
955 | |
956 | First try |
957 | |
958 | seek(GWFILE, 0, 1); |
959 | |
960 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, |
961 | but it does clear the end-of-file condition on the handle, so that the |
962 | next <GWFILE> makes Perl try again to read something. |
963 | |
964 | If that doesn't work (it relies on features of your stdio implementation), |
965 | then you need something more like this: |
966 | |
967 | for (;;) { |
968 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { |
969 | # search for some stuff and put it into files |
970 | } |
971 | # sleep for a while |
972 | seek(GWFILE, $curpos, 0); # seek to where we had been |
973 | } |
974 | |
975 | If this still doesn't work, look into the POSIX module. POSIX defines |
976 | the clearerr() method, which can remove the end of file condition on a |
977 | filehandle. The method: read until end of file, clearerr(), read some |
978 | more. Lather, rinse, repeat. |
979 | |
65acb1b1 |
980 | There's also a File::Tail module from CPAN. |
981 | |
68dc0745 |
982 | =head2 How do I dup() a filehandle in Perl? |
d74e8afc |
983 | X<dup> |
68dc0745 |
984 | |
985 | If you check L<perlfunc/open>, you'll see that several of the ways |
986 | to call open() should do the trick. For example: |
987 | |
2359510d |
988 | open(LOG, ">>/foo/logfile"); |
68dc0745 |
989 | open(STDERR, ">&LOG"); |
990 | |
991 | Or even with a literal numeric descriptor: |
992 | |
993 | $fd = $ENV{MHCONTEXTFD}; |
994 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) |
995 | |
c47ff5f1 |
996 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 |
997 | an alias. That means if you close an aliased handle, all |
197aec24 |
998 | aliases become inaccessible. This is not true with |
5a964f20 |
999 | a copied one. |
1000 | |
1001 | Error checking, as always, has been left as an exercise for the reader. |
68dc0745 |
1002 | |
1003 | =head2 How do I close a file descriptor by number? |
d74e8afc |
1004 | X<file, closing file descriptors> |
68dc0745 |
1005 | |
1006 | This should rarely be necessary, as the Perl close() function is to be |
1007 | used for things that Perl opened itself, even if it was a dup of a |
a6dd486b |
1008 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 |
1009 | to, you may be able to do this: |
1010 | |
1011 | require 'sys/syscall.ph'; |
1012 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric |
1013 | die "can't sysclose $fd: $!" unless $rc == -1; |
1014 | |
a6dd486b |
1015 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 |
1016 | |
197aec24 |
1017 | { |
1018 | local *F; |
d92eb7b0 |
1019 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
1020 | close F; |
1021 | } |
1022 | |
883f1635 |
1023 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
d74e8afc |
1024 | X<filename, DOS issues> |
68dc0745 |
1025 | |
1026 | Whoops! You just put a tab and a formfeed into that filename! |
1027 | Remember that within double quoted strings ("like\this"), the |
1028 | backslash is an escape character. The full list of these is in |
1029 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't |
1030 | have a file called "c:(tab)emp(formfeed)oo" or |
65acb1b1 |
1031 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 |
1032 | |
1033 | Either single-quote your strings, or (preferably) use forward slashes. |
46fc3d4c |
1034 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 |
1035 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b |
1036 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 |
1037 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
1038 | are more portable, too. |
68dc0745 |
1039 | |
1040 | =head2 Why doesn't glob("*.*") get all the files? |
d74e8afc |
1041 | X<glob> |
68dc0745 |
1042 | |
1043 | Because even on non-Unix ports, Perl's glob function follows standard |
46fc3d4c |
1044 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 |
1045 | files. This makes glob() portable even to legacy systems. Your |
1046 | port may include proprietary globbing functions as well. Check its |
1047 | documentation for details. |
68dc0745 |
1048 | |
1049 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? |
1050 | |
06a5f41f |
1051 | This is elaborately and painstakingly described in the |
1052 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To |
49d635f9 |
1053 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . |
68dc0745 |
1054 | |
1055 | The executive summary: learn how your filesystem works. The |
1056 | permissions on a file say what can happen to the data in that file. |
1057 | The permissions on a directory say what can happen to the list of |
1058 | files in that directory. If you delete a file, you're removing its |
1059 | name from the directory (so the operation depends on the permissions |
1060 | of the directory, not of the file). If you try to write to the file, |
1061 | the permissions of the file govern whether you're allowed to. |
1062 | |
1063 | =head2 How do I select a random line from a file? |
d74e8afc |
1064 | X<file, selecting a random line> |
68dc0745 |
1065 | |
1066 | Here's an algorithm from the Camel Book: |
1067 | |
1068 | srand; |
1069 | rand($.) < 1 && ($line = $_) while <>; |
1070 | |
49d635f9 |
1071 | This has a significant advantage in space over reading the whole file |
1072 | in. You can find a proof of this method in I<The Art of Computer |
1073 | Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. |
1074 | |
1075 | You can use the File::Random module which provides a function |
1076 | for that algorithm: |
1077 | |
1078 | use File::Random qw/random_line/; |
1079 | my $line = random_line($filename); |
1080 | |
1081 | Another way is to use the Tie::File module, which treats the entire |
1082 | file as an array. Simply access a random array element. |
68dc0745 |
1083 | |
65acb1b1 |
1084 | =head2 Why do I get weird spaces when I print an array of lines? |
1085 | |
1086 | Saying |
1087 | |
1088 | print "@lines\n"; |
1089 | |
1090 | joins together the elements of C<@lines> with a space between them. |
1091 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above |
a6dd486b |
1092 | statement would print |
65acb1b1 |
1093 | |
1094 | little fluffy clouds |
1095 | |
1096 | but if each element of C<@lines> was a line of text, ending a newline |
1097 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: |
1098 | |
1099 | little |
1100 | fluffy |
1101 | clouds |
1102 | |
1103 | If your array contains lines, just print them: |
1104 | |
1105 | print @lines; |
1106 | |
68dc0745 |
1107 | =head1 AUTHOR AND COPYRIGHT |
1108 | |
7678cced |
1109 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
1110 | other authors as noted. All rights reserved. |
5a964f20 |
1111 | |
5a7beb56 |
1112 | This documentation is free; you can redistribute it and/or modify it |
1113 | under the same terms as Perl itself. |
c8db1d39 |
1114 | |
87275199 |
1115 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 |
1116 | domain. You are permitted and encouraged to use this code and any |
1117 | derivatives thereof in your own programs for fun or for profit as you |
1118 | see fit. A simple comment in the code giving credit to the FAQ would |
1119 | be courteous but is not required. |