5 perlopentut - simple recipes for opening files and pipes in Perl
9 Whenever you do I/O on a file in Perl, you do so through what in Perl is
10 called a B<filehandle>. A filehandle is an internal name for an external
11 file. It is the job of the C<open> function to make the association
12 between the internal name and the external name, and it is the job
13 of the C<close> function to break that association.
15 For your convenience, Perl sets up a few special filehandles that are
16 already open when you run. These include C<STDIN>, C<STDOUT>, C<STDERR>,
17 and C<ARGV>. Since those are pre-opened, you can use them right away
18 without having to go to the trouble of opening them yourself:
20 print STDERR "This is a debugging message.\n";
22 print STDOUT "Please enter something: ";
23 $response = <STDIN> // die "how come no input?";
24 print STDOUT "Thank you!\n";
26 while (<ARGV>) { ... }
28 As you see from those examples, C<STDOUT> and C<STDERR> are output
29 handles, and C<STDIN> and C<ARGV> are input handles. They are
30 in all capital letters because they are reserved to Perl, much
31 like the C<@ARGV> array and the C<%ENV> hash are. Their external
32 associations were set up by your shell.
34 You will need to open every other filehandle on your own. Although there
35 are many variants, the most common way to call Perl's open() function
36 is with three arguments and one return value:
38 C< I<OK> = open(I<HANDLE>, I<MODE>, I<PATHNAME>)>
46 will be some defined value if the open succeeds, but
51 should be an undefined scalar variable to be filled in by the
52 C<open> function if it succeeds;
56 is the access mode and the encoding format to open the file with;
60 is the external name of the file you want opened.
64 Most of the complexity of the C<open> function lies in the many
65 possible values that the I<MODE> parameter can take on.
67 One last thing before we show you how to open files: opening
68 files does not (usually) automatically lock them in Perl. See
69 L<perlfaq5> for how to lock.
71 =head1 Opening Text Files
73 =head2 Opening Text Files for Reading
75 If you want to read from a text file, first open it in
76 read-only mode like this:
78 my $filename = "/some/path/to/a/textfile/goes/here";
79 my $encoding = ":encoding(UTF-8)";
80 my $handle = undef; # this will be filled in on success
82 open($handle, "< $encoding", $filename)
83 || die "$0: can't open $filename for reading: $!";
85 As with the shell, in Perl the C<< "<" >> is used to open the file in
86 read-only mode. If it succeeds, Perl allocates a brand new filehandle for
87 you and fills in your previously undefined C<$handle> argument with a
88 reference to that handle.
90 Now you may use functions like C<readline>, C<read>, C<getc>, and
91 C<sysread> on that handle. Probably the most common input function
92 is the one that looks like an operator:
94 $line = readline($handle);
95 $line = <$handle>; # same thing
97 Because the C<readline> function returns C<undef> at end of file or
98 upon error, you will sometimes see it used this way:
102 # do something with $line
105 # $line is not valid, so skip it
108 You can also just quickly C<die> on an undefined value this way:
110 $line = <$handle> // die "no input found";
112 However, if hitting EOF is an expected and normal event, you do not want to
113 exit simply because you have run out of input. Instead, you probably just want
114 to exit an input loop. You can then test to see if an actual error has caused
115 the loop to terminate, and act accordingly:
118 # do something with data in $_
121 die "unexpected error while reading from $filename: $!";
124 B<A Note on Encodings>: Having to specify the text encoding every time
125 might seem a bit of a bother. To set up a default encoding for C<open> so
126 that you don't have to supply it each time, you can use the C<open> pragma:
128 use open qw< :encoding(UTF-8) >;
130 Once you've done that, you can safely omit the encoding part of the
133 open($handle, "<", $filename)
134 || die "$0: can't open $filename for reading: $!";
136 But never use the bare C<< "<" >> without having set up a default encoding
137 first. Otherwise, Perl cannot know which of the many, many, many possible
138 flavors of text file you have, and Perl will have no idea how to correctly
139 map the data in your file into actual characters it can work with. Other
140 common encoding formats including C<"ASCII">, C<"ISO-8859-1">,
141 C<"ISO-8859-15">, C<"Windows-1252">, C<"MacRoman">, and even C<"UTF-16LE">.
142 See L<perlunitut> for more about encodings.
144 =head2 Opening Text Files for Writing
146 When you want to write to a file, you first have to decide what to do about
147 any existing contents of that file. You have two basic choices here: to
148 preserve or to clobber.
150 If you want to preserve any existing contents, then you want to open the file
151 in append mode. As in the shell, in Perl you use C<<< ">>" >>> to open an
152 existing file in append mode. C<<< ">>" >>> creates the file if it does not
156 my $filename = "/some/path/to/a/textfile/goes/here";
157 my $encoding = ":encoding(UTF-8)";
159 open($handle, ">> $encoding", $filename)
160 || die "$0: can't open $filename for appending: $!";
162 Now you can write to that filehandle using any of C<print>, C<printf>,
163 C<say>, C<write>, or C<syswrite>.
165 As noted above, if the file does not already exist, then the append-mode open
166 will create it for you. But if the file does already exist, its contents are
167 safe from harm because you will be adding your new text past the end of the
170 On the other hand, sometimes you want to clobber whatever might already be
171 there. To empty out a file before you start writing to it, you can open it
175 my $filename = "/some/path/to/a/textfile/goes/here";
176 my $encoding = ":encoding(UTF-8)";
178 open($handle, "> $encoding", $filename)
179 || die "$0: can't open $filename in write-open mode: $!";
181 Here again Perl works just like the shell in that the C<< ">" >> clobbers
184 As with the append mode, when you open a file in write-only mode,
185 you can now write to that filehandle using any of C<print>, C<printf>,
186 C<say>, C<write>, or C<syswrite>.
188 What about read-write mode? You should probably pretend it doesn't exist,
189 because opening text files in read-write mode is unlikely to do what you
190 would like. See L<perlfaq5> for details.
192 =head1 Opening Binary Files
194 If the file to be opened contains binary data instead of text characters,
195 then the C<MODE> argument to C<open> is a little different. Instead of
196 specifying the encoding, you tell Perl that your data are in raw bytes.
198 my $filename = "/some/path/to/a/binary/file/goes/here";
199 my $encoding = ":raw :bytes"
200 my $handle = undef; # this will be filled in on success
202 And then open as before, choosing C<<< "<" >>>, C<<< ">>" >>>, or
203 C<<< ">" >>> as needed:
205 open($handle, "< $encoding", $filename)
206 || die "$0: can't open $filename for reading: $!";
208 open($handle, ">> $encoding", $filename)
209 || die "$0: can't open $filename for appending: $!";
211 open($handle, "> $encoding", $filename)
212 || die "$0: can't open $filename in write-open mode: $!";
214 Alternately, you can change to binary mode on an existing handle this way:
216 binmode($handle) || die "cannot binmode handle";
218 This is especially handy for the handles that Perl has already opened for you.
220 binmode(STDIN) || die "cannot binmode STDIN";
221 binmode(STDOUT) || die "cannot binmode STDOUT";
223 You can also pass C<binmode> an explicit encoding to change it on the fly.
224 This isn't exactly "binary" mode, but we still use C<binmode> to do it:
226 binmode(STDIN, ":encoding(MacRoman)") || die "cannot binmode STDIN";
227 binmode(STDOUT, ":encoding(UTF-8)") || die "cannot binmode STDOUT";
229 Once you have your binary file properly opened in the right mode, you can
230 use all the same Perl I/O functions as you used on text files. However,
231 you may wish to use the fixed-size C<read> instead of the variable-sized
232 C<readline> for your input.
234 Here's an example of how to copy a binary file:
236 my $BUFSIZ = 64 * (2 ** 10);
237 my $name_in = "/some/input/file";
238 my $name_out = "/some/output/flie";
240 my($in_fh, $out_fh, $buffer);
242 open($in_fh, "<", $name_in)
243 || die "$0: cannot open $name_in for reading: $!";
244 open($out_fh, ">", $name_out)
245 || die "$0: cannot open $name_out for writing: $!";
247 for my $fh ($in_fh, $out_fh) {
248 binmode($fh) || die "binmode failed";
251 while (read($in_fh, $buffer, $BUFSIZ)) {
252 unless (print $out_fh $buffer) {
253 die "couldn't write to $name_out: $!";
257 close($in_fh) || die "couldn't close $name_in: $!";
258 close($out_fh) || die "couldn't close $name_out: $!";
264 =head1 Low-level File Opens via sysopen
266 To be announced. Or deleted.
272 =head1 AUTHOR and COPYRIGHT
274 Copyright 2013 Tom Christiansen.
276 This documentation is free; you can redistribute it and/or modify it under
277 the same terms as Perl itself.