| perlopentut - simple recipes for opening files and pipes in Perl |
perlopentut - simple recipes for opening files and pipes in Perl
Whenever you do I/O on a file in Perl, you do so through what in Perl is
called a filehandle. A filehandle is an internal name for an external
file. It is the job of the open function to make the association
between the internal name and the external name, and it is the job
of the close function to break that association.
For your convenience, Perl sets up a few special filehandles that are
already open when you run. These include STDIN, STDOUT, STDERR,
and ARGV. Since those are pre-opened, you can use them right away
without having to go to the trouble of opening them yourself:
print STDERR "This is a debugging message.\n";
print STDOUT "Please enter something: ";
$response = <STDIN> // die "how come no input?";
print STDOUT "Thank you!\n";
while (<ARGV>) { ... }
As you see from those examples, STDOUT and STDERR are output
handles, and STDIN and ARGV are input handles. They are
in all capital letters because they are reserved to Perl, much
like the @ARGV array and the %ENV hash are. Their external
associations were set up by your shell.
You will need to open every other filehandle on your own. Although there
are many variants, the most common way to call Perl's open() function
is with three arguments and one return value:
OK = open(HANDLE, MODE, PATHNAME)
Where:
undef if it fails;
open function if it succeeds;
Most of the complexity of the open function lies in the many
possible values that the MODE parameter can take on.
One last thing before we show you how to open files: opening files does not (usually) automatically lock them in Perl. See the perlfaq5 manpage for how to lock.
If you want to read from a text file, first open it in read-only mode like this:
my $filename = "/some/path/to/a/textfile/goes/here";
my $encoding = ":encoding(UTF-8)";
my $handle = undef; # this will be filled in on success
open($handle, "< $encoding", $filename)
|| die "$0: can't open $filename for reading: $!";
As with the shell, in Perl the "<" is used to open the file in
read-only mode. If it succeeds, Perl allocates a brand new filehandle for
you and fills in your previously undefined $handle argument with a
reference to that handle.
Now you may use functions like readline, read, getc, and
sysread on that handle. Probably the most common input function
is the one that looks like an operator:
$line = readline($handle);
$line = <$handle>; # same thing
Because the readline function returns undef at end of file or
upon error, you will sometimes see it used this way:
$line = <$handle>;
if (defined $line) {
# do something with $line
}
else {
# $line is not valid, so skip it
}
You can also just quickly die on an undefined value this way:
$line = <$handle> // die "no input found";
However, if hitting EOF is an expected and normal event, you do not want to exit simply because you have run out of input. Instead, you probably just want to exit an input loop. You can then test to see if an actual error has caused the loop to terminate, and act accordingly:
while (<$handle>) {
# do something with data in $_
}
if ($!) {
die "unexpected error while reading from $filename: $!";
}
A Note on Encodings: Having to specify the text encoding every time
might seem a bit of a bother. To set up a default encoding for open so
that you don't have to supply it each time, you can use the open pragma:
use open qw< :encoding(UTF-8) >;
Once you've done that, you can safely omit the encoding part of the open mode:
open($handle, "<", $filename)
|| die "$0: can't open $filename for reading: $!";
But never use the bare "<" without having set up a default encoding
first. Otherwise, Perl cannot know which of the many, many, many possible
flavors of text file you have, and Perl will have no idea how to correctly
map the data in your file into actual characters it can work with. Other
common encoding formats including "ASCII", "ISO-8859-1",
"ISO-8859-15", "Windows-1252", "MacRoman", and even "UTF-16LE".
See the perlunitut manpage for more about encodings.
When you want to write to a file, you first have to decide what to do about any existing contents of that file. You have two basic choices here: to preserve or to clobber.
If you want to preserve any existing contents, then you want to open the file
in append mode. As in the shell, in Perl you use ">>" to open an
existing file in append mode. ">>" creates the file if it does not
already exist.
my $handle = undef;
my $filename = "/some/path/to/a/textfile/goes/here";
my $encoding = ":encoding(UTF-8)";
open($handle, ">> $encoding", $filename)
|| die "$0: can't open $filename for appending: $!";
Now you can write to that filehandle using any of print, printf,
say, write, or syswrite.
As noted above, if the file does not already exist, then the append-mode open will create it for you. But if the file does already exist, its contents are safe from harm because you will be adding your new text past the end of the old text.
On the other hand, sometimes you want to clobber whatever might already be there. To empty out a file before you start writing to it, you can open it in write-only mode:
my $handle = undef;
my $filename = "/some/path/to/a/textfile/goes/here";
my $encoding = ":encoding(UTF-8)";
open($handle, "> $encoding", $filename)
|| die "$0: can't open $filename in write-open mode: $!";
Here again Perl works just like the shell in that the ">" clobbers
an existing file.
As with the append mode, when you open a file in write-only mode,
you can now write to that filehandle using any of print, printf,
say, write, or syswrite.
What about read-write mode? You should probably pretend it doesn't exist, because opening text files in read-write mode is unlikely to do what you would like. See the perlfaq5 manpage for details.
If the file to be opened contains binary data instead of text characters,
then the MODE argument to open is a little different. Instead of
specifying the encoding, you tell Perl that your data are in raw bytes.
my $filename = "/some/path/to/a/binary/file/goes/here";
my $encoding = ":raw :bytes"
my $handle = undef; # this will be filled in on success
And then open as before, choosing "<", ">>", or
">" as needed:
open($handle, "< $encoding", $filename)
|| die "$0: can't open $filename for reading: $!";
open($handle, ">> $encoding", $filename)
|| die "$0: can't open $filename for appending: $!";
open($handle, "> $encoding", $filename)
|| die "$0: can't open $filename in write-open mode: $!";
Alternately, you can change to binary mode on an existing handle this way:
binmode($handle) || die "cannot binmode handle";
This is especially handy for the handles that Perl has already opened for you.
binmode(STDIN) || die "cannot binmode STDIN";
binmode(STDOUT) || die "cannot binmode STDOUT";
You can also pass binmode an explicit encoding to change it on the fly.
This isn't exactly ``binary'' mode, but we still use binmode to do it:
binmode(STDIN, ":encoding(MacRoman)") || die "cannot binmode STDIN"; binmode(STDOUT, ":encoding(UTF-8)") || die "cannot binmode STDOUT";
Once you have your binary file properly opened in the right mode, you can
use all the same Perl I/O functions as you used on text files. However,
you may wish to use the fixed-size read instead of the variable-sized
readline for your input.
Here's an example of how to copy a binary file:
my $BUFSIZ = 64 * (2 ** 10);
my $name_in = "/some/input/file";
my $name_out = "/some/output/flie";
my($in_fh, $out_fh, $buffer);
open($in_fh, "<", $name_in)
|| die "$0: cannot open $name_in for reading: $!";
open($out_fh, ">", $name_out)
|| die "$0: cannot open $name_out for writing: $!";
for my $fh ($in_fh, $out_fh) {
binmode($fh) || die "binmode failed";
}
while (read($in_fh, $buffer, $BUFSIZ)) {
unless (print $out_fh $buffer) {
die "couldn't write to $name_out: $!";
}
}
close($in_fh) || die "couldn't close $name_in: $!";
close($out_fh) || die "couldn't close $name_out: $!";
To be announced.
To be announced. Or deleted.
To be announced.
Copyright 2013 Tom Christiansen.
This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.
| perlopentut - simple recipes for opening files and pipes in Perl |