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CONTENTS

NAME

perlopentut - simple recipes for opening files and pipes in Perl

DESCRIPTION

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:

OK

will be some defined value if the open succeeds, but undef if it fails;

HANDLE

should be an undefined scalar variable to be filled in by the open function if it succeeds;

MODE

is the access mode and the encoding format to open the file with;

PATHNAME

is the external name of the file you want opened.

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 perlfaq5 for how to lock.

Opening Text Files

Opening Text Files for Reading

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 perlunitut for more about encodings.

Opening Text Files for Writing

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 perlfaq5 for details.

Opening Binary Files

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: $!";

Opening Pipes

Perl also lets you open a filehandle into an external program or shell command rather than into a file. You can do this in order to pass data from your Perl program to an external command for further processing, or to receive data from another program for your own Perl program to process.

Filehandles into commands are also known as pipes, since they work on similar inter-process communication principles as Unix pipelines. Such a filehandle has an active program instead of a static file on its external end, but in every other sense it works just like a more typical file-based filehandle, with all the techniques discussed earlier in this article just as applicable.

As such, you open a pipe using the same open call that you use for opening files, setting the second (MODE) argument to special characters that indicate either an input or an output pipe. Use "-|" for a filehandle that will let your Perl program read data from an external program, and "|-" for a filehandle that will send data to that program instead.

Opening a pipe for reading

Let's say you'd like your Perl program to process data stored in a nearby directory called unsorted, which contains a number of textfiles. You'd also like your program to sort all the contents from these files into a single, alphabetically sorted list of unique lines before it starts processing them.

You could do this through opening an ordinary filehandle into each of those files, gradually building up an in-memory array of all the file contents you load this way, and finally sorting and filtering that array when you've run out of files to load. Or, you could offload all that merging and sorting into your operating system's own sort command by opening a pipe directly into its output, and get to work that much faster.

Here's how that might look:

open(my $sort_fh, '-|', 'sort -u unsorted/*.txt')
    or die "Couldn't open a pipe into sort: $!";

# And right away, we can start reading sorted lines:
while (my $line = <$sort_fh>) {
    #
    # ... Do something interesting with each $line here ...
    #
}

The second argument to open, "-|", makes it a read-pipe into a separate program, rather than an ordinary filehandle into a file.

Note that the third argument to open is a string containing the program name (sort) plus all its arguments: in this case, -u to specify unqiue sort, and then a fileglob specifying the files to sort. The resulting filehandle $sort_fh works just like a read-only ("<") filehandle, and your program can subsequently read data from it as if it were opened onto an ordinary, single file.

Opening a pipe for writing

Continuing the previous example, let's say that your program has completed its processing, and the results sit in an array called @processed. You want to print these lines to a file called numbered.txt with a neatly formatted column of line-numbers.

Certainly you could write your own code to do this — or, once again, you could kick that work over to another program. In this case, cat, running with its own -n option to activate line numbering, should do the trick:

open(my $cat_fh, '|-', 'cat -n > numbered.txt')
    or die "Couldn't open a pipe into cat: $!";

for my $line (@processed) {
    print $cat_fh $line;
}

Here, we use a second open argument of "|-", signifying that the filehandle assigned to $cat_fh should be a write-pipe. We can then use it just as we would a write-only ordinary filehandle, including the basic function of print-ing data to it.

Note that the third argument, specifying the command that we wish to pipe to, sets up cat to redirect its output via that ">" symbol into the file numbered.txt. This can start to look a little tricky, because that same symbol would have meant something entirely different had it showed it in the second argument to open! But here in the third argument, it's simply part of the shell command that Perl will open the pipe into, and Perl itself doesn't invest any special meaning to it.

Expressing the command as a list

For opening pipes, Perl offers the option to call open with a list comprising the desired command and all its own arguments as separate elements, rather than combining them into a single string as in the examples above. For instance, we could have phrased the open call in the first example like this:

open(my $sort_fh, '-|', 'sort', '-u', glob('unsorted/*.txt'))
    or die "Couldn't open a pipe into sort: $!";

When you call open this way, Perl invokes the given command directly, bypassing the shell. As such, the shell won't try to interpret any special characters within the command's argument list, which might overwise have unwanted effects. This can make for safer, less error-prone open calls, useful in cases such as passing in variables as arguments, or even just referring to filenames with spaces in them.

However, when you do want to pass a meaningful metacharacter to the shell, such with the "*" inside that final unsorted/*.txt argument here, you can't use this alternate syntax. In this case, we have worked around it via Perl's handy glob built-in function, which evaluates its argument into a list of filenames — and we can safely pass that resulting list right into open, as shown above.

Note also that representing piped-command arguments in list form like this doesn't work on every platform. It will work on any Unix-based OS that provides a real fork function (e.g. macOS or Linux), as well as on Windows when running Perl 5.22 or later.

SEE ALSO

The full documentation for open provides a thorough reference to this function, beyond the best-practice basics covered here.

AUTHOR and COPYRIGHT

Copyright 2013 Tom Christiansen; now maintained by Perl5 Porters

This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.