Ubuntu Manpage: perlopentut - simple recipes for opening files and pipes in Perl

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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.

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