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PERLOPENTUT(1)	       Perl Programmers	Reference Guide		PERLOPENTUT(1)

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
       To be announced.

Low-level File Opens via sysopen
       To be announced.	 Or deleted.

SEE ALSO
       To be announced.

AUTHOR and COPYRIGHT
       Copyright 2013 Tom Christiansen.

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

perl v5.26.0			  2017-04-19			PERLOPENTUT(1)

NAME | DESCRIPTION | Opening Text Files | Opening Binary Files | Opening Pipes | Low-level File Opens via sysopen | SEE ALSO | AUTHOR and COPYRIGHT

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