perlfaq5.pod

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=head1 NAME

perlfaq5 - Files and Formats ($Revision: 10126 $)

=head1 DESCRIPTION

This section deals with I/O and the "f" issues: filehandles, flushing,
formats, and footers.

=head2 How do I flush/unbuffer an output filehandle?  Why must I do this?
X<flush> X<buffer> X<unbuffer> X<autoflush>

Perl does not support truly unbuffered output (except insofar as you
can C<syswrite(OUT, $char, 1)>), although it does support is "command
buffering", in which a physical write is performed after every output
command.

The C standard I/O library (stdio) normally buffers characters sent to
devices so that there isn't a system call for each byte. In most stdio
implementations, the type of output buffering and the size of the
buffer varies according to the type of device. Perl's C<print()> and
C<write()> functions normally buffer output, while C<syswrite()>
bypasses buffering all together.

If you want your output to be sent immediately when you execute
C<print()> or C<write()> (for instance, for some network protocols),
you must set the handle's autoflush flag. This flag is the Perl
variable C<$|> and when it is set to a true value, Perl will flush the
handle's buffer after each C<print()> or C<write()>. Setting C<$|>
affects buffering only for the currently selected default filehandle.
You choose this handle with the one argument C<select()> call (see
L<perlvar/$E<verbar>> and L<perlfunc/select>).

Use C<select()> to choose the desired handle, then set its
per-filehandle variables.

	$old_fh = select(OUTPUT_HANDLE);
	$| = 1;
	select($old_fh);

Some modules offer object-oriented access to handles and their
variables, although they may be overkill if this is the only thing you
do with them.  You can use C<IO::Handle>:

	use IO::Handle;
	open my( $printer ), ">", "/dev/printer");   # but is this?
	$printer->autoflush(1);

or C<IO::Socket> (which inherits from C<IO::Handle>):

	use IO::Socket;		  # this one is kinda a pipe?
	my $sock = IO::Socket::INET->new( 'www.example.com:80' );

	$sock->autoflush();

You can also flush an C<IO::Handle> object without setting
C<autoflush>. Call the C<flush> method to flush the buffer yourself:

	use IO::Handle;
	open my( $printer ), ">", "/dev/printer"); 
	$printer->flush; # one time flush

	
=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
X<file, editing>

(contributed by brian d foy)

The basic idea of inserting, changing, or deleting a line from a text
file involves reading and printing the file to the point you want to
make the change, making the change, then reading and printing the rest
of the file. Perl doesn't provide random access to lines (especially
since the record input separator, C<$/>, is mutable), although modules
such as C<Tie::File> can fake it.

A Perl program to do these tasks takes the basic form of opening a
file, printing its lines, then closing the file:

	open my $in,  '<',  $file      or die "Can't read old file: $!";
	open my $out, '>', "$file.new" or die "Can't write new file: $!";

	while( <$in> )
		{
		print $out $_;
		}

   close $out;

Within that basic form, add the parts that you need to insert, change,
or delete lines.

To prepend lines to the beginning, print those lines before you enter
the loop that prints the existing lines.

	open my $in,  '<',  $file      or die "Can't read old file: $!";
	open my $out, '>', "$file.new" or die "Can't write new file: $!";

	print "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

	while( <$in> )
		{
		print $out $_;
		}

   close $out;

To change existing lines, insert the code to modify the lines inside
the C<while> loop. In this case, the code finds all lowercased
versions of "perl" and uppercases them. The happens for every line, so
be sure that you're supposed to do that on every line!

	open my $in,  '<',  $file      or die "Can't read old file: $!";
	open my $out, '>', "$file.new" or die "Can't write new file: $!";

	print "# Add this line to the top\n";

	while( <$in> )
		{
		s/\b(perl)\b/Perl/g;
		print $out $_;
		}

   close $out;

To change only a particular line, the input line number, C<$.>, is
useful. First read and print the lines up to the one you  want to
change. Next, read the single line you want to change, change it, and
print it. After that, read the rest of the lines and print those:

	while( <$in> )   # print the lines before the change
		{
		print $out $_;
		last if $. == 4; # line number before change
		}

	my $line = <$in>;
	$line =~ s/\b(perl)\b/Perl/g;
	print $out $line;

	while( <$in> )   # print the rest of the lines
		{
		print $out $_;
		}
		
To skip lines, use the looping controls. The C<next> in this example
skips comment lines, and the C<last> stops all processing once it
encounters either C<__END__> or C<__DATA__>.

	while( <$in> )
		{
		next if /^\s+#/;             # skip comment lines
		last if /^__(END|DATA)__$/;  # stop at end of code marker
		print $out $_;
		}

Do the same sort of thing to delete a particular line by using C<next>
to skip the lines you don't want to show up in the output. This
example skips every fifth line:

	while( <$in> )
		{
		next unless $. % 5;
		print $out $_;
		}

If, for some odd reason, you really want to see the whole file at once
rather than processing line by line, you can slurp it in (as long as
you can fit the whole thing in memory!):

	open my $in,  '<',  $file      or die "Can't read old file: $!"
	open my $out, '>', "$file.new" or die "Can't write new file: $!";

	my @lines = do { local $/; <$in> }; # slurp!

		# do your magic here

	print $out @lines;

Modules such as C<File::Slurp> and C<Tie::File> can help with that
too. If you can, however, avoid reading the entire file at once. Perl
won't give that memory back to the operating system until the process
finishes.

You can also use Perl one-liners to modify a file in-place. The
following changes all 'Fred' to 'Barney' in F<inFile.txt>, overwriting
the file with the new contents. With the C<-p> switch, Perl wraps a
C<while> loop around the code you specify with C<-e>, and C<-i> turns
on in-place editing. The current line is in C<$_>. With C<-p>, Perl
automatically prints the value of C<$_> at the end of the loop. See
L<perlrun> for more details.

	perl -pi -e 's/Fred/Barney/' inFile.txt

To make a backup of C<inFile.txt>, give C<-i> a file extension to add:

	perl -pi.bak -e 's/Fred/Barney/' inFile.txt

To change only the fifth line, you can add a test checking C<$.>, the
input line number, then only perform the operation when the test
passes:

	perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

To add lines before a certain line, you can add a line (or lines!)
before Perl prints C<$_>:

	perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

You can even add a line to the beginning of a file, since the current
line prints at the end of the loop:

	perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

To insert a line after one already in the file, use the C<-n> switch.
It's just like C<-p> except that it doesn't print C<$_> at the end of
the loop, so you have to do that yourself. In this case, print C<$_>
first, then print the line that you want to add.

	perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt

To delete lines, only print the ones that you want.

	perl -ni -e 'print unless /d/' inFile.txt

		... or ...

	perl -pi -e 'next unless /d/' inFile.txt

=head2 How do I count the number of lines in a file?
X<file, counting lines> X<lines> X<line>

One fairly efficient way is to count newlines in the file. The
following program uses a feature of tr///, as documented in L<perlop>.
If your text file doesn't end with a newline, then it's not really a
proper text file, so this may report one fewer line than you expect.

	$lines = 0;
	open(FILE, $filename) or die "Can't open `$filename': $!";
	while (sysread FILE, $buffer, 4096) {
		$lines += ($buffer =~ tr/\n//);
		}
	close FILE;

This assumes no funny games with newline translations.

=head2 How can I use Perl's C<-i> option from within a program?
X<-i> X<in-place>

C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
the behavior of C<< <> >>; see L<perlrun> for more details.  By
modifying the appropriate variables directly, you can get the same
behavior within a larger program.  For example:

	# ...
	{
	local($^I, @ARGV) = ('.orig', glob("*.c"));
	while (<>) {
		if ($. == 1) {
			print "This line should appear at the top of each file\n";
		}
		s/\b(p)earl\b/${1}erl/i;        # Correct typos, preserving case
		print;
		close ARGV if eof;              # Reset $.
		}
	}
	# $^I and @ARGV return to their old values here

This block modifies all the C<.c> files in the current directory,
leaving a backup of the original data from each file in a new
C<.c.orig> file.

=head2 How can I copy a file?
X<copy> X<file, copy>

(contributed by brian d foy)

Use the File::Copy module. It comes with Perl and can do a
true copy across file systems, and it does its magic in
a portable fashion.

	use File::Copy;

	copy( $original, $new_copy ) or die "Copy failed: $!";

If you can't use File::Copy, you'll have to do the work yourself:
open the original file, open the destination file, then print
to the destination file as you read the original.

=head2 How do I make a temporary file name?
X<file, temporary>

If you don't need to know the name of the file, you can use C<open()>
with C<undef> in place of the file name.  The C<open()> function
creates an anonymous temporary file.

	open my $tmp, '+>', undef or die $!;

Otherwise, you can use the File::Temp module.

	use File::Temp qw/ tempfile tempdir /;

	$dir = tempdir( CLEANUP => 1 );
	($fh, $filename) = tempfile( DIR => $dir );

	# or if you don't need to know the filename

	$fh = tempfile( DIR => $dir );

The File::Temp has been a standard module since Perl 5.6.1.  If you
don't have a modern enough Perl installed, use the C<new_tmpfile>
class method from the IO::File module to get a filehandle opened for
reading and writing.  Use it if you don't need to know the file's name:

	use IO::File;
	$fh = IO::File->new_tmpfile()
	or die "Unable to make new temporary file: $!";

If you're committed to creating a temporary file by hand, use the
process ID and/or the current time-value.  If you need to have many
temporary files in one process, use a counter:

	BEGIN {
	use Fcntl;
	my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
	my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;

	sub temp_file {
		local *FH;
		my $count = 0;
		until( defined(fileno(FH)) || $count++ > 100 ) {
			$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
			# O_EXCL is required for security reasons.
			sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT;
			}

		if( defined fileno(FH) ) {
			return (*FH, $base_name);
			}
		else {
			return ();
			}
		}
		
	}

=head2 How can I manipulate fixed-record-length files?
X<fixed-length> X<file, fixed-length records>

The most efficient way is using L<pack()|perlfunc/"pack"> and
L<unpack()|perlfunc/"unpack">.  This is faster than using
L<substr()|perlfunc/"substr"> when taking many, many strings.  It is
slower for just a few.

Here is a sample chunk of code to break up and put back together again
some fixed-format input lines, in this case from the output of a normal,
Berkeley-style ps:

	# sample input line:
	#   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
	my $PS_T = 'A6 A4 A7 A5 A*';
	open my $ps, '-|', 'ps';
	print scalar <$ps>;
	my @fields = qw( pid tt stat time command );
	while (<$ps>) {
		my %process;
		@process{@fields} = unpack($PS_T, $_);
	for my $field ( @fields ) {
		print "$field: <$process{$field}>\n";
	}
	print 'line=', pack($PS_T, @process{@fields} ), "\n";
	}

We've used a hash slice in order to easily handle the fields of each row.
Storing the keys in an array means it's easy to operate on them as a
group or loop over them with for. It also avoids polluting the program
with global variables and using symbolic references.

=head2 How can I make a filehandle local to a subroutine?  How do I pass filehandles between subroutines?  How do I make an array of filehandles?
X<filehandle, local> X<filehandle, passing> X<filehandle, reference>

As of perl5.6, open() autovivifies file and directory handles
as references if you pass it an uninitialized scalar variable.
You can then pass these references just like any other scalar,
and use them in the place of named handles.

	open my    $fh, $file_name;

	open local $fh, $file_name;

	print $fh "Hello World!\n";

	process_file( $fh );

If you like, you can store these filehandles in an array or a hash.
If you access them directly, they aren't simple scalars and you
need to give C<print> a little help by placing the filehandle
reference in braces. Perl can only figure it out on its own when
the filehandle reference is a simple scalar.

	my @fhs = ( $fh1, $fh2, $fh3 );

	for( $i = 0; $i <= $#fhs; $i++ ) {
		print {$fhs[$i]} "just another Perl answer, \n";
		}

Before perl5.6, you had to deal with various typeglob idioms
which you may see in older code.

	open FILE, "> $filename";
	process_typeglob(   *FILE );
	process_reference( \*FILE );

	sub process_typeglob  { local *FH = shift; print FH  "Typeglob!" }
	sub process_reference { local $fh = shift; print $fh "Reference!" }

If you want to create many anonymous handles, you should
check out the Symbol or IO::Handle modules.

=head2 How can I use a filehandle indirectly?
X<filehandle, indirect>

An indirect filehandle is using something other than a symbol
in a place that a filehandle is expected.  Here are ways
to get indirect filehandles:

	$fh =   SOME_FH;       # bareword is strict-subs hostile
	$fh =  "SOME_FH";      # strict-refs hostile; same package only
	$fh =  *SOME_FH;       # typeglob
	$fh = \*SOME_FH;       # ref to typeglob (bless-able)
	$fh =  *SOME_FH{IO};   # blessed IO::Handle from *SOME_FH typeglob

Or, you can use the C<new> method from one of the IO::* modules to
create an anonymous filehandle, store that in a scalar variable,
and use it as though it were a normal filehandle.

	use IO::Handle;                     # 5.004 or higher
	$fh = IO::Handle->new();

Then use any of those as you would a normal filehandle.  Anywhere that
Perl is expecting a filehandle, an indirect filehandle may be used
instead. An indirect filehandle is just a scalar variable that contains
a filehandle.  Functions like C<print>, C<open>, C<seek>, or
the C<< <FH> >> diamond operator will accept either a named filehandle