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perlfaq5 - Files and Formats ($Revision: 10126 $)
This section deals with I/O and the "f" issues: filehandles, flushing,
formats, and footers.
Perl does not support truly unbuffered output (except insofar as you
can 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 print() and
write() functions normally buffer output, while syswrite()
bypasses buffering all together.
If you want your output to be sent immediately when you execute
print() or write() (for instance, for some network protocols),
you must set the handle's autoflush flag. This flag is the Perl
variable $| and when it is set to a true value, Perl will flush the
handle's buffer after each print() or write(). Setting $|
affects buffering only for the currently selected default filehandle.
You choose this handle with the one argument select() call (see
$| in the perlvar manpage and select in the perlfunc manpage).
Use 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 IO::Handle:
use IO::Handle;
open my( $printer ), ">", "/dev/printer");
$printer->autoflush(1);
or IO::Socket (which inherits from IO::Handle):
use IO::Socket;
my $sock = IO::Socket::INET->new( 'www.example.com:80' );
$sock->autoflush();
You can also flush an IO::Handle object without setting
autoflush. Call the flush method to flush the buffer yourself:
use IO::Handle;
open my( $printer ), ">", "/dev/printer");
$printer->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, $/, is mutable), although modules
such as 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: $!";
print "# Add this line to the top\n";
while( <$in> )
{
print $out $_;
}
close $out;
To change existing lines, insert the code to modify the lines inside
the 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, $., is
useful. First read and print the lines up to the one you want to
change. Next, read the single line you want to change, change it, and
print it. After that, read the rest of the lines and print those:
while( <$in> )
{
print $out $_;
last if $. == 4;
}
my $line = <$in>;
$line =~ s/\b(perl)\b/Perl/g;
print $out $line;
while( <$in> )
{
print $out $_;
}
To skip lines, use the looping controls. The C<next> in this example
skips comment lines, and the C<last> stops all processing once it
encounters either C<
while( <$in> )
{
next if /^\s+#/;
last if /^__(END|DATA)__$/;
print $out $_;
}
Do the same sort of thing to delete a particular line by using next
to skip the lines you don't want to show up in the output. This
example skips every fifth line:
while( <$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> };
print $out @lines;
Modules such as File::Slurp and 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 inFile.txt, overwriting
the file with the new contents. With the -p switch, Perl wraps a
while loop around the code you specify with -e, and -i turns
on in-place editing. The current line is in $_. With -p, Perl
automatically prints the value of $_ at the end of the loop. See
the perlrun manpage for more details.
perl -pi -e 's/Fred/Barney/' inFile.txt
To make a backup of inFile.txt, give -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 $., 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 $_:
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 -n switch.
It's just like -p except that it doesn't print $_ at the end of
the loop, so you have to do that yourself. In this case, print $_
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
One fairly efficient way is to count newlines in the file. The
following program uses a feature of tr///, as documented in the perlop manpage.
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.
-i sets the value of Perl's $^I variable, which in turn affects
the behavior of <>; see the perlrun manpage for more details. By
modifying the appropriate variables directly, you can get the same
behavior within a larger program. For example:
{
local | 
