Prepend to a text file

First of all, I think I'd write how to prepend to the beginning of a file or how to prepend to the top of a file, but that's just nit-picking.

The real thing is that you cannot do that easily as the underlying filesystems don't make it easy. The only way you can do is to rewrite the whole file with the new content attached to the beginning of the file.

If the file is relatively small then you can read the whole file into memory into a single scalar variable, prepend whatever string you wanted to add to the beginning of the file, and then write the whole thing out to the disk.

If the file is larger than the available free memory then you'd probably need to do this in chunks using another file as a temporary file.

Let's see the first one, the easier case.

examples/prepend.pl

use strict;
use warnings;

my $str = "text to prepend\n";
my $filename = shift or die "Usage: $0 FILENAME\n";

open my $in,  '<:encoding(utf8)', $filename or die "Could not open '$filename' for reading";
local $/ = undef;  # turn on "slurp mode"
my $content = <$in>;
close $in;

open my $out, '>:encoding(utf8)', $filename or die "Could not open '$filename' for writing";
print $out $str;
print $out $content;

In this one we use the so-called slurp mode to read the content of the file into a string.

Prepend to a file with Path::Tiny

If you already know how to install CPAN modules then you can use Path::Tiny to make this shorter.

examples/prepend_with_path_tiny.pl

use strict;
use warnings;
use Path::Tiny;

my $str = "text to prepend\n";
my $filename = shift or die "Usage: $0 FILENAME\n";

my $content = path($filename)->slurp_utf8;;
path($filename)->spew_utf8($str, $content);