I was working on an example script that used a lot of calls to seek, but I kept bumping into bugs. I thought it might be a useful debugging tool to be able to read and print any section of a file given the location of the first byte and the number of bytes.

So here is a quick script I put together.

examples/read_file_section.pl

use strict;
use warnings;
use feature 'say';
use Fcntl qw(SEEK_SET);

my ($file, $start, $length) = @ARGV;
die "Usage: $0 FILE START LENGTH\n" if not $length;

open my $fh, '<', $file or die;
seek $fh, $start, SEEK_SET;
my $buffer;
read $fh, $buffer, $length;
say "'$buffer'";

The seek command allows you to move the file-reading pointer back and forth in a file so you will be abel to read the file in an arbitrary order. The Fcntl module provides various constants to help you. Using the SEEK_SET constant will tell the seek command to start from the beginning of the file.

The read command allows us to read an arbitrary number of bytes from a file into a variable that people usually call a "buffer" so we used the variable name $buffer for it.

This is a sample data file:

examples/data/planets.txt

Ceres
Charon
Earth
Jupiter
Mars
Mercury
Neptune
Pluto
Saturn
Uranus
Venus


$ perl examples/read_file_section.pl examples/data/planets.txt 48 5
'Pluto'

$ perl examples/read_file_section.pl examples/data/planets.txt 48 6
'Pluto
'