A basic CGI Counter
(how to read and write to a text file)

by pixelatedcyberdust, 1/29/04

Follow along with the comment tags so you understand the script.
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#!/usr/bin/perl
  #This is the location to perl itself on your server
use warnings;
use strict;
  # Warnings and Strict are both security modules to help protect your script. They will
  # warn you if it 'thinks' you're doing something you didn't mean to or it'll flat out
  # die and hopefully give you a reason or an idea on what's wrong.

my $file = "counter.txt";
  # Since we're using Strict, we have to globalize the variable with 'my'. All new non-special
  # system variables will have to be my'd. This is the location of the text file you want to
  # read and write to. It can have folder names such as /cgi-bin/counter.txt but be sure the
  # file already exists otherwise Warnings will kick in.

use CGI qw/:standard/;
  # Since this is a web script (one to be used and run on a web server), we're going to need to
  # use the CGI module. This module makes it easier for us to print HTML to a web page.

open( LOG, "$file" ) or die "Cannot open file $!";
  # we now begin by opening a file for read-only. We open it, give it a filehandle then give it
  # a file location. LOG is our filehandle, but in reality it can be any word you want: COUNTER,
  # NEW or even DOG, but it's best to keep descriptive filenames.

  # The location of $file near the beginning of the script. If for some reason the file cannot
  # be found or opened, "or die" will kick in, crash the script and give you a reason.

my $cnt = <LOG>;
  # Now that LOG is opened, we have to save its contents to a variable. Your variable name
  # must be assigned to the same 'filehandle' as you called it when you opened it on the
  # previous line.

close(LOG);
  # The data is stored in a variable so we no longer have a need to read the file. Close it
  # by closing it's filehandle.

++$cnt;
  # $cnt holds whatever was contained in $file, hopefully it was a number so we can increment it.
  # We are pre-incrementing (++ before the variable) so it calculates its new value before you print it
  # back to the LOG file. ++ will increase the number by 1.

open( LOG, "> $file" );
  # We now have to reopen the file, this time for WRITE only. We do this by adding ">" before the   $file
  # location. We are using the same LOG filehandle for simplicity.

print LOG $cnt;
  # Printing to a file or database is a little different than printing to the screen. For the $cnt
  # to be written to our text file, we have to print to the LOG filehandle. We just printed the new
  # value of $cnt to file.

close(LOG);
  # Since we've written the data we needed, we close the file.

print "$cnt";
  # If you pay close attention, you'll see we're not printing to a filehandle. That means $cnt is going to
  # be printed directly to your web page. A counter wouldn't do any good if the visitor (or you) can't have
  # a visual look at the counter. This prints the current count number to your screen.
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To keep track of the count, you need to use SSI on your web page to initiate this script. You can do so
using <!--#include file="counter.pl" --> in your site. Rename counter.pl to point to the directory and
file of your new script.
 

Completed script:

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#!/usr/bin/perl


use warnings;
use strict;
use CGI qw/:standard/;

my $file = "counter.txt";

open( LOG, "$file" ) or die "Cannot open file $!";
my $cnt = <LOG>;
close(LOG);

++$cnt;

open( LOG, "> $file" ) or die "Cannot open file $!";
print LOG $cnt;
close(LOG);

print "$cnt";
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