Running a CGI Script on a Web Server

For many years I have been writing Perl scripts to process ASCII files of one sort or another on my computer. I typically do this when I need to reformat or tidy up a series of HTML pages, for example.

To run a Perl script that is installed on your computer, which needs to process one or more files on your computer, and where the Perl interpreter is also installed on your computer, is very simple - you just need to double-click the perl script and it does the business - assuming that everything is set up correctly of course, for example, the location of the perl.exe program is defined in your path. You can also open a DOS window and type perl perlfile.pl to run a script (where perlfile.pl is the name of the Perl script you want to run).

However, when it comes to running a Perl script, or CGI script, on a web server, things can be a bit trickier - not too tricky, but a bit trickier.

In this article I'll look at two versions of the same script: one that will run quite happily on a local machine (by double-clicking the script, for example), and one that will run on a web server.

The script itself is very simple - it opens a file, changes some text inside the file, and then saves the file under a different name.

Version 1 - the local version

Here is version 1 of the script. This is the version that will run locally on a computer, without a web server is sight. Note that I've inserted spaces at appropriate places to prevent the code from being processed by your browser. I've done this wherever necessary in this article.

localScript.pl

$name = "before.htm" or die "cannot assign to variable: $!";
rename $name, "$name.bak" or die "cannot rename: $!";
open (IN, "<$name.bak") or die "cannot open: $!";
open (OUT, ">$name") or die "cannot create: $!";
undef $/;
while ($line = < IN >) {
$line =~ s/hello world/goodbye cruel world/s;
(print OUT $line);
}
close (OUT);
close (IN);
rename "before.htm", "after.htm";
rename "before.htm.bak", "before.htm";
 

This script opens a file called before.htm, uses a regular expression to change the string 'hello world' to 'goodbye cruel world', and writes the contents to a file called after.htm. If after.htm does not exist, it is created.

The file before.htm simply contains one line - hello world. So it's not even a proper HTML file in fact, but that doesn't matter for this exercise as it's the script that's important, not the file that's being processed.

Version 2 - the web server version

Here's the web server version of the script. It contains everything that's in version 1, plus a bit more. Again, I've inserted spaces where appropriate to ensure that the code displays correctly in your browser.

webScript.cgi

#!/usr/bin/perl -w
use CGI qw(:all);
use CGI::Carp qw(fatalsToBrowser warningsToBrowser);
warningsToBrowser(1);
use strict;
print header;
my $name = "before.htm" or die "cannot assign to variable: $!";
rename $name, "$name.bak" or die "cannot rename: $!";
open (IN, "<$name.bak") or die "cannot open: $!";
open (OUT, ">$name") or die "cannot create: $!";
undef $/;
my $line;
while ($line = < IN >) {
$line =~ s/hello world/goodbye cruel world/s;
(print OUT $line);
}
close (OUT);
close (IN);
rename "before.htm", "after.htm";
rename "before.htm.bak", "before.htm";
 

A couple of initial points to note:

1. Why does the script have a .cgi extension instead of a .pl extension? CGI is an abbreviation for Common Gateway Interface, which is a specification for transferring information between a web server and a CGI script. So CGI iteself is not a language, but CGI scripts can be written in a number of languages of which Perl is one. If you write a Perl script with a .pl extension, and then change that extension to .cgi, the script becomes a CGI script, and providing it conforms to the CGI specification, it will run on a web server.

2. If you call this script webScript.pl it will run without any problems on a local disk - just as version 1 did. That's to say, all the extra code will not prevent it from running locally.

Ok, lets go through the script line by line to see what's going on.

Running the script

To run the script you need to first upload the script and the file before.htm into the cgi-bin directory on your web server. On your web server the cgi-bin directory might be called something else, but it will probably be recognizable as the place where cgi scripts need to be located.

By default, when you upload a file onto your web server it will probably have permissions of 644. You need to change these to 755 so that the script can be run by anyone. Your ISP should provide you with a way to do this. If not, contact me at john@dixondevelopment.co.uk and I'll send you a script to change the permissions for you.

Once you have uploaded the files and changed the permissions, all you need to do is browse to the script in your favorite browser. If the browser window is blank, then everything has probably worked OK. Check in your cgi-bin directory to see if the file after.htm has been created and that it contains the words 'goodbye cruel world'.

About the Author: John Dixon is a web developer and technical author. He enjoys developing database-driven websites using PHP, Perl, and MySQL. John is also interested in anything to do with computer history. Visit http://www.computernostalgia.net/listDocumentsOutline.htm to see the current list of computer history articles on his site. To find out more about John, visit http://www.dixondevelopment.co.uk.