Hello World with plain CGI
CGI, the Common Gateway Interface was the real workhorse behind the early years of the Internet and while there are other, more modern alternatives, such as PSGI, we still encounter environments where only CGI scripts are used or only CGI script can be used.
This article for those who are stuck in such environments.
First of all you'll have to make sure your web server was properly set up to serve CGI script. Once you have that we have 3 solution here.
CGI without any module
The first one does not use any module.
#!/usr/bin/perl use strict; use warnings; print "Content-type: text/html\n\n"; print "Hello World!";
The hash-bang line, the first line in our code, has to point to the perl interpreter/compiler. Then we have the use strict; and use warnings; statements. They are not really required here, but I would not drive without seat-belts either.
Then we need to print the response HTTP header, or at least one line of it. We have to print Content-type: text/html followed by two new-lines. Actually it is the header line Content-type: text/html\n followed by an empty row which is created by the second \n.
Then we can print the HTML itself which in our case will be a simple string saying Hello World!.
In order to make this work we have to make the program executable. We do that using the following command:
$ chmod +x plain-cgi-hello-world.pl
Then, if our web server supports CGI, and if we put the file in the correct place. we can access it via the web server and see the results.
Using the CGI module
For many many years the CGI module was part of the standard distribution of Perl. Unfortunately it was marked as deprecated in version 5.19.7 and then it was removed from perl version 5.22. You can still install it though.
#!/usr/bin/perl use strict; use warnings; use CGI; my $q = CGI->new; print $q->header; print "Hello World!";
Actually it does not provide a lot of value to this example. Nevertheless it is important to see how to use this module, so later we can build on this knowledge.
The main thing it can do for us is to help creating the header line. For this we need to create an instance of the CGI class by calling new. Then we can call the header method that will return the Content-type line followed by two new lines.
If you already have to install something then probably CGI::Simple is a better choice than the old CGI module. It is smaller and it provides everything you'll need for a CGI script.
#!/usr/bin/perl use strict; use warnings; use CGI::Simple; my $q = CGI::Simple->new; print $q->header; print "Hello World!";
The solution for this simple exercise is the same. We load the CGI::Simple module. Create a new instance and call the header method to get the Content-type line.
Using CGI, or CGI::Simple might be easy, but if you'd like to make your application more future-proof, then you'd better write it using PSGI/Plack. Even if you use the CGI mode of Plack. You can see how that's done in two separate articles: Hello World in Plack CGI mode and Getting Started with PSGI/Plack.