Seminar -- Perl Programming II

	ACADEMIC COMPUTING and COMMUNICATIONS CENTER

Seminar - Perl II
0. Contents	1. Intro	2. Sources	3. References	4. Packages	5. Classes
6. Ties & DBM	7. Typeglobs	8. CGI	9. Security	10. Example

CGI

Forms and Scripts

A CGI script is just a program invoked by a web server when an appropriate URL is requested. Arguments are passed (usually, but not necessarily, from a web form) and put into the environment. The script then computes a response (usually, but not necessarily, an html page) and returns it to the user. Some issues:

Obtaining input parameters
Response options
Simultaneous access to the script
Security
Debugging

You probably already know that you can run CGI scripts at UIC by using perlwrap.

Environment Variables

One type of input comes from the web server, and is placed in the environment, accessible via the pseudo-hash %ENV. All you need to know is what's available. I usually check NCSA for a complete list, but here are some favorites:

REMOTE_HOST: Name (if available) or IP address of browser
REFERER: The refering URL
HOST_NAME: The machine you are on
SCRIPT_NAME: The path part of the url up to the script name
PATH_TRANSLATED: The path after the script, as it would refer to the local file system.

You can always write a CGI script to just print out the envrionment if you want to see what you get.

Parameter Passing

The second class of input, that from the browser request, depends on the method, GET or POST. Also, this input is encoded by one of several methods. Fortunately, there are libraries, such as CGI.pm, that do all the dirty work. So all you need do is call the library to get what you need.

Output

Whatever you send to STDOUT will be sent back to the browser. So simply compute your response and print it. But you must send it in this format:

One or more http headers. One of these headers must be either Content-type: (for defining the MIME type) or Location: (for redirection). Be sure to follow the last header with a blank line (that is, two newlines.)
The body. This is optional if you do a redirect, but should be a file of the appropriate MIME type (usually HTML) otherwise.

For example:

 print "Content-type: text/html\n\n";
 print "<html><head><title>Environment</title></head>\n";
 print "<body>\n<h1>Env</h1>\n<table>\n";
 foreach (keys %ENV) {
     print "<tr><td>$_</td><td>$ENV{$_}</td></tr>\n";
 }
 print "</table>\n</body>\n</html>\n";

CGI.pm

CGI.pm is a rather extensive library. It not only parses incoming parameters, but it helps build the output, can save state to disk, can run from the web or command line, and probably more. Details from perldoc CGI.

Personally, I don't want so much service, but I use it for two purposes:

Parse the form input.
Run from the command line, and let me type in form input on the fly. Useful for debugging when you have trouble with perl syntax.

Here's an example of how to get the value of what the user typed into the input box named "textin".

 use CGI;

 $input = new CGI;
 $textin = $input->param('textin');

Debugging

Using CGI.pm and running the script from the command line is one nice way of finding errors. But sometimes you just have to run the script through the web server. In this case, consider changing the URL to use perlwrap-d instead of perlwrap. (Or use perlwrap-auth-d instead of perlwrap-auth if you are using authentication.) The -d option will print STDERR and STDOUT back to the browser, as well as give you various information from the startup of perlwrap.

Getting STDERR this way can be invaluable, particularly if the script dies because of perl errors, before it invokes any of your debugging print statements. Check the bottom of the output for clues.

Perl II Previous: 7. Typeglobs Next: 9. Security

1999-3-3 BobG