★ wanayoo — archive 1999 http://www.wdvl.com/Authoring/Scripting/Tutorial/self_referential_scripts.html

Self-Referential Scripts

Selena Sol

Self Referential Scripts

When we built the form processor, we saw that we could use one CGI script to reference another by outputting an HTML form. This is pretty powerful since in theory, you could have a web site with no HTML files, but only CGI scripts.
However, in many CGI applications, it is more common for a CGI script to contain routines to display AND process a FORM. This is called a self-referential script because it outputs a form whose FORM tag points back to itself.
In order to determine what it should do (output a form or process a form) the script needs some extra logic.
The most common way for a CGI script to decide what it should do is to look at the state of the submit button.
The idea is this. If no submit button has been pressed, we can assum that the script is being executed for the first time. Thus, we should output the FORM. If a submit button has a value, that means that the user has already received the form and is now submitting it for processing.

Consider the following self-referential script called self-refer.cgi:

#!/usr/local/bin/perl
require "cgi-lib.pl";
&ReadParse(*form_data);
print "Content-type: text/html\n\n";

if ($form_data{'submit'} eq "")
  {
  print qq!
  <HTML>
  <HEAD>
  <TITLE>Testing Form Input</TITLE>
  </HEAD>
  <BODY>
  <FORM METHOD = "POST" ACTION = "/old?u=http%3A%2F%2Fwww.wdvl.com%2FAuthoring%2FScripting%2FTutorial%2Fself-refer.cgi&y=1999">
  <CENTER>
  <TABLE BORDER = "1">
  <TR>
  <TH>First Name</TH>
  <TD><INPUT TYPE = "text" 
                NAME = "f_name"></TD>
  </TR>

  <TR>
  <TH>Last Name</TH>
  <TD><INPUT TYPE = "text" 
                NAME = "l_name"></TD>
  </TR>

  <TR>
  <TH>Email</TH>
  <TD><INPUT TYPE = "text" 
                NAME = "email"></TD>
  </TR>

  </TABLE>
  <P>
  <INPUT TYPE = "SUBMIT" NAME = "submit">
  </CENTER>
  </FORM>
  </BODY>
  </HTML>!;
  exit;
  }

else
  {
  print qq!
  <HTML>
  <HEAD>
  <TITLE>Testing Form Input</TITLE>
  </HEAD>
  <BODY>
  <TABLE>!;

  foreach $key (keys(%form_data))
    {
    print qq!
    <TR>
    <TD>$key</TD>
    <TD>$form_data{$key}</TD>
    </TR>!;
    }

  print qq!
  </TABLE>
  </BODY>
  </HTML>!;
  }