Perl Practicum: The Camel Spins a Web

Those of you who have been living under a rock for the last twelve months may have missed out on this whole World Wide Web thing. Most of you have probably already tried your hand at some basic HTML authoring. The most interesting application of Web technology, though, is using the Web as an interface to arbitrary data from other sources such as databases and system applications. One mechanism for creating these interfaces is the Common Gateway Interface, CGI for short.

CGI Basics

Here is a trivial example

     #!/bin/perl

     print "Content-type: text/html\n\n";
     print <<"EOmyPage";
     <TITLE>"Hello World!" Page</TITLE>
     <H1>HELLO WORLD!</H1>
     EOmyPage

External Files and Applications

     #!/bin/perl

     print "Content-type: text/html\n\n";

     $visitors = 'cat countfile';
     $visitors++;
     if (open(OUT, "> countfile")) {
          print OUT $visitors;
          close(OUT);
          print <<"EOmyPage";
     <TITLE>Welcome</TITLE>
     Hello visitor number $visitors.
     EOmyPage
     }
     else {
          print "Sorry, an error occurred\n";
     }

It is almost never a good idea to abort a CGI program in the middle of execution. Remember that there is a user on the other side of the Internet who is expecting some sort of page to be returned by your script. Notice that the script above prints an error message if the open() fails rather than calling die() as you would usually.

Also keep in mind that you can manipulate the output of other programs from within your Perl script. In the example above, we used the UNIX cat program to retrieve the contents of a file, but CGI allows you to effectively extend the reach of the Web by making data from other programs available to Web browsers. For example, here is a little CGI script that gives back ps output from the machine it is run on (one could imagine this as part of a suite of remote diagnostic tools for a large network):

     #!/bin/perl
     print "Content-type: text/plain\n\n";
     if (open(PS, "ps -ef |")) {
          while (<PS>) { print; }
     }
     else {
          print "An error occurred\n";
     }

You can call just about any program. You could interface with other network information services like gopher and WAIS, or even NNTP (how about a Web-based threaded newsreader?). You could interface with pieces of your company database and write a company phone book page, or allow people to review their benefits via the Web. However, think about security before you go off and try to save the world with the Web: you may not want everybody in the world to have easy access to much of your data. Even the ps example above potentially gives away more knowledge to people outside your organization than you should be comfortable with.

The CGI Environment

     #!/bin/perl

     print "Content-type: text/plain\n\n";
     foreach $var (sort keys %ENV) {
          print "\$ENV{$var} = '$ENV{$var}'\n";
     }

The client browser can also send information to your HTTP server. Your HTTP server will put this information into your CGI program's environment using variables that are prefixed with HTTP_. In particular, the client will usually provide an identifying string such as NCSA Mosaic for the X Window System/2.4 libwww/2.12 modified in the HTTP_USER_AGENT variable. Unfortunately, there is no established format standard for user agent information, so it is nearly impossible to build a procedure which can identify an arbitrary browser from its user agent information. However, it is pretty easy to recognize most of the major browsers.

What good is identifying a browser? Remember that older browsers may not support all the latest features of the HTML specification. For example, you do not want to send a table to NCSA Mosaic 2.4 because the browser cannot format the table information, and you would not want to send an image map to a text-only browser like Lynx because the user would not be able to see the image.

Processing Forms

     <TITLE>Send Us Email!</TITLE>

     We'd love to hear from you. Enter your email address and
     comments in the spaces provided and we'll respond as quickly as we
     can!<P>

     <FORM METHOD="POST"
          ACTION="bin/process_form">
     Your E-mail address<BR>
     <INPUT NAME="email" SIZE=45 MAXLENGTH=45><BR>
     Your Message<BR>
     <TEXTAREA NAME="comments" ROWS=12 COLS=45></TEXTAREA><P>
     <INPUT TYPE="submit" VALUE="Send your comments">
     </FORM>

When the user punches the Send your comments button, the client browser bundles up all the information that the user entered in and sends that information to your HTTP server along with a request to the server to run the appropriate program from the <FORM ... ACTION=" ... ">. How your program gets the form information depends upon the <FORM METHOD="..." ...> tag. In the example form above, the form method is POST, which means that the form information will be handed to your CGI program on the standard input. You will get a blob of data whose length will be specified by the CONTENT_LENGTH environment variable. The easiest way to grab the data is with the read() function:

     #!/bin/perl

     read(STDIN,$stuff $ENV{`CONTENT_LENGTH'});
     . . .

     #!/bin/perl

     read(STDIN, $stuff, $ENV{'CONTENT_LENGTH'});
     @pairs = split(/\&/, $stuff);
     for (@pairs) {
          ($field, $val) = split(/=/);
          $field =~ s/\+/ /g;
          $field =~ s/%(\w\w)/sprintf("%c", hex($1))/eg;
          $val =~ s/\+/ /g;
          $val =~ s/%(\w\w)/sprintf("%c", hex($1))/eg;
          $entries{$field} = $val;
     }
     ...

Now that you have parsed out the input into an associative array, you can do anything with the information you like. You must return a page back to the user, however, as a result of their forms submission:

     print "Content-type: text/html\n\n";

     if (open(MAIL,
     "| /usr/lib/sendmail webmaster"))
     {
          print MAIL <<"EOdoc";
     From: The Comments Page <webmaster>
     To: webmaster
     Subject: Comments Mail

     Mail from: $entries{"email"}

     $entries{"comments"}
     EOdoc
          close(MAIL);

          print <<"EOpage";
     <TITLE>Thanks!</TITLE>
     Thanks for taking the time to send us comments!<P>
     We will be responding promptly.<P>
     EOpage
     }
     else {
          print <<"EOpage";
     <TITLE>Bummer!</TITLE>
     We encountered an error trying to send your comments.<P>
     Please send mail to <I>webmaster\@netmarket.com</I><P>
     EOpage
     }

Further Study

Sample CGI programs are available all over the Web (NCSA has a small archive of examples to get you started).

Reproduced from ;login: Vol. 20 No. 4, August 1995.

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11/27/96ah