FIT5900: Lecture 19

FIT5900 : CGI Scripts & Perl

In the previous lecture:

Working with server scripts
Using forms to supply data

In this lecture:

Writing Server Scripts
The Perl programming Language

References:

Stein, L.D.: How To Set Up and Maintain a Web Site 2nd edn, Addison Wesley 1997, Chpt9.
Wall, Larry, Tom Christiansen & Randal L. Schwartz: Programming Perl, 2nd edn., O'Reilly and Associates, Inc., 1996 (the 'Camel' book).
It is essential that some reference other than the above (eg. the WWW) be consulted on this topic, particularly in learning elementary Perl.
It is not necessary that you become a proficient Perl programmer, but an understanding of the basic syntax is required.

Limitations

Communication between script and user is indirect.
- Scripts can only send documents to users in response to a request.
- Users can only send documents containing parameters to scripts.
HTTP is a stateless protocol.
- Scripts are run anew each time.
- Information about the previous knowledge / state of the caller is not retained by the server (bit it can be stored on the client side).
Scripts can be written in a language which is
- Compiled
- Interpretted

The Content Type Field

Revise the CGI example in lecture 18.

Each document returned by a script has a MIME type which must be specified by the script as it returns the document.
The MIME type for an HTML document must be formatted as follows:

Content-type: text/html

This must be followed by a carriage return / linefeed "/r/n" before the remaining HTML code for the document is output. In most cases a single newline "/n" character will suffice but this does not strictly adhere to the CGI protocol.
The script receives its parameters via environment variables.

The Location Field

Instead of synthesizing a document and despatching it to the client, a script might just intelligently send the browser to one of several existing documents.
A script can send the browser to an existing document

Location: http://www.some.site.com/some/path.html

...followed by a blank line as before. The location field replaces the Content-type field discussed above.
Upon receipt of this header, the browser will fetch the indicated URL.

Perl

Many CGI scripts are written in Perl.

Well then? What are you waiting for? Please tell me about Perl Syntax.
(Take the link to the lecture material on Perl Syntax before you try to decipher the examples below.)

A Calendar Script

#/usr/monash/bin/perl
# A calendar script

$CAL = '/usr/bin/cal';
$YEAR = 2001;

# fetch the text of the calendar using the unix cal command
# and chop off the newline character at the end of the returned string
chop($calendar_text = `$CAL $YEAR`);

#print the output html document
print <<END
Content-type: text/html

<HTML>
<HEAD>
	<TITLE>HAL's Calendar</TITLE>
</HEAD>

<BODY bgcolor="#ffffff" text="#000000">
	<BR>
		<B>Hi Dave!</B>
	<BR>
		Here's the calendar you requested.
	<BR>
		Love HAL.
		
	<PRE>
	$calendar_text
	</PRE>
	
	<A HREF="~aland/notes/lect18.html">lecture 18</A>
</BODY>
</HTML>

END

Run the calendar script.

Random Location Script


#!/usr/monash/bin/perl
# randomFile.pl

$FILE_DIRECTORY = 'http://www.cs.monash.edu.au/~aland/notes/randomFiles';
$MAXIMUM = 1000;
$NUMBER_OF_FILES = 3;

#set random seed and select a random file name

srand(time);
$number = int(rand($MAXIMUM));
$number = $number % $NUMBER_OF_FILES;

#return the location of this file to the browser

print "Location: $FILE_DIRECTORY/file$number.html\n\n";

Run the random file script.

Print Environment Variables Script

#!/usr/monash/bin/perl
# print environment variables

&print_HTTP_header;
&print_head;
&print_body;
&print_tail;

# ---- print the HTTP content-type header
# ---- 
sub print_HTTP_header
{
	print "Content-type: text/html\n\n";
}

#  ---- print HTML stuff at head
sub print_head
{
	print <<END;
	<HTML><HEAD>
	<TITLE>Environment Variables</TITLE>
	<BODY bgcolor="#ffffff" text="#000000">
	<H4>Environment Variables</H4>
END
}

# ---- loop through env var's and print them
# ---- 
sub print_body
{
	foreach $env_var (sort keys %ENV)
	{
		print "<STRONG>$env_var:</STRONG> $ENV{$env_var}<BR>\n";
	}
}

# ---- print HTML stuff at tail
# ---- 
sub print_tail
{
	print "<BR><BR>";
	print "<A HREF = \"/~aland/notes/lect18.html\">lecture 18</A>";
	print "</BODY></HTML>";
}

Run the print environment variables script.

Here is the HTML SRC for the call:

<A HREF="/cgi-bin/cgiwrap/~aland/printenv.pl/some/path/stuff
?some+query+stuff">

Note the extra path info and the query string.

This lecture's key point(s):

The browser client may send information to the server via CGI
Perl is a popular language for CGI programming

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