Introduction to the Common Gateway Interface (CGI) on the Web

CSU - DCE 0723 - Advanced PerlCGI Operation - Fort Collins, CO

Copyright © XTR Systems, LLC

Introduction to the Common Gateway

Interface(CGI) on the Web

Instructor: Joseph DiVerdi, Ph.D.



CGI In Brief

• CGI stands for Common Gateway Interface– It allows the web server to communicate with other

programs that are running on the server. For example:

– With CGI, the server causes an external program to execute while passing user-specific data to that program.

– That program processes that data, sends a response back to the server which passes the response back to the browser.



CGI In Brief



Uses of CGI

• Web Clock

Changes from one viewing to another



Uses of CGI

• Simple Survey



Uses of CGI

• Simple Game



Uses of CGI

• Quiz



Uses of CGI

• Search Engine



Uses of CGI

• Database access



Uses of CGI

• Not limited to previously written documents • Enables web pages to be created on-the-fly

– Based on the Users’ input

• Provide users with information, collect their comments, and respond



Dynamic Page Content

• Page created dynamically via (Perl) script or• Page with Server Side Includes (SSI) or• Page with embedded call to (Perl) script• The result is still an HTML page



Dynamically Created Image

• Web Clock



Dynamic Image Inclusion

<HTML>

<HEAD>

<TITLE>Digital Clock Demo</TITLE>

</HEAD>

<BODY>

<IMG SRC="/cgi/digital_clock.pl">

</BODY>

</HTML>



CGI Interaction

• Operation is as follows:– Client requests a document which actually is a

CGI program perhaps including some user-specific data

– Server executes requested program passing data received from browser request

– Program completes execution and generates a response

– Server returns response to browser



CGI Interaction



CGI and Form Processing



CGI Database Interaction

• Combing the power of relational database management systems (RDBMS) with Web is very powerful.

• CGI program is required to decode input, assemble query, send to database, process return data from database, and create return document.



CGI Gateway to a Database



Preparing to Use CGI

• Create directory "cgi" in "html" directory• CaSe iS vErY iMpOrTaNt!• Ensure cgi directory has "rwxr-xr-x" access• CGI programs must be located here• Ensure programs have "rwxr-xr-x" access• Always test program with telnet client FIRST!• Test using Browser and sample HTML page



Simple But Useful Program

• Create a file "useful.pl"#! /usr/bin/perl -w

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

print "Hello, CGI Programmer!<BR>\n";

exit;

• Save file in "cgi"• Ensure program has "rwxr-xr-x" access• Test using telnet client• Test using browser



More Useful Program

• Modify the file "useful.pl"#! /usr/bin/perl -w

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

print "Hello, CGI Programmer!<BR>\n";

print "The current time is ", scalar localtime, "<BR>\n";

print "You are using the computer named: ", $ENV{'REMOTE_HOST'}, "<BR>\n";

exit;



Simple Shell Program

• Create a file "env.cgi"#! /bin/sh

echo "Content-type: text/html"

echo

env



Using Others' Programs

• Create a form page, e.g., Ice Cream Survey• Include the following HTML

<FORM METHOD=POST ACTION="http://www.xtrsystems.com/cgi/formmail.pl"

<INPUT TYPE=HIDDEN NAME="send_feedback_to" VALUE="your_email_address">

<INPUT TYPE=HIDDEN NAME="subject" VALUE="your_email_subject">



Using Programs

• Install formmail.pl in your account on linus• Download from class Materials Page• Upload to "cgi" directory• Check program access• Modify form HTML• Test with browser• Modify program (very carefully!!)• Have fun

Introduction to the Common Gateway Interface (CGI) on the Web

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