CGI Programming

1
CGI Programming

• The Client Side and the Server Side

2
How CGI Works

• CGI is a method allowing web pages to communicate
  with programs on an HTTP server. These programs
  can be CGI scripts, applications, or applets.
• A request is made for a URL from a location
  reserved by the HTTP server. This location,
  usually called cgi-bin, cgi, or bin, holds cgi
  scripts that can be run to send back output.
  Data can be supplied to a CGI program by
  appending it to the URL using the GET method, or
  by attaching it to a separate line using the POST
  method.

3
CGI Cont.

• A CGI script is a program, usually written in
  C,C, or Perl.
• An example of a simple CGI script written in Perl
• !/user/bin/perl
• print gtgtEND_of_HTML
• Content type text/html
• lthtmlgt
• ltheadgt
• lttitlegtHello World in Perllt/titlegt
• lt/headgt

4
Example in Perl Cont.

• ltbody bgcolorFFFFFFgt
• ltcentergtlth1gtHello, World!lt/h1gtlt/centergt
• lt/bodygt
• lt/htmlgt
• END_of_HTML
• This script is static, meaning it doesnt allow
  for any input by the user. It can only send back
  output to the user.

5
CGI Script for a Form

• The last example could only print output to a
  browser for the user to read. This example takes
  information (your name) from a form imbedded in a
  web page that is located on a server, stores the
  information, and sends back an HTML page to the
  user.

6
Form Script Cont.

• !/usr/local/perl -w
• helloyou.plx is a program to tell you hello by
  name
• set up to use the CGI.pm module
• use CGI qw(param)
• get your name you typed on the HTML form, using
  the CGI.pl module
• my yourname param("yourname")
• send the top part of the new HTML code to the
  browser
• print gtgtEND_top
• Content-type text/html

7
Form Script Cont.

• ltheadgt
• lttitlegtThe next steplt/titlegt
• lt/headgt
• ltbody bgcolor"FFFFFF"gt
• ltbrgt
• END_top
• send hello and the name from the form to the
  browser
• print ("lth1gtHello, yourname!lt/h1gt")
• lthtmlgt
• send the last part of the new HTML code to the
  browser
• print gtgtEND_bottom
• lt/bodygt
• lt/htmlgt
• END_bottom

8
HTML Form for Previous Example

• lthtmlgt
• ltheadgt
• lttitlegtSet up for Hello, YOU!lt/titlegt
• lt/headgt
• ltbody bgcolor"FFFFFF"gt
• ltform action"http//www.yoursite.com/cgi-bin/hell
  oyou.plx"gt
• lth1gtEnter your name, up to 20 letterslt/h1gtltbrgt
• ltinput type"text" name"yourname" size"20"gtltbrgt
• ltinput type"submit"gt
• lt/formgt
• lt/bodygt
• lt/htmlgt

9
Explanation of Script

• Within the HTML Form is a FORM action command.
  This command gives the location of the server
  with the cgi-bin that contains a proper script
  that can read and interpret the form. The server
  in this example is a fictious one. Examples
  could include such server locations as
  www.yahoo.com or www.okstate.edu. These
  servers have compilers these can include Java,
  C, C, Visual Basic, and in our example Perl.
  Depending on which language you use to program,
  you need that particular compiler.

10
Explanation Cont.

• Once the Submit button in the form is clicked,
  the form is sent to the server. Within the FORM
  action command is the location of the folder
  containing the script. Inside this folder is the
  Perl program where the form will be interpreted.
  In our example, the folder containing the program
  was located at www.yoursite.com/cgi-bin/, and
  the program was called hellowyou.plx.
• When the form is received, the program executes.

11
Expl. Cont.

• The program recieves the environment variable,
  designated by the . It saves the environment
  variable as its own parameter. This is denoted
  by the code, my yourname
  param(yourname)
• Next, a return HTML page is sent back to the
  user. The page will contain the string, Hello,
  yourname!

12
Scripting Languages

• CGI scripts can be written in various languages.
  The most popular of these are C and Perl, with
  Java growing in popularity. CGI programs are
  called scripts because the first programs were
  written using UNIX shell scripts and Perl.
• Perl, along with Python, a less popular scripting
  language, is an interpreted language, somewhat
  like a DOS batch file. When a Perl program is
  executed, the Perl instructions are interpreted
  and immediately compiled into machine
  instructions.

13
Languages Cont.

• C and Java, on the other hand, is compiled ahead
  of time, and the resulting executable isnt
  normally called a script. Compiled programs
  usually run faster but are more difficult to
  modify.
• Most major web sites run on Unix servers.
  However, Windows-based servers are becoming more
  and more prevelant.

14
Languages Cont.

• Windows-based servers use temporary files for
  input and output, rather than environment
  variables and standard input/output as in the
  original CGI standard.
• Variables can be accessed from the CGI programs
  written in Perl, C, C, or scripting languages
  such as Unix shell scripts, or with a one-line
  intermediate script from Java.

15
Languages Cont.

• When writing CGI scripts in Java, an intermediate
  shell script (or .bat file if on Windows) must be
  written to pass along the value of the
  QUERY_STRING variable even if your Java compiler
  generates stand-alone executables.
• This is because Java has no method to directly
  read environment variables due to the fact that
  this concept is foreign to some operating systems
  supported by Java.

16
Resources

• All information in this PowerPoint can be found
  in Core Web Programming 1998 Prentice Hall PTR
  and Using HTML 4, XML, and Java 1.2 Platinum
  Edition 1999 Que.