• CGI is a set of rules for the communication between a browser and a server for running programs to process data, perform computation, or access a database.  CGI is a protocol, not a programming language.
• A CGI program can be written in any programming language that follows the CGI protocol.  Perl is the most popular CGI programming language.   Some CGI programs are not Perl programs.
• A CGI program is stored and executed on a Web server.  A Web page can call and run a CGI program and the CGI program can return results to the browser for display on a Web page.
• A CGI program (usually called CGI script) can access data obtained from a user on a Web page, process the data, and return results to the user.
• CGI experince is in high demand from employers.  You could substantially improve your salary and job prospects by learning CGI.

• Perl is currently the most popular language for Web CGI programming.   It is the de facto programming language for dynamic and interactive web pages.
• Perl stands for Practical Extraction and Report Language.  It was created by Larry Wall in 1986.
• Perl is an interpreted language.  This means that there is no compilation process to convert the source file to an executable file.  Perl statements are directly interpreted and executed one at a time.
• Perl 5 is a powerful, general purpose programming language.  Some Perl scripts are not CGI scripts.
• Perl is particularly suited to Web-related tasks.  It is a powerful text-manupulation tool and it is cross-platform, easily ported from one platform to another.
• Perl and HTML work harmoniously together.  HTML code can create links and forms that activate Perl scripts.  Perl scripts can generate Web pages to display the results produced by the Perl scripts.
• We can write Perl scripts for processing forms, creating guestbooks, counting the number of times a Web page is visited, and much more.

• Two most popular Web servers are Apache in UNIX systems and IIS (Internet Information System) in the Mocrosoft world.
• Unix operating system is one of the most powerful and flexible operating systems.  Most people have their Web pages stored on Unix Web servers.  We will forcus on using Perl with Unix systems.
   
• Name of the Unix Web server used in this class:   cs.selu.edu
  Address of the Perl interpreter:   /usr/bin/perl

• HTML files must be stored either in the public_html directory or in a subdirectory inside the public_html directory and must have world-readable permissions (file permissions 644).  Perl cgi scripts should be in the cgi-bin subdirectory inside the public_html folder and have file permissions 755.
• You can create or edit a Perl script using either one of the following methods:
  1. Create or edit a script locally on your PC and use FTP to upload the script to the Web server.  (Using Notepad text editior)
  2. Use Telnet or Secure Shell client to logon to the Web server and directly create or edit the script on the server.  (Using Pico text editior)
  Save cgi scripts to the cgi-bin directory with the .cgi filename extension.   This filename extension makes the scripts easily identifiable to both you and the server.
• When uploading cgi scripts, you must use the ascii or text mode to send the scripts.  Do not use the binary mode.
• A Perl script must start with the following statement: #!/usr/bin/perl
  This line must be the first in the script and begin in column 1.  There must not contain even a space line before this statement.
• Perl statements must end with a semicolon, except the opening and closing lines of loops and conditional blocks.
• Perl code is case-sensitive, as are Unix commands and filenames.
• A Perl CGI script that generates output to the browser must include the following all-important MIME (Multipurpose Internet Mail Extensions) Content-type line that the browsers require:
      print "Content-type:text/html\n\n";

      print <<"heredoc-marker";  or print <<heredoc-marker;
      .
      .
      heredoc-marker

  where heredoc-marker is a word or a phrase.  The heredoc
  marker is a label to signal the end of the print statement.

  Note: 
   a. There should be no spaces between the heredoc operator << and 
      the heredoc marker.
   b. The ending heredoc-marker should start in column 1 and should contain
      nothing else, not even a trailing space or semicolon.
   c. You can enclose the heredoc-marker in single quotation marks so that
      there is no variable interpolation.

  (A) Example 1a  (Without using the heredoc operator)

  [jhu@cs cgi-bin]$
  [jhu@cs cgi-bin]$ chmod 755 example1a.cgi
  [jhu@cs cgi-bin]$ ls -la example1a.cgi
  -rwxr-xr-x    1 jhu      jhu           363 Aug 13 16:00 example1.cgi
  [jhu@cs cgi-bin]$ perl -c example1a.cgi
  example1.cgi syntax OK
  [jhu@cs cgi-bin]$ cat example1a.cgi
  #!/usr/bin/perl
  # --------------------------------------------------------
  #  Perl script to run in the UNIX command shell 
  #  (Without using the heredoc operator)
  # --------------------------------------------------------
  #  This program is not a CGI.  It is a simple Perl script.
  #
  $you = "Janice";
  print "Hello, $you.\n";
  print "Welcome to Perl CGI Programming.\n\n";
  print 'Hello, $you.\n';
  [jhu@cs cgi-bin]$ perl example1a.cgi
  Hello, Janice.
  Welcome to Perl CGI Programming.

  Hello, $you.\n

  (B) Example 1b   (Using the heredoc operator)

  [jhu@cs cgi-bin]$ perl -c example1b.cgi
  example1b.cgi syntax OK
  [jhu@cs cgi-bin]$ cat example1b.cgi
  #!/usr/bin/perl
  # --------------------------------------------------------
  #  Perl script to run in the UNIX shell 
  #  (Using the heredco operator <<)
  # --------------------------------------------------------
  #
  $you = "Janice";
  print <<"End1";
  Hello, $you.
  Welcome to Perl CGI Programming.
  End1
  print <<'End2';
  Hello, $you.
  Welcome to Perl CGI Programming.
  End2

  [jhu@cs cgi-bin]$ perl example1a.cgi
  Hello, Janice.
  Welcome to Perl CGI Programming.

  Hello, $you.\n

(A) Example 2a  (Without using the heredoc operator)

  [jhu@cs cgi-bin]$ chmod 755 example2a.cgi
  [jhu@cs cgi-bin]$ perl -c example2a.cgi
  example1.cgi syntax OK
  [jhu@cs cgi-bin]$ cat example2a.cgi
  #!/usr/bin/perl
  print "Content-type:text/html\n\n";
  #
  #-------------------------------------------------------------
  #  Perl CGI script that creates output for a browser
  #  (Without using the heredoc operator)
  #-------------------------------------------------------------
  #  Perl does not interpret HTML tags
  #  Browser does not interpret \n (Perl print escape sequence)
  #
  $you = "Janice";
  print "<html>\n<head><title>Example 2a</title></head>\n";
  print "<body>\n<br><br>";
  print "<font size=4><b>Hello, $you.</b>
  <br>Welcome to Perl CGI Programming.</font>\n";
  print </body>\n</html>\n";


  Running the script in the Unix command shell

  jhu@cs cgi-bin]$ perl example2a.cgi

  <html>
  <head><title>Example 2a</title></head>
  <body>
  <br><br><font size=4><b>Hello, Janice.</b>
  <br>Welcome to Perl CGI Programming.</font>
  </body>
  </html>

  Running the script (example2a.cgi) in the browser's window 
  (Enter the address: http://cs.selu.edu/~jhu/cgi-bin/example2a.cgi
   and the following output appears in the browser's window)
  
  Hello, Janice. 
  Welcome to Perl CGI Programming. 

  (B) Example 2b  (Using the heredoc operator)

  [jhu@cs cgi-bin]$ chmod 755 example2b.cgi
  [jhu@cs cgi-bin]$ perl -c example2b.cgi
  example2b.cgi syntax OK
  [jhu@cs cgi-bin]$ cat example2b.cgi
  #!/usr/bin/perl
  print "Content-type:text/html\n\n";
  #
  # -------------------------------------------------------
  #  Perl CGI script that creates output for a browser 
  #  (Using the heredoc operator)
  # -------------------------------------------------------
  #
  $you = "Janice";
  print <<End
  <html>
  <head><title>Example 2b</title>
  </head>
  <body>
  <br><br><font size=4><b>Hello, $you.</b>
  <br>Welcome to Perl CGI Programming.</font>
  </body>
  </html>
  End

  Running the script in the Unix shell

  [jhu@cs cgi-bin]$ perl example2b.cgi
  Content-type:text/html

  <html>
  <head><title>Example 2b</title>
  </head>
  <body>
  <br><br><font size=4><b>Hello, Janice.</b>
  <br>Welcome to Perl CGI Programming.</font>
  </body>
  </html>

  Running the script in the browser's window  
  (The following output appears in the browser's window)

  Hello, Janice. 
  Welcome to Perl CGI Programming. 

   C:\> ftp cs.selu.edu
   User: (your user name)
   Password: (Your password)
   ftp> cd public_html/cgi-bin
   ftp> ascii
   ftp> send a:\counter.cgi   
   ftp> send a:\count.dat
   ftp> cd ../
   ftp> bin
   ftp> send a:\butterfly.gif
   ftp> quit
   C:\> exit