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What is CGI ?
  • The Common Gateway Interface, or CGI, is a set of standards that define how information is exchanged between the web server and a custom script.

  • The CGI specs are currently maintained by the NCSA and NCSA defines CGI is as follows:

  • The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers.

  • The current version is CGI/1.1 and CGI/1.2 is under progress.

Web Browsing

To understand the concept of CGI, lets see what happens when we click a hyper link to browse a particular web page or URL.

  • Your browser contacts the HTTP web server and demand for the URL ie. filename.

  • Web Server will parse the URL and will look for the filename in if it finds that file then sends back to the browser otherwise sends an error message indicating that you have requested a wrong file.

  • Web browser takes response from web server and displays either the received file or error message.

However, it is possible to set up the HTTP server so that whenever a file in a certain directory is requested that file is not sent back; instead it is executed as a program, and whatever that program outputs is sent back for your browser to display. This function is called the Common Gateway Interface or CGI and the programs are called CGI scripts. These CGI programs can be a PERL Script, Shell Script, C or C++ program etc.

  • CGI Architecture Diagram
  • CGI Architecture
  • Web Server Support & Configuration
  • Before you proceed with CGI Programming, make sure that your Web Server supports CGI and it is configured to handle CGI Programs. All the CGI Programs be executed by the HTTP server are kept in a pre-configured directory. This directory is called CGI Directory and by convention it is named as /cgi-bin. By convention PERL CGI files will have extention as .cgi.

  • First CGI Program
  • Here is a simple link which is linked to a CGI script called hello.cgi This file is being kept in /cgi-bin/ directory and it has following content. Before running your CGI program make sure you have chage mode of file using chmod 755 hello.cgi UNIX command.

  • #!/usr/bin/perl
    print "Content-type:text/html\r\n\r\n";
    print '<html>';
    print '<head>';
    print '<title>Hello Word - First CGI Program</title>';
    print '</head>';
    print '<body>';
    print '<h2>Hello Word! This is my first CGI program</h2>';
    print '</body>';
    print '</html>';
    If you click hello.cgi then this produces following output:
  • Hello Word! This is my first CGI program
  • This hello.cgi script is a simple PERL script which is writing its output on STDOUT file ie. screen. There is one important and extra feature available which is first line to be printed Content-type:text/html\r\n\r\n. This line is sent back to the browser and specifiy the content type to be displayed on the browser screen. Now you must have undertood basic concept of CGI and you can write many complicated CGI programs using PERL. This script can interact with any other exertnal system also to exchange information such as RDBMS.

HTTP Header

The line Content-type:text/html\r\n\r\n is part of HTTP header which is sent to the browser to understand the content. All the HTTP header will be in the following form

HTTP Field Name: Field Content

For Example

There are few other important HTTP headers which you will use frequently in your CGI Programming.

Content-type: StringA MIME string defining the format of the file being returned. Example is Content-type:text/html
Expires: Date StringThe date the information becomes invalid. This should be used by the browser to decide when a page needs to be refreshed. A valid date string should be in the format 01 Jan 1998 12:00:00 GMT.
Location: URL StringThe URL that should be returned instead of the URL requested. You can use this filed to redirect a request to any file.
Last-modified: StringThe date of last modification of the resource.
Content-length: StringThe length, in bytes, of the data being returned. The browser uses this value to report the estimated download time for a file.
Set-Cookie: StringSet the cookie passed through the string
CGI Environment Variables

All the CGI program will have access to the following environment variables. These variables play an important role while writing any CGI program.

Variable NameDescription
CONTENT_TYPEThe data type of the content. Used when the client is sending attached content to the server. For example file upload etc.
CONTENT_LENGTHThe length of the query information. It's available only for POST requests
HTTP_COOKIEReturn the set cookies in the form of key & value pair.
HTTP_USER_AGENTThe User-Agent request-header field contains information about the user agent originating the request. Its name of the web browser.
PATH_INFOThe path for the CGI script.
QUERY_STRINGThe URL-encoded information that is sent with GET method request.
REMOTE_ADDRThe IP address of the remote host making the request. This can be useful for logging or for authentication purpose.
REMOTE_HOSTThe fully qualified name of the host making the request. If this information is not available then REMOTE_ADDR can be used to get IR address.
REQUEST_METHODThe method used to make the request. The most common methods are GET and POST.
SCRIPT_FILENAMEThe full path to the CGI script.
SCRIPT_NAMEThe name of the CGI script.
SERVER_NAMEThe server's hostname or IP Address
SERVER_SOFTWAREThe name and version of the software the server is running.

Here is small CGI program to list out all the CGI variables. Click this link to see the result Get Environment


print "Content-type: text/html\n\n";
print "<font size=+1>Environment</font>\n";
foreach (sort keys %ENV)
  print "<b>$_</b>: $ENV{$_}<br>\n";

How To Raise a "File Download" Dialog Box ?

Sometime it is desired that you want to give option where a use will click a link and it will pop up a "File Download" dialogue box to the user in stead of displaying actual content. This is very easy and will be achived through HTTP header.

This HTTP header will be different from the header mentioned in previous section.

For example,if you want make a FileName file downloadable from a given link then its syntax will be as follows.


# HTTP Header
print "Content-Type:application/octet-stream; name=\"FileName\"\r\n";
print "Content-Disposition: attachment; filename=\"FileName\"\r\n\n";

# Actual File Content will go hear.
open( FILE, "<FileName" );
while(read(FILE, $buffer, 100) )
GET and POST Methods

You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your CGI Program. Most frequently browser uses two methods two pass this information to web server. These methods are GET Method and POST Method.

Passing Information using GET method:

The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ? character as follows:

The GET method is the defualt method to pass information from browser to web server and it produces a long string that appears in your browser's Location:box. Never use the GET method if you have password or other sensitive information to pass to the server. The GET method has size limtation: only 1024 characters can be in a request string.

This information is passed using QUERY_STRING header and will be accessible in your CGI Program through QUERY_STRING environment variable

You can pass information by simply concatenating key and value pairs alongwith any URL or you can use HTML <FORM> tags to pass information using GET method.

Simple URL Example : Get Method

Here is a simple URL which will pass two values to hello_get.cgi program using GET method.

Below is hello_get.cgi script to handle input given by web browser.


    local ($buffer, @pairs, $pair, $name, $value, %FORM);
    # Read in text
    $ENV{'REQUEST_METHOD'} =~ tr/a-z/A-Z/;
    if ($ENV{'REQUEST_METHOD'} eq "GET")
	$buffer = $ENV{'QUERY_STRING'};
    # Split information into name/value pairs
    @pairs = split(/&/, $buffer);
    foreach $pair (@pairs)
	($name, $value) = split(/=/, $pair);
	$value =~ tr/+/ /;
	$value =~ s/%(..)/pack("C", hex($1))/eg;
	$FORM{$name} = $value;
    $first_name = $FORM{first_name};
    $last_name  = $FORM{last_name};

print "Content-type:text/html\r\n\r\n";
print "<html>";
print "<head>";
print "<title>Hello - Second CGI Program</title>";
print "</head>";
print "<body>";
print "<h2>Hello $first_name $last_name - Second CGI Program</h2>";
print "</body>";
print "</html>";

Simple FORM Example: GET Method

Here is a simple example which passes two values using HTML FORM and submit button. We are going to use same CGI script hello_get.cgi to handle this imput.

<FORM action="/cgi-bin/hello_get.cgi" method="GET">
First Name: <input type="text" name="first_name">  <br>

Last Name: <input type="text" name="last_name">
<input type="submit" value="Submit">

Here is the actual output of the above form, You enter First and Last Name and then click submit button to see the result.


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