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The information in this document may be outdated or incorrect. ACS provides this documentation as a courtesy for the curious.

An instantaneous introduction to CGI scripts and HTML forms

When information is collected by a browser it is sent to a HyperText Transfer Protocol (HTTP) server specified in the HTML form, and that server starts a program, also specified in the HTML form, that can process the collected information. Such programs are known as "Common Gateway Interface" programs, or CGI scripts.

This document describes the Common Gateway Interface in some detail. It focuses on the ways in which a form, a client browser, a server, and the HTTP protocol work together. To understand this complex interaction, you must first understand how a client and a server work together to deliver a "normal" HTML document. This is the "canonical" Web activity; the "usual" Web function. Then you need to understand how scripts are executed in the Web environment without mediating forms. Once these two processes are clear, the forms interface is straight-forward.

For a different approach to this same topic consult its companion piece Building blocks for CGI scripts in Perl, which provides Perl code for many common CGI tasks, making script creation fairly simple.

The Canonical Browser-Server Interaction

The following diagram shows a WWW client running on a desktop system, Computer A, interacting with two servers: An HTTP server running on Computer B and an HTTP server running on Computer C.

The client running on Computer A gets a document, stored in a file named docu1.html, from the HTTP server running on Computer B. This document contains a link to another document, stored in a file named docu2.html on Computer C. The Uniform Resource Locator (URL) for that link might look something like:

http://ComputerC.domain/docu2.html

If the user activates that link, the client retrieves the file from the HTTP server running on Computer C and displays it on the monitor connected to Computer A.

The HyperText Transfer Protocol defines communication between the client and an HTTP server. The following example shows what an HTTP exchange between a Lynx client and an HTTP server running on Computer C might look like as the client fetches docu2.html.

The client sends the following text to server:

Finally, the client sends a blank line indicating it has completed its request.

The server then responds by sending:

Things to note here:

• Client and server headers are RFC 822 compliant mail headers.
• A Client may send any number of Accept: headers and the server is expected to convert the data into a form the client can accept.

Executing "scripts"

The diagram below shows an hypertext document on Computer B with a link to a file on Computer C that holds the CGI program that will be executed if a user activates the link. This link is a "normal" http: link, but the file is stored in such a way that the HTTP server on Computer C can tell that the file contains a program that is to be run, rather than a document that is to be sent to the client as usual.

When the program runs, it prepares an HTML document on the fly, and sends that document to the client, which displays the document as it would any other HTML document.

Such programs are sometimes called HTTP scripts or "Common Gateway Interface" (CGI) scripts. Note that CGI scripts may be written in scripting languages (like Perl, TCL, etc.) or in any other programming language (like C, Pascal, Basic).

On some HTTP servers these CGI programs are stored in a directory called cgi-bin, and so they are also sometimes called "cgi-bin scripts."

Here is a simple AppleScript program that can be run by a MacHTTP server when it receives a request for the file containing the script. When it runs, this program builds an HTML document containing the current time and returns the document to the WWW client that requested it.

The program is stored in a file named "date", in a folder called "scripts". When a user activates a link that points to this script, the Web client will generate an HTTP request that might look like:

Executing a Script via an HTML Form

A set of tags was added to HTML to direct a WWW client to display a form to be filled out by a user and then forward the collected data to an HTTP server specified in the form.

Servers were modified so that they could then start the CGI program specified in the form and pass the collected data to that program, which could, in turn, prepare a response (possibly by consulting a pre-existing database) and return a WWW document to the user.

The following diagram shows the various components of the process.

In this diagram, the Web client running on Computer A acquires a form from some Web server running on Computer B. It displays the form, the user enters data, and the client sends the entered information to the HTTP server running on Computer C. There, the data is handed off to a CGI program which prepares a document and sends it to the client on Computer A. The client then displays that document.

HTML Tags Related to Forms Mode

<FORM>. . . </FORM>

Define an input form.
Attributes: ACTION, METHOD, ENCTYPE

<INPUT>

Define an input field.
Attributes: NAME, TYPE, VALUE, CHECKED, SIZE, MAXLENGTH

<SELECT> . . . </SELECT>

Define a selection list.
Attributes: NAME, MULTIPLE, SIZE

<OPTION>

Define a selection list selection (within a SELECT).
Attribute: SELECTED

<TEXTAREA> . . . </TEXTAREA>

Define a text input window.
Attribute: NAME, ROWS, COLS

An Example Form

You can select this link to see what this form looks like from your browser