CGI Programming: Part 1

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CGI Programming Part 1
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What is CGI?

• CGI Common Gateway Interface
• Provides a standardized way for web browsers to
• Call programs on a server.
• Pass data to programs on a server.
• Receive responses from programs on a server.

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What is CGI? (cont)

• CGI is the interface between server programs and
  other software.
• CGI is not a Perl specific concept.
• Any language can produce CGI programs.
• Why Perl?
• Perl provides a nice interface for creating CGI
  scripts.

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How Does CGI Work?

• Phase 1 Create.
• Phase 2 Request/Execute.
• Phase 3 Respond/Display.

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Phase 1

• A CGI script is created.
• e.g. a Perl program to do your tasks.
• The script is placed on a server.
• Made executable.
• Given appropriate permissions.
• A webpage is created and linked to the CGI
  script.
• Webpage is the scripts interface to the world.

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Phase 2

• A person visits the webpage and submits a request
  to run the script.
• Browser contacts the server with the CGI script
• Asks to run the script.
• Passes input parameters to the script.
• Server runs the script on the input parameters.

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Phase 3

• Script produces output in the form of HTML.
• Server takes the output and returns it to the web
  browser.
• Browser displays the output as an HTML page.
• Page may reference other CGI scripts.

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An Overview of the Process
Request
CGI Program
Input/ Output
Server
HTML
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Calling a CGI Program

• CGI programs are called from HTML files.
• A common/simple way is with the HREF attribute of
  the anchor (ltagt) tag.
• For example,
• lta href http//www.foo.com/cgi-bin/bar.plgt
• DO IT
• lt/agt

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Returning the Output

• CGI programs must create HTML output to return to
  the client.
• HTML is the common language between the client
  and the CGI program.
• Communication with client is through standard
  output.
• Output goes to server and then goes to screen.
• Use print function to send HTML output to client.

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HTML OUTPUT

• First line of HTML output must specify the type
• i.e. type of the content of the output.
• The type is usually text/html.
• For example,
• print Content-type text/html\n\n
• There must be a blank line after the first line.

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A Simple Example
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More Powerful Interaction

• Many webpages gather information from their
  visitors.
• A more powerful interaction between CGI program
  and browser is needed.
• This interaction is provided by forms.

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Forms

• A form is a collection of widgets in a web page.
• Solicit responses from the user.
• A form must include a submit button!
• When the submit button is clicked
• The CGI program specified in the form is
  contacted.
• A string representation of the widgets values is
  sent to the server.

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Form Values

• Remember a forms values are sent to the server
  as a string.
• Represented as a sequence of namevalue pairs
• name is the widgets name.
• value is the widgets value.
• Values are textual only!
• Other data types cannot be sent.
• Validity of values must be checked by CGI program.

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Get vs. Post

• There are two ways to send a forms values to a
  CGI program.
• Get is the default method.
• Data string is attached to URL.
• Seen in browsers address bar.
• Server removes string from URL.
• Stored in environment variable, QUERY_STRING.
• Post is the optional method.
• Data string is sent through standard input.
• CGI program can simply read it.
• Length of string is stored in environment
  variable, CONTENT_LENGTH.

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Creating a Form

• ltformgt tags are used to create a form.
• For example,
• ltform methodpost actionfile.plgt
• ltinput type submit value submit formgt
• lt/formgt