CGI Programming: Part 1
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.
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.
How Does CGI Work?
• Phase 1: Create.
• Phase 2: Request/Execute.
• Phase 3: Respond/Display.
Phase 1
• A CGI script is created.
– e.g. a Perl program to do your taxes.
• 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 script’s interface to the world.
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.
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.
An Overview of the Process
Request
Server
HTML
Input/
Output
CGI
Program
Calling a CGI Program
• CGI programs are called from HTML files.
• A common/simple way is with the HREF
attribute of the anchor (<a>) tag.
• For example,
<a href = “http://www.foo.com/cgi-bin/bar.pl”>
DO IT
</a>
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.
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.
A Simple Example
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.
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.
Form Values
• Remember a form’s values are sent to the
server as a string.
• Represented as a sequence of name=value
pairs:
– name is the widget’s name.
– value is the widget’s value.
• Values are textual only!
– Other data types cannot be sent.
• Validity of values must be checked by CGI
program.
Get vs. Post
• There are two ways to send a form’s values to a
CGI program.
• Get is the default method.
–
–
–
–
Data string is attached to URL.
Seen in browser’s 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.
Creating a Form
• <form> tags are used to create a form.
• For example,
<form method=“post” action=“file.pl”>
…
<input type = “submit” value = “submit form”>
</form>