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Common Gateway Interface (CGI)
CGI defines the way in which form data is presented to an
application program by the HTTP server.
When a user clicks the SUBMIT button on a
form, the HTTP server starts up the specified CGI program, and
makes the form data available to it.
A difference between GET and
POST is the way in which a CGI program receives
the form data. If the method was GET, the
information is usually obtained by examining the contents of an
environment variable (usually called
"QUERY_STRING) containing the URL-encoded form
data. Other environment variables contain additional useful
information.
If the method was POST, the CGI program
usually receives the form data on its standard
input stream, with any extra stuff obtained, as before,
from environment variables.
CGI programs can, as a rule, be written in any language (compiled
or interpreted) supported on the system running the HTTP
server.
On Unix servers, they are commonly written in
Perl, C or as Bourne shell
(/bin/sh) scripts.
A CGI program (almost) always generates (to standard output) a Web
page which is returned to the browser, in addition to any other
effect.
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