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Here is an example Scheme CGI script, testcgi.scm, that outputs the settings of some commonly used CGI environment variables. This information is returned as a new, freshly created, page to the browser. The returned page is simply whatever the CGI script writes to its standard output. This is how CGI scripts talk back to whoever called them -- by giving them a new page.

Note that the script first outputs the line

content-type: text/plain

followed by a blank line. This is standard ritual for a web server serving up a page. These two lines aren't part of what is actually displayed as the page. They are there to inform the browser that the page being sent is plain (ie, un-marked-up) text, so the browser can display it appropriately. If we were producing text marked up in HTML, the content-type would be text/html.

The script testcgi.scm:

#!/bin/sh
":";exec /usr/local/bin/mzscheme -r $0 "$@"

;Identify content-type as plain text.

(display "content-type: text/plain") (newline)
(newline)

;Generate a page with the requested info.  This is
;done by simply writing to standard output.

(for-each
 (lambda (env-var)
   (display env-var)
   (display " = ")
   (display (or (getenv env-var) ""))
   (newline))
 '("AUTH_TYPE"
   "CONTENT_LENGTH"
   "CONTENT_TYPE"
   "DOCUMENT_ROOT"
   "GATEWAY_INTERFACE"
   "HTTP_ACCEPT"
   "HTTP_REFERER" ; sic
   "HTTP_USER_AGENT"
   "PATH_INFO"
   "PATH_TRANSLATED"
   "QUERY_STRING"
   "REMOTE_ADDR"
   "REMOTE_HOST"
   "REMOTE_IDENT"
   "REMOTE_USER"
   "REQUEST_METHOD"
   "SCRIPT_NAME"
   "SERVER_NAME"
   "SERVER_PORT"
   "SERVER_PROTOCOL"
   "SERVER_SOFTWARE"))

testcgi.scm can be called directly by opening it on a browser. The URL is:

http://www.foo.org/cgi-bin/testcgi.scm

Alternately, testcgi.scm can occur as a link in an HTML file, which you can click. Eg,

... To view some common CGI environment variables, click
<a href="http://www.foo.org/cgi-bin/testcgi.scm">here</a>.
...

However testcgi.scm is launched, it will produce a plain text page containing the settings of the environment variables. An example output:

AUTH_TYPE =
CONTENT_LENGTH =
CONTENT_TYPE =
DOCUMENT_ROOT = /home/httpd/html
GATEWAY_INTERFACE = CGI/1.1
HTTP_ACCEPT = image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*
HTTP_REFERER =
HTTP_USER_AGENT = Mozilla/3.01Gold (X11; I; Linux 2.0.32 i586)
PATH_INFO =
PATH_TRANSLATED =
QUERY_STRING =
REMOTE_HOST = 127.0.0.1
REMOTE_ADDR = 127.0.0.1
REMOTE_IDENT =
REMOTE_USER =
REQUEST_METHOD = GET
SCRIPT_NAME = /cgi-bin/testcgi.scm
SERVER_NAME = localhost.localdomain
SERVER_PORT = 80
SERVER_PROTOCOL = HTTP/1.0
SERVER_SOFTWARE = Apache/1.2.4

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