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testcgi.scm does not take any input from the user.
A more focused script would take an argument
environment variable from the user, and output the
setting of that variable and none else. For this, we
need a mechanism for feeding arguments to CGI scripts.
The form tag of HTML provides this capability.
Here is a sample HTML page for this purpose:
<html>
<head>
<title>Form for checking environment variables</title>
</head>
<body>
<form method=get
action="http://www.foo.org/cgi-bin/testcgi2.scm">
Enter environment variable: <input type=text name=envvar size=30>
<p>
<input type=submit>
</form>
</body>
</html>
The user enters the desired environment variable (eg,
GATEWAY_INTERFACE) in the
textbox and clicks the submit button. This causes all
the information in the form -- here, the setting of
the parameter envvar to the value
GATEWAY_INTERFACE -- to be collected and sent to
the CGI script identified by the form, viz,
testcgi2.scm. The information can be sent in one
of two ways: (1) if the form's method=get (the
default), the information is sent via the environment
variable called QUERY_STRING; (2) if the form's
method=post, the information is available to the
CGI script at the latter's standard input port
(stdin). Our form uses QUERY_STRING.
It is testcgi2.scm's responsibility to extract the
information from
QUERY_STRING, and output the answer page
accordingly.
The information to the CGI script, whether arriving via
an environment variable or through stdin, is
formatted as a sequence of parameter/argument pairs.
The pairs are separated from each other by the &
character. Within a pair, the parameter occurs first
and is separated from the argument by the =
character. In this case, there is only
one parameter/argument pair, viz,
envvar=GATEWAY_INTERFACE.
The script testcgi2.scm:
#!/bin/sh
":";exec /usr/local/bin/mzscheme -r $0 "$@"
(display "content-type: text/plain") (newline)
(newline)
;string-index returns the leftmost index in string s
;that has character c
(define string-index
(lambda (s c)
(let ((n (string-length s)))
(let loop ((i 0))
(cond ((>= i n) #f)
((char=? (string-ref s i) c) i)
(else (loop (+ i 1))))))))
;split breaks string s into substrings separated by character c
(define split
(lambda (c s)
(let loop ((s s))
(if (string=? s "") '()
(let ((i (string-index s c)))
(if i (cons (substring s 0 i)
(loop (substring s (+ i 1)
(string-length s))))
(list s)))))))
(define args
(map (lambda (par-arg)
(split #\= par-arg))
(split #\& (getenv "QUERY_STRING"))))
(define envvar (cadr (assoc "envvar" args)))
(display envvar)
(display " = ")
(display (getenv envvar))
(newline)
Note the use of a helper procedure split to split
the QUERY_STRING into parameter/argument pairs
along the & character, and then splitting parameter
and argument along the = character. (If we had
used the post method rather than get, we would
have needed to extract the parameters and arguments
from the standard input.)
The <input type=text> and <input type=submit>
are but two of the many different input tags
possible in an HTML form. Consult NCS for
the full repertoire.
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