|CSC 153: Computer Science Fundamentals||Grinnell College||Spring, 2005|
|Laboratory Exercise Reading|
This reading introduces a general framework for documents which you routinely find on the World-Wide Web. The lab also begins a discussion of CGI programming, which allows a Web developer to tailor documents to an individual Web user.
As a simple example, consider the document sample.html.
For reference, the original file sample.html is shown below:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Sample HTML page<</title> </head> <body> <p> If by merely intelligible objects we mean those things which are thought through pure categories, without any schema of sensibility, such objects are impossible. For the condition of the objective employment of all our concepts of understanding is merely the mode of our sensible intuition, by which objects are given us; if we abstract from these objects, the concepts have no relation to any object. Even if we were willing to assume a kind of intuition other than this our sensible kind, the functions of our thought would still be without meaning in respect to it. </p> </body> </html>
In analyzing this material, all formatting commands are listed in angle brackets: < > . Further, the first and fourth lines are special:
Beyond these two special lines, many commands apply for a section. For example, <b> indicates the beginning of a section which should be printed in a bold type face, and </b> indicates the end of the same section. As in this example, in many cases, the formatting commands at the start and at the end of a section have the same name, but an extra slash / is added to the end marker.
The following table gives some main formatting commands illustrated in this example:
|<html >||begin an HTML document|
|<head>||begin the header section|
|<title>||begin a title|
|<body>||begin the body of the document|
|<h3>||begin a header3 section (headers can be h1, h2, h3, h4)|
|<p>||begin a new paragraph|
|<br>||break a line (begin a new line)|
|<b>||begin bold type face|
|<i>||begin italics type face|
|<hr>||draw a horizontal line|
|<blockquote>||display the section exactly as formatted|
[For more information about HTML, you might try the primer A beginner's guide to HTML, currently maintained by Marty Blase of the National Center for Supercomputing Applications.]
Both sample.shtml and virtually all of the labs for this course are static documents. That is, each document was created once with all information included at that time. Each document is static and does not adapt to user input.
In contrast, a dynamic document is produced by a program that can receive input from a user and modify the Web page based on that input. A typical mechanism for communication between a user's browser and a Web server is called the Common Gateway Interface (CGI). Altogether, the Common Gateway Interface (CGI) is a set of conventions, supported by software, that facilitate the writing of programs that generate World Wide Web documents. CGI programs can be written in almost any programming language; naturally, though, we'll use Scheme.
With CGI programming, the sequence of events for Web interaction has an extra step:
To expand on this idea, consider the following Scheme program:
(define writeln ;; This procedure writes multiple items one a line, (lambda args ;; and then goes to a new line. (for-each display args) ;; See Springer/Friedman, page 199, for details. (newline) )) (writeln "Content-type: text/html") (newline) (writeln "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\"> (writeln "<html>") (writeln "<head>") (newline) (writeln "<meta http-equiv=\"Content-Type\" content="text/html; charset=ISO-8859-1">") (writeln "<title>") (writeln "title") (writeln "</title>") (writeln "</head>") (writeln "<body>") (writeln "<h2>" "Welcome to the world of Internet programming!" "</h2>") (writeln "<p>") (writeln "<i>" "This page is created by a Scheme program." "</i>") (writeln "</body>") (writeln "</html>") (newline)When this program runs, it prints the following
Content-type: text/html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title> title <meta http-equiv=\"Content-Type\" content=\"text/html; charset=ISO-8859-1\"> </title> </head> <body> <h2>Welcome to the world of Internet programming!</h2> <p> <i>This page is created by a Scheme program.</i> </body> </html>In order to have this program run in response to a Web request to a server, our instructions to the Web server must specify running Scheme and then executing the program. This is accomplished in two main steps:
(exit)This tells Scheme to quit when after running the program.
#!/bin/csh /usr/bin/mzscheme --mute-banner --version --load /home/walker/public_html/cgi-bin/sample-script.ssWhile this file may look a little imposing, the pieces are not as bad as they may seem:
Altogether the above directions (with #!/bin/csh ) is called a cgi script which, in turn, runs a Scheme program ( sample-script.ss ).
Putting these pieces together, we run our Scheme program by typing the URL http://www.walker.cs.grinnell.edu/cgi-bin/sample-script.cgi into our browser.
By clicking on this link, we call the instructions (with the C-shell), which in turn runs our program.
By convention on Linux/Unix systems, all Web-related pages and programs are placed in a subdirectory of your home directory called public_html. Further, cgi scripts are placed in a subdirectory cgi-bin of public_html. This structure provides some level of privacy for your files. The Web server may access, display, and/or run files found within your public_html subdirectory, but it will not access files in other subdirectories of your log-in account.
Beyond this general access protocol, you will need to explicitly share your public_html directory, cgi-bin subdirectory, and any specific files that you want made available to others through the World Wide Web.
As noted earlier, cgi scripts and programs may be helpful, in that they allow a Web server to return information specifically tailored to the user.
One such cgi script is http://www.walker.cs.grinnell.edu/cgi-bin/what-i-know-script.cgi
This document is available on the World Wide Web as
created 12 October 1998
last revised 6 March 2006
|For more information, please contact Henry M. Walker at email@example.com.|