Handout 16

Overview

• Questions from last time
• The basic idea
• Simple cgi
• Forms

The basic idea

• Typically, the client requests a static document from some server. The client can request dynamic content too.
• There are a number of ways to do this: cgi, php, asp/jsp, servlets, etc. We will focus on the one you can do most easily with Perl: cgi.
• The basic idea behind cgi is that the URL includes the name of a program on the server, rather than the name of a web page. In response to the client's request, the server actually runs the program, rather than sending some static text. What's sent to the client is the result of running the program on the server.

Simple cgi

• The simplest kind of example simply prints back some text (example, code).
• Getting this to work requires some special stuff. Moreover, these additional bits are not the same on different servers.
  1. Your cgi must be put in a file with a .cgi file extension, e.g. simple1.cgi. (This is not true on all systems.)
  2. You cgi must be put in a specific directory. On the syntax server, this is public_html/cgi-bin. (This location can vary depending on the server configuration.)
  3. Your cgi program must be executable. (You can make this so with the command chmod a+x program.cgi.)
  4. If your program prints something out to be rendered in the client's browser, it must print an HTML header exactly as in the examples.
  5. Finally, the syntax server can run cgi scripts in a number of languages. To let it know that yours is in Perl, you need to include the "bang" line at the beginning of the program, as in the examples, i.e. #!/usr/bin/perl. (This specifies that the cgi program is written in perl and the location of perl on the server.)
• Running a cgi program is different from requesting a static HTML page. A static page cannot change. Your cgi program can print out different content each time it's run (example, code).
• You can also print out HTML with your cgi program (example, code). Notice that your code is now a curious blend of Perl and HTML!

Forms

• The basic idea behind forms is that HTML pages can contain graphical elements that allow you to input information to a remote cgi program.
• Let's look at a simple example and then try to understand it (form, cgi code).
• You can use this to collect data about how many people speak how many languages (form, cgi code).
• The preceding program is very tricky for several reasons. First, there must be a place to store data between invocations of the program: french.txt. Second, you must set the read/write privileges so that your cgi program can write to that file. Generally, only you can write to files in your directories, but your cgi program isn't you! Finally, there are some very tricky points about what happens if there are multiple simultaneous access of your cgi program. (See the text for details or talk to me if you think you'll be doing cgi stuff and writing to a file.)
• Forms can include lots of input devices, e.g. lines of text (type=text), boxes of text (textarea), buttons (type=submit), checkboxes (type=checkbox), and radioboxes (type=radio), etc. Here's an example that uses all of these (example, cgi code).
• You can use the standard CGI.pm module to decode all the gibberish in the query string (example, cgi code).