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Introduction to Revolution CGIs - A Tutorial

Working with Text Files

This example is a simple visitor counter which keeps a record of the number of visitors to a web page. It uses a text file located in the cgi folder to store the number of visitors. Each time the web page loads, the cgi reads the text file, adds 1 to the number it has retrieved (to account for the current visitor,) writes the new number back to the file, and returns the count to the web browser.

If you have followed the tutorial so far, you will already have a "hello.cgi" file available in the cgi folder. Edit the file to use this script:



#!revolution -ui

on startup
  put url "file:visitors.txt" into theNumber
  if theNumber = "" then put 0 into theNumber
  add 1 to theNumber
  put theNumber into url "file:visitors.txt"

  put "Content-Type: text/plain" & cr & cr
  put "Hello" && $REMOTE_ADDR && "The time is:" && the time & cr & cr
  put "You are visitor number" && theNumber && "to this page."
end startup

Before you can run this script, you must create a text file with read/write permissions (766 or 666) and store it in the cgi folder. It is not possible to create a file inside the cgi folder from within a script, as this folder has restricted permissions. To accomodate that, we must provide an existing file the script can work with. The file should have the number zero (0) as its only content. Name the file "visitors.txt".

As is the case with all Revolution file paths, the initial default directory is the one that contains the Revolution engine. Therefore, the script only needs to refer to the short file name of the text file. No additional path information is required. If the text file were stored in a different directory, then a full path would be necessary. Note that storing files within the cgi folder (or a subfolder) is more secure than storing them in other directories. It is not possible for others to access the contents of the cgi folder remotely.

Once you have both the script and the visitors.txt file installed in the cgi folder, type this into your browser's location bar:


http://localhost/cgi-bin/hello.cgi

Or if your cgi folder is on a remote web site, replace localhost with the URL of your domain. For example:


http://www.mydomain.com/cgi-bin/hello.cgi

Every time you reload the web page, you should see the visitor count increment by one. While this example returns only plain text, it could easily be altered to calculate a .jpg to display and return that to the browser instead. The result could be embedded in a page as a server-side include and would show a graphic of the number of visitors to the page.

Example: Random content

CGIs are capable of returning more than just plain text; they can also return fully formatted HTML web pages. This example returns a formatted web page that contains a random tip of the day. It also shows that you can use more than just a startup handler in your CGI script. While all CGI scripts must include a startup handler, you can follow that handler with any number of others, including both command handlers and function handlers. This example uses a function to choose a random tip.

You will need to create three files to do this example:

  • a text file that contains several lines of text, one tip per line.
  • an HTML file to serve as a template to hold the tip
  • a CGI script file

The tips file

The tips file is simply a text file with one tip entered per line. Remember to use correct line endings. Create a file similar to this:


Buy low, sell high.
Always carry an umbrella so it won't rain.
If you must do unto others, do it remotely.
Watch those line endings!
For best results, use before expiration date on package.

You can include any number of lines in the text file. Save it into the cgi folder and name it "tips.txt".

The HTML file

There are two ways to return HTML from a CGI. One is to build the HTML from within a script, and the other is to store a template file and read that, replacing only the portions you want to change. The last method is much easier, so that's what we'll use.

Create an HTML file as you would any other web page, but in the place where you want to display the tip of the day, put a placeholder. In this example, we use an asterisk surrounded by square brackets ([*]) as a placeholder.

A sample HTML template file might look like this:


	<html>
	<head>
	     <title>Tip of the Day</title>
	</head>
	<body>
	<center>
	     <i>A random tip served by a Revolution CGI:</i>
	     <p>[*]
	</center>
	</body>
	</html>

Save this file as "tipTemplate.txt" in the cgi folder on the server. Set its file permissions to 755.

The CGI file

Now create a CGI script file with the following content:


#!revolution -ui

on startup
  put "tipTemplate.txt" into theTemplateFile
  if there is not a file theTemplateFile then
    put "Error: template missing" into theData
  else
    put url ("file:"&theTemplateFile) into theData
    put offset("[*]",theData) into theStartChar
    put getTip() into char theStartChar to theStartChar+2 of theData
  end if
  
  # write minimal set of HTTP headers to stdout
  put "Content-Type: text/html" & cr
  put "Content-Length:" && the length of theData & cr & cr
  put theData
end startup


function getTip
  put "tips.txt" into theTFile
  if there is not a file theTFile then
    return "Error: file missing"
  else
    return any line of url ("file:"&theTFile)
  end if
end getTip

The script above looks for a file in the cgi folder called "tipTemplate.txt". If it does not exist, an error message is placed into the variable theData, otherwise the script loads the template file into the variable. It locates the offset of the placeholder characters and replaces them with a tip that is retrieved by the getTip function. The assembled text is then returned to the user's browser, preceded by a content-type header that specifies the content is HTML.

Save this file as "tips.cgi" in the cgi folder on the server.

When you call this CGI from your browser, you will see a random Tip of the Day, which will refresh every time you reload the page. HyperActive Software uses a similar script to display random photos on our CGI page.


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