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Home > Continuing Education > CGI Programming > Lessons > Lesson 3 > Forming URLs

Forming URLs

URL stands for Uniform Resource Locator, and it is essentially the "address" of a particular file on your web server.

URL Syntax

The URL is formed like this:

  protocol:://server/directory/file

There are several protocols that you can use in an URL:

http for HTML files
https for secure HTML files
ftp for file transfer
file for files on your local PC

The server portion corresponds to either your domain name, or the domain name of your ISP. This can also be written as an IP address. The following are all valid server names for a URL:

www.enscript.com
enscript.com
myhost.com
209.126.128.27

The directory/file portion of the URL depends on where the file resides on your web server. This is similar to the UNIX path for the file, but not exactly. Also, note that index.html is assumed if you leave off the /file portion of the URL (actually, there is a sequence of files that are looked for: index.html, index.shtml, index.cgi, default.html ...).

UNIX path vs. URL

When the ISP sets up your account, they must define a "root" HTML directory for your site. That is not the same as the UNIX root directory. The root directory for your site is the top level directory for your HTML files. Some examples:

The following diagrams depict the some of the common ways ISP set up web site directories.

Fig 1. Site Directory Setup A
Fig 2. Site Directory Setup B

In Setup A, the UNIX path for the root HTML directory is /home/ashley/www. In Setup B, it is /home/ashley/public_html. The corresponding URL, however, does not include the full UNIX path, but rather an abbreviation starting from the root HTML directory for the web site.

If you have your own domain, your root URL is something like this:

http://www.kidstuff.com

However if you are using space on a server without your own domain, it probably looks something like this:

http://www.mindspring.com/~ashley

Any subdirectory of this root HTML directory can be accessed by appending the subdirectory name to the URL, like

http://www.kidstuff.com/images

http://www.mindspring.com/~ashley/images

The distinction between UNIX path and the URL is very important in installing CGI script. Very often you will be asked to supply both. For example, from the configuration file for Links 2.0 from Gossamer Threads:


# PATH and URL of Admin CGI directory. No Trailing Slash.    
    $db_script_path   = "/home/ashley/www/links/cgi-bin/admin";
    $db_dir_url       = "http://www.enscript.com/links/cgi-bin/admin";

URL Shorthand

From within an HTML file, you can use a shorthand notation to reference other files on the same server. The root HTML directory is indicated by starting a URL with a "/", such as

<A HREF=/index.html>

This means you can now have a shortcut to your root HTML directory from any where on the same server. (If you don't have your own domain, the shortcut would be something like "/~ashley/")

You may of course also use the full URL in your HTML files:

<A HREF=http://www.kidstuff.com>

<A HREF=http://www.mindspring.com/~ashley/images>

All the shortcut lets you do is omit the protocol and server name (http://www.kidstuff.com or http://www.mindspring.com)

Absolute vs. Relative Links

An absolute link starts with "/" (the root HTML directory) and spells out the entire path to the file on your site. A relative link is more like shorthand -- if the two files are in the same directory, the link doesn't have to spell out the entire path.

For example, if you have file called test.html in the same directory as index.html, you can put

<A HREF=test.html>

inside the index.html code. You can also specify subdirectories, like

<IMG SRC=images/test.gif>

More examples:

Fig 3. Illustration of absolute vs. relative links All of these are valid ABSOLUTE HREFs inside fileA.html:
<A HREF="/fileB.html"> <IMG SRC="/mystuff/imageA.jpg"> <IMG SRC="/mystuff/someimages/imgB.jpg">
These are all valid RELATIVE HREFs inside fileA.html:
<A HREF="fileB.html"> <IMG SRC="mystuff/imageA.jpg"> <IMG SRC="mystuff/someimages/imgB.jpg">
And more complicated cases....
All of these are valid ABSOLUTE HREFs inside fileC.html:
<A HREF="/fileB.html"> <IMG SRC="/mystuff/imageA.jpg"> <IMG SRC="/mystuff/someimages/imageB.jpg">
These are all valid RELATIVE HREFs inside fileC.html:
<A HREF="../fileB.html"> <IMG SRC="imageA.jpg"> <IMG SRC="someimages/imageB.jpg">
See how the absolute tags are valid no matter which file you're in? The relative tags are dependent on the location of the HTML file.

Linking to CGI scripts

For cgi scripts, you may have to put them in a special directory in order to tell the server to "execute" them instead of just displaying them. Usually, this is in a directory called cgi-bin which has a special meaning to the server.

A URL of

http://www.kidstuff.com/cgi-bin/search.pl

will execute the search.pl CGI script from the cgi-bin directory. Note that the cgi-bin directory may not be subdirctory of the html directory, even though the URL makes it look like it is (see the differences between Figures 1 and 2 above). The actual path for the cgi-bin URL is determined by the ScriptAlias directive in the server configuration.