A Peek at Environment Variables

The term environment variable has a very Unix-ish feel to it, which is another way of saying that its meaning is unclear. So let's talk about the weather instead. At any given time the weather outside contains a number of environment variables -- in this case, environment can be taken literally. For instance, if you took a snapshot of the weather at a precise moment, that environment would contain several variables: the temperature, of course, as well as the relative humidity, the barometric pressure, the wind speed, the wind direction, and so on.

Similarly, in the event of a CGI process -- that moment when data is sent from the web browser to the web server -- there are a number of defined variables which contain information about the environment at that moment in time. These variables, created by the web server, are passed into our Perl program when it is executed via a CGI context (e.g. over the web).

Take the example of the environment variable named HTTP_USER_AGENT. This variable reflects the browser version and platform the web visitor used to access the page. Thus, the value "Mozilla/4.51 [en] (Win98; I)" means that the visitor has used Netscape Communicator 4.51 (which identifies itself as "Mozilla"), english version, on Windows 98. Different browsers identify themselves differently, so there is not one single reference for decoding the user agent value. Typically, one reads the user agent variable to determine or log the browser or platform being used ... sometimes to redirect the user to a specific page (which we'll see shortly) or to customize output messages.

In our example, we could imagine that we'd like to add the user agent information into the registration log, for users who register. This would allow us to record the browser and platform our registrants have used without explicitly asking them to enter that information. There are two ways to access a CGI environment variable from a Perl script, neither of which is inherently superior to the other:

$ENV{'HTTP_USER_AGENT'}

$cgiobject->user_agent();

The %ENV hash in Perl contains keys for each CGI environment variable. We don't need to be using the CGI module or object to access the %ENV hash, and so if you have no other need for the CGI module, you can save some memory and startup time by simply accessing this hash.

Alternatively, the CGI object possesses methods which access the same environment variable information. The method names are similar to but slightly different than the true environment variable names -- thus, the CGI object's method user_agent() is used to access the HTTP_USER_AGENT environment variable.

We can easily modify our registration log code to include the user agent data in the database:

$useragent=$cgiobject->user_agent();

print LOGFILE 
"$username\t$userphone\t$usermail\t$userZIP\t$useragent\n";

The CGI process offers a number of environment variables, but only those for which a value is known will contain data. Some environment variables are more "reliable" than others, meaning that they may or may not contain useful information. Below is a table summarizing the more commonly used CGI environment variables, the CGI object methods you can use to access them, and their reliability. You can also explore a thoroughly detailed look at CGI environment variables and associated CGI Object methods.

$ENV{'Variable'}

Registration Log
The Perl You Need to Know
Detour: Redirection