NAnt contains an impressive set of functions that can be used to get anything from the current working directory to determining whether a file exists to getting environment variables. The syntax is a bit unusual for a build script and the NAnt documentation is sparse on examples. The example below demonstrates the use of the NAnt Environment functions to get the environment variable values for the working machine.
<property name="ncover.console.exe"
value="${environment::get-variable('NCOVER_HOME')}\NCover.Console.exe" />
<property name="nunit.console.exe"
value="${environment::get-variable('NUNIT_HOME')}\bin\nunit-console.exe" />