Ant provides access to all system properties as if they had been defined using a <property> task. For example, $os.name expands to the name of the operating system.
For a list of system properties see the Javadoc of System.getProperties.
In addition, Ant has some built-in properties:
basedir the absolute path of the project's basedir (as set with the basedir attribute of <project>). ant.file the absolute path of the buildfile. ant.version the version of Ant ant.project.name the name of the project that is currently executing; it is set in the name attribute of <project>. ant.java.version the JVM version Ant detected; currently it can hold the values "1.1", "1.2", "1.3" and "1.4".