If And Unless

Since Ant 1.9.1, it is possible to add if and unless attributes on all tasks and nested elements using special namespaces.

In order to use this feature you need to add the following namespace declarations

xmlns:if="ant:if"
xmlns:unless="ant:unless"

The if and unless namespaces support the following 3 conditions:

true

true if the value of the attribute evaluates to true

blank

true if the value of the attribute is null or empty

set

true if the specified property is set

<project name="tryit"
 xmlns:if="ant:if"
 xmlns:unless="ant:unless">
 <exec executable="java">
   <arg line="-X" if:true="${showextendedparams}"/>
   <arg line="-version" unless:true="${showextendedparams}"/>
 </exec>
 <condition property="onmac">
   <os family="mac"/>
 </condition>
 <echo if:set="onmac">running on MacOS</echo>
 <echo unless:set="onmac">not running on MacOS</echo>
</project>