#10 jar file needs to be extracted

Let's say I have the following task. I need to extract
the jar file first.
cd lib
jar xvf myfile.jar

<target name="myproject" depends="jars">
<genjar jarfile="lib/myfile.jar"
destdir="lib/myfile-shrink.jar">
<class name="com/sutff" />
<classpath>
<pathelement location="lib"/>
</classpath>
</genjar>
</target>
</project>