Since none of the solutions worked for me, this is what I came up with:
Assuming you are running on Windows:
<target name="mvn">
<exec dir="." executable="cmd">
<arg line="/c mvn clean install" />
</exec>
</target>
or on UNIX:
<target name="mvn">
<exec dir="." executable="sh">
<arg line="-c 'mvn clean install'" />
</exec>
</target>
or if you want it to work on both UNIX and Windows:
<condition property="isWindows">
<os family="windows" />
</condition>
<condition property="isUnix">
<os family="unix" />
</condition>
<target name="all" depends="mvn_windows, mvn_unix"/>
<target name="mvn_windows" if="isWindows">
<exec dir="." executable="cmd">
<arg line="/c mvn clean install" />
</exec>
</target>
<target name="mvn_unix" if="isUnix">
<exec dir="." executable="sh">
<arg line="-c 'mvn clean install'" />
</exec>
</target>
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