If you want to run a different logics based on the operating system in your ANT build.xml file here are two ways to do it.

detectOSInsideTask.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--Use ANT to trigger different logic inside a task depending on what OS is being used.
This is useful if you just want to switch some of the logic inside one part of your task
depending on the OS without creating a whole other task-->
<project name="detectOS" default="showOS" >
        <!--Include new library
        If, then, else library for supporting tasks
        on how to install see: 
        https://www.google.com/search?q=how+to+install+ant-contrib.jar-->
        <taskdef resource="net/sf/antcontrib/antcontrib.properties">
            <classpath>
                <pathelement location="ant-contrib.jar"/>
            </classpath>
        </taskdef>

        <task name="showOS">
            <!--Create property based on OS-->
            <condition property="IS_WINDOWS" value="true" else="false">
                <os family="windows"/>
            </condition>
            <!--Switch statement based on OS-->
            <if>
                <equals arg1="${IS_WINDOWS}" arg2="true" />
                <then>
                    <echo message="This is a windows system" />
                </then>
                <else>
                    <echo message="This is not a windows system" />
                </else>
            </if>
        </task>
</project>

DetectOSOutsideTask.xml

<?xml version="1.0" encoding="UTF-8"?>

<!--Use ANT to trigger different tasks based on what OS is being used. See the code
snipit bellow if you don't want to create a new task just to switch some small piece
of logic depending on the operating system.-->

<project name="detectOS" default="showOS" >

    <!--Create properties based on OS
    conditions are like properties or variables that get set based on what is true 
    inside the conidtion tag. Because the condition tag is called as a child of the tag  
    project the IS_WINDOWS and IS_UNIX properties are in the global scope. This means 
    you can access these variables or properties from any task in your script. If you want 
    to create variables inside a task and not in the global scope use the example bellow 
    in the code snipit detectOSInsideTask-->
    <condition property="IS_WINDOWS">
        <os family="windows"/>
    </condition>
    <condition property="IS_UNIX">
        <os family="unix"/>
    </condition>
    
    <!--Define operating system based tasks
    When these targets get called it will only run if the variable IS_WINDOWS or 
    IS_UNIX is true-->
    <target name="show_windows" if="IS_WINDOWS" >
        <echo message="This is a Windows system " />
    </target>
    <target name="show_unix" if="IS_UNIX" >
        <echo message="This is a Unix system " />
    </target>

    <!--Run switch based on OS
    This task acts like a switch statement. It will try to run two tasks: 
    show_windows and show_unix but each task will only run if its 'if' 
    attibute is true. For example if IS_UNIX = true and IS_WINDOWS = false 
    then only antcall target="show_unix" will be run.-->
    <target name="showOS" >

        <antcall target="show_windows" />
        <antcall target="show_unix" />

    </target>

</project>

detectOSSwitchExec.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="detectOS" default="showOS" >
        <!--this is provided here for completeness. Some tags have the OS attribute on them 
        which allows you to control whether the task will be ran based on the OS. I find it 
        most usefull with the exec tag since this tag talks to the operating system-->
        <task name="showOS">
                <exec executable="sh" os="Mac OS X">
                        <arg value="-c"/>
                        <arg value="echo 'This is a Mac OSX system';"/>
                </exec>
                <exec executable="echo" os="Windows 2000,Windows NT,Windows XP">
                        <arg value="This is a windows system"/>
                </exec>
        </task>
</project>