Do I have a way to check the existence of a directory in Ant (not a file)?

Here's a small example incorporating the available element into an if test.

<!-- Test if a directory called "my_directory" is present -->
<if>
  <available file="my_directory" type="dir" />
  <then>
    <echo message="Directory exists" />
  </then>
  <else>
    <echo message="Directory does not exist" />
  </else>
</if>

Warning: you need ant-contrib.jar in your ANT_HOME\lib directory otherwise you won't have access to the if elements, and your script will fail with this error:

Problem: failed to create task or type if
Cause: The name is undefined.
Action: Check the spelling.
Action: Check that any custom tasks/types have been declared.
Action: Check that any <presetdef>/<macrodef> declarations have taken place. 

You use the available task with type set to "dir".

For example:

<available file="${dir}" type="dir"/>

The standard way to do conditional processing is with the condition task. In the example below, running doFoo will echo a message if the directory exists, whereas running doBar will echo a message unless the directory exists.

The dir.check target is required by both doFoo and doBar, it sets the dir.exists property to true or false depending on the result of the available task. The doFoo target will only run if that propery is set to true and doBar will only run if it is not set or set to false.

<?xml version="1.0"?>
<project name="test" default="doFoo" basedir=".">
  <property name="directory" value="c:\test\directory"/>

  <target name="doFoo" depends="dir.check" if="dir.exists">
    <echo>${directory} exists</echo>
  </target>

  <target name="doBar" depends="dir.check" unless="dir.exists">
    <echo>${directory} missing"</echo>
  </target>

  <target name="dir.check">
    <condition property="dir.exists">
      <available file="${directory}" type="dir"/>
    </condition>
  </target>
</project>

Antelope provides additional tasks, including an If task that can make the processing simpler (and to me, more intuitive), you can download the Antelope tasks from the download page.

Here's my solution, which doesn't require setting properties and using targets with 'if' or 'unless':

Macro:

<macrodef name="assertDirAvailable">
    <attribute name="dir" />
    <sequential>
        <fail message="The directory '@{dir}' was expected to be available but is not">
            <condition>
                <not>
                    <available file="@{dir}" type="dir" />
                </not>
            </condition>
        </fail>
    </sequential>
</macrodef>

Usage:

<assertDirAvailable dir="${dirToCheck}" />