Ant task to run an Ant target only if a file exists?

This might make a little more sense from a coding perspective (available with ant-contrib: http://ant-contrib.sourceforge.net/):

<target name="someTarget">
    <if>
        <available file="abc.txt"/>
        <then>
            ...
        </then>
        <else>
            ...
        </else>
    </if>
</target>

I think its worth referencing this similar answer: https://stackoverflow.com/a/5288804/64313

Here is a another quick solution. There are other variations possible on this using the <available> tag:

# exit with failure if no files are found
<property name="file" value="${some.path}/some.txt" />
<fail message="FILE NOT FOUND: ${file}">
    <condition><not>
        <available file="${file}" />
    </not></condition>
</fail>

Since Ant 1.8.0 there's apparently also resourceexists

From http://ant.apache.org/manual/Tasks/conditions.html

Tests a resource for existance. since Ant 1.8.0

The actual resource to test is specified as a nested element.

An example:

<resourceexists>
  <file file="${file}"/>
</resourceexists>

I was about rework the example from the above good answer to this question, and then I found this

As of Ant 1.8.0, you may instead use property expansion; a value of true (or on or yes) will enable the item, while false (or off or no) will disable it. Other values are still assumed to be property names and so the item is enabled only if the named property is defined.

Compared to the older style, this gives you additional flexibility, because you can override the condition from the command line or parent scripts:

<target name="-check-use-file" unless="file.exists">
    <available property="file.exists" file="some-file"/>
</target>
<target name="use-file" depends="-check-use-file" if="${file.exists}">
    <!-- do something requiring that file... -->
</target>
<target name="lots-of-stuff" depends="use-file,other-unconditional-stuff"/>

from the ant manual at http://ant.apache.org/manual/properties.html#if+unless

Hopefully this example is of use to some. They're not using resourceexists, but presumably you could?.....

Available and Condition

<target name="check-abc">
    <available file="abc.txt" property="abc.present"/>
</target>

<target name="do-if-abc" depends="check-abc" if="abc.present">
    ...
</target>