How to pass argument to NAnt exec task conditionally based on the property existence?

Question

I'd like to call a certain EXE from my NAnt script, and pass a property as an argument in case the property exists, or don't pass anything otherwise.

The below code seems to fit just for this case, but it doesn't work:

<exec program="notepad.exe">
  <arg line="${file}" if="${property::exists('file')}" />
</exec>

It throw Property 'file' has not been set error. Looks like it evaluates the property ignoring the condition. I would expect it to ignore the entire <arg> element in case its condition is false.

If I define the property above this block, it obviously works fine (even if the property is empty). It could be worth a workaround if it's a single case, but I have to pass a number of properties the same way. Besides, <if> is not a valid element under <exec>...

How to achieve this? Any ideas?

Recover

Answer 1

I dag out an old thread at NAnt mailing list, which actually gives the best suggestion for a workaround (to my case). It's based on the fact that <property> task works as expected. The workaround introduces an extra property to set depending on whether the original property is defined or not.

Like this:

<target name="example">
  <property name="arg.value" value="${arg}" if="${property::exists('arg')}" />
  <property name="arg.value" value="" unless="${property::exists('arg')}" />
  <exec program="program.exe">
    <arg value="${arg.value}" />
  </exec>
</target>

In my case it turns out to be easier to always set a property with default value, and even don't introduce an extra property - just pass the original property to <arg/>:

<target name="example">
  <property name="arg.value" value="default value" overwrite="false" />
  <exec program="program.exe">
    <arg value="${arg.value}" />
  </exec>
</target>

It can (and will) be overwritten in other include files.

I have also found another thread where the same question was asked, and a person involved into the NAnt development suggested to indicate whether people are interested in a patch to fix this for <arg> element. As far as I can see, no one demonstrated the interest :), hence the behavior wasn't changed.

I managed to put a little time investigating what a fix could be, and it seems that it's only about adding ExpandProperties=false to the TaskAttribute for e.g. the line property of the <arg> task. The same is done for <property value="..."> attribute where it works as expected. I didn't try it out, though - if I have some more time one of these days, I will and post an update here.

Recover

You can optionally set the default of the same variable you're reading. This seems more clear and is a single statement instead. <property name="arg" value="false" unless="${property::exists('arg')}"/> – Eadie

Based on your use case, do you mean the following for your second code snippet? <target name="example"> <property name="arg" value="default value" overwrite="false" /> <exec program="program.exe"> <arg value="${arg}" /> </exec> </target> – Rudyrudyard

Answer 2

For that case, you can just use a fileset to pass as the arguments. By default, fileset does not include items that did not match actual files on disk.

Josiahjosias

Thanks for your suggestion. Unfortunately, it's not only about files - it was just a sample. I have to pass a big number of arguments, of various nature. I'll shape what I ended up with in an answer. – Recover

Answer 3

I also ran into this issue (to be honest even more than once). The if-attribute in some cases doesn't work as you might expect it to. This is my ugly workaround:

<choose>
  <when test="${property::exists('file')}">
    <exec program="notepad.exe">
      <arg line="${file}" />
    </exec>
  </when>
  <otherwise>
    <exec program="notepad.exe" />
  </otherwise>
</choose>

Barnwell

Thanks for your suggestion. Well, that makes sense for 1 argument - but I have to pass a big number of arguments. I'll shape what I ended up with in an answer. – Recover