title: Task custom condition: does a given file exist?
body: <h1>What I'm trying to do</h1>
  <p>I have a task to execute that only makes sense if a certain config file exists. So I want to put a custom condition on the task that returns <code>true</code> if that file exists. I imagine a syntax like this would make sense:</p>
  <pre><code>condition: exists('$(projectPath)\myconfigfile.xml')
  </code></pre>
  <p>This seems like a reasonable use case for custom conditions.</p>
  <h1>What Microsoft documents</h1>
  <p><a href="https://docs.microsoft.com/en-us/azure/devops/pipelines/process/expressions?view=azure-devops#functions" rel="noreferrer">https://docs.microsoft.com/en-us/azure/devops/pipelines/process/expressions?view=azure-devops#functions</a></p>
  <p>Microsoft says the following functions are always available: <code>and</code>, <code>coalesce</code>, <code>contains</code>, <code>endsWith</code>, <code>eq</code>, <code>format</code>, <code>ge</code>, <code>gt</code>, <code>in</code>, <code>le</code>, <code>lt</code>, <code>ne</code>, <code>not</code>, <code>notIn</code>, <code>or</code>, <code>startsWith</code>, <code>xor</code>. And furthermore:</p>
  <p><a href="https://i.stack.imgur.com/C3j32.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/C3j32.jpg" alt="&quot;Depending on context, other functions may be available...&quot;" /></a></p>
  <h1>My actual question</h1>
  <p>I'm finding this a little frustrating. What &quot;other functions&quot; are there? How can I research them? And in particular, is there one that takes a filename and tells me if that file exists?</p>