title: How to search a file in subdirectories?
body: <p>I have to create a program that search the file "letter.txt" in the subdirectories of a specific directory ( folderA in the example below ).</p>
  
  <p>For example:</p>
  
  <pre><code>folderA/
     folderA1/...
     folderA2/...
     folderA3/...
     folderA4/letter.txt
     folderA5/...
  </code></pre>
  
  <p>What API do I have to use to:</p>
  
  <pre><code>- list subdirectories of a specific directory ( folderA ), 
  - open each of these subdirectories ( folderA1, folderA2, folderA3, etc ) 
  - search and open the file letter.txt ?
  </code></pre>