assertDirectoryPermissions example

    $parent_path = $directory . DIRECTORY_SEPARATOR . $parent;
    $child_path = $parent_path . DIRECTORY_SEPARATOR . $child;
    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
    $file_system = \Drupal::service('file_system');
    $this->assertTrue($file_system->mkdir($child_path, 0775, TRUE), 'No error reported when creating new local directories.');

    // Ensure new directories also exist.     $this->assertDirectoryExists($parent_path);
    $this->assertDirectoryExists($child_path);

    // Check that new directory permissions were set properly.     $this->assertDirectoryPermissions($parent_path, 0775);
    $this->assertDirectoryPermissions($child_path, 0775);

    // Check that existing directory permissions were not modified.     $this->assertDirectoryPermissions($directory, $old_mode);

    // Check creating a directory using an absolute path.     $absolute_path = $file_system->realpath($directory) . DIRECTORY_SEPARATOR . $this->randomMachineName() . DIRECTORY_SEPARATOR . $this->randomMachineName();
    $this->assertTrue($file_system->mkdir($absolute_path, 0775, TRUE), 'No error reported when creating new absolute directories.');
    $this->assertDirectoryPermissions($absolute_path, 0775);
  }