docs: function runEbookConvert(output) {
      var logger = output.getLogger();
      var options = output.getOptions();
      var format = options.get('format');
      var outputFolder = output.getRoot();
  
      if (!format) {
          return Promise(output);
      }
  
      return getConvertOptions(output)
      .then(function(options) {
          var cmd = [
              'ebook-convert',
              path.resolve(outputFolder, SUMMARY_FILE),
              path.resolve(outputFolder, 'index.' + format),
              command.optionsToShellArgs(options)
          ].join(' ');
  
          return command.exec(cmd)
          .progress(function(data) {
              logger.debug(data);
          })
          .fail(function(err) {
              if (err.code == 127) {
                  throw error.RequireInstallError({
                      cmd: 'ebook-convert',
                      install: 'Install it from Calibre: https://calibre-ebook.com'
                  });
              }
  
              throw error.EbookError(err);
          });
      })
      .thenResolve(output);
  }