Printing a list of all the credentials in the system and their IDs.

  def creds = com.cloudbees.plugins.credentials.CredentialsProvider.lookupCredentials(
      com.cloudbees.plugins.credentials.common.StandardUsernameCredentials.class,
      Jenkins.instance,
      null,
      null
  );
  for (c in creds) {
       println(c.id + ": " + c.description)
  }