import com.cloudbees.plugins.credentials.*
import com.cloudbees.plugins.credentials.common.*
def credential_id = 'test'
def id_matcher = CredentialsMatchers.withId(credential_id)
def available_credentials = CredentialsProvider.lookupCredentials(StandardCredentials.class, Jenkins.getInstance(), null, null)


String streamToSring(InputStream input) {
    Scanner s = new Scanner(input);
    StringBuilder builder = new StringBuilder();
    while (s.hasNextLine()) {
        builder.append(s.nextLine() +"\n");
    }
    return builder.toString();
}

println streamToSring(CredentialsMatchers.firstOrNull(available_credentials, id_matcher).getContent())