Jenkins_Credential_List.groovy

import jenkins.model.Jenkins;
import com.cloudbees.plugins.credentials.*
import org.jenkinsci.plugins.plaincredentials.*
import org.jenkinsci.plugins.plaincredentials.impl.*

def _cred = CredentialsProvider.lookupCredentials(
    Credentials.class,
    Jenkins.getInstance(),
    null,
    null
)
for (c in _cred) {
     if (c.getClass() == StringCredentialsImpl) {
        println "${c.id} -> ${c.getSecret()}"
    }
}