Extract secrets stored in Jenkins credentials.

get-creds-from-jenkins.groovy

import org.jenkinsci.plugins.plaincredentials.StringCredentials
import com.cloudbees.plugins.credentials.CredentialsProvider
import com.cloudbees.plugins.credentials.domains.DomainRequirement
import groovy.json.JsonBuilder

import jenkins.model.Jenkins
import hudson.model.Job

String jobName = "YourFolder/YourMultibranch/YourJob"
Job job = Jenkins.instance.getItemByFullName(jobName, Job.class)

if (job == null) {
    println("Job not found: $jobName")
    return
}

def creds = CredentialsProvider.lookupCredentials(
    StringCredentials.class, // Class for secret text credentials, you can use different classes for different contexts/secret types.
    job,
    null,
    new ArrayList<DomainRequirement>()
)

def jsonBuilder = new JsonBuilder()
def credentialsMap = [:]
creds.each { c ->
    if (c instanceof StringCredentials) {
        credentialsMap[c.id] = c.getSecret().getPlainText()
    }
}
jsonBuilder(credentialsMap)
println(jsonBuilder.toPrettyString())