getJenkinsCreds.groovy

def creds = com.cloudbees.plugins.credentials.CredentialsProvider.lookupCredentials(
      com.cloudbees.plugins.credentials.Credentials.class
)

for (c in creds) {
  println(c.id)
  println(c.class)
  if (c.properties.username) {
    println("   description: " + c.description)
  }
  if (c.properties.username) {
    println("   username: " + c.username)
  }
  if (c.properties.password) {
    println("   password: " + c.password)
  }
  if (c.properties.passphrase) {
    println("   passphrase: " + c.passphrase)
  }
  if (c.properties.secret) {
    println("   secret: " + c.secret)
  }
  if (c.properties.privateKeySource) {
    println("   privateKey: " + c.getPrivateKey())
  }
  if (c instanceof org.jenkinsci.plugins.plaincredentials.impl.FileCredentialsImpl){
    println("    "+c.getContent().getText());
  }
  if (c instanceof com.dabsquared.gitlabjenkins.connection.GitLabApiTokenImpl){
    println("    "+c.getApiToken());
  }
  println("")
}