Check credentials by name if they already exist in Jenkins

jenkins_check_ssh_credentials.groovy

import com.cloudbees.plugins.credentials.*
import com.cloudbees.plugins.credentials.common.*

def credentials_for_username(String username) {
  def username_matcher = CredentialsMatchers.withUsername(username)
  def available_credentials =
      CredentialsProvider.lookupCredentials(
        StandardUsernameCredentials.class,
        Jenkins.getInstance(),
        hudson.security.ACL.SYSTEM
      )

  return CredentialsMatchers.firstOrNull(available_credentials, username_matcher)
}

credentials_for_username("username_HERE")