Execute Jenkins Pipeline step only when building a tag

Update: As of version 1.2.8 of the Pipeline Model Definition Plugin you can now use buldingTag():

stage('Deploy') {
  when {
    buildingTag()
  }
  steps {
    echo 'Replace this with your actual deployment steps'
  }
}

---

When using the Multibranch Pipeline configuration you can use the expression condition along with the TAG_NAME environment variable provided by the underlying Branch API Plugin. Unfortunately, you can't directly check if the environment variable is defined at the Groovy level (API restrictions) so you have to test in in the shell:

stage('Deploy') {
  when { expression { sh([returnStdout: true, script: 'echo $TAG_NAME | tr -d \'\n\'']) } }
  steps {
    echo 'Replace this with your actual deployment steps'
  }
}

The above takes advantage of non-empty strings being truthy in Groovy.

An easier way may be introduced in future versions. See jenkinsci/pipeline-model-definition-plugin#240.