Not able to read Git Environment variables Jenkins using Groovy Jenkinsfile

So, anyone else who has stumbled onto this Stackoverflow issue should be aware that this is a bug with the current pipeline situation.

Lots of discussion here: https://issues.jenkins-ci.org/browse/JENKINS-35230

Essentially the plugin is not able to correctly add environment variables due to some incompatibilities with pipeline.

This is fixed in Git plugin 3.3.1

Version 3.3.1 (Jun 23, 2017)

• Print first line of commit message in console log (JENKINS-38241)
• Allow scm steps to return revision (JENKINS-26100)
• Don't require crumb for POST to /git/notifyCommit even when CSRF is enabled (JENKINS-34350)
• Fix credentials tracking null pointer exception in pipeline library use (JENKINS-44640)
• Fix credentials tracking null pointer exception in git parameters use (JENKINS-44087)

This won't work due to lack of double quotes, missing curly braces, and missing env.:

sh 'echo $BRANCH_NAME' 

This works as expected in a Jenkinsfile:

node {
    sh "echo ${env.BRANCH_NAME}"
}

Most of the pipeline examples I've found wrap the code in a "node" closure, which for some reason doesn't allow the Jenkins Git plugin to populate the environment variables.

However, if instead you wrap it like this, the environment variables are set properly:

pipeline {
  agent {
    label ('<AGENT>')
  }
  stages {
    stage('<STAGE>') {
      steps {
        <CODE>
      }
    }
  }
}

I'm not sure which closures are crucial here, but this formatting allowed me to access env variables such as env.GIT_COMMIT