How to list all `env` properties within jenkins pipeline job?

Question

Given a jenkins 2.1 build pipeline, jenkins injects a env variable into the node{}. For example, BRANCH_NAME can be accessed with

Answer 1

The answers above, are now antiquated due to new pipeline syntax. Below prints out the environment variables.

script {
        sh 'env > env.txt'
        String[] envs = readFile('env.txt').split("\r?\n")

        for(String vars: envs){
            println(vars)
        }
    }