We're using Jenkins version 1.651 with the Groovy plugin for Jenkins version 1.4.

I had real trouble getting access to the build parameters when trying to run a groovy script build step using the recommended methods of which there are many.

From the plugin documentation:

    // Retrieve parameters of the current build
    def foo = build.buildVariableResolver.resolve("FOO")
    println "FOO=$foo"

This doesn't work.

From Jenkins help:

    def thr = Thread.currentThread()
    def build = thr.executable
    // get build parameters
    def buildVariablesMap = build.buildVariables
    // get all environment variables for the build
    def buildEnvVarsMap = build.envVars

    String jobName = buildEnvVarsMap?.JOB_NAME

This also doesn't work but I believe that's because the groovy script isn't running in the same process as the Jenkins job (or so I've heard).

I also tried the Jenkins wiki that suggested:

    import hudson.model.*

    // get current thread / Executor
    def thr = Thread.currentThread()
    // get current build
    def build = thr?.executable

    // get parameters
    def parameters = build?.actions.find{ it instanceof ParametersAction }?.parameters
    parameters.each {
       println "parameter ${it.name}:"
       println it.dump()
       println "-" * 80
    }
    // ... or if you want the parameter by name ...
    def hardcoded_param = "FOOBAR"
    def resolver = build.buildVariableResolver
    def hardcoded_param_value = resolver.resolve(hardcoded_param)

    println "param ${hardcoded_param} value : ${hardcoded_param_value}"

So, to the solution...

    import hudson.model.*
    def foobar = System.getenv("FOOBAR")

It's so simple it's painful.