Creating a Jenkins environment variable using Groovy

Jenkins 1.x

The following groovy snippet should pass the version (as you've already supplied), and store it in the job's variables as 'miniVersion'.

import hudson.model.*
  
def env = System.getenv()
def version = env['currentversion']
def m = version =~/\d{1,2}/
def minVerVal = m[0]+"."+m[1]
  
def pa = new ParametersAction([
  new StringParameterValue("miniVersion", minVerVal)
])
  
// add variable to current job
Thread.currentThread().executable.addAction(pa)

The variable will then be accessible from other build steps. e.g.

echo miniVersion=%miniVersion%

Outputs:

miniVersion=12.34

I believe you'll need to use the "System Groovy Script" (on the Master node only) as opposed to the "Groovy Plugin" - https://wiki.jenkins-ci.org/display/JENKINS/Groovy+plugin#Groovyplugin-GroovyScriptvsSystemGroovyScript

Jenkins 2.x

I believe the previous (Jenkins 1.x) behaviour stopped working because of this Security Advisory...

Solution (paraphrased from the Security Advisory)

It's possible to restore the previous behaviour by setting the system property hudson.model.ParametersAction.keepUndefinedParameters to true. This is potentially very unsafe and intended as a short-term workaround only.

java -Dhudson.model.ParametersAction.keepUndefinedParameters=true -jar jenkins.war

To allow specific, known safe parameter names to be passed to builds, set the system property hudson.model.ParametersAction.safeParameters to a comma-separated list of safe parameter names.

e.g.

java -Dhudson.model.ParametersAction.safeParameters=miniVersion,FOO,BAR -jar jenkins.war

And in groovy these two lines should be written this way:

System.setProperty("hudson.model.ParametersAction.keepUndefinedParameters","true");
System.setProperty("hudson.model.ParametersAction.safeParameters","miniVersion,FOO,BAR");  

As other answers state setting new ParametersAction is the way to inject one or more environment variables, but when a job is already parameterised adding new action won't take effect. Instead you'll see two links to a build parameters pointing to the same set of parameters and the one you wanted to add will be null.

Here is a snippet updating the parameters list in both cases (a parametrised and non-parametrised job):

import hudson.model.*

def build = Thread.currentThread().executable

def env = System.getenv()
def version = env['currentversion']
def m = version =~/\d{1,2}/
def minVerVal = m[0]+"."+m[1]

def newParams = null

def pl = new ArrayList<StringParameterValue>()
pl.add(new StringParameterValue('miniVersion', miniVerVal))

def oldParams = build.getAction(ParametersAction.class)

if(oldParams != null) {
  newParams = oldParams.createUpdated(pl)
  build.actions.remove(oldParams)
} else {
  newParams = new ParametersAction(pl)
}

build.addAction(newParams)

You can also define a variable without the EnvInject Plugin within your Groovy System Script:

import hudson.model.*
def build = Thread.currentThread().executable
def pa = new ParametersAction([
  new StringParameterValue("FOO", "BAR")
])
build.addAction(pa)

Then you can access this variable in the next build step which (for example) is an windows batch command:

@echo off
Setlocal EnableDelayedExpansion
echo FOO=!FOO!

This echo will show you "FOO=BAR".

Regards

For me, the following also worked in Jenkins 2 (2.73.3)

Replace

def pa = new ParametersAction([new StringParameterValue("FOO", foo)])
build.addAction(pa)

with

def pa = new ParametersAction([new StringParameterValue("FOO", foo)], ["FOO"])
build.addAction(pa)

ParametersAction seems to have a second constructor which allows to pass in "additionalSafeParameters" https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/ParametersAction.java