In jenkins declarative pipeline, how can I set environment variable based on method?

I had exactly the same problem, and indeed it is possible to use a shared library method. But there is another solution, more simple if you do not have a shared library set-up yet, that consists in defining a groovy method before the Pipeline statement and then use it inside your pipeline like this :

def getEnvFromBranch(branch) {  if (branch == 'master') {    return 'production'  } else {    return 'staging' }}pipeline {  agent any  environment {    targetedEnv = getEnvFromBranch(env.BRANCH_NAME)  }  stages {    stage('Build') {        steps {            echo "Building in ${env.targetedEnv}"        }    } }}

You can do exactly what you're suggesting. You should create a jenkins shared library with a var (a new DSL method). These can be called to assign to a pipeline-wide environment variable. You had it basically correct. Here's a Jenkinsfile fragment to assign to an environment variable:

environment {  DEPLOY_ENV = mapBranchToDeployEnvironment()}

You don't need to pass the branch to the mapBranchToDeployEnvironment DSL method, since you can access the branch in that method. sample contents of vars/mapBranchToDeployEnvironment.groovy in shared library look like this:

def call() {  echo "branch is: ${env.BRANCH_NAME}"  if (env.BRANCH_NAME == 'master') {    return 'prod'  } else {    return 'staging'  }}

You probably shouldn't expect this to be a five minute task, but you'll get it. Good luck!

stage('Prepare env variables') {    steps {        script {            if (env.BRANCH_NAME == 'master') {                echo 'Copying project-stg.env file...';                sh 'cp /opt/project-stg.env .env';            } else {                echo 'Copying project-dev.env file...';                sh 'cp /opt/project-dev.env .env';            }        }    }}