How to execute Groovy statements in a Jenkins declarative pipeline?

Question

I want to be able to define a variable, but declarative pipelines are more restrictive than scriptive pipelines. How can I do it? It doesn't matter where I put the String CMD =... statement, I get an error, which varies depending on where I put it.

pipeline {
    agent any
    environment {
        PATH = "/usr/local/bin:$PATH"
    }
    stages {
        // The pipeline will fail it can't find terraform
        stage("Check terraform") {
            steps {
                String CMD = (env.DESTROY.toBoolean ? "destroy" : "apply")
                // other steps
            }
        }
    }
}

Answer 1

You should wrap your code into script function https://www.jenkins.io/doc/book/pipeline/syntax/#script or declare your variable before pipeline {

pipeline {
    agent any
    environment {
        PATH = "/usr/local/bin:$PATH"
    }
    stages {
        // The pipeline will fail it can't find terraform
        stage("Check terraform") {
            steps {
               script {        
                  String CMD = (env.DESTROY.toBoolean ? "destroy" : "apply")
               }
                  // other steps
            }
        }
    }
}