Jenkins Declarative Pipeline: How to read choice from input step?

Since you are using declarative pipelines we will need to do some tricks. Normally you save the return value from the input stage, like this

def returnValue = input message: 'Need some input', parameters: [string(defaultValue: '', description: '', name: 'Give me a value')]

However this is not allowed directly in declarative pipeline steps. Instead, what you need to do is wrap the input step in a script step and then propagate the value into approprierte place (env seems to work good, beware that the variable is exposed to the rest of the pipeline though).

pipeline {
    agent any
    stages {
        stage("foo") {
            steps {
                script {
                    env.RELEASE_SCOPE = input message: 'User input required', ok: 'Release!',
                            parameters: [choice(name: 'RELEASE_SCOPE', choices: 'patch\nminor\nmajor', description: 'What is the release scope?')]
                }
                echo "${env.RELEASE_SCOPE}"
            }
        }
    }
}

Note that if you have multiple parameters in the input step, then input will return a map and you need to use map references to get the entry that you want. From the snippet generator in Jenkins:

If just one parameter is listed, its value will become the value of the input step. If multiple parameters are listed, the return value will be a map keyed by the parameter names. If parameters are not requested, the step returns nothing if approved.