An example of using script block in a Declarative Jenkins Pipeline

script-in-declarative.groovy

stage('Set the build display name') {
    steps {
        script {
            CURRENT_BRANCH = sh(returnStdout: true, script: 'git rev-parse --abbrev-ref HEAD').trim()
            CURRENT_HASH = sh(returnStdout: true, script: 'git rev-parse --short HEAD').trim()
            currentBuild.displayName = "$CURRENT_BRANCH - $CURRENT_HASH"
        }
    }
}