@Override
  public void terminate() {
    shell = null;
  }
Indicates the execution runtime should be terminated. This allows it to cleanup before
returning control flow to the caller. Necessary for clean shutdowns. Copied from
{@link ExecutionStrategy#terminate()}.
geode-gfsh/src/main/java/org/apache/geode/management/internal/cli/shell/GfshExecutionStrategy.java