Answer by user2567875 for Script to check if some program is already installed

Why do you want to check it in the first place? Unless you have a good reason for it, don't do it, just apt-get install package over. If it's already installed it will be updated if there is a newer version available, if it is installed and it is up to date, nothing will happen. In case you have some configuration that needs to be applied, there are other options, like building an configuration package which depends on the package or using configuration management software like ansible.