How to install NextCloud 13 on Debian 9

We’ll show you, how to install NextCloud 13 on Debian 9. Nextcloud is an open-source software suite that allows users to store their data such as files, contacts, calendars, news feed, TODO lists and much more, on their personal servers. It is using standard protocols such as webdavm, carddav and caldav. It also provides client applications so users can easily manage and synchronize their data among Linux, MacOS, Windows platforms and smart phones, which makes Nextcloud a great free alternative to proprietary cloud services such as Dropbox, Google Drive, iCloud, etc???