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Duplicati is a free, open-source cloud backup application written in C# and distributed for Windows, MacOS X and Linux in a variety of packages. Not affiliated with any cloud storage provider, it focuses on automating and optimizing the process of making incremental backups and leaves the choice of where to store them (local media or over 20 commercial online services) up to the user. It consists of a daemon with a CLI and a web GUI with a tray icon for status updates.
SpecsUpdate
Pros
Pro An abundance of storage backends
From USB flash drives to external mechanical hard drives, shared Samba or WebDAV folders over a local network, to SFTP servers or traditional commercial cloud storage providers like Dropbox, Google Drive or Backblaze, users can select any combination they like to accomplish their data security and fault tolerance goals.
Pro Reliable outcome reporting
The local instance of the application uses a tray icon on all three platforms to inform the user of new information about configured backups, and for headless installations or managed systems a built-in e-mail generator is offered along with an XMPP client/bot. For more interactive tasks the Web UI can be port-forwarded to the public internet, allowing for real-time updates and management of tasks from anywhere in the world.
Pro Under very active development by a small, dedicated team
Duplicati 2 (the current version) has been distributed since August 2014 as a beta release and continues to receive feature updates and bug fixes averaging once every six weeks with updates on the canary channel often appearing twice monthly. While glitches and regressions aren't uncommon, the size of the user base lends itself to quickly helping newcomers troubleshoot and get comfortable.