When you log in to a Debian Linux system for the first time, before doing any work it is always a good idea to check what version of Debian is running on the machine.

Three releases of Debian are always actively maintained:

Stable - The latest officially released distribution of Debian. At the time of writing this article, the current stable distribution of Debian is version 9 (stretch). This is the version that is recommended for production environments.
Testing - The preview distribution that will become the next stable release. It contains packages that are not ready for stable release yet, but they are in the queue for that. This release is updated continually until it is frozen and released as stable.
Unstable, always codenamed sid - This is the distribution where the active development of Debian is taking place.

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