Hi.

If I'm on Debian 8 Testing and 'testing' is in the sources file (/etc/apt/sources.list) -- when Debian 9 gets released - will I automatically be switched to Debian 9 Testing?

You are not on Debian 8 Testing, Debian 8 "Jessie" is stable. If you've changed your /etc/apt/sources.list file and switched all occurrences of jessie, or stable with testing, you already are on Debian 9 testing. While it may seem to you like a great idea to have instead of distribution name in your sources.list testing, it can put you in lots of trouble. For example, when Debian 9 "Stertch" becomes stable, you will automatically switch to the next testing version. And then all kind of packages will enter into it and many of those will not be stable, will come in with various bugs and can even break your system.

If you want to have some kind of semi rolling testing release, use code names. For now stretch, when it becomes stable, wait that all those packages in testing pass some "cleaning" process and then, when things calm down a bit, change stretch for next release code name.

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if you change your source list to testing it will get updated with whatever is in that repo for whatever reasons. It will not be a stable system due to having anything that is being tested to see if it is working properly installed on your system.

This article from Debian should address your questions: https://wiki.debian.org/DebianTesting

I used Debian testing for several years. I found it quite satisfactory. Debian Testing is more stable than some distro's release versions.

(I'm not using it right now because that computer died after years of long and faithful service. I am using Debian 8 on another computer.)

A short answer:
Yes.

I used "testing" for a year and never had any problems with it. If you use, "unstable", you should be prepared for short-term breakages. As far as I can remember, all packages in unstable have to serve a fixed time without further problems being reported before they are allowed through into testing.

I know that. It was an example scenario.

I think Stable is the way to go for me. My PC is 3+ years old and I rarely buy new hardware.

Thanks for the info.

I will heed that warning.

To add if you want a rolling release you'll need to change your distro.
void
or something else that updates their repo almost on a daily basis with stable releases of whatever it updates on the days it updates its repo.

Thanks for the info guys.