One of the new configuration files systemd introduced is /etc/os-release. It replaces the multitude of per-distribution release files with a single one. Yesterday we decided to drop support for systems lacking /etc/os-release in systemd since recently the majority of the big distributions adopted /etc/os-release and many small ones did, too. It's our hope that by dropping support for non-compliant distributions we gently put some pressure on the remaining hold-outs to adopt this scheme as well.
I'd like to take the opportunity to explain a bit what the new file offers, why application developers should care, and why the distributions should adopt it. Of course, this file is pretty much a triviality in many ways, but I guess it's still one that deserves explanation.
So, you ask why this all?
- It relieves application developers who just want to know the distribution they are running on to check for a multitude of individual release files.
- It provides both a "pretty" name (i.e. one to show to the user), and machine parsable version/OS identifiers (i.e. for use in build systems).
- It is extensible, can easily learn new fields if needed. For example, since we want to print a welcome message in the color of your distribution at boot we make it possible to configure the ANSI color for that in the file.
There's already the lsb_release tool for this, why don't you just use that? Well, it's a very strange interface: a shell script you have to invoke (and hence spawn asynchronously from your C code), and it's not written to be extensible. It's an optional package in many distributions, and nothing we'd be happy to invoke as part of early boot in order to show a welcome message. (In times with sub-second userspace boot times we really don't want to invoke a huge shell script for a triviality like showing the welcome message). The lsb_release tool to us appears to be an attempt of abstracting distribution checks, where standardization of distribution checks is needed. It's simply a badly designed interface. In our opinion, it has its use as an interface to determine the LSB version itself, but not for checking the distribution or version.
Why haven't you adopted one of the generic release files, such as Fedora's /etc/system-release? Well, they are much nicer than lsb_release, so much is true. However, they are not extensible and are not really parsable, if the distribution needs to be identified programmatically or a specific version needs to be verified.
Why didn't you call this file /etc/bikeshed instead? The name /etc/os-release sucks! In a way, I think you kind of answered your own question there already.
Does this mean my distribution can now drop our equivalent of /etc/fedora-release? Unlikely, too much code exists that still checks for the individual release files, and you probably shouldn't break that. This new file makes things easy for applications, not for distributions: applications can now rely on a single file only, and use it in a nice way. Distributions will have to continue to ship the old files unless they are willing to break compatibility here.
This is so useless! My application needs to be compatible with distros from 1998, so how could I ever make use of the new file? I will have to continue using the old ones! True, if you need compatibility with really old distributions you do. But for new code this might not be an issue, and in general new APIs are new APIs. So if you decide to depend on it, you add a dependency on it. However, even if you need to stay compatible it might make sense to check /etc/os-release first and just fall back to the old files if it doesn't exist. The least it does for you is that you don't need 25+ open() attempts on modern distributions, but just one.
You evil people are forcing my beloved distro $XYZ to adopt your awful systemd schemes. I hate you! You hate too much, my friend. Also, I am pretty sure it's not difficult to see the benefit of this new file independently of systemd, and it's truly useful on systems without systemd, too.
I hate what you people do, can I just ignore this? Well, you really need to work on your constant feelings of hate, my friend. But, to a certain degree yes, you can ignore this for a while longer. But already, there are a number of applications making use of this file. You lose compatibility with those. Also, you are kinda working towards the further balkanization of the Linux landscape, but maybe that's your intention?
You guys add a new file because you think there are already too many? You guys are so confused! None of the existing files is generic and extensible enough to do what we want it to do. Hence we had to introduce a new one. We acknowledge the irony, however.
The file is extensible? Awesome! I want a new field XYZ= in it! Sure, it's extensible, and we are happy if distributions extend it. Please prefix your keys with your distribution's name however. Or even better: talk to us and we might be able update the documentation and make your field standard, if you convince us that it makes sense.
Anyway, to summarize all this: if you work on an application that needs to identify the OS it is being built on or is being run on, please consider making use of this new file, we created it for you. If you work on a distribution, and your distribution doesn't support this file yet, please consider adopting this file, too.
If you are working on a small/embedded distribution, or a legacy-free distribution we encourage you to adopt only this file and not establish any other per-distro release file.
 Yes, multitude, there's at least: /etc/redhat-release, /etc/SuSE-release, /etc/debian_version, /etc/arch-release, /etc/gentoo-release, /etc/slackware-version, /etc/frugalware-release, /etc/altlinux-release, /etc/mandriva-release, /etc/meego-release, /etc/angstrom-version, /etc/mageia-release. And some distributions even have multiple, for example Fedora has already four different files.
 To our knowledge at least OpenSUSE, Fedora, ArchLinux, Angstrom, Frugalware have adopted this. (This list is not comprehensive, there are probably more.)