Mar 30, 2022

You’ve probably seen the “I’d like to interject for a moment” quotation that is frequently attributed to Richard Stallman about how Linux should be referred to as GNU/Linux. While I disagree with that particular assertion, I do believe it is important to refer to GNU/Linux distributions as such, because GNU/Linux is a distinct operating system in the family of operating systems which use the Linux kernel, and it is technically correct to recognize this, especially as different Linux-based operating systems have different behavior, and different advantages and disadvantages.

For example, besides GNU/Linux, there are the Alpine and OpenWrt ecosystems, and last but not least, Android. All of these operating systems exist outside the GNU/Linux space and have significant differences, both between GNU/Linux and also each other.

what is GNU/Linux?

I believe part of the problem which leads people to be confused about the alternative Linux ecosystems is the lack of a cogent GNU/Linux definition, in part because many GNU/Linux distributions try to downplay that they are, in fact, GNU/Linux distributions. This may be for commercial or marketing reasons, or it may be because they do not wish to be seen as associated with the FSF. Because of this, others, who are fans of the work of the FSF, tend to overreach and claim other Linux ecosystems as being part of the GNU/Linux ecosystem, which is equally harmful.

It is therefore important to provide a technically accurate definition of GNU/Linux that provides actual useful meaning to consumers, so that they can understand the differences between GNU/Linux-based operating systems and other Linux-based operating systems. To that end, I believe a reasonable definition of the GNU/Linux ecosystem to be distributions which:

• use the GNU C Library (frequently referred to as glibc)
• use the GNU coreutils package for their base UNIX commands (such as /bin/cat and so on).

From a technical perspective, an easy way to check if you are on a GNU/Linux system would be to attempt to run the /lib/libc.so.6 command. If you are running on a GNU/Linux system, this will print the glibc version that is installed. This technical definition of GNU/Linux also provides value, because some drivers and proprietary applications, such as the nVidia proprietary graphics driver, only support GNU/Linux systems.

Given this rubric, we can easily test a few popular distributions and make some conclusions about their capabilities:

• Debian-based Linux distributions, including Debian itself, and also Ubuntu and elementary, meet the above preconditions and are therefore GNU/Linux distributions.
• Fedora and the other distributions published by Red Hat also meet the same criterion to be defined as a GNU/Linux distribution.
• ArchLinux also meets the above criterion, and therefore is also a GNU/Linux distribution. Indeed, the preferred distribution of the FSF, Parabola, describes itself as GNU/Linux and is derived from Arch.
• Alpine does not use the GNU C library, and therefore is not a GNU/Linux distribution. Compatibility with GNU/Linux programs should not be assumed. More on that in a moment.
• Similarly, OpenWrt is not a GNU/Linux distribution.
• Android is also not a GNU/Linux, nor is Replicant, despite being sponsored by the FSF.

on compatibility between distros

Even between GNU/Linux distributions, compatibility is difficult. Different GNU/Linux distributions upgrade their components at different times, and due to dynamic linking, this means that a program built against a specific set of components with a specific set of build configurations may or may not successfully run between GNU/Linux systems, but some amount of binary compatibility is otherwise possible as long as you take care to deal with that.

On top of this, there is no binary compatibility between Linux ecosystems at large. GNU/Linux binaries require the gcompat compatibility framework to run on Alpine, and it generally is not possible to run OpenWrt binaries on Alpine or vice versa. The situation is the same with Android: without a compatibility tool (such as Termux), it is not possible to run binaries from other ecosystems there.

Exacerbating the problem, developers also target specific APIs only available in their respective ecosystems:

• systemd makes use of glibc-specific APIs, which are not part of POSIX
• Android makes use of bionic-specific APIs, which are not part of POSIX
• Alpine and OpenWrt both make use of internal frameworks, and these differ between the two ecosystems (although there are active efforts to converge both ecosystems).

As a result, as a developer, it is important to note which ecosystems you are targeting, and it is important to refer to individual ecosystems, rather than saying “my program supports Linux.” There are dozens of ecosystems which make use of the Linux kernel, and it is unlikely that a program supports all of them, or that the author is even aware of them.

To conclude, it is both correct and important, to refer to GNU/Linux distributions as GNU/Linux distributions. Likewise, it is important to realize that non-GNU/Linux distributions exist, and are not necessarily compatible with the GNU/Linux ecosystem for your application. Each ecosystem is distinct, with its own strengths and weaknesses.