I know that a lot of what Nix does is working around its break from FHS, but I can imagine there are still things that seep through. Are there any unsolvable problems due to this?
I saw on this post that it is possible to use FHS on Nix. Does this solve all potential issues then?
Why doesn’t nix use fhs again?
Nix installs derivations into separate folders. A derivation can be a package, but can also be other things like configuration files, scripts or sources for packages. Nix doesn’t distinguish between these derivations by a name but rather by a hash created from their build instructions.
For example two instances of the same package with a different version are two different derivations and thus nix can have both package versions installed without them interfering with each other. But this goes beyond just a package version. It is e.g. possible to have the same package with the same version but different patches applied, or relying on different versions of dependencies. Since their build instructions differ both can be installed simultaneously.
This approach grants a variety of advantages. For example upgrading your NixOS system just installs new derivations of packages and configuration files that have changed, while keeping previous derivations until they’re garbage collected at a later time. This allows you to switch freely between both iterations of your system, for example if an update causes issues you can just revert back to before an update easily. Another advantage is that an unprivileged user can install packages they need without interfering with the rest of the system, for example an older python version or a newer one, or some software they want but the system does not provide.
The price of having this kind of isolation between packages is that nixos cannot install binaries and libraries into common locations. Effectively /usr/bin only contains the env binary. If you’re familiar with shell scripting you might have run into lines such as
. This env util will essentially search bash in your PATH variable and start it. Lines likehowever will not work, because there’s no bash installed in that location.Another case where a missing fhs is a problem is when using pre-compiled binaries. In contrast to binaries built through nix, which have their required libraries hardcoded as absolute paths, pre-compiled binaries you download usually only contain the name of the library they need, which works in a conventional fhs environment, because these libraries tend to be found in
/libor/usr/lib. On NixOS neither of those are present. There two solutions to this. Either you create an fhs environment by listing the set of derivations to be symlinked into a chroot environment which mimics an FHS. Or you can install https://github.com/Mic92/nix-ld which automatically finds the required libraries the nix way if you start such a binary. There’s also steam-run which installs an fhs with most of the dependencies necessary to start Linux games from Steam.Either you create an fhs environment by listing the set of derivations to be symlinked into a chroot environment which mimics an FHS.
Why isn’t this done on the actual system and by default? That would make it fhs compliant, no?
If your system uses 3 different Pythons as dependencies of different packages, which one gets to be /usr/bin/python?
The most recent one by default unless another is manually chosen. The nix packages can keep using their specific versions directly
Choosing the most recent one might be impossible if you have multiple installations of the same package with same version but different features enabled during the configure step.
Conflict resolution is not an impossible task. You just need to have a sensible algorithm. I get that they don’t want to do it lest people abuse it instead of using nix but there isn’t a technical challenge that can’t be overcome.
Conflict resolution was not my point. Rather the question which the “most recent” between two almost identical installations…
You may be interested in reading this post about the process of packaging Steam.
tl;dr: It’s mostly an annoyance reserved for packagers to deal with. Dynamically linked executables can be patched in a fairly universal fashion to work without FHS, so that’s the go-to approach. If the executable is statically linked, the package may have to ship a source patch instead. If the executable is statically linked & close-source, the packagers are forced to resort to simulating an FHS environment via chroot.
So that means packaging software for nix is a pain, compared to, say, gentoo or arch’s AUR, but only for a small subset of packages.
I’ll keep this in mind as I’m exploring if I should switch from Gentoo.
I would say it’s actually easier in many cases. Nix has really fantastic packaging tooling. You do have to learn a bit of the nix language, however (not become an expert).
The issue comes when trying to build from source. In most other distros, ou just follow the readme. In nix, you have to package it.
If I am packaging software for gentoo, all I have to do is translate the build instructions from the project’s documentation to gentoo’s package recipe. In nix, it seems that it is not that simple and you’ll have to do some exploration. Am I wrong?
