> Say I happen to install package X via apt and Y via nix, and both of them depend on Z (in apt and nix respectively), and Z needs to bind to a port, then I imagine both will install but one of them will break.

When you install a package with apt, rpm, etc., it has the ability to run code at install time, create init scripts/systemd unit files, etc. When you install a package with Nix, it does not have that ability. So if you install Y via Nix, it will install Z via Nix, and Nix's Z will not be started automatically. You'll need to start it yourself (or Y will need to trigger it, when you run it, etc.)

If you also install X via apt and Z via apt, apt's Z will be started automatically, and will prevent Nix's Z from starting at the same time. You'll need to stop apt's Z.

Note that there's a spectrum here - Red Hat-based distros conventionally don't start software automatically when you install it, although they do leave the configuration in place for you to enable it (with chkconfig or something). Debian-based distros do. (While I am generally a Debian fan, this is one of the things I don't like about Debian ... possibly because I lost some mail once when I did "apt-get install exim" on a production server that had died, and a bunch of email that had been in other people's queues hit the default exim and then got bounces because I hadn't configured it yet.)

Another way of thinking about it is that there are two meanings of "install GRUB". One is that the GRUB commands are available for you to use. One is that your disk has GRUB in its boot sector. Arch, Debian, Red Hat, etc. all interpret "install GRUB" as doing both of these, but doing just the first one is valid, and then it's up to you to use the GRUB commands to put GRUB on the boot sector of whatever disk you want.

> Or if I install a package on Nix that expects a certain syscall that's not in the Ubuntu kernel yet (maybe like the recent /usr/bin/sleep issue with WSL), then that either breaks Nix or my ability to keep using an Ubuntu kernel. Right?

Yes. Nix won't install a new kernel for you, unless you're using NixOS.

(Technically, I suppose you can put a new kernel down on disk with Nix, but it won't install it in the sense of changing systemwide configuration.)

> I've seen enough trainwrecks when upgrading even across OS versions that I have a hard time seeing how running 2 package managers can work on a single OS without breaking something?

This is generally due to one of two things, both of which Nix sidesteps:

- There are incompatible updates to a file, e.g., you install "mawk," which provides /usr/bin/awk, but some other package expected that to be "gawk" and uses gawk-specific features. Nix doesn't have an equivalent to /usr/bin; there are no paths that are shared across packages. Each file is in a directory which corresponds to the identity of the one package (at one specific version) that provides it, and other packages bake in those dependencies. This is the fundamental cool thing about Nix.

- Systemwide changes when you install/remove stuff, like uninstalling syslogd shutting down your syslog daemon, installing a kernel changing the default kernel you'll boot into, etc. Installing a package in Nix doesn't have any effects beyond putting down files. The most Nix will do is keep a pointer (a "profile") to some set of stuff you're interested in (again, by exact identity). You can change that pointer, but there's nothing special about your particular pointer. If you point to a set of fewer things, other people can keep pointing to the things you no longer see. If you point to more things, that doesn't cause any code to run automatically.

Combined, this means that Nix stays out of the way of a traditional package manager. (Also, it means that you can set up your system so non-root users can install/remove things with Nix - you need to do some setup as root tp install Nix in the first place, but once it's there, users can't mess with each other.)

If you're familiar with virtualenvs etc., you can sort of think of it like that (although it's not a perfect metaphor) - you can install Python packages into a specific environment, but they don't get installed systemwide, and if you install, say, a web server (gunicorn, Flask, Django, etc.) into multiple virtualenvs, none of them are started automatically. You can't meaningfully install a kernel into a virtualenv (nothing stops someone from releasing a kernel on PyPI, but "installing" it will just download the file and put it somewhere and nothing else).

This does mean that the experience of using Nix is different from using a traditional package manager - you'll need to take care of starting the services you need, etc. Basically Nix reduces the scope of a package manager to say that running services isn't part of it, and then it does a better job of implementing that reduced scope. If you're a company like Shopify that is unlikely to be happy with the built-in web server that automatically starts when they "apt-get install apache2" that shows "Congratulations, you've installed a web server" to your customers, that's totally fine, you were going to do your own service management anyway.