Depexts os distribution os family values

os-distribution value is usually the value of ID in /etc/os-release os-family value is usually the first value of ID_LIKE in /etc/os-release

e.g.

Distribution	os	os-distribution	os-family
Archlinux	linux	arch	arch
Alpine	linux	alpine	alpine
Debian	linux	debian	debian
Ubuntu	linux	ubuntu	debian
Mint	linux	linuxmint	ubuntu
Fedora	linux	fedora	fedora
CentOS	linux	centos	rhel
Rocky	linux	rocky	rhel
Oraclelinux	linux	ol	fedora
OpenSUSE(leap)	linux	opensuse-leap	suse
OpenSUSE(tumbleweed)	linux	opensuse-tumbleweed	opensuse
Gentoo	linux	gentoo	gentoo
NixOS	linux	nixos	nixos
Homebrew	macos	homebrew	homebrew
Macports	macos	macports	macports
FreeBSD	freebsd	freebsd	bsd
OpenBSD	openbsd	openbsd	bsd
NetBSD	netbsd	netbsd	bsd

For linux distributions it is generally better to use the os-family value whenever possible to make distributions derived of those ones work out of the box (e.g. PopOS for Ubuntu and Manjaro for Archlinux).

In the case of CentOS/Fedora/Oraclelinux, using os-family can get really painful with the addition of the EPEL repository so just using os-distribution in this case is usually easier.

For macOS it is better to use both os = "macos" & os-distribution = "<distro>" to avoid the very unlikely case where a linux distribution starts calling itself homebrew in the future.

For BSDs it is better to just use os as the os-family value is not very helpful for depexts purpose.

Package names to populate depexts can be looked up in various websites, like https://pkgs.org/ and https://repology.org/projects/

Windows is a bit more complicated. Quoting from https://github.com/ocaml/opam-repository/pull/25484#issuecomment-1999943746:

The Cygwin part becomes very confusing here, as it's both an OS in its own right but also an os-distribution for native Windows.

• os-family is windows for both native Windows and Cygwin (i.e. Sys.win32 || Sys.cygwin ⇒ os-family = windows)
• os is win32 for a native Windows build of opam (which includes OCaml for Windows).

For os-distribution, we then have:

Distribution	os	os-distribution	os-family	Notes
Native[^1] Windows	win32	cygwin	windows	Depexts + Unix tools provided by Cygwin
Native Windows	win32	msys2[^2]	windows	Depexts + Unix tools provided by MSYS2
Native Windows	win32	cygwinports	windows	Legacy "OCaml for Windows" (opam 2.0.10 only)
Cygwin	cygwin	cygwin	windows	Cygwin's own opam distribution

Creating conf- packages for windows needs to be done with quite some care. Quoting https://github.com/ocaml/opam-repository/pull/27996#issuecomment-2948565171:

Opam 2.2's native Windows support together with https://github.com/ocaml/opam-repository/pull/25442's new layout offer support for both MinGW and MSys2 - with an example conf-package port in https://github.com/ocaml/opam-repository/pull/26072. We've been trying to follow this example in a string of PRs to further improve Windows support: https://github.com/ocaml/opam-repository/pull/26146, https://github.com/ocaml/opam-repository/pull/26798, https://github.com/ocaml/opam-repository/pull/26811, https://github.com/ocaml/opam-repository/pull/26812, https://github.com/ocaml/opam-repository/pull/26815, https://github.com/ocaml/opam-repository/pull/26819, https://github.com/ocaml/opam-repository/pull/27553, https://github.com/ocaml/opam-repository/pull/27846, https://github.com/ocaml/opam-repository/pull/27917

[^1]: "Native" here refers to programs able to run outside of Cygwin/MSYS2, principally from the Command Prompt or Windows PowerShell. [^2]: At the moment, this is "tier 2" from opam's perspective - Cygwin is the primary supported mechanism for 2.2.0, but we're paying a lot of attention to MSYS2 partly because that's what Diskuv uses now and also because we expect we'll likely switch to recommending it across the board in future.