A command like
$ nix run nixpkgs#hello
will now build the attribute 'packages.${system}.hello' rather than
'packages.hello'. Note that this does mean that the flake needs to
export an attribute for every system type it supports, and you can't
build on unsupported systems. So 'packages' typically looks like this:
packages = nixpkgs.lib.genAttrs ["x86_64-linux" "i686-linux"] (system: {
hello = ...;
});
The 'checks', 'defaultPackage', 'devShell', 'apps' and 'defaultApp'
outputs similarly are now attrsets that map system types to
derivations/apps. 'nix flake check' checks that the derivations for
all platforms evaluate correctly, but only builds the derivations in
'checks.${system}'.
Fixes#2861. (That issue also talks about access to ~/.config/nixpkgs
and --arg, but I think it's reasonable to say that flakes shouldn't
support those.)
The alternative to attribute selection is to pass the system type as
an argument to the flake's 'outputs' function, e.g. 'outputs = { self,
nixpkgs, system }: ...'. However, that approach would be at odds with
hermetic evaluation and make it impossible to enumerate the packages
provided by a flake.
Only variables that were marked as exported are exported in the dev
shell. Also, we no longer try to parse the function section of the env
file, fixing
$ nix dev-shell
error: shell environment '/nix/store/h7ama3kahb8lypf4nvjx34z06g9ncw4h-nixops-1.7pre20190926.4c7acbb-env' has unexpected line '/^[a-z]?"""/ {'
This ensures that stdenv / setup hooks take $IN_NIX_SHELL into
account. For example, stdenv only sets
NIX_SSL_CERT_FILE=/no-cert-file.crt if we're not in a shell.
Thus
$ nix dev-shell
will now build the 'provides.devShell' attribute from the flake in the
current directory. If it doesn't exist, it falls back to
'provides.defaultPackage'.
'nix dev-shell' is intended to replace nix-shell. It supports flakes,
e.g.
$ nix dev-shell nixpkgs:hello
starts a bash shell providing an environment for building 'hello'.
Like Lorri (and unlike nix-shell), it computes the build environment
by building a modified top-level derivation that writes the
environment after running $stdenv/setup to $out and exits. This
provides some caching, so it's faster than nix-shell in some cases
(especially for packages with lots of dependencies, where the setup
script takes a long time).
There also is a command 'nix print-dev-env' that prints out shell code
for setting up the build environment in an existing shell, e.g.
$ . <(nix print-dev-env nixpkgs:hello)
https://github.com/tweag/nix/issues/21
