Nix 2.9 (2022-05-30) - Lix Reference Manual

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# Release 2.9 (2022-05-30)

- Running Nix with the new `--debugger` flag will cause it to start a repl session if an exception is thrown during evaluation, or if `builtins.break` is called. From there you can inspect the values of variables and evaluate Nix expressions. In debug mode, the following new repl commands are available:

      :env          Show env stack
      :bt           Show trace stack
      :st           Show current trace
      :st <idx>     Change to another trace in the stack
      :c            Go until end of program, exception, or builtins.break().
      :s            Go one step

  Read more about the debugger here.

- Nix now provides better integration with zsh's `run-help` feature. It is now included in the Nix installation in the form of an autoloadable shell function, `run-help-nix`. It picks up Nix subcommands from the currently typed in command and directs the user to the associated man pages.

- `nix repl` has a new build-and-link (`:bl`) command that builds a derivation while creating GC root symlinks.

- The path produced by `builtins.toFile` is now allowed to be imported or read even with restricted evaluation. Note that this will not work with a read-only store.

- `nix build` has a new `--print-out-paths` flag to print the resulting output paths. This matches the default behaviour of `nix-build`.

- You can now specify which outputs of a derivation `nix` should operate on using the syntax `installable^outputs`, e.g. `nixpkgs#glibc^dev,static` or `nixpkgs#glibc^*`. By default, `nix` will use the outputs specified by the derivation's `meta.outputsToInstall` attribute if it exists, or all outputs otherwise.

- `builtins.fetchTree` (and flake inputs) can now be used to fetch plain files over the `http(s)` and `file` protocols in addition to directory tarballs.