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.