Nix-shell, nix shell and nix develop

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I think it’d be helpful to know the story here:

nix-shell was originally designed to debug derivation build processes. You hand it a derivation, and instead of building it, it drops you into a shell very similar to the derivation’s build env, so you can walk through the stdenv phases and explore what’s actually happening.

nix-shell turned out to have 2 other significant and unforseen usecases. Over time, features were added to support them:

  • Making development environments. Rather than debugging a nix build, people would just work on their source code from within the shell. pkgs.mkShell was added to support this usecase.
  • Temporarily getting access to a package. People would use nix-shell when they wanted to run something without actually permanently installing it. nix-shell -p made this much much easier.

When the flakes-centered CLI redesign came along, one of the things people wanted to do was split this up a bit:

  • nix shell is built for that last usecase (temporary access to a package). All it does is add a package’s bin directory to PATH, without any of the development stuff. It also takes multiple package arguments, as that makes sense for this usecase. It’s similar to nix-shell -p foo when not used for development purposes, as it doesn’t do all the stdenv stuff that makes dev tools work properly, but nix-shell -p actually does.
  • nix develop takes on the other 2 usecases (debugging derivation builds and development environments). Its argument is still the package whose build environment one wants to enter, and you can still use pkgs.mkShell to create a “package” with a build environment good for development work. It’s similar to nix-shell -A foo, nix-shell foo.nix, and nix-shell.

So there’s some replication of functionality because we’re in the middle of a CLI redesign which is still experimental (nix shell and nix develop are still experimental, so nix-shell is sticking around despite doing the same thing). There are also some unforseen usecases which we’re now trying to support in less hacky ways.

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