Nixpkgs: "bad substitution" error on darwin

[Ptival]

Describe the bug
Since this commit:
https://github.com/NixOS/nixpkgs/commit/bbc5b22ad82f89d2605ab91073012b12cabe87c3
Whenever I run commands like:
nix-shell -p hello
I get the following error:

/nix/store/v94xrmhkljhb44v2shcpv74gxg1ii1f1-cctools-binutils-darwin-wrapper/nix-support/setup-hook: line 140: ${role_pre}${cmd^^}=${cmd}: bad substitution

Additional context
My default shell is zsh, which has been the cause of some issues at times.

Metadata

 - system: `"x86_64-darwin"`
 - host os: `Darwin 18.7.0, macOS 10.14.6`
 - multi-user?: `no`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): `""`
 - channels(val): `"nixpkgs-20.03pre197510.4b0508a5853"`
 - nixpkgs: `/Users/val/.nix-defexpr/channels/nixpkgs`

Paging @xzfc in particular. Let me know how I can help!

[xzfc]

You need recent bash (>=4) as macOS has outdated bash by default.

I'm not sure how nix behaves on macOS, but try either of the following:

1. If you have $NIX_BUILD_SHELL environment variable, unset it.
   unset NIX_BUILD_SHELL; nix-shell -p hello
2. export NIX_BUILD_SHELL=$(nix-build -A bashInteractive '<nixpkgs>')/bin/bash; nix-shell -p hello
3. Update your system bash or install it via nix: nix-env -iA nixpkgs.bashInteractive

[Ptival]

Thanks. I personally solved it by:

brew update
brew install bash

Which installed an ambient bash v5.0.

Feel free to close this if you think you're done tracking this concern.

[matthewbauer]

Nix should use bashInteractive from Nixpkgs. I suspect it's hitting some error and falling back here:

https://github.com/NixOS/nix/blob/7c74f075f4a7274ad38c90085cc269a19a977438/src/nix-build/nix-build.cc#L357

[themoritz]

Thanks. I personally solved it by:

brew update
brew install bash

I could also solve the issue for me with these steps.

[anirrudh]

@matthewbauer I got this issue on macOS, and installing via @xzfc's comment worked out well (which was also the solution you mentioned). I am running macOS Catalina, and wanted a pure nix/macOS machine, so for me this was a better solution.

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