nix multi-line shebang is not handled by just

[oux]

Environment:

• nix 2.20.1 or later
• just 1.25.2

justfile:

hello:
  #!/usr/bin/env nix
  #! nix shell  nixpkgs#hello nixpkgs#cowsay --command bash
  hello | cowsay

Error:

Same shebang on a simple shell script file:

Nix shebang documentation:
https://nixos.org/manual/nix/stable/command-ref/new-cli/nix.html#shebang-interpreter

[casey]

I think the reason this doesn't work is that when just extracts the body of a shebang recipe into a file to execute, just places the shabang line first, and then inserts blank lines before inserting the contents of the recipe, so that line numbers in error messages correspond to line numbers of the recipe.

So the way to fix this is to see if the next line is a shebang and put it at the top of the file.

So for example, this:

hello:
  #!/usr/bin/env nix
  #! nix shell  nixpkgs#hello nixpkgs#cowsay --command bash
  hello | cowsay

Is currently translated to this:

#!/usr/bin/env nix

#! nix shell  nixpkgs#hello nixpkgs#cowsay --command bash
hello | cowsay

When it should be translated to this:

#!/usr/bin/env nix
#! nix shell  nixpkgs#hello nixpkgs#cowsay --command bash

hello | cowsay