Add '--write' argument

[andyrichardson]

Is your feature request related to a problem? Please describe. It would be great if formatting could optionally be written to files on execution.

Tools such as prettier and eslint take a --write or --fix argument to opt-in to writing to the file.

Describe the solution you'd like

When running the following command

fmt --write flake.nix

The formatted file content would be written to the file rather than outputted to the console.

Describe alternatives you've considered

Maybe add something to the readme to make it easy for users to have this functionality (less ideal).

Something like (not tested)

find nix | grep .nix | xargs -L 1 -I {} bash -c "fmt {} > {}"  

Additional context

N/A

[zimbatm]

This is already the default. Just use nixpkgs-fmt flake.nix

[andyrichardson]

Thanks @zimbatm - looks like I was using fmt rather than nixpkgs-fmt