nix fmt should have a --dry-run option or similar

[kjeremy]

Is your feature request related to a problem? Please describe.
nix fmt is great for formatting a flake but it would be nice if we could check if nix fmt would perform any operations. This would be useful in CI for example or integrated into nix flake check.

Describe the solution you'd like
I'd like to see a --dry-run or similar flag passed to nix fmt that would return a status indicating if a nix fmt would change any files.

Describe alternatives you've considered
Calling the specified formatter manually and hoping it has this feature implemented.

Additional context
We are interested in adding this to our project CI to ensure that nix files are formatted properly when committed.

[diktomat]

Instead of --dry-run, I'd like to propose -c | --check, as that is the flag used for this purpose in all of nixfmt, nixpkgs-fmt (only long opt) and alejandra.

[fzakaria]

You can do nix fmt -- --check . now -- is that the same?
I guess running it as nix flake check would be the thing to do .