Warning
This program is experimental and its interface is subject to change.

Name

nix fmt - reformat your code in the standard style

Synopsis

nix fmt [option...] args...

Examples

# flake.nix
{
  outputs = { nixpkgs, self }: {
    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixpkgs-fmt;
  };
}

Format the current flake: $ nix fmt
Format a specific folder or file: $ nix fmt ./folder ./file.nix

With nixfmt:

# flake.nix
{
  outputs = { nixpkgs, self }: {
    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt;
  };
}

Format specific files: $ nix fmt ./file1.nix ./file2.nix

With Alejandra:

# flake.nix
{
  outputs = { nixpkgs, self }: {
    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.alejandra;
  };
}

Format the current flake: $ nix fmt
Format a specific folder or file: $ nix fmt ./folder ./file.nix

Description

nix fmt will rewrite all Nix files (*.nix) to a canonical format using the formatter specified in your flake.

Options

Common evaluation options

--arg name expr

Pass the value expr as the argument name to Nix functions.
--argstr name string

Pass the string string as the argument name to Nix functions.
--debugger

Start an interactive environment if evaluation fails.
--eval-store store-url

The URL of the Nix store to use for evaluation, i.e. to store derivations (.drv files) and inputs referenced by them.
--impure

Allow access to mutable paths and repositories.
--include / -I path

Add path to the Nix search path. The Nix search path is initialized from the colon-separated NIX_PATH environment variable, and is used to look up the location of Nix expressions using paths enclosed in angle brackets (i.e., <nixpkgs>).

For instance, passing
```
-I /home/eelco/Dev
-I /etc/nixos
```
will cause Nix to look for paths relative to /home/eelco/Dev and /etc/nixos, in that order. This is equivalent to setting the NIX_PATH environment variable to
```
/home/eelco/Dev:/etc/nixos
```
It is also possible to match paths against a prefix. For example, passing
```
-I nixpkgs=/home/eelco/Dev/nixpkgs-branch
-I /etc/nixos
```
will cause Nix to search for <nixpkgs/path> in /home/eelco/Dev/nixpkgs-branch/path and /etc/nixos/nixpkgs/path.

If a path in the Nix search path starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must consist of a single top-level directory. For example, passing
```
-I nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
```
tells Nix to download and use the current contents of the master branch in the nixpkgs repository.

The URLs of the tarballs from the official nixos.org channels (see the manual page for nix-channel) can be abbreviated as channel:<channel-name>. For instance, the following two flags are equivalent:
```
-I nixpkgs=channel:nixos-21.05
-I nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
```
You can also fetch source trees using flake URLs and add them to the search path. For instance,
```
-I nixpkgs=flake:nixpkgs
```
specifies that the prefix nixpkgs shall refer to the source tree downloaded from the nixpkgs entry in the flake registry. Similarly,
```
-I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05
```
makes <nixpkgs> refer to a particular branch of the NixOS/nixpkgs repository on GitHub.
--override-flake original-ref resolved-ref

Override the flake registries, redirecting original-ref to resolved-ref.

--commit-lock-file

Commit changes to the flake's lock file.
--inputs-from flake-url

Use the inputs of the specified flake as registry entries.
--no-registries

Don't allow lookups in the flake registries.

DEPRECATED

Use --no-use-registries instead.
--no-update-lock-file

Do not allow any updates to the flake's lock file.
--no-write-lock-file

Do not write the flake's newly generated lock file.
--output-lock-file flake-lock-path

Write the given lock file instead of flake.lock within the top-level flake.
--override-input input-path flake-url

Override a specific flake input (e.g. dwarffs/nixpkgs). This implies --no-write-lock-file.
--recreate-lock-file

Recreate the flake's lock file from scratch.

DEPRECATED

Use nix flake update instead.
--reference-lock-file flake-lock-path

Read the given lock file instead of flake.lock within the top-level flake.
--update-input input-path

Update a specific flake input (ignoring its previous entry in the lock file).

DEPRECATED

Use nix flake update instead.

--debug

Set the logging verbosity level to 'debug'.
--log-format format

Set the format of log output; one of raw, internal-json, bar or bar-with-logs.
--print-build-logs / -L

Print full build logs on standard error.
--quiet

Decrease the logging verbosity level.
--verbose / -v

Increase the logging verbosity level.

Miscellaneous global options

--help

Show usage information.
--offline

Disable substituters and consider all previously downloaded files up-to-date.
--option name value

Set the Nix configuration setting name to value (overriding nix.conf).
--refresh

Consider all previously downloaded files out-of-date.
--repair

During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths.
--version

Show version information.

Options that change the interpretation of installables

--expr expr

Interpret installables as attribute paths relative to the Nix expression expr.
--file / -f file

Interpret installables as attribute paths relative to the Nix expression stored in file. If file is the character -, then a Nix expression will be read from standard input. Implies --impure.

Note

See man nix.conf for overriding configuration settings with command line flags.

Nix Reference Manual