nix3-eval

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

Name

nix eval - evaluate a Nix expression

Synopsis

nix eval [option…] installable

Examples

•

Evaluate a Nix expression given on the command line:

# nix eval --expr ’1 + 2’

•

Evaluate a Nix expression to JSON:

# nix eval --json --expr ’{ x = 1; }’
{"x":1}

•

Evaluate a Nix expression from a file:

# nix eval -f ./my-nixpkgs hello.name

•

Get the current version of the nixpkgs flake:

# nix eval --raw nixpkgs#lib.version

•

Print the store path of the Hello package:

# nix eval --raw nixpkgs#hello

•

Get a list of checks in the nix flake:

# nix eval nix#checks.x86_64-linux --apply builtins.attrNames

•

Generate a directory with the specified contents:

# nix eval --write-to ./out --expr ’{ foo = "bar"; subdir.bla = "123"; }’
# cat ./out/foo
bar
# cat ./out/subdir/bla
123

Description

This command evaluates the Nix expression installable and prints the result on standard output.

Output format

nix eval can produce output in several formats:

	•		By default, the evaluation result is printed as a Nix expression.
	•		With --json, the evaluation result is printed in JSON format. Note that this fails if the result contains values that are not representable as JSON, such as functions.
	•		With --raw, the evaluation result must be a string, which is printed verbatim, without any quoting.
	•		With --write-to path, the evaluation result must be a string or a nested attribute set whose leaf values are strings. These strings are written to files named path/attrpath. path must not already exist.

Options

•

--apply expr

Apply the function expr to each argument.

•

--json

Produce output in JSON format, suitable for consumption by another program.

•

--raw

Print strings without quotes or escaping.

•

--read-only

Do not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation.

•

--write-to path

Write a string or attrset of strings to path.

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.

•

--eval-store store-url

The Nix store to use for evaluations.

•

--impure

Allow access to mutable paths and repositories.

•

--include / -I path

Add path to the list of locations used to look up <...> file names.

•

--override-flake original-ref resolved-ref

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

Common flake-related options:

•

--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. This option is deprecated; use --no-use-registries.

•

--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.

•

--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.

•

--update-input input-path

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

Options that change the interpretation of installables:

•

--derivation

Operate on the store derivation rather than its outputs.

•

--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.