Reference

nix3-eval(1)

evaluate a Nix expression

Warning
This program is experimental ⟨@docroot@/development/experimental-features.md#xp-feature-nix-command⟩ 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 --file ./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 given Nix expression, and prints the result on standard output.

It also evaluates any nested attribute values and list items.

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 ⟨#opt-apply⟩ expr

Apply the function expr to each argument.

•

--json ⟨#opt-json⟩

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

•

--raw ⟨#opt-raw⟩

Print strings without quotes or escaping.

•

--read-only ⟨#opt-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 ⟨#opt-write-to⟩ path

Write a string or attrset of strings to path.

Common evaluation options

•

--arg ⟨#opt-arg⟩ name expr

Pass the value expr as the argument name to Nix functions.

•

--arg-from-file ⟨#opt-arg-from-file⟩ name path

Pass the contents of file path as the argument name to Nix functions.

•

--arg-from-stdin ⟨#opt-arg-from-stdin⟩ name

Pass the contents of stdin as the argument name to Nix functions.

•

--argstr ⟨#opt-argstr⟩ name string

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

•

--debugger ⟨#opt-debugger⟩

Start an interactive environment if evaluation fails.

•

--eval-store ⟨#opt-eval-store⟩ store-url

The URL of the Nix store ⟨@docroot@/store/types/index.md#store-url-format⟩ to use for evaluation, i.e. to store derivations (.drv files) and inputs referenced by them.

•

--impure ⟨#opt-impure⟩

Allow access to mutable paths and repositories.

•

--include ⟨#opt-include⟩ / -I path

Add path to search path entries used to resolve lookup paths ⟨@docroot@/language/constructs/lookup-path.md⟩

This option may be given multiple times.

Paths added through -I take precedence over the nix-path configuration setting ⟨@docroot@/command-ref/conf-file.md#conf-nix-path⟩ and the NIX_PATH environment variable ⟨@docroot@/command-ref/env-common.md#env-NIX_PATH⟩\c .

•

--override-flake ⟨#opt-override-flake⟩ original-ref resolved-ref

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

Common flake-related options

•

--commit-lock-file ⟨#opt-commit-lock-file⟩

Commit changes to the flake’s lock file.

•

--inputs-from ⟨#opt-inputs-from⟩ flake-url

Use the inputs of the specified flake as registry entries.

•

--no-registries ⟨#opt-no-registries⟩

Don’t allow lookups in the flake registries.

DEPRECATED

Use --no-use-registries ⟨@docroot@/command-ref/conf-file.md#conf-use-registries⟩ instead.

•

--no-update-lock-file ⟨#opt-no-update-lock-file⟩

Do not allow any updates to the flake’s lock file.

•

--no-write-lock-file ⟨#opt-no-write-lock-file⟩

Do not write the flake’s newly generated lock file.

•

--output-lock-file ⟨#opt-output-lock-file⟩ flake-lock-path

Write the given lock file instead of flake.lock within the top-level flake.

•

--override-input ⟨#opt-override-input⟩ input-path flake-url

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

•

--recreate-lock-file ⟨#opt-recreate-lock-file⟩

Recreate the flake’s lock file from scratch.

DEPRECATED

Use nix flake update ⟨@docroot@/command-ref/new-cli/nix3-flake-update.md⟩ instead.

•

--reference-lock-file ⟨#opt-reference-lock-file⟩ flake-lock-path

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

•

--update-input ⟨#opt-update-input⟩ input-path

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

DEPRECATED

Use nix flake update ⟨@docroot@/command-ref/new-cli/nix3-flake-update.md⟩ instead.

Logging-related options

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--debug ⟨#opt-debug⟩

Set the logging verbosity level to ‘debug’.

•

--log-format ⟨#opt-log-format⟩ format

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

•

--print-build-logs ⟨#opt-print-build-logs⟩ / -L

Print full build logs on standard error.

•

--quiet ⟨#opt-quiet⟩

Decrease the logging verbosity level.

•

--verbose ⟨#opt-verbose⟩ / -v

Increase the logging verbosity level.

Miscellaneous global options

•

--help ⟨#opt-help⟩

Show usage information.

•

--offline ⟨#opt-offline⟩

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

•

--option ⟨#opt-option⟩ name value

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

•

--refresh ⟨#opt-refresh⟩

Consider all previously downloaded files out-of-date.

•

--repair ⟨#opt-repair⟩

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

•

--version ⟨#opt-version⟩

Show version information.

Options that change the interpretation of installables

•

--expr ⟨#opt-expr⟩ expr

Interpret installables ⟨@docroot@/command-ref/new-cli/nix.md#installables⟩ as attribute paths relative to the Nix expression expr.

•

--file ⟨#opt-file⟩ / -f file

Interpret installables ⟨@docroot@/command-ref/new-cli/nix.md#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 ⟨@docroot@/command-ref/conf-file.md#command-line-flags⟩ for overriding configuration settings with command line flags.

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