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

Name

nix eval - evaluate a Nix expression

Synopsis

nix eval [option...] installable

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

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

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.

--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.

--arg name expr

Pass the value expr as the argument name to Nix functions.
--arg-from-file name path

Pass the contents of file path as the argument name to Nix functions.
--arg-from-stdin name

Pass the contents of stdin 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 search path entries used to resolve lookup paths

This option may be given multiple times.

Paths added through -I take precedence over the nix-path configuration setting and the NIX_PATH environment variable.
--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.

--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.

--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.