nix3-eval(1)

    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 --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 expr

    Apply the function expr to each argument.

•   --json

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

•   --no-pretty

    Print compact JSON output on a single line, even when the output is a terminal. Some commands may print multiple JSON objects on separate lines.

                See `--pretty`.

•   --pretty

    Print multi-line, indented JSON output for readability.

                Default: indent if output is to a terminal.
                This option is only effective when `--json` is also specified.

•   --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.

•   --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.

  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.

        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). The input path must not be empty. 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.

  Logging-related options

•   --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 is read from standard input. Implies --impure.

    Note

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