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.

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.

  • --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.

  • --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.