nix3-eval(1) General Commands Manual nix3-eval(1)

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

nix eval - evaluate a Nix expression

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 -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

This command evaluates the Nix expression installable 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.

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.