nix-eval

evaluates Nix expressions

TLDR

Evaluate an expression

$ nix eval --expr "[1 + 1]"

Evaluate a flake attribute

$ nix eval [.#packages.x86_64-linux.default.name]

Get a package version from nixpkgs

$ nix eval --raw nixpkgs#[hello].version

Apply a function to the result

$ nix eval --apply builtins.attrNames --expr "{a=1; b=2;}"

Output as JSON

$ nix eval --json --expr "[{a = 1;}]"

Evaluate a file

$ nix eval -f [file.nix]

Evaluate an attribute from a file

$ nix eval -f [default.nix] [attribute]

Print a string without quotes

$ nix eval --raw --expr "\"hello\""

Read a NixOS configuration option from a flake

$ nix eval .#nixosConfigurations.[hostname].config.[networking.hostName]

SYNOPSIS

nix eval [options] [installable]

nix eval evaluates the given Nix expression or installable and prints the result on standard output. Nested attribute values and list items are evaluated too.By default the result is printed as a Nix expression; --json and --raw give output suitable for scripts. It is the new-CLI replacement for nix-instantiate --eval.

PARAMETERS

INSTALLABLE

Flake output attribute, or attribute path when used with --expr or --file.

--expr EXPR

Interpret installables as attribute paths relative to this Nix expression.

-f, --file FILE

Interpret installables as attribute paths relative to the expression in FILE (- reads stdin). Implies --impure.

--json

Output as JSON. Fails if the result contains functions.

--raw

Print strings without quotes or escaping. The result must be a string.

--apply FUNC

Apply function to result.

--write-to PATH

Write a string, or a nested attribute set of strings, as files under PATH.

--read-only

Do not instantiate evaluated derivations. Faster, but can fail when store paths of derivations are accessed.

--impure

Allow access to mutable paths, environment variables and the current time.

--arg NAME EXPR, --argstr NAME STRING

Pass arguments to Nix functions.

--debugger

Start an interactive debugger if evaluation fails.

--help

Display help information.

INSTALL

sudo apt install nix-bin

sudo dnf install nix

sudo pacman -S nix

sudo apk add nix

sudo zypper install nix

nix profile install nixpkgs#nix

CAVEATS

Part of the experimental new Nix CLI: requires the nix-command experimental feature, plus flakes for flake references. Evaluation is pure by default, so builtins.getEnv, builtins.currentTime and paths outside a flake need --impure.

HISTORY

nix eval appeared with the new nix command in Nix 2.0 (2018) and was reworked for flakes and installables in Nix 2.4 (2021).

RESOURCES

Source code · Documentation