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 -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

Description¶

This command evaluates the Nix expression installable 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.
• --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.