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 -f ./my-nixpkgs hello.name -
Get the current version of the
nixpkgsflake:# 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
nixflake:# 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-topath, 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
-
--applyexpr
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-topath
Write a string or attrset of strings to path.
Common evaluation options:
-
--argname expr
Pass the value expr as the argument name to Nix functions. -
--argstrname string
Pass the string string as the argument name to Nix functions. -
--eval-storestore-url
The Nix store to use for evaluations. -
--impure
Allow access to mutable paths and repositories. -
--include/-Ipath
Add path to the list of locations used to look up<...>file names. -
--override-flakeoriginal-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-fromflake-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-inputinput-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-inputinput-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. -
--exprexpr
Interpret installables as attribute paths relative to the Nix expression expr. -
--file/-ffile
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.