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 --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

This command evaluates the given Nix expression 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.

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--apply expr
Apply the function expr to each argument.
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--json
Produce output in JSON format, suitable for consumption by another program.
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--raw
Print strings without quotes or escaping.
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--read-only
Do not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation.
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--write-to path
Write a string or attrset of strings to path.

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--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.
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--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.
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--debugger
Start an interactive environment if evaluation fails.
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--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.
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--impure
Allow access to mutable paths and repositories.
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--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.
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--override-flake original-ref resolved-ref
Override the flake registries, redirecting original-ref to resolved-ref.
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--commit-lock-file
Commit changes to the flake’s lock file.
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--inputs-from flake-url
Use the inputs of the specified flake as registry entries.
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--no-registries
Don’t allow lookups in the flake registries.
DEPRECATED
Use --no-use-registries instead.
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--no-update-lock-file
Do not allow any updates to the flake’s lock file.
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--no-write-lock-file
Do not write the flake’s newly generated lock file.
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--output-lock-file flake-lock-path
Write the given lock file instead of flake.lock within the top-level flake.
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--override-input input-path flake-url
Override a specific flake input (e.g. dwarffs/nixpkgs). This implies --no-write-lock-file.
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--recreate-lock-file
Recreate the flake’s lock file from scratch.
DEPRECATED
Use nix flake update instead.
•
--reference-lock-file flake-lock-path
Read the given lock file instead of flake.lock within the top-level flake.
•
--update-input input-path
Update a specific flake input (ignoring its previous entry in the lock file).
DEPRECATED
Use nix flake update instead.
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--debug
Set the logging verbosity level to ‘debug’.
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--log-format format
Set the format of log output; one of raw, internal-json, bar or bar-with-logs.
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--print-build-logs / -L
Print full build logs on standard error.
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--quiet
Decrease the logging verbosity level.
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--verbose / -v
Increase the logging verbosity level.

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--help
Show usage information.
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--offline
Disable substituters and consider all previously downloaded files up-to-date.
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--option name value
Set the Nix configuration setting name to value (overriding nix.conf).
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--refresh
Consider all previously downloaded files out-of-date.
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--repair
During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths.
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--version
Show version information.

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--expr expr
Interpret installables as attribute paths relative to the Nix expression expr.
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--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.