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

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 --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

Description

This command evaluates the given Nix expression, and prints the result on standard output.

It also evaluates any nested attribute values and list items.

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.
•
--arg-from-file name path
Pass the contents of file path as the argument name to Nix functions.
•
--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.
•
--debugger
Start an interactive environment if evaluation fails.
•
--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.
•
--impure
Allow access to mutable paths and repositories.
•
--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.
•
--override-flake original-ref resolved-ref
Override the flake registries, redirecting original-ref to resolved-ref.
•
--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.
DEPRECATED
Use --no-use-registries instead.
•
--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.
•
--output-lock-file flake-lock-path
Write the given lock file instead of flake.lock within the top-level flake.
•
--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.
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.
•
--debug
Set the logging verbosity level to ‘debug’.
•
--log-format format
Set the format of log output; one of raw, internal-json, bar or bar-with-logs.
•
--print-build-logs / -L
Print full build logs on standard error.
•
--quiet
Decrease the logging verbosity level.
•
--verbose / -v
Increase the logging verbosity level.

Miscellaneous global options

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Consider all previously downloaded files out-of-date.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

Options that change the interpretation of installables

•
--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. Implies --impure.

Note

See man nix.conf for overriding configuration settings with command line flags.