table of contents
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.
- 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.
- 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.
Common flake-related options¶
- Commit changes to the flake’s lock file.
- •
- --inputs-from flake-url
- Use the inputs of the specified flake as registry entries.
- Don’t allow lookups in the flake registries.
- DEPRECATED
- Use --no-use-registries instead.
- Do not allow any updates to the flake’s 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 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.
Logging-related options¶
- •
- --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.
- 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).
- 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.
- 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.