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nix3-eval(1) General Commands Manual nix3-eval(1) Warning This program is experimental <../../development/experimental- features.md#xp-feature-nix-command> 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 repre- sentable 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 ex- ist. Options • --apply <#opt-apply> expr Apply the function expr to each argument. • --json <#opt-json> Produce output in JSON format, suitable for consumption by another program. • --no-pretty <#opt-no-pretty> Print compact JSON output on a single line, even when the output is a terminal. Some commands may print multiple JSON objects on sepa- rate lines. See `--pretty`. • --pretty <#opt-pretty> Print multi-line, indented JSON output for readability. Default: indent if output is to a terminal. This option is only effective when `--json` is also specified. • --raw <#opt-raw> Print strings without quotes or escaping. • --read-only <#opt-read-only> Do not instantiate each evaluated derivation. This improves perfor- mance, but can cause errors when accessing store paths of deriva- tions during evaluation. • --write-to <#opt-write-to> path Write a string or attrset of strings to path. Common evaluation options • --arg <#opt-arg> name expr Pass the value expr as the argument name to Nix functions. • --arg-from-file <#opt-arg-from-file> name path Pass the contents of file path as the argument name to Nix func- tions. • --arg-from-stdin <#opt-arg-from-stdin> name Pass the contents of stdin as the argument name to Nix functions. • --argstr <#opt-argstr> name string Pass the string string as the argument name to Nix functions. • --debugger <#opt-debugger> Start an interactive environment if evaluation fails. • --eval-store <#opt-eval-store> store-url The URL of the Nix store <../../store/types/index.md#store-url- format> to use for evaluation, i.e. to store derivations (.drv files) and inputs referenced by them. • --impure <#opt-impure> Allow access to mutable paths and repositories. • --include <#opt-include> / -I path Add path to search path entries used to resolve lookup paths <../../language/constructs/lookup-path.md> This option may be given multiple times. Paths added through -I take precedence over the nix-path configura- tion setting <../../command-ref/conf-file.md#conf-nix-path> and the NIX_PATH environment variable <../../command-ref/env-common.md#env- NIX_PATH>. • --override-flake <#opt-override-flake> original-ref resolved-ref Override the flake registries, redirecting original-ref to resolved- ref. Common flake-related options • --commit-lock-file <#opt-commit-lock-file> Commit changes to the flake's lock file. • --inputs-from <#opt-inputs-from> flake-url Use the inputs of the specified flake as registry entries. • --no-registries <#opt-no-registries> Don't allow lookups in the flake registries. DEPRECATED Use --no-use-registries <../../command-ref/conf-file.md#conf- use-registries> instead. • --no-update-lock-file <#opt-no-update-lock-file> Do not allow any updates to the flake's lock file. • --no-write-lock-file <#opt-no-write-lock-file> Do not write the flake's newly generated lock file. • --output-lock-file <#opt-output-lock-file> flake-lock-path Write the given lock file instead of flake.lock within the top-level flake. • --override-input <#opt-override-input> input-path flake-url Override a specific flake input (e.g. dwarffs/nixpkgs). This implies --no-write-lock-file. • --recreate-lock-file <#opt-recreate-lock-file> Recreate the flake's lock file from scratch. DEPRECATED Use nix flake update <../../command-ref/new-cli/nix3-flake- update.md> instead. • --reference-lock-file <#opt-reference-lock-file> flake-lock-path Read the given lock file instead of flake.lock within the top-level flake. • --update-input <#opt-update-input> input-path Update a specific flake input (ignoring its previous entry in the lock file). DEPRECATED Use nix flake update <../../command-ref/new-cli/nix3-flake- update.md> instead. Logging-related options • --debug <#opt-debug> Set the logging verbosity level to `debug'. • --log-format <#opt-log-format> format Set the format of log output; one of raw, internal-json, bar or bar- with-logs. • --print-build-logs <#opt-print-build-logs> / -L Print full build logs on standard error. • --quiet <#opt-quiet> Decrease the logging verbosity level. • --verbose <#opt-verbose> / -v Increase the logging verbosity level. Miscellaneous global options • --help <#opt-help> Show usage information. • --offline <#opt-offline> Disable substituters and consider all previously downloaded files up-to-date. • --option <#opt-option> name value Set the Nix configuration setting name to value (overriding nix.conf). • --refresh <#opt-refresh> Consider all previously downloaded files out-of-date. • --repair <#opt-repair> During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths. • --version <#opt-version> Show version information. Options that change the interpretation of installables • --expr <#opt-expr> expr Interpret installables <../../command-ref/new- cli/nix.md#installables> as attribute paths relative to the Nix ex- pression expr. • --file <#opt-file> / -f file Interpret installables <../../command-ref/new- cli/nix.md#installables> as attribute paths relative to the Nix ex- pression stored in file. If file is the character -, then a Nix ex- pression is read from standard input. Implies --impure. Note See man nix.conf <../../command-ref/conf-file.md#command-line-flags> for overriding configuration settings with command line flags. nix3-eval(1)
Name | Synopsis | Examples | Description | Output format | Options
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