nix3-eval(1)

📦
version 2.35.3 · 13 versions
, in fedora 44

💅 nix3-eval — like, the manual page bestie

okay so grab a snack and sit down, because we're doing nix3-eval(1) from the General Commands Manual and honestly? she's iconic. 🌸

⚠️ Warning ⚠️ This program is experimental and its interface is subject to change. Like, it WILL ghost you and change its whole personality. no cap.

✨ Name ✨

nix eval — evaluate a Nix expression 🧚

🎀 Synopsis 🎀

nix eval [option…] installable

🌈 Examples 🌈

okay these are the vibes, bestie, eat them up:

  • 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 Nix expression you hand it, and prints the result on standard output. that's the whole job. that's the assignment. 💅

It also evaluates any nested attribute values and list items. like, it goes deep. main character energy.

🦋 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.
  • --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 separate lines. See --pretty.
  • --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 — 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). The input path must not be empty. 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 is read from standard input. Implies --impure.
Note 📌 See man nix.conf for overriding configuration settings with command line flags.

and that's the whole thing, bestie, you ate. 🫶 this page came from nix3-eval(1) — General Commands Manual, dated (you know the date, it's literally in the footer of the original) — so yeah, go forth and eval responsibly. periodt. 💖

📦 where this page comes from