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.

   Common flake-related options
       •  --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.

   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.

       •  --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.

                                                                                                    nix3-eval(1)