R""(

• 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
  

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

It also evaluates any nested attribute values and list items.

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.

)""