lix/src/nix/eval.md

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Note: this command's interface is based heavily around installables, which you may want to read about first (nix --help).

Examples

• Evaluate a Nix expression given on the command line:

  # nix eval --expr '1 + 2'
  

• Evaluate a Nix expression to JSON using the short-form expression flag:

  # nix eval --json -E '{ 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.

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.

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