nix-super/src/nix/eval.md
2020-12-21 13:32:28 +01:00

1.6 KiB

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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 -f ./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 installable 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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