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.

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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