Manpages - nix3-eval.1

nix eval - evaluate a Nix expression

nix eval [/option/…] installable

• 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.

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.

• --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.