2af9fd20c6
the term was hard to discover, as its definition and explanation were in a very long document lacking an overview section. search did not help because it occurs so often. - clarify wording in the definition - add an overview of installable types - add "installable" to glossary - link to definition from occurrences of the term - be more precise about where store derivation outputs are processed - installable Nix expressions must evaluate to a derivation Co-authored-by: Adam Joseph <54836058+amjoseph-nixpkgs@users.noreply.github.com>
1.6 KiB
R""(
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
nixpkgsflake:# 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
nixflake:# 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-topath, 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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