Warning
This program is experimental and its interface is subject to change.
Name
nix eval - evaluate a Nix expression
Synopsis
nix eval [option...] installable
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
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.
Options
-
--applyexpr Apply the function expr to each argument. -
--jsonProduce output in JSON format, suitable for consumption by another program. -
--rawPrint strings without quotes or escaping. -
--read-onlyDo not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation. -
--write-topath Write a string or attrset of strings to path.
Common evaluation options:
-
--argname expr Pass the value expr as the argument name to Nix functions. -
--argstrname string Pass the string string as the argument name to Nix functions. -
--debuggerStart an interactive environment if evaluation fails. -
--eval-storestore-url The URL of the Nix store to use for evaluation, i.e. to store derivations (.drvfiles) and inputs referenced by them. -
--impureAllow access to mutable paths and repositories. -
--include/-Ipath Add path to the Nix search path. The Nix search path is initialized from the colon-separatedNIX_PATHenvironment variable, and is used to look up the location of Nix expressions using paths enclosed in angle brackets (i.e.,<nixpkgs>).For instance, passing
-I /home/eelco/Dev -I /etc/nixoswill cause Lix to look for paths relative to
/home/eelco/Devand/etc/nixos, in that order. This is equivalent to setting theNIX_PATHenvironment variable to/home/eelco/Dev:/etc/nixosIt is also possible to match paths against a prefix. For example, passing
-I nixpkgs=/home/eelco/Dev/nixpkgs-branch -I /etc/nixoswill cause Lix to search for
<nixpkgs/path>in/home/eelco/Dev/nixpkgs-branch/pathand/etc/nixos/nixpkgs/path.If a path in the Nix search path starts with
http://orhttps://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must consist of a single top-level directory. For example, passing-I nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gztells Lix to download and use the current contents of the
masterbranch in thenixpkgsrepository.The URLs of the tarballs from the official
nixos.orgchannels (see the manual page fornix-channel) can be abbreviated aschannel:<channel-name>. For instance, the following two flags are equivalent:-I nixpkgs=channel:nixos-21.05 -I nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xzYou can also fetch source trees using flake URLs and add them to the search path. For instance,
-I nixpkgs=flake:nixpkgsspecifies that the prefix
nixpkgsshall refer to the source tree downloaded from thenixpkgsentry in the flake registry. Similarly,-I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05makes
<nixpkgs>refer to a particular branch of theNixOS/nixpkgsrepository on GitHub. -
--override-flakeoriginal-ref resolved-ref Override the flake registries, redirecting original-ref to resolved-ref.
Common flake-related options:
-
--commit-lock-fileCommit changes to the flake's lock file. -
--inputs-fromflake-url Use the inputs of the specified flake as registry entries. -
--no-registriesDon't allow lookups in the flake registries. This option is deprecated; use--no-use-registries. -
--no-update-lock-fileDo not allow any updates to the flake's lock file. -
--no-write-lock-fileDo not write the flake's newly generated lock file. -
--output-lock-fileflake-lock-path Write the given lock file instead offlake.lockwithin the top-level flake. -
--override-inputinput-path flake-url Override a specific flake input (e.g.dwarffs/nixpkgs). This implies--no-write-lock-file. -
--reference-lock-fileflake-lock-path Read the given lock file instead offlake.lockwithin the top-level flake.
Logging-related options:
-
--debugSet the logging verbosity level to 'debug'. -
--log-formatformat Set the format of log output; one ofraw,internal-json,bar,bar-with-logs,multilineormultiline-with-logs. -
--print-build-logs/-LPrint full build logs on standard error. -
--quietDecrease the logging verbosity level. -
--verbose/-vIncrease the logging verbosity level.
Miscellaneous global options:
-
--helpShow usage information. -
--offlineDisable substituters and consider all previously downloaded files up-to-date. -
--optionname value Set the Lix configuration setting name to value (overridingnix.conf). -
--refreshConsider all previously downloaded files out-of-date. -
--repairDuring evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths. -
--versionShow version information.
Options that change the interpretation of installables:
-
--expr/-Eexpr Interpret installables as attribute paths relative to the Nix expression expr. -
--file/-ffile Interpret installables as attribute paths relative to the Nix expression stored in file. If file is the character -, then a Nix expression will be read from standard input. Implies--impure.
Note
See
man nix.conffor overriding configuration settings with command line flags.