Warning
This program is experimental and its interface is subject to change.
Name
nix eval - evaluate a Nix expression
Synopsis
nix eval [option...] installable
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 --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
- 
--applyexprApply the function expr to each argument.
 - 
Produce output in JSON format, suitable for consumption by another program.
 - 
Print strings without quotes or escaping.
 - 
Do not instantiate each evaluated derivation. This improves performance, but can cause errors when accessing store paths of derivations during evaluation.
 - 
--write-topathWrite a string or attrset of strings to path.
 
Common evaluation options
- 
--argname exprPass the value expr as the argument name to Nix functions.
 - 
--argstrname stringPass the string string as the argument name to Nix functions.
 - 
Start an interactive environment if evaluation fails.
 - 
--eval-storestore-urlThe URL of the Nix store to use for evaluation, i.e. to store derivations (
.drvfiles) and inputs referenced by them. - 
Allow access to mutable paths and repositories.
 - 
--include/-IpathAdd path to the Nix search path. The Nix search path is initialized from the colon-separated
NIX_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 Nix 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 Nix 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 Nix 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-refOverride the flake registries, redirecting original-ref to resolved-ref.
 
Common flake-related options
- 
Commit changes to the flake's lock file.
 - 
--inputs-fromflake-urlUse the inputs of the specified flake as registry entries.
 - 
Don't allow lookups in the flake registries.
DEPRECATED
Use
--no-use-registriesinstead. - 
Do not allow any updates to the flake's lock file.
 - 
Do not write the flake's newly generated lock file.
 - 
--output-lock-fileflake-lock-pathWrite the given lock file instead of
flake.lockwithin the top-level flake. - 
--override-inputinput-path flake-urlOverride a specific flake input (e.g.
dwarffs/nixpkgs). This implies--no-write-lock-file. - 
Recreate the flake's lock file from scratch.
DEPRECATED
Use
nix flake updateinstead. - 
--reference-lock-fileflake-lock-pathRead the given lock file instead of
flake.lockwithin the top-level flake. - 
--update-inputinput-pathUpdate a specific flake input (ignoring its previous entry in the lock file).
DEPRECATED
Use
nix flake updateinstead. 
Logging-related options
- 
Set the logging verbosity level to 'debug'.
 - 
--log-formatformatSet the format of log output; one of
raw,internal-json,barorbar-with-logs. - 
--print-build-logs/-LPrint full build logs on standard error.
 - 
Decrease the logging verbosity level.
 - 
--verbose/-vIncrease the logging verbosity level.
 
Miscellaneous global options
- 
Show usage information.
 - 
Disable substituters and consider all previously downloaded files up-to-date.
 - 
--optionname valueSet the Nix configuration setting name to value (overriding
nix.conf). - 
Consider all previously downloaded files out-of-date.
 - 
During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths.
 - 
Show version information.
 
Options that change the interpretation of installables
- 
--exprexprInterpret installables as attribute paths relative to the Nix expression expr.
 - 
--file/-ffileInterpret 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.