nix eval
Evaluate a Nix expression.
Warning
This program is experimental and its interface is subject to change.
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
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:
```console ## 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-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.
Options
-
--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.
Common evaluation options:
-
--arg
name expr Pass the value expr as the argument name to Nix functions. -
--argstr
name string Pass the string string as the argument name to Nix functions. -
--debugger
Start an interactive environment if evaluation fails. -
--eval-store
store-url The URL of the Nix store to use for evaluation, i.e. to store derivations (.drv
files) and inputs referenced by them. -
--impure
Allow access to mutable paths and repositories. -
--include
/-I
path Add path to the Nix search path. The Nix search path is initialized from the colon-separatedNIX_PATH
environment 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/nixos
will cause Lix to look for paths relative to /home/eelco/Dev
and
/etc/nixos
, in that order. This is equivalent to setting the
NIX_PATH
environment variable to
/home/eelco/Dev:/etc/nixos
It is also possible to match paths against a prefix. For example, passing
-I nixpkgs=/home/eelco/Dev/nixpkgs-branch
-I /etc/nixos
will cause Lix to search for <nixpkgs/path>
in
/home/eelco/Dev/nixpkgs-branch/path
and /etc/nixos/nixpkgs/path
.
If a path in the Nix search path starts with http://
or https://
,
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.gz
tells Lix to download and use the current contents of the master
branch in the nixpkgs
repository.
The URLs of the tarballs from the official nixos.org
channels
(see the manual page for nix-channel
) can be
abbreviated as channel:<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.xz
You can also fetch source trees using flake URLs and add them to the search path. For instance,
-I nixpkgs=flake:nixpkgs
specifies that the prefix nixpkgs
shall refer to the source tree
downloaded from the nixpkgs
entry in the flake registry. Similarly,
-I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05
makes <nixpkgs>
refer to a particular branch of the
NixOS/nixpkgs
repository on GitHub.
--override-flake
original-ref resolved-ref Override the flake registries, redirecting original-ref to resolved-ref.
Common flake-related options:
-
--commit-lock-file
Commit changes to the flake's lock file. -
--inputs-from
flake-url Use the inputs of the specified flake as registry entries. -
--no-registries
Don't allow lookups in the flake registries. This option is deprecated; use--no-use-registries
. -
--no-update-lock-file
Do not allow any updates to the flake's lock file. -
--no-write-lock-file
Do not write the flake's newly generated lock file. -
--output-lock-file
flake-lock-path Write the given lock file instead offlake.lock
within the top-level flake. -
--override-input
input-path flake-url Override a specific flake input (e.g.dwarffs/nixpkgs
). This implies--no-write-lock-file
. -
--reference-lock-file
flake-lock-path Read the given lock file instead offlake.lock
within the top-level flake.
Logging-related options:
-
--debug
Set the logging verbosity level to 'debug'. -
--log-format
format Set the format of log output; one ofraw
,internal-json
,bar
orbar-with-logs
. -
--print-build-logs
/-L
Print full build logs on standard error. -
--quiet
Decrease the logging verbosity level. -
--verbose
/-v
Increase the logging verbosity level.
Miscellaneous global options:
-
--help
Show usage information. -
--offline
Disable substituters and consider all previously downloaded files up-to-date. -
--option
name value Set the Lix configuration setting name to value (overridingnix.conf
). -
--refresh
Consider all previously downloaded files out-of-date. -
--repair
During evaluation, rewrite missing or corrupted files in the Nix store. During building, rebuild missing or corrupted store paths. -
--version
Show version information.
Options that change the interpretation of installables:
-
--expr
/-E
expr Interpret installables as attribute paths relative to the Nix expression expr. -
--file
/-f
file 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.conf
for overriding configuration settings with command line flags.