Warning
This program is
experimental
and its interface is subject to change.
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.
- •
- --arg
name expr
- Pass the value expr as the argument name to Nix
functions.
- •
- --arg-from-file
name path
- Pass the contents of file path as the argument name to Nix
functions.
- •
- --arg-from-stdin
name
- Pass the contents of stdin 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 search path entries used to resolve
lookup
paths
- This option may be given multiple times.
- Paths added through -I take precedence over the
nix-path
configuration setting and the
NIX_PATH
environment variable.
- •
- --override-flake
original-ref resolved-ref
- Override the flake registries, redirecting original-ref to
resolved-ref.
- •
- --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.
- •
- --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 of flake.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.
- •
- --recreate-lock-file
- Recreate the flake’s lock file from scratch.
- •
- --reference-lock-file
flake-lock-path
- Read the given lock file instead of flake.lock
within the top-level flake.
- •
- --update-input
input-path
- Update a specific flake input (ignoring its previous entry in the lock
file).
- •
- --debug
- Set the logging verbosity level to ‘debug’.
- •
- --log-format
format
- Set the format of log output; one of raw,
internal-json, bar or
bar-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.
- •
- --help
- Show usage information.
- •
- --offline
- Disable substituters and consider all previously downloaded files
up-to-date.
- •
- --option
name value
- Set the Nix configuration setting name to value (overriding
nix.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.
- •
- --expr
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.