Provided by: nix-bin_2.18.1+dfsg-1ubuntu5_amd64 
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 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
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-
separated NIX_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 Nix 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 Nix 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 Nix 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 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).
Logging-related options:
• --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.
Miscellaneous global options:
• --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.
Options that change the interpretation of installables:
• --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.
Options to override configuration settings:
• --accept-flake-config
Enable the accept-flake-config setting.
• --access-tokens value
Set the access-tokens setting.
• --allow-dirty
Enable the allow-dirty setting.
• --allow-import-from-derivation
Enable the allow-import-from-derivation setting.
• --allow-new-privileges
Enable the allow-new-privileges setting.
• --allow-symlinked-store
Enable the allow-symlinked-store setting.
• --allow-unsafe-native-code-during-evaluation
Enable the allow-unsafe-native-code-during-evaluation setting.
• --allowed-impure-host-deps value
Set the allowed-impure-host-deps setting.
• --allowed-uris value
Set the allowed-uris setting.
• --allowed-users value
Set the allowed-users setting.
• --auto-allocate-uids
Enable the auto-allocate-uids setting.
• --auto-optimise-store
Enable the auto-optimise-store setting.
• --bash-prompt value
Set the bash-prompt setting.
• --bash-prompt-prefix value
Set the bash-prompt-prefix setting.
• --bash-prompt-suffix value
Set the bash-prompt-suffix setting.
• --build-hook value
Set the build-hook setting.
• --build-poll-interval value
Set the build-poll-interval setting.
• --build-users-group value
Set the build-users-group setting.
• --builders value
Set the builders setting.
• --builders-use-substitutes
Enable the builders-use-substitutes setting.
• --commit-lockfile-summary value
Set the commit-lockfile-summary setting.
• --compress-build-log
Enable the compress-build-log setting.
• --connect-timeout value
Set the connect-timeout setting.
• --cores value
Set the cores setting.
• --diff-hook value
Set the diff-hook setting.
• --download-attempts value
Set the download-attempts setting.
• --download-speed value
Set the download-speed setting.
• --eval-cache
Enable the eval-cache setting.
• --experimental-features value
Set the experimental-features setting.
• --extra-access-tokens value
Append to the access-tokens setting.
• --extra-allowed-impure-host-deps value
Append to the allowed-impure-host-deps setting.
• --extra-allowed-uris value
Append to the allowed-uris setting.
• --extra-allowed-users value
Append to the allowed-users setting.
• --extra-build-hook value
Append to the build-hook setting.
• --extra-experimental-features value
Append to the experimental-features setting.
• --extra-extra-platforms value
Append to the extra-platforms setting.
• --extra-hashed-mirrors value
Append to the hashed-mirrors setting.
• --extra-ignored-acls value
Append to the ignored-acls setting.
• --extra-nix-path value
Append to the nix-path setting.
• --extra-platforms value
Set the extra-platforms setting.
• --extra-plugin-files value
Append to the plugin-files setting.
• --extra-sandbox-paths value
Append to the sandbox-paths setting.
• --extra-secret-key-files value
Append to the secret-key-files setting.
• --extra-substituters value
Append to the substituters setting.
• --extra-system-features value
Append to the system-features setting.
• --extra-trusted-public-keys value
Append to the trusted-public-keys setting.
• --extra-trusted-substituters value
Append to the trusted-substituters setting.
• --extra-trusted-users value
Append to the trusted-users setting.
• --fallback
Enable the fallback setting.
• --filter-syscalls
Enable the filter-syscalls setting.
• --flake-registry value
Set the flake-registry setting.
• --fsync-metadata
Enable the fsync-metadata setting.
• --gc-reserved-space value
Set the gc-reserved-space setting.
• --hashed-mirrors value
Set the hashed-mirrors setting.
• --http-connections value
Set the http-connections setting.
• --http2
Enable the http2 setting.
• --id-count value
Set the id-count setting.
• --ignore-try
Enable the ignore-try setting.
• --ignored-acls value
Set the ignored-acls setting.
• --impersonate-linux-26
Enable the impersonate-linux-26 setting.
• --keep-build-log
Enable the keep-build-log setting.
• --keep-derivations
Enable the keep-derivations setting.
• --keep-env-derivations
Enable the keep-env-derivations setting.
• --keep-failed
Enable the keep-failed setting.
• --keep-going
Enable the keep-going setting.
• --keep-outputs
Enable the keep-outputs setting.
• --log-lines value
Set the log-lines setting.
• --max-build-log-size value
Set the max-build-log-size setting.
• --max-free value
Set the max-free setting.
• --max-jobs value
Set the max-jobs setting.
• --max-silent-time value
Set the max-silent-time setting.
• --max-substitution-jobs value
Set the max-substitution-jobs setting.
• --min-free value
Set the min-free setting.
• --min-free-check-interval value
Set the min-free-check-interval setting.
• --nar-buffer-size value
Set the nar-buffer-size setting.
• --narinfo-cache-negative-ttl value
Set the narinfo-cache-negative-ttl setting.
• --narinfo-cache-positive-ttl value
Set the narinfo-cache-positive-ttl setting.
• --netrc-file value
Set the netrc-file setting.
• --nix-path value
Set the nix-path setting.
• --no-accept-flake-config
Disable the accept-flake-config setting.
• --no-allow-dirty
Disable the allow-dirty setting.
• --no-allow-import-from-derivation
Disable the allow-import-from-derivation setting.
• --no-allow-new-privileges
Disable the allow-new-privileges setting.
• --no-allow-symlinked-store
Disable the allow-symlinked-store setting.
• --no-allow-unsafe-native-code-during-evaluation
Disable the allow-unsafe-native-code-during-evaluation setting.
• --no-auto-allocate-uids
Disable the auto-allocate-uids setting.
• --no-auto-optimise-store
Disable the auto-optimise-store setting.
• --no-builders-use-substitutes
Disable the builders-use-substitutes setting.
• --no-compress-build-log
Disable the compress-build-log setting.
• --no-eval-cache
Disable the eval-cache setting.
• --no-fallback
Disable the fallback setting.
• --no-filter-syscalls
Disable the filter-syscalls setting.
• --no-fsync-metadata
Disable the fsync-metadata setting.
• --no-http2
Disable the http2 setting.
• --no-ignore-try
Disable the ignore-try setting.
• --no-impersonate-linux-26
Disable the impersonate-linux-26 setting.
• --no-keep-build-log
Disable the keep-build-log setting.
• --no-keep-derivations
Disable the keep-derivations setting.
• --no-keep-env-derivations
Disable the keep-env-derivations setting.
• --no-keep-failed
Disable the keep-failed setting.
• --no-keep-going
Disable the keep-going setting.
• --no-keep-outputs
Disable the keep-outputs setting.
• --no-preallocate-contents
Disable the preallocate-contents setting.
• --no-print-missing
Disable the print-missing setting.
• --no-pure-eval
Disable the pure-eval setting.
• --no-require-drop-supplementary-groups
Disable the require-drop-supplementary-groups setting.
• --no-require-sigs
Disable the require-sigs setting.
• --no-restrict-eval
Disable the restrict-eval setting.
• --no-run-diff-hook
Disable the run-diff-hook setting.
• --no-sandbox
Disable sandboxing.
• --no-sandbox-fallback
Disable the sandbox-fallback setting.
• --no-show-trace
Disable the show-trace setting.
• --no-substitute
Disable the substitute setting.
• --no-sync-before-registering
Disable the sync-before-registering setting.
• --no-trace-function-calls
Disable the trace-function-calls setting.
• --no-trace-verbose
Disable the trace-verbose setting.
• --no-use-case-hack
Disable the use-case-hack setting.
• --no-use-cgroups
Disable the use-cgroups setting.
• --no-use-registries
Disable the use-registries setting.
• --no-use-sqlite-wal
Disable the use-sqlite-wal setting.
• --no-use-xdg-base-directories
Disable the use-xdg-base-directories setting.
• --no-warn-dirty
Disable the warn-dirty setting.
• --plugin-files value
Set the plugin-files setting.
• --post-build-hook value
Set the post-build-hook setting.
• --pre-build-hook value
Set the pre-build-hook setting.
• --preallocate-contents
Enable the preallocate-contents setting.
• --print-missing
Enable the print-missing setting.
• --pure-eval
Enable the pure-eval setting.
• --relaxed-sandbox
Enable sandboxing, but allow builds to disable it.
• --require-drop-supplementary-groups
Enable the require-drop-supplementary-groups setting.
• --require-sigs
Enable the require-sigs setting.
• --restrict-eval
Enable the restrict-eval setting.
• --run-diff-hook
Enable the run-diff-hook setting.
• --sandbox
Enable sandboxing.
• --sandbox-build-dir value
Set the sandbox-build-dir setting.
• --sandbox-dev-shm-size value
Set the sandbox-dev-shm-size setting.
• --sandbox-fallback
Enable the sandbox-fallback setting.
• --sandbox-paths value
Set the sandbox-paths setting.
• --secret-key-files value
Set the secret-key-files setting.
• --show-trace
Enable the show-trace setting.
• --ssl-cert-file value
Set the ssl-cert-file setting.
• --stalled-download-timeout value
Set the stalled-download-timeout setting.
• --start-id value
Set the start-id setting.
• --store value
Set the store setting.
• --substitute
Enable the substitute setting.
• --substituters value
Set the substituters setting.
• --sync-before-registering
Enable the sync-before-registering setting.
• --system value
Set the system setting.
• --system-features value
Set the system-features setting.
• --tarball-ttl value
Set the tarball-ttl setting.
• --timeout value
Set the timeout setting.
• --trace-function-calls
Enable the trace-function-calls setting.
• --trace-verbose
Enable the trace-verbose setting.
• --trusted-public-keys value
Set the trusted-public-keys setting.
• --trusted-substituters value
Set the trusted-substituters setting.
• --trusted-users value
Set the trusted-users setting.
• --use-case-hack
Enable the use-case-hack setting.
• --use-cgroups
Enable the use-cgroups setting.
• --use-registries
Enable the use-registries setting.
• --use-sqlite-wal
Enable the use-sqlite-wal setting.
• --use-xdg-base-directories
Enable the use-xdg-base-directories setting.
• --user-agent-suffix value
Set the user-agent-suffix setting.
• --warn-dirty
Enable the warn-dirty setting.
nix3-eval(1)