Name

nix.conf - Nix configuration file

Description

Nix supports a variety of configuration settings, which are read from configuration files or taken as command line flags.

keep-outputs = true       # Nice for developers
keep-derivations = true   # Idem

substituters = a b
extra-substituters = c d

Available settings

•

abort-on-warn

If set to true, builtins.warn will throw an error when logging a warning.

This will give you a stack trace that leads to the location of the warning.

This is useful for finding information about warnings in third-party Nix code when you can not start the interactive debugger, such as when Nix is called from a non-interactive script. See debugger-on-warn.

Currently, a stack trace can only be produced when the debugger is enabled, or when evaluation is aborted.

This option can be enabled by setting NIX_ABORT_ON_WARN=1 in the environment.

Default: false

•

accept-flake-config

extra-experimental-features = flakes
accept-flake-config = ...

Whether to accept nix configuration from a flake without prompting.

Default: false

•

access-tokens

Access tokens used to access protected GitHub, GitLab, or other locations requiring token-based authentication.

Access tokens are specified as a string made up of space-separated host=token values. The specific token used is selected by matching the host portion against the “host” specification of the input. The actual use of the token value is determined by the type of resource being accessed:

Example ~/.config/nix/nix.conf:

access-tokens = github.com=23ac...b289 gitlab.mycompany.com=PAT:A123Bp_Cd..EfG gitlab.com=OAuth2:1jklw3jk

Example ~/code/flake.nix:

input.foo = {
type = "gitlab";
host = "gitlab.mycompany.com";
owner = "mycompany";
repo = "pro";
};

This example specifies three tokens, one each for accessing github.com, gitlab.mycompany.com, and gitlab.com.

The input.foo uses the “gitlab” fetcher, which might requires specifying the token type along with the token value.

Default: empty

•

allow-dirty

Whether to allow dirty Git/Mercurial trees.

Default: true

•

allow-import-from-derivation

By default, Nix allows Import from Derivation.

With this option set to false, Nix will throw an error when evaluating an expression that uses this feature, even when the required store object is readily available. This ensures that evaluation will not require any builds to take place, regardless of the state of the store.

Default: true

•

allow-new-privileges

(Linux-specific.) By default, builders on Linux cannot acquire new privileges by calling setuid/setgid programs or programs that have file capabilities. For example, programs such as sudo or ping will fail. (Note that in sandbox builds, no such programs are available unless you bind-mount them into the sandbox via the sandbox-paths option.) You can allow the use of such programs by enabling this option. This is impure and usually undesirable, but may be useful in certain scenarios (e.g. to spin up containers or set up userspace network interfaces in tests).

Default: false

•

allow-symlinked-store

If set to true, Nix will stop complaining if the store directory (typically /nix/store) contains symlink components.

This risks making some builds “impure” because builders sometimes “canonicalise” paths by resolving all symlink components. Problems occur if those builds are then deployed to machines where /nix/store resolves to a different location from that of the build machine. You can enable this setting if you are sure you’re not going to do that.

Default: false

•

allow-unsafe-native-code-during-evaluation

Enable built-in functions that allow executing native code.

In particular, this adds:

Opens dynamic shared object (DSO) at path, loads the function with the symbol name symbol from it and runs it. The loaded function must have the following signature: cpp extern "C" typedef void (*ValueInitialiser) (EvalState & state, Value & v);

The Nix C++ API documentation has more details on evaluator internals.

Execute a program, where arguments are specified as a list of strings, and parse its output as a Nix expression.

Default: false

•

allowed-impure-host-deps

Which prefixes to allow derivations to ask for access to (primarily for Darwin).

Default: empty

•

allowed-uris

A list of URI prefixes to which access is allowed in restricted evaluation mode. For example, when set to https://github.com/NixOS, builtin functions such as fetchGit are allowed to access https://github.com/NixOS/patchelf.git.

Access is granted when

Default: empty

•

allowed-users

A list user names, separated by whitespace. These users are allowed to connect to the Nix daemon.

You can specify groups by prefixing names with @. For instance, @wheel means all users in the wheel group. Also, you can allow all users by specifying *.

Default: *

•

always-allow-substitutes

If set to true, Nix will ignore the allowSubstitutes attribute in derivations and always attempt to use available substituters.

Default: false

•

auto-allocate-uids

extra-experimental-features = auto-allocate-uids
auto-allocate-uids = ...

Whether to select UIDs for builds automatically, instead of using the users in build-users-group.

UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.

Default: false

•

auto-optimise-store

If set to true, Nix automatically detects files in the store that have identical contents, and replaces them with hard links to a single copy. This saves disk space. If set to false (the default), you can still run nix-store --optimise to get rid of duplicate files.

Default: false

•

bash-prompt

The bash prompt (PS1) in nix develop shells.

Default: empty

•

bash-prompt-prefix

Prefix prepended to the PS1 environment variable in nix develop shells.

Default: empty

•

bash-prompt-suffix

Suffix appended to the PS1 environment variable in nix develop shells.

Default: empty

•

build-dir

The directory on the host, in which derivations’ temporary build directories are created.

If not set, Nix will use the system temporary directory indicated by the TMPDIR environment variable. Note that builds are often performed by the Nix daemon, so its TMPDIR is used, and not that of the Nix command line interface.

This is also the location where --keep-failed leaves its files.

If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g. macOS), then the builder’s environment will contain this directory, instead of the virtual location sandbox-build-dir.

Default: ``

•

build-hook

The path to the helper program that executes remote builds.

Nix communicates with the build hook over stdio using a custom protocol to request builds that cannot be performed directly by the Nix daemon. The default value is the internal Nix binary that implements remote building.

Default: empty

•

build-poll-interval

How often (in seconds) to poll for locks.

Default: 5

•

build-users-group

This options specifies the Unix group containing the Nix build user accounts. In multi-user Nix installations, builds should not be performed by the Nix account since that would allow users to arbitrarily modify the Nix store and database by supplying specially crafted builders; and they cannot be performed by the calling user since that would allow him/her to influence the build result.

Therefore, if this option is non-empty and specifies a valid group, builds will be performed under the user accounts that are a member of the group specified here (as listed in /etc/group). Those user accounts should not be used for any other purpose!

Nix will never run two builds under the same user account at the same time. This is to prevent an obvious security hole: a malicious user writing a Nix expression that modifies the build result of a legitimate Nix expression being built by another user. Therefore it is good to have as many Nix build user accounts as you can spare. (Remember: uids are cheap.)

The build users should have permission to create files in the Nix store, but not delete them. Therefore, /nix/store should be owned by the Nix account, its group should be the group specified here, and its mode should be 1775.

If the build users group is empty, builds will be performed under the uid of the Nix process (that is, the uid of the caller if NIX_REMOTE is empty, the uid under which the Nix daemon runs if NIX_REMOTE is daemon). Obviously, this should not be used with a nix daemon accessible to untrusted clients.

Defaults to nixbld when running as root, empty otherwise.

Default: machine-specific

•

builders

A semicolon- or newline-separated list of build machines.

In addition to the usual ways of setting configuration options, the value can be read from a file by prefixing its absolute path with @.

builders = @/etc/nix/machines

Each machine specification consists of the following elements, separated by spaces. Only the first element is required. To leave a field at its default, set it to -.

--builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'

nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 8 1 kvm
nix@itchy.labs.cs.uu.nl    i686-linux /home/nix/.ssh/id_scratchy 8 2
nix@poochie.labs.cs.uu.nl  i686-linux /home/nix/.ssh/id_scratchy 1 2 kvm benchmark

requiredSystemFeatures = [ "benchmark" ];

requiredSystemFeatures = [ "benchmark" "kvm" ];

For Nix to use substituters, the calling user must be in the trusted-users list.

To build only on remote machines and disable local builds, set max-jobs to 0.

If you want the remote machines to use substituters, set builders-use-substitutes to true.

Default: machine-specific

•

builders-use-substitutes

If set to true, Nix will instruct remote build machines to use their own substituters if available.

It means that remote build hosts will fetch as many dependencies as possible from their own substituters (e.g, from cache.nixos.org) instead of waiting for the local machine to upload them all. This can drastically reduce build times if the network connection between the local machine and the remote build host is slow.

Default: false

•

commit-lock-file-summary

extra-experimental-features = flakes
commit-lock-file-summary = ...

The commit summary to use when committing changed flake lock files. If empty, the summary is generated based on the action performed.

Default: empty

Deprecated alias: commit-lockfile-summary

•

compress-build-log

If set to true (the default), build logs written to /nix/var/log/nix/drvs will be compressed on the fly using bzip2. Otherwise, they will not be compressed.

Default: true

Deprecated alias: build-compress-log

•

connect-timeout

The timeout (in seconds) for establishing connections in the binary cache substituter. It corresponds to curl’s --connect-timeout option. A value of 0 means no limit.

Default: 0

•

cores

Sets the value of the NIX_BUILD_CORES environment variable in the invocation of the builder executable of a derivation. The builder executable can use this variable to control its own maximum amount of parallelism.

For instance, in Nixpkgs, if the attribute enableParallelBuilding for the mkDerivation build helper is set to true, it will pass the -j${NIX_BUILD_CORES} flag to GNU Make.

The value 0 means that the builder should use all available CPU cores in the system.

Default: machine-specific

Deprecated alias: build-cores

•

debugger-on-trace

If set to true and the --debugger flag is given, the following functions will enter the debugger like builtins.break.

This is useful for debugging warnings in third-party Nix code.

Default: false

•

debugger-on-warn

If set to true and the --debugger flag is given, builtins.warn will enter the debugger like builtins.break.

This is useful for debugging warnings in third-party Nix code.

Use debugger-on-trace to also enter the debugger on legacy warnings that are logged with builtins.trace.

Default: false

•

diff-hook

Absolute path to an executable capable of diffing build results. The hook is executed if run-diff-hook is true, and the output of a build is known to not be the same. This program is not executed to determine if two results are the same.

The diff hook is executed by the same user and group who ran the build. However, the diff hook does not have write access to the store path just built.

The diff hook program receives three parameters:

The stderr and stdout output from the diff hook will not be displayed to the user. Instead, it will print to the nix-daemon’s log.

When using the Nix daemon, diff-hook must be set in the nix.conf configuration file, and cannot be passed at the command line.

Default: ``

•

download-attempts

How often Nix will attempt to download a file before giving up.

Default: 5

•

download-buffer-size

The size of Nix’s internal download buffer during curl transfers. If data is not processed quickly enough to exceed the size of this buffer, downloads may stall.

Default: 67108864

•

download-speed

Specify the maximum transfer rate in kilobytes per second you want Nix to use for downloads.

Default: 0

•

eval-cache

Whether to use the flake evaluation cache.

Default: true

•

eval-system

This option defines builtins.currentSystem in the Nix language if it is set as a non-empty string. Otherwise, if it is defined as the empty string (the default), the value of the system configuration setting is used instead.

Unlike system, this setting does not change what kind of derivations can be built locally. This is useful for evaluating Nix code on one system to produce derivations to be built on another type of system.

Default: empty

•

experimental-features

Experimental features that are enabled.

Example:

experimental-features = nix-command flakes

The following experimental features are available:

Experimental features are further documented in the manual.

Default: empty

•

extra-platforms

System types of executables that can be run on this machine.

Nix will only build a given derivation locally when its system attribute equals any of the values specified here or in the system option.

Setting this can be useful to build derivations locally on compatible machines:

Build systems will usually detect the target platform to be the current physical system and therefore produce machine code incompatible with what may be intended in the derivation. You should design your derivation’s builder accordingly and cross-check the results when using this option against natively-built versions of your derivation.

Default: machine-specific

•

fallback

If set to true, Nix will fall back to building from source if a binary substitute fails. This is equivalent to the --fallback flag. The default is false.

Default: false

Deprecated alias: build-fallback

•

filter-syscalls

Whether to prevent certain dangerous system calls, such as creation of setuid/setgid files or adding ACLs or extended attributes. Only disable this if you’re aware of the security implications.

Default: true

•

flake-registry

extra-experimental-features = flakes
flake-registry = ...

Path or URI of the global flake registry.

When empty, disables the global flake registry.

Default: https://channels.nixos.org/flake-registry.json

•

fsync-metadata

If set to true, changes to the Nix store metadata (in /nix/var/nix/db) are synchronously flushed to disk. This improves robustness in case of system crashes, but reduces performance. The default is true.

Default: true

•

gc-reserved-space

Amount of reserved disk space for the garbage collector.

Default: 8388608

•

hashed-mirrors

A list of web servers used by builtins.fetchurl to obtain files by hash. Given a hash algorithm ha and a base-16 hash h, Nix will try to download the file from hashed-mirror/ha/h. This allows files to be downloaded even if they have disappeared from their original URI. For example, given an example mirror http://tarballs.nixos.org/, when building the derivation

builtins.fetchurl {
url = "https://example.org/foo-1.2.3.tar.xz";
sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
}

Nix will attempt to download this file from http://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae first. If it is not available there, if will try the original URI.

Default: empty

•

http-connections

The maximum number of parallel TCP connections used to fetch files from binary caches and by other downloads. It defaults to 25. 0 means no limit.

Default: 25

Deprecated alias: binary-caches-parallel-connections

•

http2

Whether to enable HTTP/2 support.

Default: true

•

id-count

The number of UIDs/GIDs to use for dynamic ID allocation.

Default: 8388608

•

ignore-try

If set to true, ignore exceptions inside ‘tryEval’ calls when evaluating nix expressions in debug mode (using the –debugger flag). By default the debugger will pause on all exceptions.

Default: false

•

ignored-acls

A list of ACLs that should be ignored, normally Nix attempts to remove all ACLs from files and directories in the Nix store, but some ACLs like security.selinux or system.nfs4_acl can’t be removed even by root. Therefore it’s best to just ignore them.

Default: security.csm security.selinux system.nfs4_acl

•

impersonate-linux-26

Whether to impersonate a Linux 2.6 machine on newer kernels.

Default: false

Deprecated alias: build-impersonate-linux-26

•

impure-env

extra-experimental-features = configurable-impure-env
impure-env = ...

A list of items, each in the format of:

If the user is trusted (see trusted-users option), when building a fixed-output derivation, environment variables set in this option will be passed to the builder if they are listed in impureEnvVars.

This option is useful for, e.g., setting https_proxy for fixed-output derivations and in a multi-user Nix installation, or setting private access tokens when fetching a private repository.

Default: empty

•

keep-build-log

If set to true (the default), Nix will write the build log of a derivation (i.e. the standard output and error of its builder) to the directory /nix/var/log/nix/drvs. The build log can be retrieved using the command nix-store -l path.

Default: true

Deprecated alias: build-keep-log

•

keep-derivations

If true (default), the garbage collector will keep the derivations from which non-garbage store paths were built. If false, they will be deleted unless explicitly registered as a root (or reachable from other roots).

Keeping derivation around is useful for querying and traceability (e.g., it allows you to ask with what dependencies or options a store path was built), so by default this option is on. Turn it off to save a bit of disk space (or a lot if keep-outputs is also turned on).

Default: true

Deprecated alias: gc-keep-derivations

•

keep-env-derivations

If false (default), derivations are not stored in Nix user environments. That is, the derivations of any build-time-only dependencies may be garbage-collected.

If true, when you add a Nix derivation to a user environment, the path of the derivation is stored in the user environment. Thus, the derivation will not be garbage-collected until the user environment generation is deleted (nix-env --delete-generations). To prevent build-time-only dependencies from being collected, you should also turn on keep-outputs.

The difference between this option and keep-derivations is that this one is “sticky”: it applies to any user environment created while this option was enabled, while keep-derivations only applies at the moment the garbage collector is run.

Default: false

Deprecated alias: env-keep-derivations

•

keep-failed

Whether to keep temporary directories of failed builds.

Default: false

•

keep-going

Whether to keep building derivations when another build fails.

Default: false

•

keep-outputs

If true, the garbage collector will keep the outputs of non-garbage derivations. If false (default), outputs will be deleted unless they are GC roots themselves (or reachable from other roots).

In general, outputs must be registered as roots separately. However, even if the output of a derivation is registered as a root, the collector will still delete store paths that are used only at build time (e.g., the C compiler, or source tarballs downloaded from the network). To prevent it from doing so, set this option to true.

Default: false

Deprecated alias: gc-keep-outputs

•

log-lines

The number of lines of the tail of the log to show if a build fails.

Default: 25

•

max-build-log-size

This option defines the maximum number of bytes that a builder can write to its stdout/stderr. If the builder exceeds this limit, it’s killed. A value of 0 (the default) means that there is no limit.

Default: 0

Deprecated alias: build-max-log-size

•

max-call-depth

The maximum function call depth to allow before erroring.

Default: 10000

•

max-free

When a garbage collection is triggered by the min-free option, it stops as soon as max-free bytes are available. The default is infinity (i.e. delete all garbage).

Default: -1

•

max-jobs

Maximum number of jobs that Nix will try to build locally in parallel.

The special value auto causes Nix to use the number of CPUs in your system. Use 0 to disable local builds and directly use the remote machines specified in builders. This will not affect derivations that have preferLocalBuild = true, which are always built locally.

The setting can be overridden using the --max-jobs (-j) command line switch.

Default: 1

Deprecated alias: build-max-jobs

•

max-silent-time

This option defines the maximum number of seconds that a builder can go without producing any data on standard output or standard error. This is useful (for instance in an automated build system) to catch builds that are stuck in an infinite loop, or to catch remote builds that are hanging due to network problems. It can be overridden using the --max-silent-time command line switch.

The value 0 means that there is no timeout. This is also the default.

Default: 0

Deprecated alias: build-max-silent-time

•

max-substitution-jobs

This option defines the maximum number of substitution jobs that Nix will try to run in parallel. The default is 16. The minimum value one can choose is 1 and lower values will be interpreted as 1.

Default: 16

Deprecated alias: substitution-max-jobs

•

min-free

When free disk space in /nix/store drops below min-free during a build, Nix performs a garbage-collection until max-free bytes are available or there is no more garbage. A value of 0 (the default) disables this feature.

Default: 0

•

min-free-check-interval

Number of seconds between checking free disk space.

Default: 5

•

nar-buffer-size

Maximum size of NARs before spilling them to disk.

Default: 33554432

•

narinfo-cache-negative-ttl

The TTL in seconds for negative lookups. If a store path is queried from a substituter but was not found, there will be a negative lookup cached in the local disk cache database for the specified duration.

Set to 0 to force updating the lookup cache.

To wipe the lookup cache completely:

$ rm $HOME/.cache/nix/binary-cache-v*.sqlite*
# rm /root/.cache/nix/binary-cache-v*.sqlite*

Default: 3600

•

narinfo-cache-positive-ttl

The TTL in seconds for positive lookups. If a store path is queried from a substituter, the result of the query will be cached in the local disk cache database including some of the NAR metadata. The default TTL is a month, setting a shorter TTL for positive lookups can be useful for binary caches that have frequent garbage collection, in which case having a more frequent cache invalidation would prevent trying to pull the path again and failing with a hash mismatch if the build isn’t reproducible.

Default: 2592000

•

netrc-file

If set to an absolute path to a netrc file, Nix will use the HTTP authentication credentials in this file when trying to download from a remote host through HTTP or HTTPS. Defaults to $NIX_CONF_DIR/netrc.

The netrc file consists of a list of accounts in the following format:

machine my-machine login my-username password my-password

For the exact syntax, see the curl documentation.

Default: /dummy/netrc

•

nix-path

List of search paths to use for lookup path resolution. This setting determines the value of builtins.nixPath and can be used with builtins.findFile.

If the respective paths are accessible, the default values are:

See NIX_STATE_DIR for details.

Default: machine-specific

•

nix-shell-always-looks-for-shell-nix

Before Nix 2.24, nix-shell would only look at shell.nix if it was in the working directory - when no file was specified.

Since Nix 2.24, nix-shell always looks for a shell.nix, whether that’s in the working directory, or in a directory that was passed as an argument.

You may set this to false to temporarily revert to the behavior of Nix 2.23 and older.

Using this setting is not recommended. It will be deprecated and removed.

Default: true

•

nix-shell-shebang-arguments-relative-to-script

Before Nix 2.24, relative file path expressions in arguments in a nix-shell shebang were resolved relative to the working directory.

Since Nix 2.24, nix-shell resolves these paths in a manner that is relative to the base directory, defined as the script’s directory.

You may set this to false to temporarily revert to the behavior of Nix 2.23 and older.

Using this setting is not recommended. It will be deprecated and removed.

Default: true

•

plugin-files

A list of plugin files to be loaded by Nix. Each of these files will be dlopened by Nix. If they contain the symbol nix_plugin_entry(), this symbol will be called. Alternatively, they can affect execution through static initialization. In particular, these plugins may construct static instances of RegisterPrimOp to add new primops or constants to the expression language, RegisterStoreImplementation to add new store implementations, RegisterCommand to add new subcommands to the nix command, and RegisterSetting to add new nix config settings. See the constructors for those types for more details.

Warning! These APIs are inherently unstable and may change from release to release.

Since these files are loaded into the same address space as Nix itself, they must be DSOs compatible with the instance of Nix running at the time (i.e. compiled against the same headers, not linked to any incompatible libraries). They should not be linked to any Nix libs directly, as those will be available already at load time.

If an entry in the list is a directory, all files in the directory are loaded as plugins (non-recursively).

Default: empty

•

post-build-hook

Optional. The path to a program to execute after each build.

This option is only settable in the global nix.conf, or on the command line by trusted users.

When using the nix-daemon, the daemon executes the hook as root. If the nix-daemon is not involved, the hook runs as the user executing the nix-build.

The program executes with no arguments. The program’s environment contains the following environment variables:

Default: empty

•

pre-build-hook

If set, the path to a program that can set extra derivation-specific settings for this system. This is used for settings that can’t be captured by the derivation model itself and are too variable between different versions of the same system to be hard-coded into nix.

The hook is passed the derivation path and, if sandboxes are enabled, the sandbox directory. It can then modify the sandbox and send a series of commands to modify various settings to stdout. The currently recognized commands are:

Default: empty

•

preallocate-contents

Whether to preallocate files when writing objects with known size.

Default: false

•

print-missing

Whether to print what paths need to be built or downloaded.

Default: true

•

pure-eval

Pure evaluation mode ensures that the result of Nix expressions is fully determined by explicitly declared inputs, and not influenced by external state:

Default: false

•

require-drop-supplementary-groups

Following the principle of least privilege, Nix will attempt to drop supplementary groups when building with sandboxing.

However this can fail under some circumstances. For example, if the user lacks the CAP_SETGID capability. Search setgroups(2) for EPERM to find more detailed information on this.

If you encounter such a failure, setting this option to false will let you ignore it and continue. But before doing so, you should consider the security implications carefully. Not dropping supplementary groups means the build sandbox will be less restricted than intended.

This option defaults to true when the user is root (since root usually has permissions to call setgroups) and false otherwise.

Default: false

•

require-sigs

If set to true (the default), any non-content-addressed path added or copied to the Nix store (e.g. when substituting from a binary cache) must have a signature by a trusted key. A trusted key is one listed in trusted-public-keys, or a public key counterpart to a private key stored in a file listed in secret-key-files.

Set to false to disable signature checking and trust all non-content-addressed paths unconditionally.

(Content-addressed paths are inherently trustworthy and thus unaffected by this configuration option.)

Default: true

•

restrict-eval

If set to true, the Nix evaluator will not allow access to any files outside of builtins.nixPath, or to URIs outside of allowed-uris.

Default: false

•

run-diff-hook

If true, enable the execution of the diff-hook program.

When using the Nix daemon, run-diff-hook must be set in the nix.conf configuration file, and cannot be passed at the command line.

Default: false

•

sandbox

If set to true, builds will be performed in a sandboxed environment, i.e., they’re isolated from the normal file system hierarchy and will only see their dependencies in the Nix store, the temporary build directory, private versions of /proc, /dev, /dev/shm and /dev/pts (on Linux), and the paths configured with the sandbox-paths option. This is useful to prevent undeclared dependencies on files in directories such as /usr/bin. In addition, on Linux, builds run in private PID, mount, network, IPC and UTS namespaces to isolate them from other processes in the system (except that fixed-output derivations do not run in private network namespace to ensure they can access the network).

Currently, sandboxing only work on Linux and macOS. The use of a sandbox requires that Nix is run as root (so you should use the “build users” feature to perform the actual builds under different users than root).

If this option is set to relaxed, then fixed-output derivations and derivations that have the __noChroot attribute set to true do not run in sandboxes.

The default is true on Linux and false on all other platforms.

Default: true

Deprecated alias: build-use-chroot, build-use-sandbox

•

sandbox-build-dir

Linux only

The build directory inside the sandbox.

This directory is backed by build-dir on the host.

Default: /build

•

sandbox-dev-shm-size

Linux only

This option determines the maximum size of the tmpfs filesystem mounted on /dev/shm in Linux sandboxes. For the format, see the description of the size option of tmpfs in mount(8). The default is 50%.

Default: 50%

•

sandbox-fallback

Whether to disable sandboxing when the kernel doesn’t allow it.

Default: true

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sandbox-paths

A list of paths bind-mounted into Nix sandbox environments. You can use the syntax target=source to mount a path in a different location in the sandbox; for instance, /bin=/nix-bin will mount the path /nix-bin as /bin inside the sandbox. If source is followed by ?, then it is not an error if source does not exist; for example, /dev/nvidiactl? specifies that /dev/nvidiactl will only be mounted in the sandbox if it exists in the host filesystem.

If the source is in the Nix store, then its closure will be added to the sandbox as well.

Depending on how Nix was built, the default value for this option may be empty or provide /bin/sh as a bind-mount of bash.

Default: empty

Deprecated alias: build-chroot-dirs, build-sandbox-paths

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secret-key-files

A whitespace-separated list of files containing secret (private) keys. These are used to sign locally-built paths. They can be generated using nix-store --generate-binary-cache-key. The corresponding public key can be distributed to other users, who can add it to trusted-public-keys in their nix.conf.

Default: empty

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show-trace

Whether Nix should print out a stack trace in case of Nix expression evaluation errors.

Default: false

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ssl-cert-file

The path of a file containing CA certificates used to authenticate https:// downloads. Nix by default will use the first of the following files that exists:

The path can be overridden by the following environment variables, in order of precedence:

Default: /etc/ssl/certs/ca-certificates.crt

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stalled-download-timeout

The timeout (in seconds) for receiving data from servers during download. Nix cancels idle downloads after this timeout’s duration.

Default: 300

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start-id

The first UID and GID to use for dynamic ID allocation.

Default: 872415232

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store

The URL of the Nix store to use for most operations. See the Store Types section of the manual for supported store types and settings.

Default: auto

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substitute

If set to true (default), Nix will use binary substitutes if available. This option can be disabled to force building from source.

Default: true

Deprecated alias: build-use-substitutes

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substituters

A list of URLs of Nix stores to be used as substituters, separated by whitespace. A substituter is an additional store from which Nix can obtain store objects instead of building them.

Substituters are tried based on their priority value, which each substituter can set independently. Lower value means higher priority. The default is https://cache.nixos.org, which has a priority of 40.

At least one of the following conditions must be met for Nix to use a substituter:

In addition, each store path should be trusted as described in trusted-public-keys

Default: https://cache.nixos.org/

Deprecated alias: binary-caches

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sync-before-registering

Whether to call sync() before registering a path as valid.

Default: false

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system

The system type of the current Nix installation. Nix will only build a given derivation locally when its system attribute equals any of the values specified here or in extra-platforms.

The default value is set when Nix itself is compiled for the system it will run on. The following system types are widely used, as Nix is actively supported on these platforms:

In general, you do not have to modify this setting. While you can force Nix to run a Darwin-specific builder executable on a Linux machine, the result would obviously be wrong.

This value is available in the Nix language as builtins.currentSystem if the eval-system configuration option is set as the empty string.

Default: i686-linux

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system-features

A set of system “features” supported by this machine.

This complements the system and extra-platforms configuration options and the corresponding system attribute on derivations.

A derivation can require system features in the requiredSystemFeatures attribute, and the machine to build the derivation must have them.

System features are user-defined, but Nix sets the following defaults:

Included on Darwin if virtualization is available.

Included on Linux if /dev/kvm is accessible.

These historical pseudo-features are always enabled for backwards compatibility, as they are used in Nixpkgs to route Hydra builds to specific machines.

Included by default if the ca-derivations experimental feature is enabled.

This system feature is implicitly required by derivations with the __contentAddressed attribute.

Included by default if the recursive-nix experimental feature is enabled.

On Linux, Nix can run builds in a user namespace where they run as root (UID 0) and have 65,536 UIDs available. This is primarily useful for running containers such as systemd-nspawn inside a Nix build. For an example, see [tests/systemd-nspawn/nix][nspawn].

[nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.

Included by default on Linux if the auto-allocate-uids setting is enabled.

Default: machine-specific

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tarball-ttl

The number of seconds a downloaded tarball is considered fresh. If the cached tarball is stale, Nix will check whether it is still up to date using the ETag header. Nix will download a new version if the ETag header is unsupported, or the cached ETag doesn’t match.

Setting the TTL to 0 forces Nix to always check if the tarball is up to date.

Nix caches tarballs in $XDG_CACHE_HOME/nix/tarballs.

Files fetched via NIX_PATH, fetchGit, fetchMercurial, fetchTarball, and fetchurl respect this TTL.

Default: 3600

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timeout

This option defines the maximum number of seconds that a builder can run. This is useful (for instance in an automated build system) to catch builds that are stuck in an infinite loop but keep writing to their standard output or standard error. It can be overridden using the --timeout command line switch.

The value 0 means that there is no timeout. This is also the default.

Default: 0

Deprecated alias: build-timeout

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trace-function-calls

If set to true, the Nix evaluator will trace every function call. Nix will print a log message at the “vomit” level for every function entrance and function exit.

function-trace entered undefined position at 1565795816999559622 function-trace exited undefined position at 1565795816999581277 function-trace entered /nix/store/…/example.nix:226:41 at 1565795253249935150 function-trace exited /nix/store/…/example.nix:226:41 at 1565795253249941684

The undefined position means the function call is a builtin.

Use the contrib/stack-collapse.py script distributed with the Nix source code to convert the trace logs in to a format suitable for flamegraph.pl.

Default: false

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trace-verbose

Whether builtins.traceVerbose should trace its first argument when evaluated.

Default: false

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trust-tarballs-from-git-forges

If enabled (the default), Nix will consider tarballs from GitHub and similar Git forges to be locked if a Git revision is specified, e.g. github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f. This requires Nix to trust that the provider will return the correct contents for the specified Git revision.

If disabled, such tarballs are only considered locked if a narHash attribute is specified, e.g. github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f?narHash=sha256-PPXqKY2hJng4DBVE0I4xshv/vGLUskL7jl53roB8UdU%3D.

Default: true

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trusted-public-keys

A whitespace-separated list of public keys.

At least one of the following condition must be met for Nix to accept copying a store object from another Nix store (such as a substituter):

Default: cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=

Deprecated alias: binary-cache-public-keys

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trusted-substituters

A list of Nix store URLs, separated by whitespace. These are not used by default, but users of the Nix daemon can enable them by specifying substituters.

Unprivileged users (those set in only allowed-users but not trusted-users) can pass as substituters only those URLs listed in trusted-substituters.

Default: empty

Deprecated alias: trusted-binary-caches

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trusted-users

A list of user names, separated by whitespace. These users will have additional rights when connecting to the Nix daemon, such as the ability to specify additional substituters, or to import unsigned realisations or unsigned input-addressed store objects.

You can also specify groups by prefixing names with @. For instance, @wheel means all users in the wheel group.

Default: root

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upgrade-nix-store-path-url

Used by nix upgrade-nix, the URL of the file that contains the store paths of the latest Nix release.

Default: https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix

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use-case-hack

Whether to enable a macOS-specific hack for dealing with file name case collisions.

Default: false

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use-cgroups

Whether to execute builds inside cgroups. This is only supported on Linux.

Cgroups are required and enabled automatically for derivations that require the uid-range system feature.

Default: false

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use-registries

extra-experimental-features = flakes
use-registries = ...

Whether to use flake registries to resolve flake references.

Default: true

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use-sqlite-wal

Whether SQLite should use WAL mode.

Default: true

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use-xdg-base-directories

If set to true, Nix will conform to the XDG Base Directory Specification for files in $HOME. The environment variables used to implement this are documented in the Environment Variables section.

In particular, the following locations change:

Old	New
~/.nix-profile	$XDG_STATE_HOME/nix/profile
~/.nix-defexpr	$XDG_STATE_HOME/nix/defexpr
~/.nix-channels	$XDG_STATE_HOME/nix/channels

If you already have Nix installed and are using profiles or channels, you should migrate manually when you enable this option. If $XDG_STATE_HOME is not set, use $HOME/.local/state/nix instead of $XDG_STATE_HOME/nix. This can be achieved with the following shell commands:

nix_state_home=${XDG_STATE_HOME-$HOME/.local/state}/nix
mkdir -p $nix_state_home
mv $HOME/.nix-profile $nix_state_home/profile
mv $HOME/.nix-defexpr $nix_state_home/defexpr
mv $HOME/.nix-channels $nix_state_home/channels

Default: false

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user-agent-suffix

String appended to the user agent in HTTP requests.

Default: empty

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warn-dirty

Whether to warn about dirty Git/Mercurial trees.

Default: true

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warn-large-path-threshold

Warn when copying a path larger than this number of bytes to the Nix store (as determined by its NAR serialisation).

Default: -1