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# Common Environment Variables
Most Nix commands interpret the following environment variables:
- <span id="env-IN_NIX_SHELL">[`IN_NIX_SHELL`](#env-IN_NIX_SHELL)</span>\
Indicator that tells if the current environment was set up by
`nix-shell`. It can have the values `pure` or `impure`.
- <span id="env-NIX_PATH">[`NIX_PATH`](#env-NIX_PATH)</span>\
A colon-separated list of directories used to look up the location of Nix
expressions using [paths](@docroot@/language/values.md#type-path)
enclosed in angle brackets (i.e., `<path>`),
e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the
[`-I` option](@docroot@/command-ref/opt-common.md#opt-I).
If `NIX_PATH` is not set at all, Nix will fall back to the following list in [impure](@docroot@/command-ref/conf-file.md#conf-pure-eval) and [unrestricted](@docroot@/command-ref/conf-file.md#conf-restrict-eval) evaluation mode:
1. `$HOME/.nix-defexpr/channels`
2. `nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixpkgs`
3. `/nix/var/nix/profiles/per-user/root/channels`
If `NIX_PATH` is set to an empty string, resolving search paths will always fail.
For example, attempting to use `<nixpkgs>` will produce:
error: file 'nixpkgs' was not found in the Nix search path
- <span id="env-NIX_IGNORE_SYMLINK_STORE">[`NIX_IGNORE_SYMLINK_STORE`](#env-NIX_IGNORE_SYMLINK_STORE)</span>\
Normally, the Nix store directory (typically `/nix/store`) is not
allowed to contain any symlink components. This is to prevent
“impure” builds. Builders sometimes “canonicalise” paths by
resolving all symlink components. Thus, builds on different machines
(with `/nix/store` resolving to different locations) could yield
different results. This is generally not a problem, except when
builds are deployed to machines where `/nix/store` resolves
differently. If you are sure that you’re not going to do that, you
can set `NIX_IGNORE_SYMLINK_STORE` to `1`.
Note that if you’re symlinking the Nix store so that you can put it
on another file system than the root file system, on Linux you’re
better off using `bind` mount points, e.g.,
```console
$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix
```
Consult the mount 8 manual page for details.
- <span id="env-NIX_STORE_DIR">[`NIX_STORE_DIR`](#env-NIX_STORE_DIR)</span>\
Overrides the location of the Nix store (default `prefix/store`).
- <span id="env-NIX_DATA_DIR">[`NIX_DATA_DIR`](#env-NIX_DATA_DIR)</span>\
Overrides the location of the Nix static data directory (default
`prefix/share`).
- <span id="env-NIX_LOG_DIR">[`NIX_LOG_DIR`](#env-NIX_LOG_DIR)</span>\
Overrides the location of the Nix log directory (default
`prefix/var/log/nix`).
- <span id="env-NIX_STATE_DIR">[`NIX_STATE_DIR`](#env-NIX_STATE_DIR)</span>\
Overrides the location of the Nix state directory (default
`prefix/var/nix`).
- <span id="env-NIX_CONF_DIR">[`NIX_CONF_DIR`](#env-NIX_CONF_DIR)</span>\
Overrides the location of the system Nix configuration directory
(default `prefix/etc/nix`).
- <span id="env-NIX_CONFIG">[`NIX_CONFIG`](#env-NIX_CONFIG)</span>\
Applies settings from Nix configuration from the environment.
The content is treated as if it was read from a Nix configuration file.
Settings are separated by the newline character.
- <span id="env-NIX_USER_CONF_FILES">[`NIX_USER_CONF_FILES`](#env-NIX_USER_CONF_FILES)</span>\
Overrides the location of the Nix user configuration files to load from.
The default are the locations according to the [XDG Base Directory Specification].
See the [XDG Base Directories](#xdg-base-directories) sub-section for details.
The variable is treated as a list separated by the `:` token.
- <span id="env-TMPDIR">[`TMPDIR`](#env-TMPDIR)</span>\
Use the specified directory to store temporary files. In particular,
this includes temporary build directories; these can take up
substantial amounts of disk space. The default is `/tmp`.
- <span id="env-NIX_REMOTE">[`NIX_REMOTE`](#env-NIX_REMOTE)</span>\
This variable should be set to `daemon` if you want to use the Nix
daemon to execute Nix operations. This is necessary in [multi-user
Nix installations](@docroot@/installation/multi-user.md). If the Nix
daemon's Unix socket is at some non-standard path, this variable
should be set to `unix://path/to/socket`. Otherwise, it should be
left unset.
- <span id="env-NIX_SHOW_STATS">[`NIX_SHOW_STATS`](#env-NIX_SHOW_STATS)</span>\
If set to `1`, Nix will print some evaluation statistics, such as
the number of values allocated.
- <span id="env-NIX_COUNT_CALLS">[`NIX_COUNT_CALLS`](#env-NIX_COUNT_CALLS)</span>\
If set to `1`, Nix will print how often functions were called during
Nix expression evaluation. This is useful for profiling your Nix
expressions.
- <span id="env-GC_INITIAL_HEAP_SIZE">[`GC_INITIAL_HEAP_SIZE`](#env-GC_INITIAL_HEAP_SIZE)</span>\
If Nix has been configured to use the Boehm garbage collector, this
variable sets the initial size of the heap in bytes. It defaults to
384 MiB. Setting it to a low value reduces memory consumption, but
will increase runtime due to the overhead of garbage collection.
## XDG Base Directories
Nix follows the [XDG Base Directory Specification].
For backwards compatibility, Nix commands will follow the standard only when [`use-xdg-base-directories`] is enabled.
[New Nix commands](@docroot@/command-ref/new-cli/nix.md) (experimental) conform to the standard by default.
The following environment variables are used to determine locations of various state and configuration files:
- `XDG_CONFIG_HOME` (default `~/.config`)
- `XDG_STATE_HOME` (default `~/.local/state`)
- `XDG_CACHE_HOME` (default `~/.cache`)
[XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories
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