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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 search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
This environment variable overrides the value of the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path).
It can be extended using the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I).
> **Example**
>
> ```bash
> $ export NIX_PATH=`/home/eelco/Dev:nixos-config=/etc/nixos
> ```
If `NIX_PATH` is set to an empty string, resolving search paths will always fail.
> **Example**
>
> ```bash
> $ NIX_PATH= nix-instantiate --eval '<nixpkgs>'
> error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
> ```
- <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`]{#env-XDG_CONFIG_HOME} (default `~/.config`)
- [`XDG_STATE_HOME`]{#env-XDG_STATE_HOME} (default `~/.local/state`)
- [`XDG_CACHE_HOME`]{#env-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
In addition, setting the following environment variables overrides the XDG base directories:
- [`NIX_CONFIG_HOME`]{#env-NIX_CONFIG_HOME} (default `$XDG_CONFIG_HOME/nix`)
- [`NIX_STATE_HOME`]{#env-NIX_STATE_HOME} (default `$XDG_STATE_HOME/nix`)
- [`NIX_CACHE_HOME`]{#env-NIX_CACHE_HOME} (default `$XDG_CACHE_HOME/nix`)
When [`use-xdg-base-directories`] is enabled, the configuration directory is:
1. `$NIX_CONFIG_HOME`, if it is defined
2. Otherwise, `$XDG_CONFIG_HOME/nix`, if `XDG_CONFIG_HOME` is defined
3. Otherwise, `~/.config/nix`.
Likewise for the state and cache directories.
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