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# Common Environment Variables
Most Nix commands interpret the following environment variables:
- [`IN_NIX_SHELL`]{#env-IN_NIX_SHELL}\
Indicator that tells if the current environment was set up by
`nix-shell`. It can have the values `pure` or `impure`.
- [`NIX_PATH`]{#env-NIX_PATH}\
A colon-separated list of directories used to look up the location of Nix
expressions using [paths](../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](./opt-common.md#opt-I).
If `NIX_PATH` is not set at all, Nix will fall back to the following list in [impure](./conf-file.md#conf-pure-eval) and [unrestricted](./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
- [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
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.
- [`NIX_STORE_DIR`]{#env-NIX_STORE_DIR}\
Overrides the location of the Nix store (default `prefix/store`).
- [`NIX_DATA_DIR`]{#env-NIX_DATA_DIR}\
Overrides the location of the Nix static data directory (default
`prefix/share`).
- [`NIX_LOG_DIR`]{#env-NIX_LOG_DIR}\
Overrides the location of the Nix log directory (default
`prefix/var/log/nix`).
- [`NIX_STATE_DIR`]{#env-NIX_STATE_DIR}\
Overrides the location of the Nix state directory (default
`prefix/var/nix`).
- [`NIX_CONF_DIR`]{#env-NIX_CONF_DIR}\
Overrides the location of the system Nix configuration directory
(default `prefix/etc/nix`).
- [`NIX_CONFIG`]{#env-NIX_CONFIG}\
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.
- [`NIX_USER_CONF_FILES`]{#env-NIX_USER_CONF_FILES}\
Overrides the location of the user Nix configuration files to load
from (defaults to the XDG spec locations). The variable is treated
as a list separated by the `:` token.
- [`TMPDIR`]{#env-TMPDIR}\
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`.
- [`NIX_REMOTE`]{#env-NIX_REMOTE}\
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](../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.
- [`NIX_SHOW_STATS`]{#env-NIX_SHOW_STATS}\
If set to `1`, Nix will print some evaluation statistics, such as
the number of values allocated.
- [`NIX_COUNT_CALLS`]{#env-NIX_COUNT_CALLS}\
If set to `1`, Nix will print how often functions were called during
Nix expression evaluation. This is useful for profiling your Nix
expressions.
- [`GC_INITIAL_HEAP_SIZE`]{#env-GC_INITIAL_HEAP_SIZE}\
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 Directory
New Nix commands conform to the [XDG Base Directory Specification], and use the following environment variables 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`)
Classic Nix commands can also be made to follow this standard using the [`use-xdg-base-directories`] configuration option.
[XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
[`use-xdg-base-directories`]: ../command-ref/conf-file.md#conf-use-xdg-base-directories
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