# 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 user Nix configuration files to load
    from (defaults to the XDG spec locations). 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 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`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories