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# Common Environment Variables

Most Nix commands interpret the following environment variables:

- `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`

  A colon-separated list of search path entries used to resolve lookup paths.

  This environment variable overrides the value of the `nix-path` configuration setting.

  It can be extended using the `-I` option.

  > Example

  >     $ 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

  >     $ NIX_PATH= nix-instantiate --eval '<nixpkgs>'
  >     error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

- `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.,

      $ mkdir /nix
      $ mount -o bind /mnt/otherdisk/nix /nix

  Consult the mount 8 manual page for details.

- `NIX_STORE_DIR`

  Overrides the location of the Nix store (default `prefix/store`).

- `NIX_DATA_DIR`

  Overrides the location of the Nix static data directory (default `prefix/share`).

- `NIX_LOG_DIR`

  Overrides the location of the Nix log directory (default `prefix/var/log/nix`).

- `NIX_STATE_DIR`

  Overrides the location of the Nix state directory (default `prefix/var/nix`).

- `NIX_CONF_DIR`

  Overrides the location of the system Nix configuration directory (default `sysconfdir/nix`, i.e. `/etc/nix` on most systems).

- `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`

  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 sub-section for details.

  The variable is treated as a list separated by the `:` token.

- `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`

  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. 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`

  If set to `1`, Nix will print some evaluation statistics, such as the number of values allocated.

- `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`

  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 (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`)

In addition, setting the following environment variables overrides the XDG base directories:

- `NIX_CONFIG_HOME` (default `$XDG_CONFIG_HOME/nix`)
- `NIX_STATE_HOME` (default `$XDG_STATE_HOME/nix`)
- `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.