Common Environment Variables - Lix Reference Manual

# Common Environment Variables

Most commands in Lix 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 directories used to look up the location of Nix expressions using paths enclosed in angle brackets (i.e., `<path>`), e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the `-I` option.

  If `NIX_PATH` is not set at all, Lix will fall back to the following list in impure and unrestricted 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`
  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 Lix static data directory (default `prefix/share`).

- `NIX_LOG_DIR`
  Overrides the location of the Lix log directory (default `prefix/var/log/nix`).

- `NIX_STATE_DIR`
  Overrides the location of the Lix state directory (default `prefix/var/nix`).

- `NIX_CONF_DIR`
  Overrides the location of the system Lix configuration directory (default `prefix/etc/nix`).

- `NIX_CONFIG`
  Applies settings from Lix configuration from the environment. The content is treated as if it was read from a Lix configuration file. Settings are separated by the newline character.

- `NIX_USER_CONF_FILES`
  Overrides the location of the Lix 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 Lix daemon to execute Nix operations. This is necessary in multi-user Nix installations. If the Lix 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`, Lix will print some evaluation statistics, such as the number of values allocated.

- `NIX_COUNT_CALLS`
  If set to `1`, Lix 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

Lix follows the XDG Base Directory Specification.

For backwards compatibility, commands in Lix 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`)