# 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`. Since Nix 2.0 the values are `"pure"` and `"impure"`

- `NIX_PATH`
  A colon-separated list of directories used to look up Nix expressions enclosed in angle brackets (i.e., `<path>`). For instance, the value

      /home/eelco/Dev:/etc/nixos

  will cause Nix to look for paths relative to `/home/eelco/Dev` and `/etc/nixos`, in this order. It is also possible to match paths against a prefix. For example, the value

      nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos

  will cause Nix to search for `<nixpkgs/path>` in `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.

  If a path in the Nix search path starts with `http://` or `https://`, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must consist of a single top-level directory. For example, setting `NIX_PATH` to

      nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz

  tells Nix to download and use the current contents of the `master` branch in the `nixpkgs` repository.

  The URLs of the tarballs from the official nixos.org channels (see the manual for `nix-channel`) can be abbreviated as `channel:<channel-name>`. For instance, the following two values of `NIX_PATH` are equivalent:

      nixpkgs=channel:nixos-21.05
      nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz

  The Nix search path can also be extended using the `-I` option to many Nix commands, which takes precedence over `NIX_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 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 `prefix/etc/nix`).

- `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 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`
  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.