# 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., ``). 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 `` 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/nixos-15.09.tar.gz tells Nix to download the latest revision in the Nixpkgs/NixOS 15.09 channel. A following shorthand can be used to refer to the official channels: nixpkgs=channel:nixos-15.09 The search path can be extended using the `-I` option, 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., ```console $ 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_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](../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` 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.