Nix/doc/manual/src/command-ref/env-common.md

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.