nix-super/doc/manual/source/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. 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 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 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]{#env-XDG_CONFIG_HOME} (default ~/.config)
• [XDG_STATE_HOME]{#env-XDG_STATE_HOME} (default ~/.local/state)
• [XDG_CACHE_HOME]{#env-XDG_CACHE_HOME} (default ~/.cache)

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

• [NIX_CONFIG_HOME]{#env-NIX_CONFIG_HOME} (default $XDG_CONFIG_HOME/nix)
• [NIX_STATE_HOME]{#env-NIX_STATE_HOME} (default $XDG_STATE_HOME/nix)
• [NIX_CACHE_HOME]{#env-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.