nix-super/doc/manual/src/command-ref/env-common.md

Common Environment Variables

Most Nix commands interpret the following environment variables:

• [IN_NIX_SHELL]{#env-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]{#env-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.

• [NIX_IGNORE_SYMLINK_STORE]{#env-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]{#env-NIX_STORE_DIR}
  Overrides the location of the Nix store (default prefix/store).

• [NIX_DATA_DIR]{#env-NIX_DATA_DIR}
  Overrides the location of the Nix static data directory (default prefix/share).

• [NIX_LOG_DIR]{#env-NIX_LOG_DIR}
  Overrides the location of the Nix log directory (default prefix/var/log/nix).

• [NIX_STATE_DIR]{#env-NIX_STATE_DIR}
  Overrides the location of the Nix state directory (default prefix/var/nix).

• [NIX_CONF_DIR]{#env-NIX_CONF_DIR}
  Overrides the location of the system Nix configuration directory (default prefix/etc/nix).

• [NIX_CONFIG]{#env-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]{#env-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]{#env-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]{#env-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]{#env-NIX_SHOW_STATS}
  If set to 1, Nix will print some evaluation statistics, such as the number of values allocated.

• [NIX_COUNT_CALLS]{#env-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]{#env-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 Directory

New Nix commands conform to the XDG Base Directory Specification, and use the following environment variables 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)

Classic Nix commands can also be made to follow this standard using the use-xdg-base-directories configuration option.