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/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.,
```
$ 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. 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.

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Common Environment Variables

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