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# 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](#ssec-multi-user). 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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