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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.