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<h1 id="built-in-constants">Built-in Constants</h1>
<p>These constants are built into the Nix language evaluator:</p>
<dl>
<dt id="builtins-builtins">
<a href="#builtins-builtins"><code>builtins</code></a> (set)
</dt>
<dd>
Contains all the [built-in functions](../language/builtins.md) and values.
Since built-in functions were added over time, [testing for attributes](./operators.md#has-attribute) in `builtins` can be used for graceful fallback on older Nix installations:
<div class="highlight"><pre><span></span><code><span class="c1">## if hasContext is not available, we assume `s` has a context</span>
<span class="k">if</span> <span class="nb">builtins</span> <span class="o">?</span> hasContext <span class="k">then</span> <span class="nb">builtins</span><span class="o">.</span>hasContext s <span class="k">else</span> <span class="no">true</span>
</code></pre></div>
</dd>
<dt id="builtins-currentSystem">
<a href="#builtins-currentSystem"><code>currentSystem</code></a> (string)
</dt>
<dd>
The value of the
[`eval-system`](../Command-Reference/conf-file.md#conf-eval-system)
or else
[`system`](../Command-Reference/conf-file.md#conf-system)
configuration option.
It can be used to set the `system` attribute for [`builtins.derivation`](../language/derivations.md) such that the resulting derivation can be built on the same system that evaluates the Nix expression:
<div class="highlight"><pre><span></span><code> <span class="nb">builtins</span><span class="o">.</span><span class="nb">derivation</span> <span class="p">{</span>
<span class="c1">## ...</span>
<span class="ss">system</span> <span class="o">=</span> <span class="nb">builtins</span><span class="o">.</span>currentSystem<span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
It can be overridden in order to create derivations for different system than the current one:
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>nix-instantiate<span class="w"> </span>--system<span class="w"> </span><span class="s2">&quot;mips64-linux&quot;</span><span class="w"> </span>--eval<span class="w"> </span>--expr<span class="w"> </span><span class="s1">&#39;builtins.currentSystem&#39;</span>
<span class="go">&quot;mips64-linux&quot;</span>
</code></pre></div>
!!! note
Not available in [pure evaluation mode](../Command-Reference/conf-file.md#conf-pure-eval).
</dd>
<dt id="builtins-currentTime">
<a href="#builtins-currentTime"><code>currentTime</code></a> (integer)
</dt>
<dd>
Return the [Unix time](https://en.wikipedia.org/wiki/Unix_time) at first evaluation.
Repeated references to that name will re-use the initially obtained value.
Example:
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>nix<span class="w"> </span>repl
<span class="go">Welcome to Nix 2.15.1 Type :? for help.</span>
<span class="go">nix-repl&gt; builtins.currentTime</span>
<span class="go">1683705525</span>
<span class="go">nix-repl&gt; builtins.currentTime</span>
<span class="go">1683705525</span>
</code></pre></div>
The [store path](../glossary.md#gloss-store-path) of a derivation depending on `currentTime` will differ for each evaluation, unless both evaluate `builtins.currentTime` in the same second.
!!! note
Not available in [pure evaluation mode](../Command-Reference/conf-file.md#conf-pure-eval).
</dd>
<dt id="builtins-false">
<a href="#builtins-false"><code>false</code></a> (Boolean)
</dt>
<dd>
Primitive value.
It can be returned by
[comparison operators](../language/operators.md#Comparison)
and used in
[conditional expressions](../language/constructs.md#Conditionals).
The name `false` is not special, and can be shadowed:
<div class="highlight"><pre><span></span><code>nix-repl&gt; let false = 1; in false
1
</code></pre></div>
</dd>
<dt id="builtins-langVersion">
<a href="#builtins-langVersion"><code>langVersion</code></a> (integer)
</dt>
<dd>
The legacy version of the Nix language. Always is `6` on Lix,
matching Nix 2.18.
Code in the Nix language should use other means of feature detection
like detecting the presence of builtins, rather than trying to find
the version of the Nix implementation, as there may be other Nix
implementations with different feature combinations.
If the feature you want to write compatibility code for cannot be
detected by any means, please file a Lix bug.
</dd>
<dt id="builtins-nixPath">
<a href="#builtins-nixPath"><code>nixPath</code></a> (list)
</dt>
<dd>
The search path used to resolve angle bracket path lookups.
Angle bracket expressions can be
[desugared](https://en.wikipedia.org/wiki/Syntactic_sugar)
using this and
[`builtins.findFile`](./builtins.html#builtins-findFile):
<div class="highlight"><pre><span></span><code><span class="l">&lt;nixpkgs&gt;</span>
</code></pre></div>
is equivalent to:
<div class="highlight"><pre><span></span><code><span class="nb">builtins</span><span class="o">.</span>findFile <span class="nb">builtins</span><span class="o">.</span>nixPath <span class="s2">&quot;nixpkgs&quot;</span>
</code></pre></div>
</dd>
<dt id="builtins-nixVersion">
<a href="#builtins-nixVersion"><code>nixVersion</code></a> (string)
</dt>
<dd>
Legacy version of Nix. Always returns "2.18.3-lix" on Lix.
Code in the Nix language should use other means of feature detection
like detecting the presence of builtins, rather than trying to find
the version of the Nix implementation, as there may be other Nix
implementations with different feature combinations.
If the feature you want to write compatibility code for cannot be
detected by any means, please file a Lix bug.
</dd>
<dt id="builtins-null">
<a href="#builtins-null"><code>null</code></a> (null)
</dt>
<dd>
Primitive value.
The name `null` is not special, and can be shadowed:
<div class="highlight"><pre><span></span><code>nix-repl&gt; let null = 1; in null
1
</code></pre></div>
</dd>
<dt id="builtins-storeDir">
<a href="#builtins-storeDir"><code>storeDir</code></a> (string)
</dt>
<dd>
Logical file system location of the [Nix store](../glossary.md#gloss-store) currently in use.
This value is determined by the `store` parameter in [Store URLs](../Command-Reference/New-CLI/nix3-help-stores.md):
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>nix-instantiate<span class="w"> </span>--store<span class="w"> </span><span class="s1">&#39;dummy://?store=/blah&#39;</span><span class="w"> </span>--eval<span class="w"> </span>--expr<span class="w"> </span>builtins.storeDir
<span class="go">&quot;/blah&quot;</span>
</code></pre></div>
</dd>
<dt id="builtins-true">
<a href="#builtins-true"><code>true</code></a> (Boolean)
</dt>
<dd>
Primitive value.
It can be returned by
[comparison operators](../language/operators.md#Comparison)
and used in
[conditional expressions](../language/constructs.md#Conditionals).
The name `true` is not special, and can be shadowed:
<div class="highlight"><pre><span></span><code>nix-repl&gt; let true = 1; in true
1
</code></pre></div>
</dd>
</dl>
<h3 id="things-which-might-be-mistaken-for-constants">Things which might be mistaken for constants</h3>
<dl>
<dt><code>__curPos</code></dt>
<dd>
This is not a constant but a [context-dependent keyword](../language/constructs.md#keywords-__curPos)
</dd>
</dl>
</article>
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