Nix Cheat Sheet

The world of Nix has a bit of a steep learning curve. As I write blog posts on the subject, I’ll refer to some of the tips and tricks from the cheat sheet here.

Documentation

A list of Nix documentation I refer to, along with some getting started pages:

• Nixpkgs manual
• NixOS manual
• nix.dev
• Nix Pills
• Zero to Nix

Resources

Here are a list of resources I use to search for Nix options, services, and any Nix built-in functions.

• NixOS Search
• MyNixOS
• noogle

Functions (and Lambdas)

In some languages you typically have a formal declaration of a function (or lambda), such as for Python:

def double_num(num_to_double)
  return num_to_double * 2

Nixlang is a functional language, and a typical behaviour of these is to define lambdas and assign them to variables.

double_num = num_to_double: num_to_double * 2

The lambda is defined when we create parameters for that lambda, which are defined by having a variable name prefixed by a colon. From the example above, the parameter for the double_num lambda is num_to_double.

Once all the parameters are defined we move onto the body, in which the final statement is returned, similarly to the implicit return in Ruby. Here we’re just using the maths expression num_to_double * 2 to have the intended outcome.

Check if attribute is in attrset shorthand

You can use the built in function hasAttr to determine if an attribute exists within an attrset.

$ attrSet = { key1 = "key1"; };

$ builtins.hasAttr "key1" attrSet
true

$ builtins.hasAttr "key2" attrSet
false

You can use the ? operator as an alternate way of doing the same.

$ attrSet ? "key1"
true

$ attrSet ? "key2"
false

Merge two attrsets shorthand

You can use the // operator to merge the contents of the attrset on the right into the attrset on the left.

$ newAttrset = { key1 = "key1"; } // { key2 = "key2"; }

$ newAttrset
{ key1 = "key1"; key2 = "key2"; }

If the attribute already exists in the left attrset, then it is overwritten.

$ attrSet = { key1 = "key1"; }

$ newAttrSet = attrSet // { key1 = "newKey1"; }

$ newAttrSet
{ key1 = "newKey1"; }

Add an attribute to an attrset

Given an attrset with this structure:

$ items = {
  keyName = {
    value1 = "value";
    value2 = "value";
  }
}

We can use the map function to loop over each attrName (attribute name (aka key)) and its value in items to create a new attrset with its attrName added to it:

$ newItems = map
    (key: items."${key}" // { name = key; })
    (builtins.attrNames items);

The map function takes two parameters:

• a lambda that should be performed against each element
  • (key: items."${key}" // { name = key; })
    • This lambda takes a parameter key, and returns the attribute value at items.key, and merges it with the attrset { name = key; } using the [[#Merge two attrsets shorthand]].
• a list of elements
  • (builtins.attrNames items)
  • builtins.attrNames is a function that takes a attrset and returns all the attribute names (keys) for it

So newItems will equal the below:

$ newItems
[
  {
    name = "keyName";
    value1 = "value";
    value2 = "value";
  }
];

The map function is also useful for converting an attrset to a list, since a list is what is returns by default.

$ items_list = map
    (key: items."${key}")
    (builtins.attrNames items);

$ items_list
[
  {
    value1 = "value";
    value2 = "value";
  }
]

Converting a list to an attribute set

When we [[#Add an attribute to an attrset|add an attribute to an attset]], the map function returns a list, when we may want to have an attrset back instead.

We can use the builtins.listToAttrs function to convert it to one. It takes one argument, a list of attrsets that contain two attributes; name and value - the former becomes the attrName and the latter its value.

$ list = [
    {
        name = "keyName";
        value = {
            value1 = "value";
            value2 = "value";
        };
    }
    {
        name = "keyName2";
        value = {
            value3 = "value";
        };
    }
]

$ attrSet = builtins.listToAttrs list

attrSet will then equal the below:

attrSet = {
    keyName = {
        value1 = "value";
        value2 = "value";
    };
    keyName2 = {
        value3 = "value";
    };
}