What does {{ }} mean in YAML?

Last Updated : 10 Sep, 2024

YAML (YAML Ain't Markup Language) is a popular human-readable data serialization format often used for configuration files and data exchange. YAML's simplicity and readability have made it a favorite among developers and system administrators. While YAML is generally straightforward, there are times when you encounter unfamiliar constructs, such as `{{ }}`. In this article, we'll delve into what `{{ }}` means in YAML, provide examples, and cover every aspect of its usage.

YAML Basics

Before diving into {{ }}, let's briefly recap some YAML basics.

YAML uses indentation to define the structure of data.
It supports key-value pairs, lists, and nested structures.
Comments start with the `#` character and are used for explanatory notes.
YAML is whitespace-sensitive, meaning that proper indentation is crucial.

Understanding `{{ }}` in YAML

The `{{ }}` construct in YAML is used to denote a "tagged scalar." It's a way to specify that a string should be treated as a specific data type or to apply a transformation to it. Essentially, `{{ }}` allows you to assign additional metadata or behaviors to a value within a YAML document.

Examples of `{{ }}`

Let's explore some common use cases for {{ }} in YAML with examples:

Example 1: Tagging Scalars

age: !!int 20

In this example, we have a YAML key-value pair where the key is "age," and the value is `!!int 20`.

`!!int` inside `{{ }}` is a YAML tag that tells the YAML parser to interpret the value as an integer.
The `20` is a scalar value (a plain number) without any quotes or special notation.

When this YAML document is parsed, the YAML parser will recognize that the `age` value is tagged as an integer (`!!int`), ensuring that it's treated as a numeric data type rather than a string. This can be useful when you want to maintain the data type's integrity.

Example 2: Applying Transformations

name: !!str {{ name.toUpperCase() }}

In this example, we have a YAML key-value pair where the key is "name," and the value is `{{ name.toUpperCase() }}`.

`{{ }}` is used to apply a transformation to the value of `name`.
name.toUpperCase() is a JavaScript-like expression enclosed within `{{ }}`. It transforms the `name` variable to uppercase.

When the YAML document is processed, it will evaluate the expression `name.toUpperCase()` and assign the result (the uppercase version of the `name` variable) as the value for the "name" key.

This is a powerful way to modify or preprocess data within YAML files, especially when dealing with configuration files or data transformations.

Example 3: Environment Variable Substitution

database_url: {{ env('DB_URL') }}

In this example, we have a YAML key-value pair where the key is "database_url," and the value is`{{ env('DB_URL') }}`.

`{{ env('DB_URL') }}` is a common use case for `{{ }}` in configuration files. It's used to substitute the value with the contents of the `DB_URL` environment variable.

When the YAML document is processed, the `{{ env('DB_URL') }}` expression is replaced with the actual value of the `DB_URL` environment variable, effectively setting the "database_url" key to the database URL stored in the environment variable.

This is a valuable feature for injecting dynamic or sensitive data into configuration files without hardcoding it, making the configuration more flexible and secure.

Nested `{{ }}`

value: !!int {{ env('SOME_ENV_VARIABLE').toUpperCase() }}

In this example, we have a YAML key-value pair where the key is "value," and the value is a combination of nested {{ }} constructs.

`{{ env('SOME_ENV_VARIABLE') }}` retrieves the value of the "SOME_ENV_VARIABLE" environment variable.
`.toUpperCase()` is applied to the result of `{{ env('SOME_ENV_VARIABLE') }}` to convert it to uppercase.
Finally, `!!int` tags the entire expression as an integer.

When processed, this YAML document will:

Retrieve the value of the "SOME_ENV_VARIABLE" environment variable.
Convert it to uppercase.
Treat the result as an integer.

The resulting integer value is then assigned to the "value" key.

This example illustrates the flexibility of `{{ }}` in allowing you to perform multiple operations on a value within a YAML document.

Conclusion

In this artcile we disscude `{{ }}` in YAML which is a versatile feature that can be used to tag values with data types, apply transformations, and perform dynamic substitutions, enhancing the flexibility and functionality of YAML documents, especially in configuration and data management contexts.