What is Fn::Join in CloudFormation: Simple Explanation and Example

Fn::Join is a CloudFormation function that combines a list of strings into one string using a specified separator. It helps you build strings like URLs, file paths, or resource names by joining parts together in your template.

How It Works

Fn::Join takes two inputs: a separator string and a list of strings. It then connects all the strings in the list into one single string, placing the separator between each part. Think of it like using a glue stick to stick words together with a chosen space or symbol in between.

For example, if you want to create a full website address from parts like "https://", "www.example", and ".com", Fn::Join can combine them with no spaces or with dots. This makes your CloudFormation templates cleaner and easier to manage because you don’t have to write the full string manually.

Example

This example shows how to join parts of a URL using Fn::Join in a CloudFormation template.

yaml

Resources:
  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Join ["-", ["my", "example", "bucket"]]

Outputs:
  BucketURL:
    Value: !Join ["", ["https://", !Ref MyBucket, ".s3.amazonaws.com"]]
    Description: "The URL of the S3 bucket"

Output

Outputs:
  BucketURL:
    Value: https://my-example-bucket.s3.amazonaws.com

When to Use

Use Fn::Join when you need to build strings dynamically from parts in your CloudFormation templates. This is common when creating resource names, URLs, or paths that depend on parameters or other resource names.

For example, if you want to create a unique bucket name by joining a project name and environment, or build an endpoint URL from parts, Fn::Join helps you do this cleanly without hardcoding values.

Key Points

• Fn::Join combines a list of strings into one string with a separator.
• The separator can be any string, including empty for no space.
• It helps create dynamic names and URLs in templates.
• It improves template readability and reduces errors from manual string building.

Key Takeaways

Fn::Join merges multiple strings into one using a chosen separator.

It is useful for building dynamic resource names and URLs in CloudFormation.

The separator can be any string, including empty for no spaces.

Using Fn::Join makes templates cleaner and easier to maintain.