packer-cn/website/pages/docs/from-1.5/blocks/build/source.mdx

---
description: >
  A source block nested in a build block allows you to use an already defined
  source and to set specific fields.
layout: docs
page_title: source - build - Blocks
sidebar_title: <tt>source</tt>
---

# The `source` block

`@include 'from-1.5/beta-hcl2-note.mdx'`

A `source` block nested in a `build` block allows to use an already defined
source and to set specific fields.

```hcl
# builds.pkr.hcl
build {
  source "amazon-ebs.example" {
    output = "specific-value"
  }
  #  ...
}
```

-> **Note:** It is **not allowed** to set the same field in a top-level source
  block and in a used source block. For example, if in the above example, the
  top-level "amazon-ebs.example" source block also had an `output` field;
  Packer would error.