---
description: >
  A `source` block nested in a `build` block reuses a builder defined
  elsewhere in the configuration. Learn how to configure a nested `source` block.
page_title: source block reference
---

# `source` block

This topic provides reference information about `source` blocks nested in a `build` block.

## Description

Add a `source` block to your `build` block to reuse a builder defined elsewhere in the configuration. Packer fills in the source fields that are not already set in the top-level `source` block.

Build-level source blocks are implemented by merging their contents with the
corresponding top-level source block, and a `packer build` command fails when it
encounters the ambiguity that arises when a source parameter is defined twice.

For example, in the below example, if the top-level `lxd.arch` source block
also defined an `output_image` field or if one of the build-level source blocks
redefined and image field, Packer would error.

```hcl
# file: builds.pkr.hcl
source "lxd" "arch" {
  image = "archlinux"
}

build {
  # Use the singular `source` block set specific fields.
  # Note that fields cannot be overwritten, in other words, you cannot
  # set the 'image' field from the top-level source block in here, as well as
  # the 'name' and 'output_image' fields cannot be set in the top-level source block.
  source "lxd.arch" {
    # Setting the name field allows to rename the source only for this build section.
    name = "nomad"
    output_image = "nomad"
  }

  provisioner "shell" {
    inline = [ "echo installing nomad" ]
  }
}

build {
  source "lxd.arch" {
    name = "consul"
    output_image = "consul"
  }

  provisioner "shell" {
    inline = [ "echo installing consul" ]
  }
}
```
