---
description: >
  A source block nested in a build block allows you to use an already defined
  source and to set specific fields which aren't already set in the top-level
  source block.
page_title: source - build - Blocks
---

# The `source` block

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

A `source` block nested in a `build` block allows you to use an already defined
source and to "fill in" those fields _which aren't 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 will fail 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" ]
  }
}
```