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layout: "docs"
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# Custom Provisioner Development

Provisioners are the components of Packer that install and configure
software into a running machine prior to turning that machine into an
image. An example of a provisioner is the [shell provisioner](/docs/provisioners/shell.html),
which runs shell scripts within the machines.

Prior to reading this page, it is assumed you have read the page on
[plugin development basics](/docs/extend/developing-plugins.html).

Provisioner plugins implement the `packer.Provisioner` interface and
are served using the `plugin.ServeProvisioner` function.

<div class="alert alert-block">
  <strong>Warning!</strong> This is an advanced topic. If you're new to Packer,
  we recommend getting a bit more comfortable before you dive into writing
  plugins.
</div>


## The Interface

The interface that must be implemented for a provisioner is the
`packer.Provisioner` interface. It is reproduced below for easy reference.
The actual interface in the source code contains some basic documentation as well explaining
what each method should do.


<pre class="prettyprint">
type Provisioner interface {
	Prepare(...interface{}) error
	Provision(Ui, Communicator) error
}
</pre>

### The "Prepare" Method

The `Prepare` method for each provisioner is called prior to any runs with
the configuration that was given in the template. This is passed in as
an array of `interface{}` types, but is generally `map[string]interface{}`. The prepare
method is responsible for translating this configuration into an internal
structure, validating it, and returning any errors.

For multiple parameters, they should be merged together into the final
configuration, with later parameters overwriting any previous configuration.
The exact semantics of the merge are left to the builder author.

For decoding the `interface{}` into a meaningful structure, the
[mapstructure](https://github.com/mitchellh/mapstructure) library is recommended.
Mapstructure will take an `interface{}` and decode it into an arbitrarily
complex struct. If there are any errors, it generates very human friendly
errors that can be returned directly from the prepare method.

While it is not actively enforced, **no side effects** should occur from
running the `Prepare` method. Specifically, don't create files, don't launch
virtual machines, etc. Prepare's purpose is solely to configure the builder
and validate the configuration.

The `Prepare` method is called very early in the build process so that
errors may be displayed to the user before anything actually happens.

### The "Provision" Method

The `Provision` method is called when a machine is running and ready
to be provisioned. The provisioner should do its real work here.

The method takes two parameters: a `packer.Ui` and a `packer.Communicator`.
The UI can be used to communicate with the user what is going on. The
communicator is used to communicate with the running machine, and is
guaranteed to be connected at this point.

The provision method should not return until provisioning is complete.

## Using the Communicator

The `packer.Communicator` parameter and interface is used to communicate
with running machine. The machine may be local (in a virtual machine or
container of some sort) or it may be remote (in a cloud). The communicator
interface abstracts this away so that communication is the same overall.

The documentation around the [code itself](https://github.com/mitchellh/packer/blob/master/packer/communicator.go)
is really great as an overview of how to use the interface. You should begin
by reading this. Once you have read it, you can see some example usage below:

<pre class="prettyprint">
// Build the remote command.
var cmd packer.RemoteCmd
cmd.Command = "echo foo"

// We care about stdout, so lets collect that into a buffer. Since
// we don't set stderr, that will just be discarded.
var stdout bytes.Buffer
cmd.Stdout = &stdout

// Start the command
if err := comm.Start(&cmd); err != nil {
  panic(err)
}

// Wait for it to complete
cmd.Wait()

// Read the stdout!
fmt.Printf("Command output: %s", stdout.String())
</pre>
