packer-cn/website/source/intro/getting-started/provision.html.markdown

---
layout: "intro"
page_title: "Provision"
prev_url: "/intro/getting-started/build-image.html"
next_url: "/intro/getting-started/parallel-builds.html"
next_title: "Parallel Builds"
---

# Provision

In the previous page of this guide, you created your first image with
Packer. The image you just built, however, was basically just a repackaging
of a previously existing base AMI. The real utility of Packer comes from
being able to install and configure software into the images as well.
This stage is also known as the _provision_ step. Packer fully supports
automated provisioning in order to install software onto the machines prior
to turning them into images.

In this section, we're going to complete our image by installing
Redis on it. This way, the image we end up building actually contains
Redis pre-installed. Although Redis is a small, simple example, this should
give you an idea of what it may be like to install many more packages into
the image.

Historically, pre-baked images have been frowned upon because changing
them has been so tedious and slow. Because Packer is completely automated,
including provisioning, images can be changed quickly and integrated with
modern configuration management tools such as Chef or Puppet.

## Configuring Provisioners

Provisioners are configured as part of the template. We'll use the built-in
shell provisioner that comes with Packer to install Redis. Modify the
`example.json` template we made previously and add the following. We'll
explain the various parts of the new configuration following the code
block below.

<pre class="prettyprint">
{
  "variables": [...],
  "builders": [...],

  "provisioners": [{
    "type": "shell",
    "inline": [
      "sleep 30",
      "sudo apt-get update",
      "sudo apt-get install -y redis-server"
    ]
  }]
}
</pre>

<div class="alert alert-block alert-info">
<strong>Note:</strong> The <code>sleep 30</code> in the example above is
very important. Because Packer is able to detect and SSH into the instance
as soon as SSH is available, Ubuntu actually doesn't get proper amounts
of time to initialize. The sleep makes sure that the OS properly initializes.
</div>

Hopefully it is obvious, but the `builders` section shouldn't actually
contain "...", it should be the contents setup in the previous page
of the getting started guide.

To configure the provisioners, we add a new section `provisioners` to the
template, alongside the `builders` configuration. The provisioners section
is an array of provisioners to run. If multiple provisioners are specified, they
are run in the order given.

By default, each provisioner is run for every builder defined. So if we had
two builders defined in our template, such as both Amazon and DigitalOcean, then
the shell script would run as part of both builds. There are ways to restrict
provisioners to certain builds, but it is outside the scope of this getting
started guide. It is covered in more detail in the complete
[documentation](/docs).

The one provisioner we defined has a type of `shell`. This provisioner
ships with Packer and runs shell scripts on the running machine. In our
case, we specify two inline commands to run in order to install Redis.

## Build

With the provisioner configured, give it a pass once again through
`packer validate` to verify everything is okay, then build it using
`packer build example.json`. The output should look similar to when you
built your first image, except this time there will be a new step where
the provisioning is run.

The output from the provisioner is too verbose to include in this
guide, since it contains all the output from the shell scripts. But you
should see Redis successfully install. After that, Packer once again
turns the machine into an AMI.

If you were to launch this AMI, Redis would be pre-installed. Cool!

This is just a basic example. In a real world use case, you may be provisioning
an image with the entire stack necessary to run your application. Or maybe
just the web stack so that you can have an image for web servers pre-built.
This saves tons of time later as you launch these images since everything
is pre-installed. Additionally, since everything is pre-installed, you
can test the images as they're built and know that when they go into
production, they'll be functional.