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layout: "docs"
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# Custom Command Development

Commands are the components of Packer that add functionality to the
`packer` application. Packer comes with a set of commands out of the
box, such as `build`. Commands are invoked as `packer <COMMAND>`.
Custom commands allow you to add new commands to Packer to perhaps
perform new functionality.

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

Command plugins implement the `packer.Command` interface and are served
using the `plugin.ServeCommand` function. Commands actually have no control
over what keyword invokes the command with the `packer` binary. The keyword
to invoke the command depends on how the plugin is installed and configured
in the core Packer configuration.

<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 command is the `packer.Command`
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 Command interface {
	Help() string
	Run(env Environment, args []string) int
	Synopsis() string
}
</pre>

### The "Help" Method

The `Help` method returns long-form help. This help is most commonly
shown when a command is invoked with the `--help` or `-h` option.
The help should document all the available command line flags, purpose
of the command, etc.

Packer commands generally follow the following format for help, but
it is not required. You're allowed to make the help look like anything
you please.

```
Usage: packer COMMAND [options] ARGS...

  Brief one or two sentence about the function of the command.

Options:

  -foo=bar                  A description of the flag.
  -another                  Another description.
```

### The "Run" Method

`Run` is what is called when the command is actually invoked. It is given
the `packer.Environment`, which has access to almost all components of
the current Packer run, such as UI, builders, other plugins, etc. In addition
to the environment, the remaining command line args are given. These command
line args have already been stripped of the command name, so they can be
passed directly into something like the standard Go `flag` package for
command-line flag parsing.

The return value of `Run` is the exit status for the command. If everything
ran successfully, this should be 0. If any errors occured, it should be any
positive integer.

### The "Synopsis" Method

The `Synopsis` method should return a short single-line description
of what the command does. This is used when `packer` is invoked on its own
in order to show a brief summary of the commands that Packer supports.

The synopsis should be no longer than around 50 characters, since it is
already appearing on a line with other text.
