# Cookbook

{% hint style="warning" %}
We use Gazebo version Fortress in our local experiment but the linked documentation may involve a different version of Gazebo.
{% endhint %}

## How are the model files resolved?

For large world, it's impossible to store all the model definition in the .sdf world file. Instead, models are commonly stored in a subdirectory `models`.  For example, in the aws-robomaker-bookstore-world, we have the following project structure

```
~/workspace/gazebo-projects/aws-robomaker-bookstore-world$ tree -L 1
.
├── CMakeLists.txt
├── CODE_OF_CONDUCT.md
├── CONTRIBUTING.md
├── docs
├── launch
├── LICENSE
├── maps
├── models
├── my_note.txt
├── package.xml
├── README.md
├── routes
└── worlds

```

And the models directory looks like the following:

```
~/workspace/gazebo-projects/aws-robomaker-bookstore-world/models$ tree -L 2 | head -n 15
.
├── aws_robomaker_retail_AirConditionerC_01
│   ├── materials
│   ├── meshes
│   ├── model.config
│   └── model.sdf
├── aws_robomaker_retail_Bicycle_01
│   ├── materials
│   ├── meshes
│   ├── model.config
│   └── model.sdf

```

These models are reference in the world file. For instance:

```xml
<model name="AirConditionerC_01_001">
    <include>
        <uri>model://aws_robomaker_retail_AirConditionerC_01</uri>
    </include>
    <pose frame="">7.458202 1.422453 3.290944 0 -0 -1.564217</pose>
</model>
```

As we can see, the model path is a relative path. Therefore, when launching the Gazebo, we need to set the model path. Depending on the Gazebo version, there may be many environment variables available. According to the [documentation](http://classic.gazebosim.org/tutorials?tut=components\&cat=get_started), we have the following variables:

* GAZEBO\_MODEL\_PATH: colon-separated set of directories where Gazebo will search for models
* GAZEBO\_RESOURCE\_PATH: colon-separated set of directories where Gazebo will search for other resources such as world and media files.
* GAZEBO\_MASTER\_URI: URI of the Gazebo master. This specifies the IP and port where the server will be started and tells the clients where to connect to.
* GAZEBO\_PLUGIN\_PATH: colon-separated set of directories where Gazebo will search for the plugin shared libraries at runtime.
* GAZEBO\_MODEL\_DATABASE\_URI: URI of the online model database where Gazebo will download models from.

For Gazebo Fortress, we need to set the variable IGN\_GAZEBO\_RESOURCE\_PATH to the  `models` directory. This variable is documented on this [page](https://gazebosim.org/api/gazebo/4.0/resources.html), Models, Lights, actors section.

## Where are the Gazebo plugins installed on Ubuntu?

* Check the environment variable `IGN_GAZEBO_SYSTEM_PLUGIN_PATH`
* Check directory similar to /usr/lib/x86\_64-linux-gnu/gazebo-7.0/plugins


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