# Using the devicefarm-cli tool in custom test environments
<a name="custom-test-environments-hosts-software-cli"></a>

The test host uses a standardized version management tool called ` devicefarm-cli` to select software versions. This tool is separate from the AWS CLI and only available on the Device Farm test host. With `devicefarm-cli`, you can switch to any pre-installed software version on the test host. This provides a straightforward way to maintain your Device Farm test spec file over time and gives you a predictable mechanism to upgrade software versions in the future.

**Important**  
 This command line tool is not available on legacy iOS hosts. For more information, see the topic on the [Legacy iOS test host](custom-test-environments-hosts-ios.md#legacy-ios-host). 

The snippet below shows the `help` page of `devicefarm-cli`:

```
$ devicefarm-cli help
 Usage: devicefarm-cli COMMAND [ARGS]
     
     Commands:
         help                         Prints this usage message.
         list                         Lists all versions of software configurable
                                      via this CLI.
         use <software> <version>     Configures the software for usage within the
                                      current shell's environment.
```

Let's review a couple of examples using `devicefarm-cli`. To use the tool to change the Python version from *3.10* to *3.9* in your test spec file, run the following commands:

```
$ python --version
Python 3.10.12
$ devicefarm-cli use python 3.9
$ python --version
Python 3.9.17
```

To change the Appium version from *1* to *2* :

```
$ appium --version
1.22.3
$ devicefarm-cli use appium 2
$ appium --version
2.1.2
```

**Tip**  
Note that when you select a software version, `devicefarm-cli` also switches the supporting tools for those languages, such as `pip` for Python and `npm` for NodeJS. 

For more information about the preinstalled software on the test host, see [Supported software within custom test environments](custom-test-environments-hosts-software.md).