# The three pillars of the Arduino CLI

The Arduino CLI is an open source Command Line Application written in [Golang] that can be used from a terminal to
compile, verify and upload sketches to Arduino boards and that’s capable of managing all the software and tools needed
in the process. But don’t get fooled by its name: Arduino CLI can do much more than the average console application, as
shown by the [Arduino Pro IDE] and [Arduino Create], which rely on it for similar purposes but each one in a completely
different way from the other. In this article we introduce the three pillars of the Arduino CLI, explaining how we
designed the software so that it can be effectively leveraged under different scenarios.

## The first pillar: command line interface

### Console applications for humans

As you might expect, the first way to use the Arduino CLI is from a terminal and by a human, and user experience plays a
key role here. The UX is under a continuous improvement process as we want the tool to be powerful without being too
complicated. We heavily rely on sub-commands to provide a rich set of different operations logically grouped together,
so that users can easily explore the interface while getting very specific contextual help.

![contextual help screenshot][]

### Console applications for robots

Humans are not the only type of customers we want to support and the Arduino CLI was also designed to be used
programmatically - think about automation pipelines or a [CI][continuous integration]/[CD][continuous deployment]
system. There are some niceties to observe when you write software that’s supposed to be easy to run when unattended and
one in particular is the ability to run without a configuration file. This is possible because every configuration
option you find in the arduino-cli.yaml configuration file can be provided either through a command line flag or by
setting an environment variable. To give an example, the following commands are all equivalent and will fetch the
package index used to work with unstable experimental versions of Arduino's cores:

![configuration methods screenshot][]

See the [configuration documentation] for details about Arduino CLI's configuration system.

Consistent with the previous paragraph, when it comes to providing output the Arduino CLI aims to be user friendly but
also slightly verbose, something that doesn’t play well with robots. This is why we added an option to provide output
that’s easy to parse. For example, the following figure shows what getting the software version in [JSON] format looks
like.

![JSON output screenshot][]

Even if not related to software design, one last feature that’s worth mentioning is the availability of a one-line
[installation script] that can be used to make the latest version of the Arduino CLI available on most systems with an
HTTP client like curl or wget and a shell like bash.

For more information on Arduino CLI's command line interface, see the [command reference].

## The second pillar: gRPC interface

[gRPC] is a high performance [RPC] framework that can efficiently connect client and server applications. The Arduino
CLI can act as a gRPC server (we call it [daemon mode]), exposing a set of procedures that implement the very same set
of features of the command line interface and waiting for clients to connect and use them. To give an idea, the
following is some [Golang] code capable of retrieving the version number of a remote running Arduino CLI server
instance:

![gRPC interface screenshot][]

gRPC is language agnostic: even if the example is written in Golang, the programming language used for the client can be
Python, JavaScript or any of the many [supported ones][grpc supported languages], leading to a variety of possible
scenarios. The new [Arduino Pro IDE] is a good example of how to leverage the daemon mode of the Arduino CLI with a
clean separation of concerns: the Pro IDE knows nothing about how to download a core, compile a sketch or talk to an
Arduino board and it demands all these features of an Arduino CLI instance. Conversely, the Arduino CLI doesn’t even
know that the client that’s connected is the Pro IDE, and neither does it care.

For more information on Arduino CLI's gRPC interface, see the [gRPC interface reference].

## The third pillar: embedding

Arduino CLI is written in [Golang] and the code is organized in a way that makes it easy to use it as a library by
including the modules you need in another Golang application at compile time. Both the first and second pillars rely on
a common Golang API, a set of functions that abstract all the functionalities offered by the Arduino CLI, so that when
we provide a fix or a new feature, they are automatically available to both the command line and gRPC interfaces. The
source modules implementing this API can be imported in other Golang programs to embed a full-fledged Arduino CLI. For
example, this is how some backend services powering [Arduino Create] can compile sketches and manage libraries. Just to
give you a taste of what it means to embed the Arduino CLI, here is how to search for a core using the API:

![Go library interface screenshot][]

Embedding the Arduino CLI is limited to Golang applications and requires a deep knowledge of its internals. For the
average use case, the gRPC interface might be a better alternative. Nevertheless, this remains a valid option that we
use and provide support for.

## Conclusions

You can start playing with the Arduino CLI right away. The code is open source and [the repo][arduino cli repository]
contains [example code showing how to implement a gRPC client][grpc client example]. If you’re curious about how we
designed the low level API, have a look at the [commands package] and don’t hesitate to leave feedback on the [issue
tracker] if you’ve got a use case that doesn’t fit one of the three pillars.

[golang]: https://golang.org/
[arduino pro ide]: https://www.arduino.cc/pro/arduino-pro-ide
[arduino create]: https://create.arduino.cc
[continuous integration]: https://en.wikipedia.org/wiki/Continuous_integration
[continuous deployment]: https://en.wikipedia.org/wiki/Continuous_deployment
[configuration documentation]: configuration.md
[json]: https://www.json.org
[installation script]: installation.md#use-the-install-script
[command reference]: commands/arduino-cli.md
[grpc]: https://grpc.io/
[rpc]: https://en.wikipedia.org/wiki/Remote_procedure_call
[daemon mode]: commands/arduino-cli_daemon.md
[grpc interface reference]: ../rpc/commands
[grpc supported languages]: https://grpc.io/docs/languages/
[arduino cli repository]: https://github.com/arduino/arduino-cli
[grpc client example]: https://github.com/arduino/arduino-cli/blob/master/client_example
[commands package]: https://github.com/arduino/arduino-cli/tree/master/commands
[issue tracker]: https://github.com/arduino/arduino-cli/issues
[contextual help screenshot]: img/CLI_contextual_help_screenshot.png
[configuration methods screenshot]: img/CLI_configuration_methods_screenshot.png
[json output screenshot]: img/CLI_JSON_output_screenshot.png
[grpc interface screenshot]: img/CLI_gRPC_interface_screenshot.png
[go library interface screenshot]: img/CLI_Go_library_interface_screenshot.png