The Command Line Interface (CLI) provides the UX for FFX. It is responsible for:

  • Parsing user parameters (CLI Parameters)
  • Communicating with the daemon (starting it if necessary)
  • Routing parsed parameters and requested FIDL proxies to the proper code path for execution

Parsing CLI Parameters

FFX uses the Rust crate Argh to manage CLI parameter definitions and parsing. As Argh complies with the Google standard, code is able to be structured into decoupled plugins for FFX. Each subcommand in the FFX call can be represented by a plugin, so for example:

$fx ffx component run /core/ffx-laboratory:hello_world_rust "fuchsia-pkg://"

The part of the command ‘component run’ is a subcommand that routes the code execution to the ‘component run’ plugin. More on this in the routing section.

Communicating with the daemon

FFX uses a daemon to run on the host to facilitate long running tasks. By running these tasks in a daemon in the background, up-to-date data can be supplied to the CLI as soon as it is requested instead of waiting for target devices to respond.

When the CLI needs to communicate with the daemon, it first checks to see if the process is running on the host. If the daemon is not running, the CLI spawns the daemon process and waits for a connection. So it is possible to see longer than average run times on the initial run of FFX.


The CLI is designed as an extensible architecture. In this architecture, plugins provide the code execution. Plugins are defined by a mixture of GN build rules and Rust attributes. Plugins are designed to be as decoupled from the internal workings of FFX as possible. If you are interested in developing a plugin for FFX, please visit the Integrating With FFX page.