Building a CLI for Firmware Projects using Invoke

Building a small (or large) command line interface (CLI) for a project is a great way to get an entire team to build, test, debug, and work with a project in the same way using the same set of tools. This post goes into detail about how to think about a project’s CLI and implementing one using the Invoke Python package.

Table of Contents

A Project CLI?

Every project should have a single way for any developer, script, continuous integration system, or QA technician, to perform the most basic tasks. Whether this is building the project or documentation, running tests, or flashing a device, there should be only one easy way to do it.

For example, instead of having to remember that JLinkGDBServer must be launched in the following manner:

$ JLinkGDBServer -device Cortex-M4 -if SWD -speed 4000 -RTOSPlugin_FreeRTOS

I can make it:

$ invoke jlink

Similarly, flashing our firmware through GDB can go from:

$ cd products/blinky/gcc
$ arm-none-eabi-gdb-py -x gdbinit.jlink ../build/product.elf -x ../../common_gdb_scripts.gdbinit

to:

$ invoke flash

A web service’s REST API needs to be stable, easy to use, and self documenting. Your project’s CLI should meet the same requirements.

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When to Build a Project CLI

If you or your teammates have experienced any of the following more than once, it may be time to write a CLI for your project.

  • You copy/paste commands from a Wiki page or an old Slack message
  • You found the command you were looking for but it’s now out-of-date
  • It has become common for teammates to ask in Slack “What is that command…“
  • Scripts have undocumented arguments and environment variable overrides
  • Two or more teammates have written similar scripts for themselves but a common shared version doesn’t exist, or is broken.
  • You check out a 3 month-old revision of your project and there is no longer documentation about about this ancient version (factory builds anyone?).

I could go on and on, but I hope the idea is clear. Building a CLI using Invoke helps with all of the above issues and more.

Why Invoke and Python

My favorite tool to build a CLI for my firmware projects is Invoke. Some reasons to use Invoke as your CLI language are:

  • Configuration management through use of environment variables, arguments, and config files 1
  • Automatic help menus and argument parsing with validation 2
  • Tasks can be run from any directory under the project root
  • Invoke and Python can be more easily used in a cross-platform environment
  • Easier debugging using the Python debugger and more flexible printing
  • Python is more widely known and understood among developers
  • Python gives access to a wide variety of packages to use

Why not Make

Since many firmware projects use Make as a build system, engineers tend to reach for it when they need additional CLI commands. Make is great at dependency checking and simple recipes, but it should not be used as a CLI. Let’s go over a few reasons why.

Make does not handle arguments well

In the nRF5 SDK’s Makefile, this is the recipe for flash.

# Flash the program
flash: default
    @echo Flashing: $(OUTPUT_DIRECTORY)/nrf52840_xxaa.hex
    nrfjprog -f nrf52 --program $(OUTPUT_DIRECTORY)/nrf52840_xxaa.hex --sectorerase
    nrfjprog -f nrf52 --reset

This uses the nrfjprog to flash and reset the device with the hard coded .hex file. This is great if that’s all we wanted to do, but in the future…

  • What if we want to flash a different .hex file?
  • What if we have two devices plugged in and want to select a certain one?
  • What if we allowed the developer to choose between flashing with GDB or nrfjprog?

As you can see, this would require a few global variables that can be overridden using make flash ELF=<path/to/hex> METHOD=gdb, and a developer would never know about these unless a Wiki page is written and kept up-to-date or the source code is read frequently.

Makes does not come with batteries included

The lack of argument parsing in Make is just the start. CLI commands which use Make will depend on system packages to do the heavy lifting.

  • Want to interact with a web API? You need to have cURL installed.
  • Want to parse a text or configuration file? You’ll need awk or sed.
  • Want to make the CLI cross platform? You’ll need to handle differences between Windows, Mac and Linux packages and their shell quirks.

The list goes on.

Make is hard to debug

Debugging a CLI is mostly about debugging the variables that get passed down into the shell commands or into the later Python scripts that get called.

To debug an Invoke task, there are three common methods:

  1. Printing out the ctx object to get the state and variables
  2. Invoke with the --echo flag, which prints out the exact command run within ctx.run().
  3. If all else fails, a developer can drop into Python’s built in debugger pdb3
@task
def env(ctx):
    """Print out the `ctx` variable"""
    # Print out the Context, which contains most information you need
    print(ctx)

    # Print out the exact invocation
    ctx.run('make', echo=True)

    # Set a breakpoint to get a debugger
    import pdb
    pdb.set_trace()

To debug a Make task, there is really only one easy way, and that is using $(info ...) statements.

env:
    $(info $(ARG_ELF_FILE))
    # ...

Make commands aren’t documented

When working in a team environment, documentation is critical. When using Make, the Makefile and other source code is all the documentation that is provided.

With Invoke, documentation is built in. We can print all commands available as well as drill down into each to find out more information. This makes it easy for developers to discover new commands and find what they are looking for.

$ inv --list
Available tasks:

  build       Build the project
  console     Start an RTT console session
  debug       Spawn GDB and attach to the device without flashing
  flash       Spawn GDB and flash application & softdevice
  gdbserver   Start JLinkGDBServer
$ inv flash --help
Usage: inv[oke] [--core-opts] flash [--options] [other tasks here ...]

Docstring:
  Spawn GDB and flash application & softdevice

  Examples:
      # Flash the default ELF file over JLink
      $ inv flash

      # Flash a given binary over a JLink on the given port
      $ inv flash --elf /path/to/file.elf --port 12345

Options:
  -e STRING, --elf=STRING   The elf to flash
  -p INT, --port=INT        The GDB port to attach to

Getting Started with Invoke

Let’s move onto our example CLI using Invoke to interface with the Nordic nRF5 SDK “Blinky” project.

Installing Invoke

I highly recommend using a virtual environment4, which is a way to sandbox your python environment for your project. This guide is a good starting point. In this example, I will be using Python 3.6, which has the virtualenv command available already.

$ virtualenv venv
$ source venv/bin/activate
(venv)
$ pip install invoke

We can quickly test if it was properly installed by running invoke in our shell.

(venv)
$ invoke --list
Can't find any collection named 'tasks'!

We’ve confirmed it’s installed. To start adding tasks to Invoke, one needs to add a tasks.py file to the root of a project. No matter where the user is within the project, Invoke will search upwards until it finds a tasks.py file.

Minimal Invoke script

For purposes of this example, we are going to use the Nordic nRF5 SDK version 15.2.0, which can be downloaded here, and within that, the Blinky app located at nrf5_sdk/examples/peripheral/blinky/pca10056/blank/armgcc/.

The following is a simple tasks.py we have written to build the Blinky project.

# /nrf5_sdk/tasks.py

import os

from invoke import Collection, task

ROOT_DIR = os.path.dirname(__file__)
BLINKY_DIR = os.path.join("nrf5_sdk", "examples", "peripheral",
                          "blinky", "pca10056", "blank", "armgcc")

@task
def build(ctx):
    """Build the project"""
    with ctx.cd(PCA10056_BLINKY_ARMGCC_DIR):
        ctx.run("make")

This sets up the proper paths, cd’s into the directory, and runs make.

$ inv --echo build
cd /Users/tyler/dev/memfault/interrupt/example/invoke-basic/nrf5_sdk/examples/peripheral/blinky/pca10056/blank/armgcc && make
mkdir _build
cd _build && mkdir nrf52840_xxaa
Assembling file: gcc_startup_nrf52840.S
Compiling file: main.c
...

Some of you might immediately think this wrapper or abstraction is unnecessary, and maybe it is in the simplest case, but let’s dive a bit into the inevitable future of this build command.

Iterating on our CLI

Adding Parallel Builds

A few weeks pass on the project and our build time slips from a few seconds to 30 seconds. We now want everyone working on the project to use Make’s -j option to enable parallel builds. To do so using our Invoke command, we’d only have to add -j4 to our build command.

@task
def build(ctx):
    """Build the project"""
    with ctx.cd(PCA10056_BLINKY_ARMGCC_DIR):
        ctx.run("make build -j4")

Now everyone running invoke build will automatically use parallel make.

Detecting and using ccache

A few more weeks, and our build time slowed down again. This time, we’ve thought to enable ccache, and we want every developer to use this. It appears that the nRF5 SDK somewhat enables this feature, but it depends on the existence of a /usr/bin/ccache binary. On my macOS machine, ccache is installed to the path /usr/local/bin/ccache, so we’d rather have this variable be automatically set!

We can change the SDK from := to ?= to allow the CCACHE variable to be overridden by our environment, and now we can define this variable ourselves from our Invoke task!

# ⁨nRF5_SDK_15.2.0/components⁩/toolchain⁩/gcc⁩/Makefile.common

CCACHE ?= $(if $(filter Windows%,$(OS)),, \
               $(if $(wildcard /usr/bin/ccache),ccache))
from shutil import which

@task
def build(ctx, ccache=True):
    """Build the project"""
    env = {}
    if ccache:
      env = {'CCACHE': which(ccache)}

    with ctx.cd(PCA10056_BLINKY_ARMGCC_DIR):
        ctx.run("make build -j4", env=env)

A few things are happening above now.

  1. We’ve imported the which Python function to help us find where a binary in our $PATH is located.
  2. We create an env variable and pass that to our Invoke task. Invoke will take the variables defined and export them into the environment while running that task.
  3. We’ve added a ccache argument, which our CI system can use to disable ccache in CI since we want to guarantee a fresh build. Our CI system would use this as invoke build --no-ccache

What’s neat about everything here is that even though developers keep running invoke build, new functionality gets added without them knowing.

Checking environment for ccache

A few more weeks later, we realize some developers are not using ccache because it isn’t installed on their machines. One could go around from computer to computer checking that each developer installs it and keeps it installed, but an Invoke ‘pre’ task is a better option.

@task
def check_ccache(ctx):
    ccache = which(ccache)
    if not ccache:
        msg = (
            "Couldn't find `ccache`.\n"
            "Please install it using the following command:\n\n"
            ">  `brew install ccache` OR `apt install ccache`"
        )
        raise Exit(msg)


@task(pre=[check_ccache])
def build(ctx, ccache=True):
    """Build the project"""
    ...

Using these check_ style routines along with @task(pre=[...]) is an easy way to ensure that a developer’s local environment is set up and remains set up correctly even as project requirements and versions change. I would argue it’s very much required when a project upgrades gcc versions!

A full Invoke example for Blinky

Here, I provide a more production ready example for the Blinky app to give a better idea of how Invoke should be used for a project. The contents here can be placed in a tasks.py file in the project root and Invoke will automatically pick up the tasks!

This source code can be found on Github

import os, shutil, sys
from invoke import Collection, Config, Exit, task
from shutil import which

# Constants
ROOT_DIR = os.path.dirname(__file__)
NRF_SDK_DIR = os.path.join(ROOT_DIR, "nrf5_sdk")
BLINKY_DIR = os.path.join(NRF_SDK_DIR, "examples", "peripheral", "blinky",
                          "pca10056", "blank", "armgcc")
BLINKY_ELF = os.path.join(BLINKY_DIR, "_build", "nrf52840_xxaa.out")

GDB_EXE = "arm-none-eabi-gdb-py"
GCC_EXE = "arm-none-eabi-gcc"

JLINK_GDB_PORT = 2331
JLINK_TELNET_PORT = 19021


def _check_exe(exe, instructions_url):
    exe_path = which(exe)
    if not exe_path:
        msg = (
            "Couldn't find `{}`.\n"
            "This tool can be found here:\n\n"
            ">  {}".format(exe, instructions_url)
        )
        raise Exit(msg)

@task
def check_toolchain(ctx):
    """Run as a `pre` task to check for the presence of the ARM toolchain"""
    url = "https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads"
    _check_exe(GCC_EXE, url)
    _check_exe(GDB_EXE, url)


@task
def check_segger_tools(ctx):
    """Run as a `pre` task to check for the presence of the Segger tools"""
    url = "https://www.segger.com/downloads/jlink/#J-LinkSoftwareAndDocumentationPack"
    _check_exe("JLinkGDBServer", url)


@task(pre=[check_toolchain])
def build(ctx, ccache=True):
    """Build the project"""
    with ctx.cd(BLINKY_DIR):
        # To get nrf52 SDK to pick up GCC in our $PATH
        env = {"GNU_INSTALL_ROOT": ""}
        if ccache:
            env.update({"CCACHE": which("ccache")})
        ctx.run("make -j4", env=env)


@task(pre=[check_toolchain], help={
    "elf": "The elf to flash",
    "port": "The GDB port to attach to"
})
def flash(ctx, elf=BLINKY_ELF, port=JLINK_GDB_PORT):
    """Spawn GDB and flash application & softdevice

    Examples:
        # Flash the default ELF file over JLink
        $ invoke flash

        # Flash a given binary over a JLink on the given port
        $ invoke flash --elf /path/to/file.elf --port 12345

    """
    cmd = f'{GDB_EXE} --eval-command="target remote localhost:{JLINK_GDB_PORT}"' \
          f' --ex="mon reset" --ex="load" --ex="mon reset" --se={BLINKY_ELF}'
    ctx.run(cmd)


@task(pre=[check_toolchain], help={
        "elf": "The ELF file to pull symbols from",
        "port": "The GDB port to attach to"
})
def debug(ctx, elf=BLINKY_ELF, port=JLINK_GDB_PORT):
    """Spawn GDB and attach to the device without flashing"""
    cmd = f'{GDB_EXE} --eval-command="target remote localhost:{JLINK_GDB_PORT}"' \
          f' --se={BLINKY_ELF}'
    ctx.run(cmd)


@task(pre=[check_segger_tools], help={
    "gdb": "The GDB port to publish to",
    "telnet": "The Telnet port to publish to"
})
def gdbserver(ctx, gdb=JLINK_GDB_PORT, telnet=JLINK_TELNET_PORT):
    """Start JLinkGDBServer"""
    ctx.run(f"JLinkGDBServer -if swd -device nRF52840_xxAA -speed auto "
            f"-port {gdb} -RTTTelnetPort {telnet}")


@task(pre=[check_segger_tools])
def console(ctx, telnet=JLINK_TELNET_PORT):
    """Start an RTT console session"""
    ctx.run(f"JLinkRTTClient -LocalEcho Off -RTTTelnetPort {telnet}")


# Add all tasks to the namespace
ns = Collection(build, console, debug, flash, gdbserver)
# Configure every task to act as a shell command
#   (will print colors, allow interactive CLI)
# Add our extra configuration file for the project
config = Config(defaults={"run": {"pty": True}})
ns.configure(config)

Given the full (but incredibly minimal) example above, we have the following tasks and features:

  • We can run:
    • invoke build to build the project with ccache and parallelized
    • invoke console will connect to the device’s serial console
    • invoke gdbserver will spawn JLinkGDBServer with the correct configuration
    • invoke flash will flash the binary through GDB and give us a prompt to the device
    • invoke debug will attach to a running device using GDB without flashing
  • For each command, we will check that the proper binaries/packages are installed using pre tasks
  • We can run inv --list and inv <command> --help for help menus.

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Final Thoughts

I’ve thoroughly enjoyed using Invoke in my previous and current job. In Memfault’s codebase, we have around 100 tasks, most of which are namespaced under general top-level modules. This provides our team with a centralized place for all of the common tasks, such as building the firmware SDK, publishing documentation, running automated tests on our devices, pushing new versions of our service, performing database migrations…everything. The self-documenting nature of the commands is indispensable.

All the code used in this blog post is available on Github. See anything you’d like to change? Submit a pull request!

Tips & Further Reading

  • I encourage the exploration of the Invoke documentation to learn more about all the features.
  • For a production example of Invoke tasks, look no further than the Invoke source code. It provides a good example of how to import tasks from other modules to create a centralized tasks list.
  • I like to use a combination of Invoke and Click for better pretty printing and colors.
  • Invoke also supports tab-completion support.
Tyler Hoffman has worked on the embedded software teams at Pebble and Fitbit. He is now a founder at Memfault.