There are many features I desire in a CLI, and working in zsh every day makes it hard to give up my fancy autocomplete, history, and fuzzy find. That is probably asking too much of an embedded system anyhow, so we’ll have to make some compromises.

Basic Shell Functionality

Echo - anything you type in is printed back to the screen so you can see what you’re doing.

Backspace - Echo lets you see the mistake but supporting backspace lets you fix it.

Help messages - a quick reference on what is available and how it works

Language

We are developing for embedded systems and mostly target C, so a C (or maybe C++) implementation is preferred. Additionally, a small library, adding only one or two files to the project, is ideal so we’ll look for implementations that are one c source and one header file.

Function Pointers

While being transport layer agnostic is a must-have the implementation of that as passing function pointers to the library module is just a desired feature. There are many ways to allow a developer to provide data to your library but function pointers enforce an agreed upon interface, enforce one entry/exit point for data, and allow the user of the library great flexibility in implementing that function. For that reason, we’ll look for libraries that use function pointers.

Unit Testing

Here at Dojo Five, we’re always trying to do more and better unit testing, so finding a CLI that has already implemented its own unit tests would save us the time of starting from scratch and give some confidence in the stability of the library.

Static Allocation

Generally, a best practice in an embedded system is to avoid dynamic memory allocation, so we’ll look for libraries that use static memory allocation.

Minimal Files

Some people can do ingenious (and crazy) things with a single header file, but no CLIs that I found were header-only implementations; but that’s ok, a two file implementation is just as good. Two files simplify inclusion into projects.

Built-in Help

Treating help information as an integral part of the CLI makes it easier for those unfamiliar with the system to start interacting with it. Native support for help messages and printing available functions is a plus.

Runtime Command Registration

Ideally, your CLI implementation wouldn’t need to include every header file to gain access to the functions to call, but with compile-time command registration, this is often required. Runtime command registration is where the CLI is made aware of each command at runtime via function calls to set up the command. This enables individual modules to add commands to the CLI during their own initialization by calling the appropriate function from the CLI library. Instead of the CLI including all header files, each module that wants to add commands to the CLI includes the CLI header. This also allows runtime reconfigurable CLIs for more advanced use cases.

Say you have an advanced or manufacturing mode for the CLI – you can dynamically add those functions at runtime in response to a variety of inputs. Perhaps pogo pins connect a net to ground or maybe a password is required to add additional functionality. CLI libraries with runtime command registration make this feature possible without rebuilding the code and having extensive compile time #IFDEFS around which commands are available.

Nice to Have