#Rust2018 and the Great CLI Awakening

This is a response to the #Rust2018 call for blog posts with a little bit of my experience and how I see the 2018 year moving forward.

The Experience of Writing a CLI in Rust

spoiler: simultaniously refreshing and painful

Rust is a fantastic language for writing a Command Line Application (CLI). For the ergonomics of hacking, it has one of the best argument parsers ever, has seriously the best serialization library ever and it compiles to almost any target and goes fast when it runs.

For testing and robustness, its type system prevents you from making stupid errors, hooking into fuzz-testing is trivial, and there are better and better libraries for testing CLI applications. It is simply a fantastic language and will be my goto for writing CLIs from now on (and I come from python!).

However, there are some serious difficulties. The std library is small, and while adding functionality from crates with cargo-edit is a simple cargo add away, there are so many crates that make up a typical CLI, each one requring you to add extern crate foo to your main.rs and then have to write use foo at the top of every file you want to use it.

Probably the most significant paper cut is that rust’s error handling semantics of Result<T, E> don’t work well for the CLI use case. In most cases a CLI works like a compiler: I want to show the user all the things that are wrong, not fail on the first error I find! There needs to be a standard crate for handling a “stream of errors and warnings”, probably via a channel, which I can query “were there errors in the last function?”.

Filling the Gaps

I have a strong desire to fill the gaps that exist in the ecosystem today. I am currently doing a complete rewrite of my command line and web application artifact, and am using that use case to “fill the holes” that I see.

Here are some of the things I am working on to fill the gaps. I would like help from the community as well.

Gap 1: Working with Paths

I found working directly with paths to be extremely annoying. Does my path exist? Is it equal to this other path? Is my path a file or a directory and why can’t a type tell me? Why is creating/writing/appending a string to my path not possible in a single line of code?

To solve this I wrote the crate path_abs. It exports types which guarantee that all paths exist and are absolute. Even better, it has PathFile and PathDir types with commonly used methods attached to them.

Oh and did I mention that you can serialize the paths with serde? That was one of my biggest pain points – if I have a path I can’t stick it in a struct which I want to send anywhere. This library solves that and more.

This was one of my biggest pain points developing a CLI in rust, and it was filled (by me…) in early 2018 – so the year is already looking good for CLI ergonomics!

Gap 2: writing tests

Rust makes it extremely simple to write unit tests, but once you want to do a more end-to-end test there isn’t a lot out there. Unfortunately, testing a CLI application requires end to end tests to be confident you are giving the user the experience you expected.

If all your text is plain (not colored or formatted) then assert_cli is the crate for you. However, the second you put style or color – or even a table – in your application it becomes a tedious affair of editing a string of raw bytes every time you make a minor change to your application.

I’m currently working on a CLI testing framework built in rust, using the new crate I wrote called termstyle to aid in being able to easily express your styles (when you write your app) and then make it easy to test them as well. The goal is to be able to write your CLI tests in a simple YAML file and get clear diffs against the expected vs result. One of the benefits: while you are writing your tests there is no compilation time if you are not touching your source code – and running the tests takes almost no time at all!

Shoutout to pretty_assertions for making reading diffs in regular rust tests SO much easier. This is a life saver for CLI applications!

Also shoutout to the "{:#?}" pretty printing formatter. When I discovered it I felt like the largest pain point of rust just vanished.

Gap 3: working with errors

This one is a little bit more difficult and I am currently trying out solutions in my rewrite of artifact.

The basic idea is this:

  • Instead of returning errors, I get passed a Sender object where I push errors/warnings/info to that may or may not end up getting displayed to the user. (note: I use a sender so that multithreading is trivial)
  • If I don’t get any useful work done, I return None, but there might have been several errors that contributed to that instead of only one.
  • The function above me checks for errors and can choose to exit early. At the highest levels, the errors are sorted and displayed to the user.

What I would like to do is leverage something like slog for this purpose. slog has a similar design: it wants you to pass a “log sender” into every function/struct and log to that. What I need is a way to query what logs have been passed (in particular whether they are errors). More investigation is necessary, but I (personally) found slog very difficult to get running. Maybe simply improving slog’s ergonomics could fill this gap and simultaniously provide built-in logging?

Gap 4: bringing it all together

The fourth major gap is the ergonomics of actually using the ecosystem. It is often stated that rust’s ecosystem is immature. While this is somewhat true, the real issue is in finding and using the pieces you need.

My final major project this year (along with artifact itself) is to flush out the stdcli crate. The goal (unlike the similar crate stdx) is to have a specific use case in mind – which is to create a Command Line SDK.

I have made significant progress already, and will flush out the first release as I rewrite artifact, but the primary goal is to reduce the number of use foo and external crate foo statements that have to be made (especially regarding traits), by providing a standard prelude for developing CLI applications. The idea is that developing a CLI using stdcli should almost feel like a completely different language. Everything you need should be directly within reach, and the only crates you need to add should be application specific.

Unlike stdx, I plan on using the >=X.Y.Z syntax for all dependencies. I then plan on setting up something similar to Crater. If your application depends on stdcli and you have a solid set of tests then you can have your project run automated tests with it’s dependencies continuously updated. This will give you a feeling of security that you can stay on the cutting edge, because stdcli will be detecting any regressions and opening bugs with the relevant libraries.

Gap 5 (stretch): Distributing Your Application

This one is a bit of a stretch, but I would like to have a simple way to distribute applications written in rust that doesn’t require me to maintain a build systems on Travis and Appveyor for 3 platforms. I am seriously looking at the Nix project as the savior to all my woes. I don’t have much more to say except that the rust team’s goal of making it easier to integrate cargo into build system is much appreciated.


I think CLI applications are (perhaps surprisingly) one of rust’s strong suites, and it needs only a few relatively minor tweaks to make it the best language for developing CLIs bar none.

The introduction of webassembly and being able to write client-side web applications will only make this case stronger. Imagine being able to expose a CLI application that can launch a static (or dynamic+backend) webpage whenever that makes sense. Imagine ps (for monitoring processes), but you could do ps serve and it would pop open a dynamically updating processor status in your browser, with complex searching and graphing functionality built in.

Rust will push this frontier. We are going to see more and more hybrid CLI/Web applications being built every day and rust could be the language of choice. I think it will be, and I think #Rust2018 will be what turns the tide.