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How to Write a Command-Line Tool with Kotlin Multiplatform

Jean-Michel Fayard πŸ‡«πŸ‡·πŸ‡©πŸ‡ͺπŸ‡¬πŸ‡§πŸ‡ͺπŸ‡ΈπŸ‡¨πŸ‡΄
Weeks of programming can save hours of planning
・6 min read

Being able to write your own command-line tools is a great skill to have: automate all the things!

Unfortunately, writing CLI tools is still most often done with Bash. My strong opinion - weakly held - is that there are only two good kinds of Bash scripts:

  • the ones that are five lines long or less
  • the ones that are written and maintained by others

Fortunately, retiring Bash is pretty simple: you just have to use your favorite programming language instead!

I've played around with Kotlin Multiplatform and not only built a cool CLI tool with it, but I've packaged it as a template project that you can leverage to get started faster!

GitHub logo jmfayard / kotlin-cli-starter

Life is too short for Bash programming

A starter project to build command-line tools in Kotlin Multiplatform

Contains a re-implementation of a real world CLI tool: git-standup

jmfayard_kotlin-cli-starter__Life_is_too_short_for_Bash_programming_and_Telegram_and_GitHub_Desktop

Installation

You can install using one of the options listed below

Source Command
Node npm install -g kotlin-cli-starter
Installer curl -L https://raw.githubusercontent.com/jmfayard/kotlin-cli-starter/main/installer.sh | sudo sh
Tests ./gradlew allTests
Kotlin All Platforms Run ./gradlew allRun
Kotlin JVM Run ./gradlew run
Kotlin Native Run ./gradlew install then $ git standup
Kotlin Node.JS Run ./gradlew jsNodeRun

Why?

Being able to write your own command-line tools is a great skill to have. Automate all the things!

You can write the CLI tools in Kotlin and reap the benefits of using

  • a modern programming language
  • modern IDE support
  • modern practices such as unit testing and continuous integration
  • leverage Kotlin multiplatform libraries
  • run your code on the JVM and benefit from a wealth of Java libraries
  • or build a native executable, which starts very fast and…

git-standup, Kotlin Multiplatform edition

git-standup is a smart little tool for people like me who often panic before the stand-up. What the hell did I do yesterday? That command refreshes my mind by browsing all git repositories and printing the commits that I made yesterday. Try it out, it may well become part of your workflow.

Clone the repo:

git clone https://github.com/jmfayard/kotlin-cli-starter
cd kotlin-cli-starter
./gradlew install
git standup
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./gradlew install will install the tool as a native executable.

There are two more options available:

  • ./gradlew run will run it on the JVM
  • ./gradlew jsNodeRun will run it on Node.js

Same common code, multiple platforms.

git standup

Modern Programming Practices

A traditional Bash script contains

  • a syntax where you can't remember how to do an if/else of a for loop
  • unreadable code due to Code golf : ztf -P -c -D -a
  • no dependency management (apart from an error message: ztf not found)
  • one file that does everything
  • no build system
  • no IDE support (vim should be enough for everybody)
  • no unit tests
  • no CI/CD

By choosing a modern programming language like Kotlin instead, we get to have

  • a modern syntax
  • readable code
  • separation of concerns in multiple functions, files, class and packages
  • a modern build system like Gradle
  • dependency management (here with gradle refreshVersions)
  • a modern IDE like JetBrains IntelliJ
  • unit tests. Here, they are run on all platforms with ./gradlew allRun
  • CI-CD. Here, the unit tests are run on GitHub Actions. See workflow

That may sound obvious, yet people still write their CLI tools in Bash, so it's worth repeating.

Why support multiple platforms?

The three platforms have different characteristics, and by making your code available on all platforms, you can decide which one works better for you:

  • The JVM has a plethora of available libraries, superb tooling support ; it lacks a package manager and is slow to start
  • A native stand-alone executable starts very fast, it can be distributed by Homebrew for example - I make a Homebrew recipe
  • There are also a plethora of libraries on Node.js, and packaging it is easy with npm-publish. My version of git-standup is available at https://www.npmjs.com/package/kotlin-cli-starter and I find it fascinating that I could do so without writing a line of JavaScript.

But mostly it's a good opportunity to learn Kotlin/Multiplatform, a fascinating technology, but one that can also be complex. By writing a CLI tool with Kotlin/Multiplatform, you start with something simple.

What is it like to write Kotlin Multiplatform code?

It feels like regular Kotlin once you have the abstractions you need in place, and all you need is to modify commonMain.

It gets interesting when you have to connect to platform-specific APIs with the expect/actual mechanism.

For example, I needed to check if a file exists:

// commonMain
expect fun fileIsReadable(filePath: String): Boolean
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Implementing this on the JVM is straightforward

// jvmMain
actual fun fileIsReadable(filePath: String): Boolean =
    File(filePath).canRead()
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But how to implement this with Kotlin Native?

Since Kotlin Native gives us access to the underlying C APIs, the question is: how would they do that in C?

Let's google "how to check a file is readable C".

The first answer is about the access system call.

We read the friendly manual with man access:

man_access__Users_jmfayard_and_Untitled

Which leads us directly to this implementation:

// nativeMain
actual fun fileIsReadable(filePath: String): Boolean =
    access(filePath, R_OK ) == 0
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Interesting, strange feeling that we are writing C in Kotlin.

It works, but wait, there is a better solution!

Use an existing Kotlin Multiplatform library

I discovered later that okio has already implemented the multiplatform primitives I needed to work with files.

I was glad I could delete my platform-specific code:

// commonMain
expect val fileSystem: FileSystem

fun fileIsReadable(filePath: String): Boolean =
    fileSystem.exists(filePath.toPath())

fun readAllText(filePath: String): String =
    fileSystem.read(filePath.toPath()) {
        readUtf8()
    }

fun writeAllText(filePath: String, text: String): Unit =
    fileSystem.write(filePath.toPath()) {
        writeUtf8(text)
    }
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See commit

Which multiplatform libraries should I be aware of?

The most common use cases are covered with Kotlin Multiplatform libraries: parsing arguments, JSON, databases, IO, DI, coroutines and testing.

Bookmark those libraries:

I especially liked CliKt for arguments parsing. It has this killer feature that it can generate automatically not only the help message, but also shell completions for Bash, Zsh, and Fish.

The drawback: it takes time to set everything up

Building this project was a lot of fun, and I learned a lot, but it also took lots of time.

What should I do next time if I don't have so much time available?

Go back to Bash?

kotlin-cli-starter : starter project to build CLI tools

I really didn't want to go back to Bash.

This is when I decided to go another route, and transform my experiment in a starter project that I, and everyone, could re-use.

Click Use this template and you get a head start for writing your own CLI tool.

jmfayard_kotlin-cli-starter__Life_is_too_short_for_Bash_programming_and_Telegram_and_GitHub_Desktop

I have put comments starting with CUSTOMIZE_ME in all places you need to customize

Find them with Edit > File > Find in Files

Conclusion

Automate your workflow by using command-line tools. But think twice before using Bash like in the previous century. Kotlin and other modern programming language could well be a far superior solution.

Using Kotlin Multiplatform gives you the strength of each platform, like a wealth of libraries for the JVM, a fast startup for native executables and a package manager (npm) for Node.js.

It is not strictly necessary, but if you want to learn Kotlin Multiplatform, this is a great point to start.

It could take a while to set up from scratch, though.

This is why I transformed my project to a starter project / GitHub template that you can reuse.

GitHub logo jmfayard / kotlin-cli-starter

Life is too short for Bash programming

A starter project to build command-line tools in Kotlin Multiplatform

Contains a re-implementation of a real world CLI tool: git-standup

jmfayard_kotlin-cli-starter__Life_is_too_short_for_Bash_programming_and_Telegram_and_GitHub_Desktop

Installation

You can install using one of the options listed below

Source Command
Node npm install -g kotlin-cli-starter
Installer curl -L https://raw.githubusercontent.com/jmfayard/kotlin-cli-starter/main/installer.sh | sudo sh
Tests ./gradlew allTests
Kotlin All Platforms Run ./gradlew allRun
Kotlin JVM Run ./gradlew run
Kotlin Native Run ./gradlew install then $ git standup
Kotlin Node.JS Run ./gradlew jsNodeRun

Why?

Being able to write your own command-line tools is a great skill to have. Automate all the things!

You can write the CLI tools in Kotlin and reap the benefits of using

  • a modern programming language
  • modern IDE support
  • modern practices such as unit testing and continuous integration
  • leverage Kotlin multiplatform libraries
  • run your code on the JVM and benefit from a wealth of Java libraries
  • or build a native executable, which starts very fast and…

If you build something with the kotlin-cli-starter, I would be glad if you let me know!

Discussion (8)

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alanmasciangelo profile image
Alan Masciangelo

None of these things are strictly true.

no dependency management (apart from an error message: ztf not found)
one file that does everything
no build system
no IDE support (vim should be enough for everybody)
no unit tests
no CI/CD

The biggest issue with writing a CLI in a JVM language is no one wants to run a CLI via a JVM, download Gradle (even with the wrapper), etc which wasn't really addressed here. The selling point of bash is it runs natively.

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jcshartz profile image
jcshartz

I'm confused "biggest issue with writing a CLI in a JVM language" comment. If a dev is already doing Kotlin wouldn't writing CLI tools in a language they are familiar with be useful? When the target is native, you don't need a JVM to run the resultant code.

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alanmasciangelo profile image
Alan Masciangelo

Aren't you fairly limited in the libraries you can use when building native though? Is being restrict to C library wrappers really better than canonic GNU programs?

Thread Thread
jcshartz profile image
jcshartz • Edited

I would say this is a matter of how comfortable one is using bash versus how comfortable one is using Kotlin. As someone who uses Kotlin on the regular, I can visualize the business logic better in that language and the developer tools are more familiar to me. Additionally, if your CLI tool is doing less interaction with GNU tools and more processing and you are very comfortable with Kotlin then Kotlin CLI might be more for you... especially if you don't need to write a lot of CLI tools. Also if you are writing a tool that is targeted to multi-platform then this is exactly what you want to do if Kotlin is your primary language as you would only need to write the business logic once.

But I agree with your statement in general.

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jmfayard profile image
Jean-Michel Fayard πŸ‡«πŸ‡·πŸ‡©πŸ‡ͺπŸ‡¬πŸ‡§πŸ‡ͺπŸ‡ΈπŸ‡¨πŸ‡΄ Author

There are shell scripts who do all of this, and there are flying fishes, but that's not the majority of the species!

Gradle and the JVM are only needed for building the tool.
After, if you use Kotlin/JS, only node is required, and if you use Kotlin/native, nothing is required

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karanpratapsingh profile image
Karan Pratap Singh

this is quite interesting, kotlin seems quite underrated (outside android ecosystem)

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jmfayard profile image
Jean-Michel Fayard πŸ‡«πŸ‡·πŸ‡©πŸ‡ͺπŸ‡¬πŸ‡§πŸ‡ͺπŸ‡ΈπŸ‡¨πŸ‡΄ Author

Thanks a lot Karan!

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mr3ytheprogrammer profile image
M R 3 Y

Thanks for sharing Jean!