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A shell powered by Kotlin
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Kash is a shell written in and powered by Kotlin.

The philosophy of Kash is to provide a minimal layer of compatibility with existing shells and then allow the user to write scripts in Kotlin for anything that requires more logic. This document is split in two parts:

  1. Kash's shell compabitility layer
  2. Kash and Kotlin

Running Kash

Because Kash is still Alpha, releases are not yet available. In order to run it, you will need a JDK version greater or equal to 9 and to clone the repository:

$ git clone
$ cd kash
$ export JAVA_HOME=... # JDK >= 9
$ ./run    # Build and run Kash

Kash as a shell

Kash supports the following features:

  • Redirections (stdout, stdin, stderr):
$ ls > a.txt 2>error.txt
$ cat < a.txt
  • Pipes:
$ ls | wc
11      11     86       
  • Environment variables
$ echo $PATH

Note: how to set environment variables is covered in the Kotlin section

  • Changing directories
$ cd ..
$ cd -    # go back to the previous directory
$ cd ~    # go home
  • Wildcards
$ ls *kts
  • Tab completion
$ ls kash<TAB>
kash         kash-debug

Kash and Kotlin

Kotlin evaluation

When a line is entered and Kash determines it's not a shell command, Kash will evaluate the line as a Kotlin expression:

$ fun h() = "hello"
$ h()

Shell and Kotlin can be embedded within each other with the ` (backtick) character. You can either embed Kotlin in a shell command or embed a shell command within a Kotlin expression:

$ val a=""
$ ls `a`

Predefined functions

Kash favors calling Kotlin functions for as many operations as possible.


$ path()
$ path("/usr/local/bin")
$ path()

Current directory

$ pwd()


The environment is controlled by the function and value kenv (to avoid colliding with env, which is likely to be on your path):

$ kenv("foo", "bar")
$ kenv("foo")
$ echo $foo   # Note that you can access environment variables the shell way too
$ kenv   # display the whole environment


The prompt is changed with the prompt() function. You can pass either a simple string to that function, or a Kotlin expression surrounded by `, in which case, that expression will be evaluated each time:

$ prompt("test$ ")
test$ prompt("`java.util.Date().toString() + '$'`")
Mon Jun 03 21:38:55 PDT 2019$      # <press RETURN> 
Mon Jun 03 21:39:29 PDT 2019$

See the .kash.kts section for a more complex prompt implementation.


A file called ~/.kash.kts in the user's home directory will automatically be read by Kash when starting, allowing users to define their own functions and environment. For example, here is how to create a prompt that contains the current directory in short form followed by the current git branch, if in a Git directory:

// ~/.kash.kts

 * Display the current git branch if in a git directory or an empty string otherwise
fun gitBranch() = File(pwd(), ".git/HEAD").let { head ->
    if (head.exists()) {
        val segments = head.readLines()[0].split("/")
        " [" + segments[segments.size - 1] + "]"
    } else {

 * Display the last two segments of the current directory
fun currentDir() = Paths.get(pwd()).let { path ->
    path.nameCount.let { size ->
        if (size > 2) path.getName(size - 2).toString() + "/" + path.getName(size - 1).toString()
        else path.toString()

fun myPrompt() = currentDir() + gitBranch() + "\u001B[32m$ "


The prompt now looks like this:

kotlin/kash [master]$ git checkout dev
kotlin/kash [dev]$ cd ..


All the functions supported by Kash are defined in the file Predef.kts, which is shipped inside of Kash. You can browse this file yourself to get an idea of what other functionalities are available.

Programming Kash

Kash offers a variety of functionalities to let you program it in Kotlin.

Kash configuration files

Kash can be configured with two different files: ~/.kash.json and ~/.kash.kts.


This file is a JSON file that lets you configure Kash with a few parameters:

  • classPath: An array of strings pointing to the classpath that Kash will be started with.
  • scriptPath: An array of strings pointing to directories where scripts are located.

Here is a sample ~/.kash.json:

    "classPath": [
    "scriptPath": [


This file contains valid Kotlin code and will be run at start up, allowing you to define functions and variables that you need.

Writing scripts with Kash

Kash script files are regular Kotlin Script files but with a few additions. We recommend using the suffix .kash.kts for your Kash files, which will provide additional support in IDEA for these files.

Here is an example Kash script named a.kash.kts:

// a.kash.kts
fun hi(s: String = "Unknown") = println("Hello, $s")

if (args.size > 0) hi(args[0]) else hi()

Assuming you have defined your ~/.kash.json as shown above, save this file as ~/.kash-scripts/a.kash.kts. All you need to do now is just type a in Kash and this code will be executed:

$ a
Hello, Unknown

Since this script file contains some code to parse the args parameter, you can actually pass it parameters:

$ a Cedric
Hello, Cedric

This logic is executed by the following code from the script file:

if (args.size > 0) hi(args[0]) else hi()

Of course, as soon as you load the script, the function hi() gets defined, so you can still invoke it directly if you prefer:

$ hi()
Hello, Unknown
$ hi("Cedric")
Hello, Cedric

Kash additions

In addition to regular .kts files, Kash has a few additional functionalities.

Annotation @file:DependsOn

This annotation allows you to tell Kash that your script depends on extra libraries which are defined by Maven coordinates. Here is a small example that automatically loads the log4j library:

// log.kash.kts
val log = org.apache.log4j.Logger.getRootLogger()

One way to invoke this file:

$ . log.kash.kts

Of course, you can also put this file on your scriptPath and then just invoke it with log.


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