A small library for executing stuff


Guillotine makes it easy to work with shell processes in Scala, with simple interpolation-based definitions of commands, and type-based interpretation of their output. For example, a directory listing may be obtained with sh"ls $path".exec[List[String]]().



Guillotine is available as a binary for Scala 3.5.0-RC1 and later, from Maven Central. To include it in an sbt build, use the coordinates:

libraryDependencies += "dev.soundness" % "guillotine-core" % "0.1.0"

Getting Started


Shell commands are created using the sh"" interpolator, which will interpret (at compiletime) a command and its arguments, correctly interpreting single- and double-quoted arguments and escaped characters. Unclosed quotes will result in a compile error.

Substitutions of values of a variety of different types may be made into an sh interpolator, and may be read as either "single-argument" (such as String or Int) or "multi-argument" (such as List[Int] or Set[String]).

Multi-argument substitutions will be interpreted as multiple arguments to the shell command unless they are enclosed within quotes, in which case they will be interpreted as a space-separated string.

Substitutions should normally be surrounded by spaces, otherwise they will be prepended or appended to adjacent arguments.


Two commands may be combined using the pipe operator (|), for example,

sh"cat /home/work/file" | sh"grep $query" | sh"wc -l"

which is equivalent to the single shell command, cat /home/work/file | grep $query | wc -l, with the appropriate substitution of query being made.

While this expression will seem very familiar from a shell-command perspective, it may also be written in function application style as,

sh"wc -l"(sh"grep $query"(sh"cat /home/work/file"))

and the two versions are equivalent.


Substitutions of a variety of different types may be made into an interpolated sh command. Any type for which a gossamer.Show typeclass exists will be inserted as a single parameter, and any sequence of one of these types will be inserted as multiple arguments. A Command instance may also be substituted into another, for example,

val echo = sh"echo Hello World"
sh"sh -c '$echo'"

where the quotes are required around '$echo' so that the command is passed to sh -c as a single argument, rather than multiple arguments (of which only the first would be used).


Execution requires an Environment instance specifying a map of environment variables and a working directory as a String, and should be specified as a contextual value, for example,

given Environment(Map("PATH" -> "/usr/bin:/usr/sbin"), "/home/work")

however it is common to use the enclosing environment. That is, to pass the environment in which the JVM was started to its subprocess, ensuring that processes started by Guillotine behave as they would if started directly from the shell. There may, however, be security implications when doing this, so it must be explicitly enabled with:

given Environment = environments.enclosing


Two methods are provided for starting execution of a process: fork and exec, both taking a type parameter which determines the type of the return value, and may also affect how execution is handled.

val result: String = sh"echo Hello World".exec[String]()

The exec method will return a value synchronously, when that value is ready. This may happen only when the process completes execution, if the entire output is caputured, for example if the return-type is String, or may happen earlier if a streaming return type, such as LazyList[String], is specified.

The fork method always starts the process asynchronously, and returns an instance of Process[T], where T is the specified return type.

val process: Process[String] = sh"locate lostfile".fork[String]()

Process implements a few useful methods for working with a running process:

The synchronous exec[T]() method is always equivalent to fork[T]().await().

Result interpretation

Different shell processes may behave differently in how their results should be interpreted. Those differences include the interpretation of the exit status—where different nonzero codes may be interpreted as different types of failure—and which stream contains the important output, STDOUT or STDERR.

How these differences are interpreted is determined by the choice of return type: nonzero return types may be presented as thrown exceptions, or interpreted as a different sort of "success". Furthermore, the return type will determine whether the result may be return before the shell process terminates, or whether (in the case of a streaming response) it may be returned earlier.

The Executor[T] typeclass provides support for producing different return types. Executors for the following types are provided:

Custom executors may be provided by implementing the Executor trait with the single abstract method,

def interpret(process: java.lang.Process): T

or by mapping across an existing Executor, for example,

given Executor[Int] = summon[Executor[String]].map(_.toInt)

since all Executors are functors.


Guillotine is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.