Guillotine

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]]().

Features

• lightweight syntax for expressing shell commands, e.g. sh"ls"
• typeclasses define how different types are substituted into commands
• compile-time checking of command syntax, with correct handling of quotes and substitutions
• typeclass-based execution strategies for commands
• shell command results may be interpreted as String, Int, Unit or streamed as LazyList
• processes may be started synchronously or asynchronously
• commands may be piped to other commands with the | operator, or applied like functions

Availability

Getting Started

Commands

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.

Piping

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

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).

Environment

Execution requires an Env 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 Env(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 Env = envs.enclosing

Execution

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:

• await() which waits until the process completes, and returns its result of type T
• abort() which stopes execution, by delegating to Java's Process#destroy
• kill() which stopes execution, by delegating to Java's Process#destroyForcibly
• pid which returns a Pid instance representing the OS-dependent process ID
• stdout and stderr methods for directly accessing the process's output streams; these methods both take an integer parameter limiting the number of bytes that may be read from the stream, defaulting to 10MB
• stdin(in) which accepts a stream of bytes (LazyList[IArray[Byte]]) as standard input to the 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:

• String, which interprets the response using the system encoding and returns a value after the process terminates
• LazyList[String], which provides a stream of lines of text (without the newline character)
• LazyList[IArray[Byte]], which returns a stream of byte arrays
• Unit, to be used when the result is not important
• ExitStatus, an enumeration of Ok or Fail(status) where status is a nonzero positive integer

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.

Status

Guillotine is classified as maturescent. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Guillotine is designed to be small. Its entire source code currently consists of 644 lines of code.

Building

Guillotine will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Guillotine?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Guillotine's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Guillotine and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of guillotine. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Guillotine's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributing

Contributors to Guillotine are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Guillotine easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Author

Guillotine was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Name

Guillotine is named after the Eighteenth Century execution apparatus, as both perform executions.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

Logo

The logo shows the principal parts of a guillotine: the lunette and the blade.

License

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