A Swift framework for shell scripting.
Swift Shell


Build Status Platforms Carthage compatible

Swift 3 | Swift 2


A library for creating command-line applications and running shell commands in Swift. It lets you:

  • run shell commands
  • run them asynchronously, and be notified when output is available
  • access the context in which your application is running, like environment variables, standard input, standard output, standard error, the current directory and the command line arguments
  • create new such contexts you can run commands in
  • handle errors
  • read and write files

See also


Print line numbers

import SwiftShell

do {
    // If there is an argument, try opening it as a file. Otherwise use standard input.
    let input = try main.arguments.first.map {try open($0)} ?? main.stdin

        .enumerated().forEach { (linenr,line) in print(linenr+1, ":", line) }

    // Add a newline at the end
} catch {

Launched with e.g. cat long.txt | print_linenumbers.swift or print_linenumbers.swift long.txt this will print the line number at the beginning of each line.

Run commands

Print output

try runAndPrint(bash: "cmd1 arg | cmd2 arg") 

Run a shell command just like you would in the terminal. The name may seem a bit cumbersome, but it explains exactly what it does. SwiftShell never prints anything without explicitly being told to.


let date: String = run("date", "-u")
print("Today's date in UTC is " + date)

Similar to $(cmd) in bash, this just returns the output from the command as a string, ignoring any errors.


let command = runAsync("cmd", "-n", 245)
// do something with command.stderror or command.stdout
try command.finish()

Launch a command and continue before it's finished. You can process standard output and standard error, and optionally wait until it's finished and handle any errors.

If you read all of command.stderror or command.stdout it will automatically wait for the command to finish running. You can still call finish() to check for errors.


The 3 run functions above take 2 different types of parameters:

(_ executable: String, _ args: Any ...)

If the path to the executable is without any /, SwiftShell will try to find the full path using the which shell command.

The array of arguments can contain any type, since everything is convertible to strings in Swift. If it contains any arrays it will be flattened so only the elements will be used, not the arrays themselves.

try runAndPrint("echo", "We are", 4, "arguments")
// echo "We are" 4 arguments

let array = ["But", "we", "are"]
try runAndPrint("echo", array, array.count + 2, "arguments")
// echo But we are 5 arguments

(bash bashcommand: String)

These are the commands you normally use in the Terminal. You can use pipes and redirection and all that good stuff. Support for other shell interpreters can easily be added.


If the command provided to runAsync could not be launched for any reason the program will print the error to standard error and exit, as is usual in scripts (it is quite possible SwiftShell should be less usual here).

The runAsync("cmd").finish() method on the other hand throws an error if the exit code of the command is anything but 0:

let command = runAsync("cmd", "-n", 245)
// do something with command.stderror or command.stdout
do {
    try command.finish()
} catch ShellError.ReturnedErrorCode(let error) {
    // use error.command or error.errorcode

The runAndPrint command can also throw this error, in addition to this one if the command could not be launched:

} catch ShellError.InAccessibleExecutable(let path) {
    // ‘path’ is the full path to the executable

Instead of dealing with the values from these errors you can just print them:

} catch {

... or if they are sufficiently serious you can print them to standard error and exit:

} catch {


When launched from the top level you don't need to catch any errors, but you still have to use try.


main.stdout is for normal output and main.stderror for errors. You can also write to a file:

main.stdout.print("everything is fine")
main.stderror.print("no wait, something went wrong ...")

let file = try open(forWriting: path)

.write doesn't add a newline, and you can change the text encoding with .encoding.


Use main.stdin to read from standard input, or you can read from a file:

let input: String? = main.stdin.readSome() // read what is available, don't wait for end of file 

let file = try open(path)
let contents: String = file.read() // read everything

Using .readSome() you can read piecewise instead of waiting for the input to be finished and then reading everything at once. You can change the text encoding with .encoding.


So what else can main do? It is the only global value in SwiftShell and contains all the contextual information about the outside world:

var encoding: UInt
lazy var env: [String : String]

lazy var stdin: ReadableStream
lazy var stdout: WriteableStream
lazy var stderror: WriteableStream

var currentdirectory: String
lazy var tempdirectory: String

lazy var arguments: [String]
lazy var name: String

Everything is mutable, so you can set e.g. the text encoding or reroute standard error to a file.


One of the goals of SwiftShell is to be able to run single .swift files directly, like you do with bash and Python files. This is possible now, but every time you upgrade Xcode or Swift you have to recompile all the third party frameworks your Swift script files use (including the SwiftShell framework). This will continue to be a problem until Swift achieves ABI stability in (hopefully) version 4. For now it is more practical to precompile the script into a self-contained executable.

Pre-compiled executable

If you put Misc/swiftshell-init somewhere in your $PATH you can create a new project with swiftshell-init <name>. This creates a new folder, initialises a Swift Package Manager executable folder structure, downloads the latest version of SwiftShell, creates an Xcode project and opens it. After running swift build you can find the compiled executable at .build/debug/<name>.

Shell script

  • In the Terminal, go to where you want to download SwiftShell.
  • Run

    git clone https://github.com/kareman/SwiftShell.git
    cd SwiftShell
  • Copy/link Misc/swiftshell to your bin folder or anywhere in your PATH.

  • To install the framework itself, run xcodebuild and copy the resulting framework from the build folder to your library folder of choice. If that is not "~/Library/Frameworks" or "/Library/Frameworks" then make sure the folder is listed in $SWIFTSHELL_FRAMEWORK_PATH.

Then include this in the beginning of each script:

#!/usr/bin/env swiftshell

import SwiftShell

Swift Package Manager

Add .Package(url: "https://github.com/kareman/SwiftShell", "3.0.0-beta") to your Package.swift:

import PackageDescription

let package = Package(
    name: "somecommandlineapp",
    dependencies: [
        .Package(url: "https://github.com/kareman/SwiftShell.git", "3.0.0-beta")

and run swift build.


Add github "kareman/SwiftShell" "master" to your Cartfile, then run carthage update and add the resulting framework to the "Embedded Binaries" section of the application. See Carthage's README for further instructions.


Released under the MIT License (MIT), http://opensource.org/licenses/MIT

Some files are covered by other licences, see included works.

Kåre Morstøl, NotTooBad Software