Scripting and Compiling Swift on the Command Line

Apple’s Swift programming language is most widely used for iOS and macOS development in Xcode, however, it can also be used in a manner more like a traditional Unix shell script or even compiled into an executable using the command line. Below, I give some examples of these alternative uses.

For scripting, like other shell scripts, the first line of your script’s .swift file should be a “shebang”, which points to the swift executable. Following that, you can write Swift code as usual:

#!/usr/bin/swift

func fibonacci(_ n: Int) -> Int {
    if n <= 2 {
        return 1
    } else {
        return fibonacci(n - 1) + fibonacci(n - 2)
    }
}

print(fibonacci(10))

You can make the script executable and run it as you would any other shell script:

% chmod +x fibonacci.swift
% ./fibonacci.swift
55

Printing the 10th integer in the Fibonacci sequence is hard coded above, but what if you want to print other values? We can modify the script to take a single command line argument, the index of the Fibonacci number you want to print. Replace the last line above with the following:

let arg = CommandLine.arguments[1]  // Store first argument.
if let n = Int(arg) {               // If it converts to Int,
    print(fibonacci(n))             // print fibonacci(n).
} else {                            // Otherwise,
    print("Usage: fibonacci <n>")   // print usage and exit.
}

Then invoke the script as follows:

% ./fibonacci.swift 36
14930352
% ./fibonacci.swift junk
Usage: fibonacci <n>

As with modern scripting languages such as Python and Ruby, you can also run Swift interactively (with Xcode 6.1 and later). The interactive command-line interface is called the Swift Read Eval Print Loop (REPL):

% swift
Welcome to Apple Swift version 3.1 (swiftlang-802.0.48 clang-802.0.48). Type :help for assistance.
  1> 1 + 2
$R0: Int = 3
  2> print("hello, world")
hello, world

Finally, you can also compile Swift code and run it as a binary:

% swiftc -o fibonacci fibonacci.swift
% ./fibonacci 7
13

As with C and other languages, you can compile multiple Swift files:

% swiftc first.swift second.swift third.swift -o program

Note: These instructions are valid on macOS Sierra with Xcode 8.3 and Swift 3.1, but as the Swift language evolves they may change.