Swift scripts (swift shebang), command-line tools

Why Swift scripts?

Shebang basics

// First line (shebang) when saved to a file:
// #!/usr/bin/env swift
// Then mark file executable: chmod +x tool.swift
// Run directly: ./tool.swift

print("Hello from a Swift script!")

Arguments parsing

// Parse arguments: CommandLine.arguments[0] is the script path.
let args = CommandLine.arguments.dropFirst()  // ignore script name
if args.isEmpty {
    print("Usage: greet.swift <name>")
} else {
    let name = args[args.startIndex]
    print("Hello, \\(name)!")
}

Tiny options

// Very small options parser: --shout and --times
struct Options {
    var shout = false
    var times = 1
    var rest: [String] = []
}

func parse(_ raw: ArraySlice<String>) -> Options {
    var o = Options()
    var it = raw.makeIterator()
    while let token = it.next() {
        switch token {
        case "--help", "-h":
            print("Usage: echo.swift [--shout] [--times N] <text>")
            exit(0)
        case "--shout":
            o.shout = true
        case "--times":
            if let nStr = it.next(), let n = Int(nStr), n > 0 { o.times = n }
        default:
            o.rest.append(token)
        }
    }
    return o
}

let options = parse(CommandLine.arguments.dropFirst())
let text = options.rest.joined(separator: " ").isEmpty ? "echo" : options.rest.joined(separator: " ")
let output = options.shout ? text.uppercased() : text
for _ in 0..<options.times { print(output) }

Exit status

import Foundation

enum CLIError: Error { case invalidInput }

func validateNumber(_ s: String) throws -> Int {
    guard let n = Int(s) else { throw CLIError.invalidInput }
    return n
}

let args2 = CommandLine.arguments.dropFirst()
do {
    guard let first = args2.first else {
        print("Usage: num.swift <int>"); exit(64) // EX_USAGE
    }
    let n = try validateNumber(first)
    print("ok:", n)
    exit(0)
} catch {
    fputs("error: \\(error)\\n", stderr)
    exit(1)
}

Compile & distribute

// You can compile a single-file tool:
// swiftc tool.swift -o tool
// ./tool --help
print("Tip: use swiftc to build a tiny binary; for multi-file CLIs prefer SwiftPM (swift package init).")

Shebang purpose

Recap