01 · Setup & First Program

Install the Swift toolchain

Swift ships built-in on macOS via Xcode (or the smaller Xcode Command Line Tools), and is available as a standalone toolchain on Linux and Windows.

# macOS -- install the command line tools (includes swiftc, swift)
xcode-select --install

# Or install full Xcode from the App Store for iOS/macOS app development

# Linux (Ubuntu) -- download a toolchain from swift.org, or use swiftly:
curl -s https://swift.org/install.sh | bash

# Windows: installer from https://www.swift.org/install/windows/

Verify the install:

swift --version
# Swift version 5.10 (or later)

swiftc is the compiler; swift runs the interactive REPL or executes source files directly without a separate compile step.

The swift REPL

The REPL (Read-Eval-Print Loop) is the fastest way to try out small snippets:

swift

1> print("Hello, world!")
Hello, world!
2> let x = 10
x: Int = 10
3> x * 2
$R0: Int = 20
4> :quit

Every line you type is evaluated immediately and its result printed — great for experimenting with a new API before committing it to a file.

Running a .swift file

Create hello.swift:

// hello.swift
print("Hello, world!")

Run it directly — Swift compiles in memory and executes immediately:

swift hello.swift
# Hello, world!

Notice there's no public class wrapper and no explicit main function — top-level code in a single file is the entry point. This is one of Swift's biggest ergonomic wins for scripting and learning.

Compiling to a binary

For anything you'll run more than once, compile an optimized executable:

swiftc hello.swift -o hello
# produces the "hello" binary

./hello
# Hello, world!

swiftc -O hello.swift -o hello adds optimizations, closer to what a release build does.

Anatomy of the program

Piece	Meaning
print(...)	Writes text followed by a newline to standard output.
No semicolons required	A newline ends a statement (semicolons are optional, used only to put two statements on one line).
No main function needed	Top-level statements in a single-file script run top to bottom, in order.
//	Starts a single-line comment; /* ... */ for multi-line.

Choosing an editor

Xcode (macOS only, free) gives the richest experience — autocomplete, inline diagnostics, a debugger, and Swift Playgrounds for live-executing snippets as you type. On Linux or for lightweight editing anywhere, VS Code with the official "Swift" extension (powered by SourceKit-LSP) works well. Either is fine for this course — pick one and move on.

How It Actually Works

When you run swift file.swift or hit Run in Xcode, several distinct tools chain together before a single instruction executes:

• The Swift compiler (swiftc) parses your source into an AST, then lowers it to SIL (Swift Intermediate Language) — a Swift-specific IR that's where the compiler enforces exclusivity checking and performs whole-module optimizations (generic specialization, inlining, ARC optimization passes) before handing off to LLVM IR and finally to machine code via LLVM's backend.
• The Swift runtime is a separate shared library (libswiftCore) linked into every binary. It provides metadata for types (used for reflection, String description, and dynamic casts like as?), the memory allocator hooks for reference counting, and the existential container machinery for protocol types.
• Package/toolchain resolution: swift build, Xcode, and SwiftPM all resolve which toolchain (Swift version, SDK, target triple) to use before invoking the same underlying swiftc — this is why swift --version and your Xcode's "Swift Language Version" setting must agree, or you'll see ABI-mismatch style errors.
• On Apple platforms, the compiled binary also embeds a Swift ABI stability marker; since Swift 5, the compiler emits code that talks to a fixed runtime ABI baked into the OS, which is why apps don't have to bundle the whole Swift runtime themselves on modern OS versions (it ships in the OS instead).

🔀 See this in another language

• Kotlin — Setup & First Program
• Shell/Bash — Setup & First Script
• C — Setup & First Program

Exercise

Write a file greeter.swift that prints a greeting for three different names, one per line, using print. Run it two ways: directly with swift greeter.swift, and compiled with swiftc greeter.swift -o greeter followed by ./greeter.