Shebang Explained: What #!/usr/bin/env Does

The #!/usr/bin/env swift line at the top of a script file is called a shebang (or hashbang). It tells your system which interpreter should execute the code.

Anatomy of the Shebang

#!/usr/bin/env swift

Part	Purpose
#!	Signal to the kernel: “This is a script”
/usr/bin/env	Utility that searches PATH for the program
swift	The interpreter to use (swift, python, bash, zsh)

Why Use /usr/bin/env?

Method	Example	Behavior
Direct Path	#!/usr/bin/swift	Breaks if Swift is installed elsewhere
Env Path	#!/usr/bin/env swift	Finds Swift anywhere in PATH

Using env makes scripts portable. If someone installs Swift via Homebrew (/opt/homebrew/bin) or a version manager, the script still works.

Examples by Language

Bash Script

#!/usr/bin/env bash

echo "Running with Bash"
echo "Current shell: $BASH_VERSION"

Zsh Script

#!/usr/bin/env zsh

echo "Running with Zsh"
# Zsh arrays start at index 1
my_list=("Apple" "Banana")
echo "First item: $my_list[1]"

Python Script

#!/usr/bin/env python3

import sys
print(f"Python version: {sys.version}")
print("Hello from Python!")

Swift Script

#!/usr/bin/env swift

import Foundation

// Access command line arguments
let args = CommandLine.arguments

if args.count > 1 {
    print("Hello, \(args[1])!")
} else {
    print("Hello, World!")
}

// Use Foundation APIs
let currentDir = FileManager.default.currentDirectoryPath
print("Running in: \(currentDir)")

Running Scripts

Make the script executable, then run it directly.

chmod +x myscript.swift
./myscript.swift "iOS Developer"

Output:

> Hello, iOS Developer!
> Running in: /Users/mehmet/Dev

How It Works

When you execute ./myscript.swift:

1. The kernel reads the first two bytes (#!)
2. It identifies the file as a script, not a binary
3. It reads the interpreter path from the shebang line
4. It launches the interpreter and passes the script as input

The command ./myscript.swift becomes swift myscript.swift automatically.

Quick Reference

Language	Shebang
Bash	#!/usr/bin/env bash
Zsh	#!/usr/bin/env zsh
Python 3	#!/usr/bin/env python3
Node.js	#!/usr/bin/env node
Swift	#!/usr/bin/env swift

For Swift scripts with external dependencies, consider using swift-sh which allows importing Swift packages directly in script files.