A playground to understand how to find the current username on macOS.

Usernames.swift

// Some Swift code for different ways to find the current username on macOS.
//
// You can run this in a Playground, or compile it with `swiftc`. Compiling
// it will let you experiment with different ways of running the command.

print("********** Foundation Usernames **********")
// If you are only importing Foundation you have a few options to get the current user.
// Note that if run from the CLI they will report the CLI user running them.
import Foundation

// NSUserName() is the simplest by far.
let nsUsername = NSUserName()
print("Current user name: \(nsUsername)")

// ProcessInfo() is a close second.
let piUser = ProcessInfo.processInfo.userName
print("Current user name: \(piUser)")

// If you really want to you can also read it from the environment variables of the current process.
if let foundationUser = ProcessInfo().environment["USER"] {
    print("Current user name: \(foundationUser)")
}

print("\n********** Cocoa Usernames **********")
// If you have Cocoa, or CoreGraphics, imported you can be pretty precise by finding who the user
// of the current CoreGraphics session is.
//
// Since Cocoa is all about the GUI session, this will report the name of the logged in user.
import Cocoa

if let cgSession = CGSessionCopyCurrentDictionary() as? [String: Any],
    let cgUsername = cgSession["kCGSSessionUserNameKey"] {
        print("Current user name: \(cgUsername)")
} else {
    print("Could not get current CGSession.")
}

print("\n********** SystemConfiguration Usernames **********")
// If you want to replicate the approach that many scripters take with `scutil` you can import
// SystemConfiguration and then get the current console user from that.
//
// Like Cocoa, this will report the user who has a logged in GUI session.
import SystemConfiguration

if let csUsername = SCDynamicStoreCopyConsoleUser(nil, nil, nil) as String? {
    print("Current user user: \(csUsername)")
} else {
    print("Could not get current console user.")
}

print("\n********** POSIX Usernames **********")
// If you really wanted to, you can always import Darwin and use the POSIX tools.
//
// Like the foundation methods this will report the owner of process.
import Darwin

if let pw = getpwuid(getuid()), let name = pw.pointee.pw_name {
    let pwUsername = String(cString: name)
    print("Current user name: \(pwUsername)")
}

Example output from Playground or Terminal as current user:

********** Foundation Usernames **********
Current user name: joshwisenbaker
Current user name: joshwisenbaker
Current user name: joshwisenbaker

********** Cocoa Usernames **********
Current user name: joshwisenbaker

********** SystemConfiguration Usernames **********
Current user user: joshwisenbaker

********** POSIX Usernames **********
Current user name: joshwisenbaker

Example output when running as a different user with sudo:

sudo ./usernames
Password:
********** Foundation Usernames **********
Current user name: root
Current user name: root
Current user name: root

********** Cocoa Usernames **********
Current user name: joshwisenbaker

********** SystemConfiguration Usernames **********
Current user user: joshwisenbaker

********** POSIX Usernames **********
Current user name: root